TESTIA-UK-TRA-PT-EN Non-Destructive Testing PDF

Number: TESTIA-UK-TRA-PT-003-EN Issue: 3.0 Date: 17/07/2022 Approvals: Compiled By: Approved By:...

Number: TESTIA-UK-TRA-PT-003-EN Issue: 3.0 Date: 17/07/2022 Approvals: Compiled By: Approved By: Authorisation: Rob Bilney Keith Phillips Henry Corr Amendment History: Issue No Reason Date 1.0 Initial release 29/01/2014 2.0 Modify template 6/7/2016 3.0 Transfer to new template 17/7/2022 Template Ref.: TESTIA-UK-GEN-006-F01-EN issue 2 UNCONTROLLED WHEN PRINTED Page 1 of 2 © Testia Ltd. – all rights reserved  Number: TESTIA-UK-TRA-PT-003-EN Issue: 3.0 Date: 17/07/2022 Chapter no. Title: Chapter 1 INTRODUCTION AND HISTORY Chapter 2 BASIC PENETRANT PROCEDURE – MATERIALS THAT CAN BE PROCESSED – CATEGORY OF PENETRANT MATERIALS – TYPES OF PENETRANTS – PENETRANT CHARACTERISTICS. Chapter 3 SURFACE PREPARATION – DIFFERENT TYPES OF CLEANERS. Chapter 4 PENETRANT APPLICATION – POST EMULSIFIABLE, WATER WASHABLE, COLOUR CONTRAST & THIXOTROPIC – POST CLEANING – PENETRANT LAYOUT – PORTABLE KITS- CHOICE OF PENETRANT. Chapter 5 PROPERTIES OF A PENETRANT – PENETRANT APPLICATION – SAFETY – ADVANTAGES & DIS- ADVANTAGES OF THE PENETRANT SYSTEM. Chapter 6 PENETRANT REMOVAL- TYPES OF EMULSIFIERS – SOLVENT REMOVERS. Chapter 7 DRYING OF PARTS – DEVELOPER APPLICATION – TYPES OF DEVELOPERS - ADVANTAGES & DIS-ADVANTAGES OF THE DIFFERENT DEVELOPERS. Chapter 8 WHY FLUORESCENT PENETRANT FLUORESCE – ELECTRO-MAGNETIC SPECTRUM – UV LIGHT & WHITE LIGHT – OPTICAL AIDS- PENETRANT DEGRADATION – TYPES OF INDICATIONS – INDICATION VERIFICATION – HOW TO MEASURE AN INDICATION. Chapter 9 QUALITY CONTROL - SYSTEMS CHECK – SYSTEMS CHECK FAILURE – OTHER CHECKS – LIGHT INTENSITIES – TYPICAL PENETRANT INDICATIONS. Chapter 10 WORK INSTRUCTION WRITING AND REPORTING. Template Ref.: TESTIA-UK-GEN-006-F01-EN issue 2 UNCONTROLLED WHEN PRINTED Page 2 of 2 © Testia Ltd. – all rights reserved  Chapter 1 Page 1 of 78 CHAPTER 1 1. What is non-destructive testing. (NDT)? Non-destructive testing (or NDT as I shall be referring to it from now on) is a method of examining a part either externally or internally for defects that may make the part “unfit for purpose” without actually harming the part(s). 2. History From the early days of technology it is reasonable to assume that some form of visual inspection was carried out. Whether it was the use of soot being sprinkled onto glazed china to find defects in the smooth surface to the inspection of steam engine axles using the oil & whiting method. We all want our products to be fit for purpose. The oil & whiting method is considered to be the forerunner of the present day penetrant system. The method itself used “dirty” machine oil diluted with kerosene as the penetrant & a white chalk suspended in alcohol as the developer. Basically the parts were submerged in the oil for a certain time. The excess oil was wiped off using rags. The part was then “painted” with a solution of chalk in alcohol (Whiting). The part was then struck with a hammer. This would cause the oil to seep out onto the surface & stain the whiting. As the industrial revolution took off so did the requirement for bigger & faster machines and the disasters were also bigger and lives were being lost. Dee-railing bridge collapse Boiler explosion 1850 1879 Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 3 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 1 Page 2 of 78 It became evident that a more sensitive form of inspection was needed; eventually dyes were added to the liquid and by the 1940’s fluorescent or visible penetrant were available. Significant advances have taken place in technology and science since the 1940’s, in particular the development of the jet engine and engineering structures etc. New and existing lighter alloys are now being used more & more in engineering and industry required even more reliable means of testing these materials for shallow cracks initiating fatigue type cracks. The answer came in the form of a separate emulsifier and penetrant. This had two immediate advantages: First it was possible to formulate a penetrant which actually resisted water and second it was possible to dissolve more colour or fluorescent particles in such a penetrant. The system using a non-water washable penetrant followed by the application of an emulsifier became known as the “post emulsifiable penetrant process”. It was immediately successful in answering the special requirement of finding small shallow defects. Although great advancements have been made in the world of NDT, disasters still happen. Common methods of NDT that are in use to-day:- Penetrant Testing. (PT) Magnetic Particle Testing. (MT) Radiographic Testing. (RT) Eddy Current Testing. (ET) Ultrasonic Testing. (UT) Visual Testing. (VT) Thermography Testing. (TT) Acoustic Emission. (AE) 3. Magnetic Particle Testing. (MT) A magnetic field is established in a component made from ferromagnetic material. The magnetic lines of force travel through the material, and exit and re-enter the material at the poles. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 3 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 1 Page 3 of 78 Component Defect Magnetic ___Field Current Defects such as cracks or voids cannot support as much flux, and force some of the flux outside of the part. Magnetic particles distributed over the component will be attracted to areas of flux leakage and produce a visible indication. a. Advantages Rapid and simple, adaptable to low speed automation. Relatively inexpensive to operate. No elaborate surface pre-cleaning necessary. Sensitive for fine surface cracks. Inspects components with thin non-magnetic coatings. b. Disadvantages Requires good accessibility and surface contact, limited to surface and immediate sub-surface defects in Ferro-magnetic materials. Limited effectiveness for complex parts. Surface must be clean and dry. Post cleaning and de-magnetisation is required. Reveals only defects transverse to magnetic field. 4. Eddy Current. (ET) Alternating electrical current is passed through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces current flow in the material. These currents travel in closed loops and are called eddy currents. Eddy currents produce their own magnetic field that can be measured and used to find flaws and characterize conductivity, permeability, and dimensional features Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 3 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 1 Page 4 of 78 a. Advantages Inexpensive, immediate test results, testing contact not required. Capable of providing quantities information. Can be adapted for portable of for high speed automation. Capable of measuring many physical properties. Surface contaminants not normally a problem. Good sensitivity for surface cracks. b. Disadvantages Limited to surface and sub-surface defects of electrical-conductive materials. Results influenced by a large number of variables including testing frequency, testing distance and material permeability and conductivity. Tests are hindered by complex geometric shapes. 5. Ultrasonic Inspection (UT) Ultrasonic inspection (UT) uses high frequency transducer to produce sound waves in a component and monitors how they travel in the material. A transducer converts electrical energy into mechanical energy to send signals into the test piece, working in reverse for returned signals. A contact transducer may be used with a coupling gel on the surface of the part, or immersion transducers if the sample is in a water bath. During an ultrasonic test, sound waves travel through the material and are reflected back from surfaces or flaws. Reflected sound energy is displayed versus time, and the inspector can visualize a cross div of the specimen revealing the depth of various features a. Advantages Inexpensive, immediate test results, testing contact not required. Capable of providing quantities information. Can be adapted for portable of for high speed automation. Capable of measuring many physical properties. Surface contaminants not normally a problem. Good sensitivity for surface cracks. