Non-Destructive Testing (NDT) Methods

• Non-destructive testing (NDT) methods evaluate materials and components without causing damage or impairing their intended use
• NDT is primarily used to detect faults in materials, assessing their existing state and quality for acceptance or rejection

Types of Faults

• External faults include surface finish issues, machining marks, hardening cracks, blow holes, and surface inclusions
• Internal faults encompass overlaps, fish-tails, piping, micro-segregation, and hydrogen embrittlement

NDT Methods

• Visual Testing
• Hydrostatic Testing
• Dye Penetrant Testing
• Magnetic Particle Testing
• X- & Gamma Ray Radiography
• Ultrasonic Testing
• Eddy Current Testing

Visual Testing

• Common examination method involving direct viewing, often aided by magnifying tools and light sources
• It is a quick, inexpensive way to find cracks, but its effectiveness depends on the inspector's skill and experience
• Optical inspection probes enable visual checks in areas with limited access, like ducts and pipes
• Rigid probes range from 2mm to 20mm in diameter, while flexible probes, using fiber optics, range from 4mm to 15mm

Hydrostatic Testing

• Detects defects through the flow of gas or liquid into or through leaks
• Commonly used to find leaks in items like inner tubes by pressurizing them with gas and looking for bubble formation when immersed
• Hydrostatic testing checks for leaks in welded pressure vessels, piping, or valve sections by applying water pressure 1.5 to 2 times the working pressure
• Leakage is identified by water or gas seepage, or by changes in liquid or gas pressure
• Helium, an inert gas, is used for highly sensitive leak tests with a helium leak detector

Dye Penetrant Testing

• Useful for detecting small cracks or discontinuities not visible through normal inspection
• Relies on a highly penetrating liquid seeping into any surface discontinuities
• Liquid penetrant process is best for detecting defects open to the surface of homogeneous materials
• Equally effective on metals, plastics, glass, and ceramic
• Defect orientation does not matter
• This method can be applied without complex or expensive equipment

Types of Penetrants

• Classified into visible and fluorescent groups
• Visible penetrants contain a bright dye, usually red, for viewing under white light
• Fluorescent penetrants have a dye that glows under ultraviolet light (black light)
• Each group is divided into three types based on penetrant removal method: Water-washable, Post-emulsified, and Solvent-removable

Penetrant Procedures (Water-Washable System)

• Clean the test surface
• Apply the penetrant
• Remove excess penetrant
• Apply the developer
• Perform inspection and interpret results

Dye Penetrant Precautions

• Prevent penetrants and solvents from contacting skin or clothing to avoid irritation
• Ensure good ventilation to avoid toxic fumes from penetrants or solvents
• Most materials in portable penetrant kits are flammable
• Avoid looking directly at black-light sources to prevent temporary vision issues

Magnetic Particle Inspection

• Detects discontinuities at or near the surface of ferromagnetic materials
• A leakage field indicates discontinuities which is detected by applying finely divided ferromagnetic particles to the surface

Methods

• Longitudinal Magnetisation
  • Using permanent magnets or yokes on a steel surface creates a magnetic flux, good for finding cracks between 90° and 45° to the flux path
  • Coils induce a longitudinal field, achieved via fixed or portable current-carrying cables
• Circular magnetization
  • Achieved by applying electrical contacts to allow current to flow directly
  • Prod Contacts: Used on parts too large for end contacts

Current for Magnetization

• Magnetizing current depends on metal permeability, test piece shape and thickness, and discontinuity type

Magnetic Particles

• Wet particles: Suspension of magnetic particles in light petroleum distillate or oil. Applied by dipping, immersing, or spraying and are colored
• Dry particles: Available in powder form in various colors. Applied using hand shakers, spray bulbs, or air streams

Demagnetization

• Ferrous materials retain residual magnetism post-magnetization, causing issues like instrument errors or interference with machining
• Achieved by subjecting the part to an alternating, diminishing magnetic field