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 3 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 1 Page 5 of 78 b. Disadvantages Limited to surface and sub-surface defects of electrical-conductive materials. Results influenced by a large number of variables including testing frequency, testing distance and material permeability and conductivity. Tests are hindered by complex geometric shapes. 6. Radiographic Testing. (RT) X-rays and gamma radiation have wavelengths shorter than 100 nanometres (nm). Energy, at these wavelengths, will penetrate solid material. The shorter the wavelength, the greater the penetration. Like visible light, X-rays and Gamma radiation also have a photochemical effect on silver halide and can therefore produce an image on film. Thus, by passing penetrating radiation through an object, and recording the emerging radiation on a film, a two dimensional picture of the differences in thickness or density of the object can be obtained. Hence, flaws in the object can be detected. a. Advantages. Provides a permanent record and can be used for internal evaluation of most materials. Surface conditions poses no problems. Excellent capability for detecting voids, porosity and inclusions. No contact required and generally accepted standards. b. Disadvantages High equipment and testing costs, sensitivity limited to 1% to 2% of material thickness. Flaw orientation critical. Test thickness limited and flaw depth not readily apparent. Radiation hazard and access to opposite sides of object required. Complex geometrical shapes can cause problems. 7. Visual Testing. (VT) Visual inspection is one of the simplest forms of NDT. But it can also be one of the most useful All the indications you will see have to be assessed using visual inspection. I.e. The inspector’s eye sight has the final decision. Whether it is on the actual part itself or displayed on a screen. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 3 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 1 Page 6 of 78 More sophisticated visual aids are readily available to the actual NDT technician that were at one time only available in the laboratories. These include such items as, Borescopes, Video equipment, CCTV monitors. Lap top adaptor. a. Advantages A rapid method that does not involve any sophisticated techniques. Can be used in most environments b. Disadvantages Relies on the ability of the inspectors to interpret what they see. Surface condition & accessibility are very important. It is purely a surface inspection and cannot be expected to identify internal defect 8. Thermography Thermal NDT methods involve the measurement or mapping of surface temperatures as heat flows to, from and/or through an object. The simplest thermal measurements involve making point measurements with a thermocouple. This type of measurement might be useful in locating hot spots, such as a bearing that is wearing out and starting to heat up due to an increase in friction. In its more advanced form, the use of thermal imaging systems allow thermal information to be very rapidly collected over a wide area and in a non-contact mode. Thermal imaging systems are instruments that create pictures of heat flow rather than of light. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 3 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 1 Page 7 of 78 a. Advantages The thermal imaging device looks at the equipment while it is in operation. The equipment does not have to be halted for the duration of the inspection. Because the inspection can cover a large area(s) at once, the cost of inspection is reduced. b. Disadvantages Specialist equipment is used to survey the parts. Heat must be either radiating from the part or induced into the part so that the heat distribution can be mapped. 9. Acoustic Emission Unlike conventional ultrasonic testing, AE tools are designed for monitoring acoustic emissions produced within the material during failure or stress, rather than actively transmitting waves, then collecting them after they have travelled through the material. a. Advantages Parts can be monitored whilst the aircraft is flying. Defects can be monitored & the corrective action taken before the part(s) fail. b. Disadvantages Part must be under stress when the inspection is carried out 10. Training and Certification in the UK To ensure that personnel are trained correctly certain specification/documentation are published to provide guide lines as to the Training & Certification of NDT personnel. The approval body in Europe is the, European Airworthy Safety Agency (EASA). The body for the United Kingdom is the Civil Aviation Authority (CAA). The CAA are answerable to EASA. The specification approved for the training & certification of NDT personnel by EASA is EN 4179. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 3 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 1 Page 8 of 78 Training & EASA Certification of NDT personnel. CAA CAP 747 GR23 There are 4 levels of NDT Inspector. These are:- Level 1. Level 1 Ltd. Level 2. Level 3. Trainees and auditors are also noted but the grades are not listed in the EN4179 specification. There are three aims when performing penetrant testing; Revealing the nature of the discontinuity without degrading the material. Obtaining a visual representation of a discontinuity on the surface of the test item. Acquiring a method of separating acceptable and unacceptable material in accordance with pre-determined standards. An evaluation of the results obtained is necessary before any test is complete. This is dependent on the inspector having a firm understanding of the objective of the test, as well as knowledge relating to the material from which the component is made, its manufacturing process, discontinuities which are inherent to that manufacturing process, assembly, geometry and the ability to interpret and implement the relevant procedures and acceptance criteria. The purpose of these notes is to help you to understand the principles of the penetrant process and its limitations and to:- a. Ascertain that the proper test technique or combination of techniques is used to assure the quality of the products. b. Interpret, evaluate and make a sound decision with regards to the test results. c. Recognise those areas exhibiting doubtful test results that require either re-test or assistance in their interpretation & evaluation. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 3 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 2 Page 9 of 78 CHAPTER 2 1. The Basic Penetrant Process Clean & dry penetrant Apply penetrant Remove excess Dry Apply developer Inspect Ensure the part(s) are dry & clean. The defect(s) must be “open to the surface” otherwise the penetrant will not be able to enter the defect(s). Apply the penetrant for the stipulated time. (10 – 60 minutes). Ensure all surfaces are covered as required. Remove excess penetrant. This operation is critical. A “slight” fluorescence is a good indication that just the right amount of excess penetrant has been removed. If there is no background, too much penetrant has been removed & the part(s) will have to be reprocessed from the beginning. Dry the part(s). This takes place in an air circulating oven. The maximum temperature permitted is 70°C. The time will be governed by the size, material & complexity of the part(s). Ensure the part(s) are dry before applying the developer. The part(s) are placed into a “storm cabinet”. This is a cabinet which is capable of blowing the developer over all the surfaces in one operation. A thin layer of developer is best as this will not smother the indications. Inspect. When using fluorescent penetrant a Ultra-Violet (UV) light (also known as “black” light) is used to make the penetrant fluoresce. If colour contrast penetrants are used, white light is used to inspect the parts. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 2 Page 10 of 78 2. What materials can be processed using the penetrant process The following materials can be processed:- Aluminium. Magnesium. Stainless Steel. Titanium. Glass. Ceramics. Composites. Brass. Steel. Providing the material is “Non-Porous” it can be processed using the penetrant system. The materials used in the actual penetrant process can be corrosive on certain materials, therefore the parts need to be cleaned & protected against corrosion once the process has been completed. The penetrant, emulsifiers, removers and developers used in the penetrant process have to meet certain requirements to ensure they can do what they are supposed to do. The specification that controls this is AMS 2644. Once a product has been accepted it is then placed on the Qualified Product List QPL AMS 2644. Only products on this list can be used in the aerospace industry. The products used in the penetrant system will also be placed into the appropriate category for that item. The listing is as follows:- Penetrant Penetrant Sensitivity Type 1 - Fluorescent dye Level ½ - Ultra low Type 2 - Visible dye Level 1 - Low Method A - Water washable Level 2 - Medium Method B - Post Emulsifiable Lipophilic Level 3 - High Method C - Solvent Removable Level 4 - Ultra High Method D - Post Emulsifiable, Hydrophilic Developer Removers Form a - Dry Developer Class 1 - Halogenated Form b - Water Soluble Class 2 - Non-Halogenated Form c - Water Suspended Class 3 - Special Form d - Non-Aqueous Solvent Based Form e - Non-Aqueous Visible Dye Form f - Special Application. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 2 Page 11 of 78 Example. Type 1D3a Type 2A3e Type 1 - Fluorescent Dye Type 2 - Visible Dye. D - Post Emulsifiable. Hydrophilic A - Water Washable. 3 - Sensitivity Level High. 3 - Sensitivity Level High. a - Dry Developer. e - Non-Aqueous Visible dye. Solvent Based 3. Penetrant Classification 3.1. Visible dye penetrant Visible penetrant (as the name suggests) are penetrants that are visible under normal lighting conditions. Red is the most common colour, but different colours are available. 3.2. Fluorescent Penetrant Fluorescent penetrant contain a “fluorescent” dye and are only visible when viewed under Ultra- Violet light. 3.3. Dual Sensitivity Penetrant Dual sensitivity penetrant can be either viewed using a white light source, but to make the penetrant fluoresce you still have to use an ultra-violet light source. These penetrants are not as sensitive as the fluorescent penetrant & when viewed under white light there is not such a clear definition as with ordinary visible penetrants. 4. Properties of a penetrant There are a number of factors which go towards making a good penetrant. These are:- I. Surface tension, II. Contact angle. III. Capillary action. (This is broken down into Adhesion & Cohesion). IV. Viscosity. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 2 Page 12 of 78 4.1. Surface Tension This controls how the penetrant will move over the surface of the part. If the surface tension is “HIGH” the penetrant will not spread over the surface of the part, it will form patches on the surface. HIGH surface tension. Penetrant will not spread over the surface of the part. LOW surface tension. Penetrant will spread evenly over the surface of the part. LOW SURFACE TENSION GIVES GOOD WETABILTY. 4.2. Contact Angle The contact angle is an angle that a liquid creates with a solid surface of a material when both materials come in contact together. This angle is determined by both properties of the solid and the liquid and the interaction and repulsion forces between liquid and solid and by the three phase interface properties (gas, liquid and solid). Those interactions are described by cohesion and adhesion forces which are intermolecular forces. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 2 Page 13 of 78 Contact angle less than 90° Contact angle greater than 90° LOW CONTACT ANGLE – GOOD WETABILITY – GOOD PENETRANT. 4.3. Capillary Action. Is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces, such as gravity. Capillary action can be seen when a corner of a paper towel is placed into a glass of water. The water soon spreads into other parts of the towel due to the loose fibres which have small spaces between them which act as capillary tubes. Capillary action is dependent on two things. These are adhesion and cohesion. Cohesion is the mutual attractive forces that exist between like molecules of a particular liquid (Low Cohesion). Adhesion is the tendency of dissimilar particles or surfaces to cling to one another (High Adhesion). The figure below shows that the same liquid will flow further up a narrow diameter tube than a larger diameter tube. 4.4. Viscosity. Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 2 Page 14 of 78 A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion. When a liquid is heated its viscosity is lowered. I.e. it will flow much more easily. Low Viscosity - High Viscosity - Liquid will flow Liquid will not flow easier. so easy. LOW Viscosity = Good penetrant HIGH Viscosity = Poor penetrant 4.5. Specific Gravity (SG) Specific gravity is the ratio of the density (thickness) of a substance to the density (mass of the same unit volume) of a reference substance. The specific gravity of a Penetrant is compared to that of water (1 one). SG 1 New Penetrant 0.95 Old Penetrant 0.85 Temperature °C Specific Gravity of a liquid is measured using a “Hydrometer”. (left). The correct way to take a reading, is shown on the right. If the reading falls outside of the chart above. The penetrant is discarded. The reference charts are available from the manufacturer of the penetrant. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 2 Page 15 of 78 A good penetrant must have:- High adhesion Low cohesion Low contact angle Low viscosity Low surface tension High wettability 5. Penetrant Dwell Time Also known as, Penetrant contact time, Total contact time, Penetrant application time. Whichever term is used the penetrant dwell time starts from when the penetrant is applied to when it is washed off. Dwell times will vary depending on a number of factors:- I. Type of defect II. Temperature III. Surface finish IV. Type of penetrant You must also take into consideration Customer & Company requirements. Typical contact times for penetrant are between 10 to 60 minutes. Type of defect(s) - The tighter the defect the longer the penetrant dwell time. The dwell time for grinding cracks will be longer than for porosity, as grinding cracks are much tighter than porosity. Temperature - The temperature of the part or the temperature of the penetrant will influence the penetrant dwell time as temperature will alter the viscosity of the penetrant. The higher the temperature the lower the viscosity. The lower the temperature the “higher” the viscosity of the penetrant. Therefore, depending on the temperature of the part and/or the temperature that the penetrant is applied will also have an influence on the penetrant dwell time. Surface finish - The surface “roughness” will also influence the penetrant to use. It will be more difficult to remove the penetrant from a cast surface than from a machined surface. Castings are normally processed using a water washable penetrant where a machined part will be processed using a post emulsifiable penetrant. Type of penetrant. Post emulsifiable penetrants are “thinner” in their make than water washable (as water washable have a built in emulsifier).Therefore when using a post emulsifiable penetrant you can shorten the penetrant dwell time. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 2 Page 16 of 78 Penetrant can be categorised by their removal method:- I. Water washable penetrant. (WW) II. Post Emulsifiable penetrants (PE) III. Solvent removable penetrant. All of the penetrants are available as either, fluorescent or visible penetrants. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 3 Page 17 of 78 CHAPTER 3 1. SURFACE PREPARATION As stated before, penetrant inspection will only find defects that are “open” to the surface, therefore it stands to reason that cleaning is one of the most important stages of the penetrant process. Surface contaminates are considered any contaminates that “hinder” the penetrant entering a discontinuity. Common contaminates are (but not limited to):- Dirt Rust Grease Oil Water Water is the most common contaminate as it is used in the cleaning process and therefore it is imperative that the parts are dry before the application of the penetrant. The reason that the parts need to be dry is that the penetrant is lighter than the water (and will not readily mix with the water) and the penetrant will “float” over the top of the water that is trapped in a discontinuity. Another reason is that if water is not completely removed from the surface it will contaminate the penetrant which will reduce the ability of the penetrant to “find or enter” any discontinuities. 2. CLEANERS There are a number of cleaning methods approved for use with the penetrant system. These are (but not limited to):- a. Vapour degreasers b. Aqueous cleaners c. High pressure steam cleaners d. De-ruster/paint removers e. Ultrasonic cleaners f. Acid etch g. Mechanical Cleaning Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 3 Page 18 of 78 2.1.1.Vapour Degreasers There are a number of substances that are banned for use in the EU. A list is of banned substances are available on the internet via the Health & Safety Executive. (H&SE). http://www.hse.gov.uk/. Check this list before purchasing and/or using a product. Safe use of a vapour degreaser: i. Perform all the safety checks required before operating the unit. Do not use a unit that does not comply or is found faulty. ii. Ensure you are wearing the correct personal protection equipment (PPE). iii. Place the part(s) into the boil sump for the required time. iv. Remove from the boil sump & place into the rinse sump. v. Remove the part(s) from the rinse sump & place them in the vapour area for the stipulated time. vi. Place the part into the “freeboard” area, again for the stipulated time. vii. Allow the part(s) to cool to the required temperature before applying the penetrant. viii. Vapour degreasers tend to us a hydrocarbon as the cleaning solution and for this reason they are not considered “environmentally friendly”. ix. The lower tank containing dirty fluid is heated causing the fluid to evaporate. At the top of the machine there is a refrigeration coil. Fluid condenses on the coil and falls into the upper tank. The upper tank eventually overflows and clean fluid runs into the work tank where the cleaning takes place. Purchase price is higher than simpler machines, but such machines are economical in the long run. The same fluid can be reused many times, minimising wastage and pollution. 2.1.2.Aqueous Cleaners Aqueous cleaners are replacing vapour degreasers due to environmental issues. For surface cleaning in the metal processing industry alkaline media with a pH value >7 are mostly employed. They remove both organic contaminants such as grease, oil and wax as well as inorganic residues like metal chips and dirt. Strongly alkaline cleaners display a greater dissolving ability for organic contaminants than weaker alkalis. Alkaline agents are diluted to the required strength. This will vary greatly depending on the condition of the parts being cleaned. Prior to use tests must be carried out to ensure that the alkaline cleaning agent does not attack the surface of the parts they are cleaning. As these cleaners are water based extra drying is required to ensure the parts are dry before applying penetrant to the surface of the parts. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 3 Page 19 of 78 Typical aqueous cleaner 2.1.3.High pressure steam cleaner High pressure steam cleaners are used to remove very heavy contaminations from the surface of a part(s). Further cleaning may be required such as, vapour degrease. 2.1.4.De-ruster & Paint removers Both of these use some form of acidic/alkaline solutions to remove the contaminates. A number of factors need to be taken into consideration when using these solutions. Such as:- 1. Strength of the solution. 2. Temperature. (Hot or cold soak). 3. Time in the solution. 4. Material of the part being cleaned. (Steel, Aluminium etc.) Once cleaning has been completed water is used to neutralize the solution, therefore drying is required prior to the application of the penetrant. 2.1.5.Ultrasonic cleaners Although we have mentioned ultrasonic cleaner there are so many on the market, (a few of which are illustrated below) the principles are the same whatever size of tank is used. A frequency of between 20 to 400 KHz is generally used to “vibrate” the solution. This tends to “shake” the particle loose & helps greatly with the cleaning action. The size of the tank will depend Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 3 Page 20 of 78 on the items that are being processed. Ultrasonic cleaners are used as cleaners for variety of items ranging from, jewellery to large aerospace parts. 2.1.6.Acid Etch Acid etch is one of the most thorough cleaners as it actually removes material from the surface of the part. A diluted solution of acid is applied to the surface of the part(s) as material is actually removed from the surface of the part you must ensure that the part stays within the drawing tolerances. If you go outside of the permitted tolerances you may end up scrapping the part. The acid used will depend on the material being processed. Stages of acid etching:- I. Clean the surface of the part as acid will not remove oils/grease II. Apply or dip the part(s) into the acid solution for the stipulated time III. Remove from the acid & neutralize (usually with water) IV. Dry the part(s) Ensure the parts are dry before applying the penetrant. Note: Certain acids attack the grain boundaries of the metal & can cause “embrittlement”, which weakens the structure of the material and can cause pre-mature failure of that part. For this reason parts may have to go through a “de-embrittlement” stage. This involves heat treating the part(s) at a certain temperature (135°C is typical) for a specific time (one hour is normal. But anything up to 36 hours is not unknown). As you can see this adds a tremendous amount of time to the processing of parts and subsequently cost. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 3 Page 21 of 78 2.1.7.Mechanical cleaning Mechanical cleaning actually involves displacing material on the surface of the part being cleaned. Such methods are (but not limited to):- Vapour blasting. Grit blasting. Shot blasting Glass ball peening. Simpler forms of metal removal may also cause displacement, such as the use of emery cloth, files and wire brushes. The latter are commonly used when cleaning off contamination, such as rust from welds prior to being penetrant inspected. All of the above will cause metal to be displaced and may actually “peen” or “smear” over (close over) cracks etc. Prior to being vapour blasted Part being vapour blasted. After vapour blasting the defect the defect is “open” to the has been “peened” over. i.e. no surface. longer open to the surface. 