X and Gamma Radiography

• Radiography creates images on fluorescent screens or photographic material using short wave radiation of X-rays and Gamma (γ) rays

X-Radiography

• X-rays: electromagnetic waves from 10 Å to 0.01 Å form via high-energy electrons emitted from a cathode filament accelerated to strike an anode
• Only 1% of electron energy converts to radiation

Gamma Radiography

• Gamma (γ) rays are emitted from decomposing radioactive substances
• Gamma rays have shorter wavelengths (0.1 to 0.005 Å) than X-rays
• Common radioisotopes: Cobalt 60, Caesium 137, and Iridium 192

Film Density

• Film density measures film darkness, determined by light transmission
• D = log10(Io/It), where Io is incident light intensity and It is transmitted light intensity
• A film density of 2.0 transmits 1% of the incident light

Definition

• Definition in radiography: Sharpness and clarity of an image outline.

Influencing Factors

• Film Unsharpness: Depends on film and radiation energy
• Geometric Unsharpness: Affected by relative positions of the specimen and film, and focal spot size
• Minimize source size, maximize source-to-specimen distance, minimize specimen-to-film distance
• Direct radiation rays perpendicular to the film; align film and specimen in parallel

Radiographic Sensitivity

• Sensitivity measures thinnest detectable discontinuity on a radiograph

Image Quality Indicator (IQI)

• Sensitivity quantified via IQIs, such as wire or step hole types detailed in British Standards BS 3971

Ultrasonic Testing

• Ultrasonic flaw detection employs sound waves above 20 kHz
• Intensity is undiminished traveling through meters of material unlike X-rays
• Ultrasonic energy reflects at boundary surfaces

Ultrasonic Wave Generation

• Common method uses piezoelectric materials converting electrical energy to mechanical
• Common Materials: Quartz Crystal, Barium Titanate, and Lithium Sulphate

Ultrasonic Techniques

• Pulse-Echo
  • Ultrasound pulses travel through a specimen, reflecting back from interfaces to a transducer
  • Reflected sound produces an amplitude (y-axis) on a cathode ray oscilloscope
  • Distance is registered along the x-axis based on known ultrasound velocity
• Through Transmission
  • Shares equipment with pulse-echo but uses separate transducers on opposing sides of the specimen
  • Measures transmitted ultrasound amplitude variations to detect structural discontinuities based on reduced amplitude

Eddy Current Testing

• Definition: Circulating electrical current induced in a conducting material by an alternating magnetic field
• As magnetic field changes (reverses), so does the eddy current

Eddy Current Principle

• The eddy current induces a magnetic field opposing the original field
• Impedance of the coil near specimen changes due to induced eddy current
• The path of this eddy current is distorted by defects or inhomogeneities.

Key Information

• Presence of a defect alters the coil's apparent impedance, indicating physical, chemical, or metallurgical differences
• The induced eddy current concentrates near the specimen's surface, known as the 'skin effect'

Applications

• Metal sorting
• Detection of cracks, voids, and inclusions
• Determination of coating thickness
• Measurement of thickness of non-conducting films on conductive materials

Factors Affecting Magnitude

• Alternating current magnitude and frequency
• Specimen factors: Electrical conductivity, magnetic permeability, and shape.
• Relative positioning of coil and specimen
• Discontinuities or inhomogeneities presence

Test Coils and Probes

• Concentric or encircling coil: completely surrounds the specimen
• Point probe or surface coil: small coil placed near the surface
• Inside / bobbin type coil: moved through a tube or pipe

NDT

• Selection depends on cost, defect type/origin, material type, and object shape/size

Examples

• Each method suits specific defects
• Comprehensive examination may require multiple methods

Crack examples

• Thread cracks in bolts: often caused by fatigue from vibration stressing thread roots
• Stress corrosion cracks: worsened by high stress levels
• Heat-treatment cracks: result from rapid thickness changes causing localized stress during heating and cooling
• Corrosion wall thinning: corrosion reduces thickness, especially serious in pipelines
• Gas porosity: porosity caused by gas entrapment in welds.