3. WHAT TO DO IF A PART(S) HAVE BEEN SUBJECTED TO THIS FORM OF CLEANING? The normal cause of action to take is to apply acid etch the part(s) prior to the application of the penetrant. The procedure will remove material from the surface of the part(s) & will open the surface(s) so that the penetrant may enter. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 3 Page 22 of 78 This procedure may be applied to 100% of the part, or to local areas as shown below:- Procedure 100% Local Apply acid to the surface. This may be applied locally or by immersing the complete part in acid. (Contact time is normally 2 mins). Neutralise the acid. This is normally accomplished by immersing the part(s) in water. For local application the area may be Completely dry the part. The area must be dry prior to applying the penetrant. Parts may be placed in an oven. With certain acids it may be a requirement that the parts are de- embrittled after the parts have been inspected. Process the part(s) as normal Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 4 Page 23 of 78 Chapter 4 1. WATER WASH PENETRANTS (WW) Clean Ensure parts are clean and dry before applying the penetrant. Apply penetrant Apply penetrant for the stipulated time. Remove excess penetrant. This is normally accomplished by using an Remove excess Air/water mix or just water. Carried out under UV light when using penetrant fluorescent penetrants. Dry The part is normally placed in an air circulating oven at a pre-set temperature. Developer may be applied by a number of ways.Dust cabinets Apply developer being the most common in aerospace. Carried out under UV light for fluorescent penetrant or white light Inspect when using colour contrast (visible) penetrants. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 4 Page 24 of 78 2. POST EMULSIFIABLE PENETRANTS HYDROPHILIC (PE) Clean Ensure parts are clean & dry before applying the penetrant. Apply penetrant Apply penetrant for the stipulated time. Remove excess penetrant. This is normally accomplished by using an Remove excess Air/water mix or just water. Carried out under UV light when using penetrant fluorescent penetrants. Emulsifier is normally applied by immersion (or spray) for the Apply emulsifier stipulated time. Remove excess penetrant. This is normally accomplished by using an Remove excess Air/water mix or just water. Carried out under UV light when using penetrant fluorescent penetrants. The part is normally placed in an air circulating oven at a pre-set Dry temperature. Developer may be applied by a number of ways.Dust cabinets being Apply developer the most common in aerospace. Carried out under UV light for fluorescent penetrant or white light Inspect when using colour contrast (visible) penetrants. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 4 Page 25 of 78 3. POST EMULSIFIABLE LIPOPHILIC PENETRANTS (PE) Clean Ensure parts are clean & dry before applying the penetrant. Apply penetrant Apply penetrant for the stipulated time. Apply emulsifier Emulsifier applied by immersion for the stipulated time. Remove excess penetrant. This is normally accomplished by using an Air/water Remove excess mix or just water. Carried out under UV light when using fluorescent penetrant penetrants. Dry The part is normally placed in an air circulating oven at a pre-set temperature Developer may be applied by a number of ways.Dust cabinets being the most Apply developer common in aerospace. Carried out under UV light for fluorescent penetrant or white light when using Inspect colour contrast (visible) penetrants. Note: With lipophilic penetrant there is no wash operation before applying the emulsifier. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 4 Page 26 of 78 4. VISIBLE SOLVENT PENETRANTS (LOCAL APPLICATION) Clean Area cleaned by spraying with a solvent. Apply penetrant Spray or Brush. Remove excess Using a lint free tissue, remove excess penetrant. Then moisten tissue penetrant with solvent & remove remaining penetrant. Dry Allow solvent to dry. Apply developer Spray a light coating of non-aqueous developer. Inspect Inspect under white light. 5. THIXOTROPIC PENETRANTS MATERIALS Are not in general use compared to other penetrant materials. These penetrant materials were invented by the Norman Henry HYAM (1918-2000) who filed a patent in 1973, and have been placed on the market this same year. A common everyday example of a Thixotropic solution is “non-drip paint”. As with non-drip paint as you move the brush along the surface, only the paint that is in immediate contact with the surface becomes a liquid, the rest remains in a semi-solid state. This is the same phenomena with penetrants, which makes thixotropic penetrants ideal when beneath large structures. The excess penetrant is removed by wiping with clean, lint-free tissue paper or rag either dampened with water or, if using a post-emulsified thixotropic penetrant, soaked with a thixotropic emulsifier. The developers are the same as those used for non-thixotropic penetrants. Inspect under UV or white light depending on the type of penetrant used..i.e. Type 1 or 2. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 4 Page 27 of 78 6. POST CLEANING The residues of penetrant testing can adversely affect subsequent processing and service of a component. Penetrant and developer residues may interfere with the application of surface finishes such as paint, anodizing, plating etc. For components that will make contact with oxygen or liquid oxygen, (LOX) special attention is required as, if oil contacts oxygen there will be an explosion. Residues may also affect weld quality. Developer residues can retain and absorb moisture resulting in corrosion; it can also disrupt the function of moving or wear surfaces. Residues can be removed with an appropriate method already mentioned under pre-cleaning methods. 7. BASIC PENETRANT LINE Post emulsifiable system only Pre-Cleaner (1) Penetrant (2) Drain Wash (3) Emulsifier (4) Wash (5) Inspection Developer Post cleaner (9) Dryer (6) booth (8) application (7) (1) Pre-cleaner This may be separate to the rest of the system or incorporated within the same system. (2) Penetrant application station & drain Normal method of application is by dipping the part(s) into the penetrant & then draining the part(s) so that the excess penetrant is returned to the penetrant application tank. (3) Wash Station (Pre-Wash) Excess penetrant is removed by either an air/water mix or just water. This operation is carried out under UV light when using fluorescent penetrants. (4) Emulsifier application Dip application. Only used with post emulsifiable penetrants. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 4 Page 28 of 78 (5) Wash station (Post Wash) Excess penetrant is removed by either an air/water mix or just water. This operation is carried out under UV light when using fluorescent penetrants. Only required when using post emulsifiable penetrant. (6) Drier (Oven) An air circulating oven. (7) Developer application Also known as a “storm cabinet”. This is an enclosed cabinet. Air is blown into the cabinet which ensures the developer cover s all surfaces. This will normally have a lock mechanism in place so that the lid cannot be opened until all the developer has settled or been extracted as with some systems. (8) Inspection Booth When using fluorescent penetrants. This will be a darkened booth with UV light used for inspection of the part(s). Will also have a white light which is used to verify an indication. When using visible penetrants. White light is used for inspection. The inspection booth needs to be well ventilated and comfortable for the inspector. (9) Post cleaner This may be separate to the rest of the system or incorporated within the same system. Typical commercially available penetrant line. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 4 Page 29 of 78 8. PORTABLE FLUORESCENT KITS Cleaner (Pre & Post Cleaning) Penetrant ( Water Washable or Solvent Removable) Developer (Non-Aqueous) Portable UV light White Light Misc. Equipment:- Wipes – Brush – Marker Pen – UV / White light meters 9. PORTABLE COLOUR CONTRAST (VISIBLE) KITS Cleaner (Pre & Post Cleaning) Penetrant ( Water Washable or Solvent Removable) Developer (Non-Aqueous) Portable White Light Misc. Equipment:- Wipes – Brush – Marker Pen – UV / White light meters 10. WHICH PENETRANT TO USE? The penetrant to use will depend on a number of factors, such as:- I. Type of defect sought. Grinding cracks, porosity, shrinkage. II. Size & configuration of the part(s). III. Surface finish. Cast or machined. IV. Number of items to be tested. V. Where is the process/inspection to take place? Shop floor or on-site. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 4 Page 30 of 78 VI. Amenities. Is air or water required, if so is it readily available? 11. WATER-WASHABLE PENETRANTS Water Washable penetrants work best on rough surface parts, threaded or grooved parts and those with holes and orifices, which may be difficult to remove with the post emulsification method. It is especially suitable for automation as no emulsification stage is required. It is less expensive than the post emulsifiable system, the process time is quicker, as there are fewer processing steps and timing variables are reduced. 12. POST-EMULSIFIABLE PENETRANTS When removal of the penetrant from the defect due to over-washing of the part is a concern, a post-emulsifiable penetrant system can be used. These parts are usually smoothed surfaced and perform critical functions that require higher sensitivity to smaller defects. Post-emulsifiable penetrants require a separate emulsifier to break the penetrant down and make it water washable. It is more expensive than the water washable system, the process time is longer as there are more stages than the water wash system. Also known as a “two step” system due to the extra stages (Emulsification & Pre-wash). Post emulsifiable penetrants tend to be “thinner” as they do not have an emulsifier built in. Also you are governed by what the penetrant procedure states. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 5 Page 31 of 78 CHAPTER 5 1. PROPERTIES OF A PENETRANT Hold dye materials in suspension. You do not want the dye to separate from the carrier when the penetrant is in the tanks. Spread evenly over all the surfaces. So that the penetrant will not just stay in the one place, it will seep into all the nook & crannies. Enter small discontinuities by capillary action without the dye separating from the carrier. Not easily over washed. Draw back out of the discontinuities when the developer is applied. Easily be removed from all surfaces once the process has been completed. Not be harmful to the parts under test. As you can see there are a number of things we require from our penetrant, not to mention that we also want it to fluoresce under UV light or be easily seen when using colour contrast penetrants, are numerous & quite demanding. 2. PENETRANT APPLICATION Penetrant may be applied by either:- Immersion Brush Spray 1.1. Immersion This is probably the most common way that penetrant is actually applied to parts and is also the most wasteful, as more penetrant is applied than is actually required. This also means extra washing, which in itself is also time consuming. 1.2. Brush This is normally applicable to local inspection only as it would be too time consuming the brush penetrant onto large or complicated shaped parts. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 5 Page 32 of 78 1.3. Spray There are two types of spray used today: 1. Low pressure spray 2. Electro-static spray Electro-static spray was originally used by the paint industry. It was found that by using an electro-static charge, that the coating thickness of the paint can be controlled. The advantages of electro-static spray for the application of penetrants was soon realised, especially when processing large castings, as large tanks are required which also require large quantities of penetrant to fill them. The tanks are expensive to fill & require topping up on a regular basis due to, the large amount of “drag out”. Although expensive to initially install the cost is soon recuperated by the amount of penetrant saved & also the amount of time saved when processing parts. 3. THE PRINCIPLES OF ELECTRO-STATIC SPRAY. What is electro-static energy? There are many examples of electrostatic phenomena, from those as simple as the attraction of the plastic wrap to your hand after you remove it from a package, and the attraction of paper to a charged scale. When you were a child you may have (or seen) children rubbing a balloon against their hair and as they move the balloon away the hair follows the balloon. This is caused by electro-static energy. Electrostatics involves the build-up of charge on the surface of objects due to contact with other surfaces. Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. This is because the charges that transfer to or from the highly resistive surface are more or less trapped there for a long enough time for their effects to be observed. These charges then remain on the object until they either bleed off to ground or are quickly neutralized by a discharge: e.g., the familiar phenomenon of a static 'shock' is caused by the neutralization of charge built up in the body from contact with insulated surfaces Electro-static spray relies upon those principles. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 5 Page 33 of 78 Paper strips attracted by a charged CD. Lightning. Examples of static electricity. Like poles repel. Unlike poles attract. An electro-static charge (40Kv – 100Kv) is applied to the penetrant particles via the tip of the spray gun. The part(s) being processed are ground so that as the penetrant charged particle come into close contact with the parts being processed they are attracted to the part(s). This method uses less penetrant & there is also less waste due to what is known as the “wrap around effect”. Feed line. This is also a very good method of applying penetrant to parts that have small openings, as it is difficult to remove the excess penetrant from these areas. Penetrant that is held in this type of opening will “bleed out” and may cover over small cracks propagating from the small openings. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 5 Page 34 of 78 Typical “air cooling” holes that will trap penetrant & make it very difficult to remove. 4. SAFETY As with all materials used to-day it is important that they are used correctly & also that they are disposed of as required by the Local Council Environmental requirements. All the materials used in the NDT industry will have Health & Safety (H&S) labels adhered to them. You must ensure that you understand what these labels mean. If in doubt speak to your H&S department representative or the Local Council Environmental office as these will have the information you require. 5. ADVANTAGES & DIS-ADVANTAGES OF THE PENETRANT SYSTEM. Advantages:- The method has high sensitivity to small surface discontinuities. The method has few material limitations, i.e. metallic and non-metallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected. Large areas and large volumes of parts/materials can be inspected rapidly and at low cost. Parts with complex geometric shapes are routinely inspected. Indications are produced directly on the surface of the part and constitute a visual representation of the flaw. Aerosol spray cans make penetrant materials very portable. Penetrant materials and associated equipment are relatively inexpensive. Disadvantages:- Only surface breaking defects can be detected. Only materials with a relatively nonporous surface can be inspected. Pre-cleaning is critical since contaminants can mask defects. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 5 Page 35 of 78 Metal smearing from machining, grinding, and grit or vapour blasting must be removed prior to LPI. The inspector must have direct access to the surface being inspected. Surface finish and roughness can affect inspection sensitivity. Multiple process operations must be performed and controlled. Post cleaning of acceptable parts or materials is required. Chemical handling and proper disposal is required. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 6 Page 36 of 78 CHAPTER 6. 1. PENETRANT REMOVAL The method for removing penetrant will depend upon the penetrant used. There are three types of penetrant in common use to-day & these are:- a. Water washable penetrant (WW) use either a water/air mixture or just water. b. Post emulsifiable penetrant (PE) require the application of an emulsifier. c. Solvent removable penetrant only removable with a solvent. When using WW or PE the following will apply: Water wash: Water washing or spray rinsing is normally carried out in a stationary rinse tank which has a hose, nozzle, drain and a UV-A light in the case of fluorescent penetrant. The rinsing procedures used for the removal of water washable penetrant, Method A, and post-emulsified penetrant, Method B (after emulsification) are the same. Controlling of the rinse & emulsification times are the only differences. The rinse times for Method A penetrants are critical as the entrapped water washable penetrants can be removed from within discontinuities, if the time is not controlled. Entrapped post emulsified penetrants that have not been diffused with emulsifier resist removal and rinse times are not as critical. The conditions and procedures described below are applicable to both water-washable and post-emulsifiable penetrants. Water droplet size Excess surface penetrant removal depends on the mechanical force of the water impacting the component surface. The impact force consists of the droplet mass and velocity at impact. These two factors are related and an increase in either will produce a higher mechanical force. There are limits on both size and pressure. If the droplet size is small or the pressure is too high, the result will be a fog or mist with little removal ability. Solid streams of water are not desirable as they cover only a small area at one time. 2. WATER PRESSURE Increasing the water pressure, increases the speed of removal. Although, excessive pressure can atomize the water into a fog and this is useless for removal. Maximum water pressure is normally 40 psi. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 6 Page 37 of 78 3. WATER TEMPERATURE. Water temperature will also affect the wash time. If the temperature is too high the penetrant will become less viscous and if the temperature is low this will cause the penetrant to be more viscous. Either of these variations will affect the wash time. Typical water wash temperatures are between 10°C to 38°C. The pressure & temperatures stated are typical of the pressures & temperature used on most penetrant systems. But there will be variations & the requirements of each specification must be checked to ensure you are within the required specification that you are working to. The best way to judge that you have removed just the correct amount of penetrant (when using fluorescent penetrants) is to carry out the water wash procedure under UV light. There should be a slight background visible. Slight surface penetrant evident. Part has not been over washed. No surface penetrant evident. That penetrant has been removed from the discontinuity. Part has been ”over washed”. 4. EMULSIFIERS (PE PENETRANTS ONLY) Immiscible liquids do not mix together. For example, if you add oil to water, the oil floats on the surface of the water. Then, if you shake the two together, tiny droplets of one liquid become spread through the other liquid; this forms a mixture called an emulsion. In an emulsion, the oil and water gradually separate out again. Tiny droplets join together until eventually, the oil is floating on the water again. To stop the two liquids separating, we need a substance called an emulsifier Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 6 Page 38 of 78 Phase 1 Phase 2 (A) (B) (C) (D) (A) Two immiscible liquids, not yet emulsified (B) An emulsion of Phase II dispersed in Phase I (C) The unstable emulsion progressively separates (D) The surfactant (outline around particles) positions itself on the interfaces between Phase 2 and Phase 1, stabilizing the emulsion. The diagram of a micelle of oil in aqueous suspension, such as might occur in an emulsion of oil in water. In this example the surfactant molecules' oil-soluble tails project into the oil, while the water-soluble ends remain in contact with the water phase At the wash stage, the penetrant particles are literally “dragged” off the surface of the part. There are two basic emulsifiers used in the penetrant method, these are lipophilic & hydrophilic. Lipo from the Greek meaning oil or fat & Hydro meaning water. Philic meaning an affinity to. Lipophilic is an oil or fat based material and hydrophilic is a water based solution. In these notes, the words “emulsifier” will be used when referring to lipophilic material and the word “remover” will be used when discussing hydrophilic material. This is the practice generally used by the industry. Emulsifiers/Removers are applied by immersion or spray. Slight agitation is permitted when applying either. The one thing that both the emulsifier & removers have in common is that they shall not be applied by brush. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 6 Page 39 of 78 The reason for this is that that both the emulsifier and or removers become more aggressive with movement & as each person will apply using different brush movement, pressures & even different brushes as this is not controllable it is not permitted. The normal application time (often referred to as the emulsification time) is two minutes. The time starts from when the emulsifier or remover comes into contact with the part(s) to when it is washed off. As this is normally performed using either a water or air/water mix. When large parts are processed it is important that the solutions are neutralised within the two minutes. The normal practice when processing large parts, is to immerse the part(s) into a tank of water, this neutralises the solution immediately. 5. MAIN DIFFERENCE BETWEEN LIPOPHILIC & HYDROPHILIC Lipophilic differences: I. Lipophilic emulsifiers actually mix with the penetrant & use both chemical & mechanical means to remove the penetrant from the surface. They are true emulsifiers II. You are not able to dilute Lipophilic emulsifiers III. As you cannot alter the strength, you cannot alter the aggressiveness of the emulsifier Hydrophilic differences: IV. Hydrophilic emulsifiers act more like a “detergent”. That is they do not “mix” with the penetrant they break the penetrant up into smaller manageable quantities that are removed during the water wash process. V. The strength of the solution can be altered by diluting the emulsifier with water. VI. The hydrophilic emulsifiers are more aggressive in the undiluted state, but they can be made less aggressive by diluting them with water. NOTE: In the past both the lipophilic and hydrophilic were referred to as emulsifiers. Nowadays the hydrophilic are referred to as “Removers” as they are not true emulsifiers. But you will still hear people referring to both types as emulsifiers. 6. LIPOPHILIC METHOD Lipophilic emulsifiers (Method B) were introduced in the late 1950's and work with both a chemical and mechanical action. After the emulsifier has coated the surface of the object, mechanical action starts to remove some of the excess penetrant as the mixture drains from the part. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 6 Page 40 of 78 During the emulsification time, the emulsifier diffuses into the remaining penetrant and the resulting mixture is easily removed with a water spray. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 6 Page 41 of 78 7. HYDROPHILIC METHOD Hydrophilic emulsifiers (Method D) also remove the excess penetrant with mechanical and chemical action but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basically detergents that contain solvents and surfactants. The hydrophilic emulsifier breaks up the penetrant into small quantities and prevents these pieces from recombining or reattaching to the surface of the part. The mechanical action of the rinse water removes the displaced penetrant from the part and causes fresh remover to contact and lift newly exposed penetrant from the surface The hydrophilic post-emulsifiable method (Method D) was introduced in the mid 1970's. Since it is more sensitive than the lipophilic post emulsifiable method it has made the later method virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsification time should be controlled as closely as possible, a variation of one minute or more in the contact time will have little effect on flaw detectability when a hydrophilic emulsifier is used. However, a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used. Typical concentration levels are anything from 5% to 20%. The strength required will be ascertained by experimenting with the solutions at different strengths. The concentration levels have to be maintained but this will be covered later. 8. SOLVENT REMOVABLE PENETRANTS Although solvent removable penetrants are available in the same quantities as any of the other penetrants, they are mainly used for “Local” inspection using a colour contrast penetrants penetrant (Also known as Colour Contrast, Visible or Red dye penetrants). The correct way to remove the excess penetrant that has been locally applied is:- a) Using a clean lint free tissue remove as much of the excess penetrant as possible. b) Moisten a clean piece of lint free tissue and remove the remaining excess penetrant. c) Lightly spray with a non-aqueous developer over the area. Typically you should still see the parent material through the developer. When spraying the developer, start the spray just before the area you are inspecting & complete the run in one clean action, stop the flow just after the area you are inspection. Try not to have areas where there are heavy deposits of developer, as this may obscure fine cracks. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 6 Page 42 of 78 Never spray the cleaner directly onto the area you are inspecting as this will, dilute the penetrant (as penetrant is solvent based) & in extreme cases make the penetrant “less” visible. 9. WHEN TO USE POST EMULSIFIABLE PENETRANTS (PE) PE penetrants are used when there is a possibility that you may wash away any discontinuities when using Water Washable (WW) penetrant. The ruling with regards to which penetrant to use will depend on a number of factors already discussed. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 7 Page 43 of 78 CHAPTER 7. 1. DRYING PROCEDURE The figure opposite shows a typical air circulating oven that is used for the drying of parts. The size of the oven will depend on the maximum size of the parts that are being processed. The actual temperature controller & the temperature indicator may be separate items or a combined item. The temperature shall be controlled with a calibrated device capable of maintaining the temperature at ±15°F [8.3°C] of the temperature for which it is set. The oven shall not exceed160°F [71°C]. The temperature indicator shall be accurate to ±10°F [5.6°C] of the actual oven temperature. The time it takes to dry a part(s) will depend on a number of factors. These are:- Size of the part or area of a number of parts Shape. Is it a simple shape like a blade or a more complicated casting Material. Aluminium or steel Specification. A number of specifications state a maximum time that the part(s) can remain in the oven A clean air blast (25 psi) can be used to remove the surplus water. 2. DEVELOPER A developer has basically three functions. These are:- a) Withdraw the penetrant out of the discontinuity b) Enlarge the indication c) Make the indication brighter Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 7 Page 44 of 78 Actual crack (bright area) Enlarged area Developers actually help to withdraw the penetrant out of the defect by capillary action as illustrated below. These areas between each particle form tubes which the penetrant is drawn up. This is one of the reasons why the developer needs to be of a uniform size, so that these “tube ways” are not blocked by smaller sized particles b a a = Actual width of crack. b = Width of defect as seen by the inspector. The developer must be able to: a) Reflect UV & or white light b) Non-toxic to other materials c) Safe for the operator d) Environmentally friendly Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 7 Page 45 of 78 3. DEVELOPER CLASSIFICATION Developer may be applied by:- Form a - Dry powder Form b – Water soluble Form c – Water suspendable Form d – Non-Aqueous for Type I Fluorescent penetrant Form e – Non-Aqueous for Type II Visible dye Form f - Special application 3.1. Form “A” Dry Powder Normal applied by:- Storm cabinet. Electro-static spray. Main advantage is that the coating thickness can be controlled. The application by an electro- static spray are the same principles as with electro-static penetrant. Storm Cabinet. Electro-static application. Electro-static equipment. 3.2. Form “b” Water soluble The developer is mixed in a container and allowed to dissolve with the water to form a slurry. The slurry is then gradually added to the water in the tank and gently mixed using some form of agitation. When using this form of developer the parts can be dipped or sprayed with the developer & then dried in the oven, as the water evaporates it leaves behind the reformed developer powder. The action is very similar to when sugar is dissolved in water, once the water has evaporated the sugar crystals are left behind. Document Number: TESTIA - UK - TRA - PT – 003A - EN - Issue 2 Registered in England and Wales Testia Limited Company No. 08360678 Airbus Main Gate, Golf Course Lane, Building 07D, Filton, Bristol, UK BS34 7QQ  Chapter 7 Page 46 of 78 The main problem with this type of developer is that it can leave “runs” on the surface of the parts which may hide any discontinuities present in the part(s). 3.3. Form “c” Water suspendable This is very similar to Form b but instead of the powder dissolving with the water, it uses the water as a carrier. When using this form of developer the parts can be dipped or sprayed with the developer. The parts are then dried in the oven, as the water evaporates it leaves behind the developer powder. The powder uses the water as a carrier (much like plastic balls floating on the water) and once the solution has been dried it leaves behind the powder. Again the main problem with this