Composite Inspection Methods PDF

Composite Inspection Composite Inspection Methods Today’s composite inspection techniques and non-destructive testing (NDT) methods typically involve the use of multiple methods to accurately determine the airworthiness of the structure. Fortunately, many metal inspection and NDT methods transfer to...

Composite Inspection Composite Inspection Methods Today’s composite inspection techniques and non-destructive testing (NDT) methods typically involve the use of multiple methods to accurately determine the airworthiness of the structure. Fortunately, many metal inspection and NDT methods transfer to composite applications. Composite structures require ongoing inspection intervals along with non-scheduled damage inspection and testing. When a composite structure is damaged, it must first be thoroughly inspected to determine the extent of the damage, which often extends beyond the immediately apparent defect. Proper inspection and testing methods help determine the classification of damage, which is whether the damage is repairable or the part must be replaced. In addition, classifying the damage helps to determine the proper method of repair. The manufacturer’s structural repair manual outlines inspection procedures, damage classification factors, and recommended repair methods. Inspection of composite blades on a gas turbine engine 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 116 of 229 Visual Inspection Visual inspection is the most frequently used inspection method in aviation. Ideally, pilots, ground crew and maintenance technicians visually inspect the aircraft on a daily basis. This method of inspection is generally used to detect resin-rich areas, resin starvation, edge delamination, fibre break-out, cracks, blistering and other types of surface irregularities. A strong light and magnifying glass are useful tools for visual inspection. In extremely critical cases, a small microscope is helpful for determining whether the fibres in a cracked surface are broken or if the crack affects the resin only. Visual inspection 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 117 of 229 Tap Testing A tap testing inspection is an effective way to discover if a finished job/repair has a good bond line or has air pockets or other inclusions that may have caused a disbond. Tap testing inspection Tap testing using a small tap testing tool 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 118 of 229 Ultrasonic Inspection Ultrasonic inspection is the most common instrumental NDT method used on composites today. An ultrasonic tester is useful for detecting internal damage such as delaminations, core crush and other subsurface defects. Two common methods of ultrasonic testing include the pulse echo and through-transmission methods. Ultrasonic inspection In the pulse echo method, the tester generates ultrasonic pulses, sends them through the part and receives the return echo. The echo patterns are displayed on an oscilloscope. An advantage to the pulse echo method is that it requires access to only one side of the structure. The through-transmission method uses two transducers. One emits ultrasonic waves through the part and the other receives them. Defects located at multiple levels throughout the structure are more easily detected because the receiver, located on the backside of the part, receives the reduced number of sound waves that pass through the defects. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 119 of 229 Radiographic Inspection Radiography or x-ray inspection is used to detect differences in the thickness or physical density when compared to the surrounding material of a composite. Unlike ultrasonic or tap testing, it can be used to detect surface or internal cracks, entrapped water inside honeycomb core cells, foreign objects in the composite structure, and the extent and size of the damage. Radiographic inspection Thermography Thermography locates flaws by detecting temperature variations at the surface of a damaged part. Heat is applied to the part and the temperature gradients are measured using an infrared camera. Thermography requires knowledge of the thermal conductivity of the test specimen and a reference standard for comparison purposes. https://www.infratec.eu/thermography/industries-applications/aerospace-industry/ Thermography image of an aircraft after landing 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 120 of 229 Thermography is also used to identify water ingress into aircraft structures, by imaging the aircraft in flight or post flight. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 121 of 229 Composites Defects and Damage Damage Classifications Depending on the manufacturer of the aircraft, damage is usually classified according to three categories: Negligible – damage that may be corrected by a simple repair procedure with no restrictions on flight operations Repairable – damage to the skin, bond or core that cannot exist without placing restrictions on flight operations, but can be repaired Non-repairable – damage to the structure or component that cannot be repaired (component must be replaced). Areas of condensation remaining on a structure after the aircraft has warmed up following coldsoaking at altitude might indicate water ingress and should be investigated. Damage to composite fibres 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 122 of 229 Cosmetic Defects A cosmetic defect is a defect on the outer surface skin that does not involve damage to the structural reinforcing fibres. Cosmetic defects 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 123 of 229 Impact Damage Impact damage occurs when a foreign object strikes the part. The degree of damage may range from slight to severe. The most common cause of impact damage is careless handling during transportation or storage, or standing parts on their edge without adequate protection. The thin face sheets on a sandwich panel are very susceptible to impact damage. An area that has been subjected to impact damage should also be inspected for delamination around the impacted area. Nicking, chipping, cracking or breaking away pieces of the edge or corner can also be caused by improper handling. Impact damage 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 124 of 229 Delamination Delamination is the separation of fabric layers of material in a laminate. Delamination can occur with no visible indications from the outer skin. To compound the problem, delamination often accompanies other types of damage, particularly impact damage. This damage occurs as the result of several causes, including impact, moisture ingress in the fabric or lightning strikes. Delamination Disbonds Disbonding is a separation of the composite material from another material to which it has been adhesively bonded. This is different to a delamination, which refers to a similar separation between any plies or layers of the composite. Separation between the skin and core of a composite sandwich structure is separately referred to as a core disbond. A disbond may be the consequence of poor adhesion, service loading or impact damage. The disbond may not be visible externally and, if tight or weakly bonded, may be difficult to detect using NDT methods. This type of defect is known as a kissing bond. Disbonding is particularly important to avoid in joins such as end connections. Disbonds 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 125 of 229 Cracks Cracks can occur in composite structures just as in metallic ones. Cracking is a common form of damage in composites arising in manufacture or under service conditions. It has a significant effect on the integrity of the composite, allowing environmental ingress and damage to extend under service loading. Cracking is often associated with the final stages of in-service failure. Sometimes cracks can be detected visually, but other times they may require more advanced methods of non-destructive inspection (NDI). A thorough inspection should be made to determine the extent of each crack. ATSB Transport Safety Report, Occurrence Investigation AO-2014-032 Composite material damage on the vertical stabiliser of an ATR 72 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 126 of 229 Cracking Resin Matrix Damage Damage to the resin matrix can be caused by many things, such as fire or excessive heat, UV rays, paint stripper, impacts, etc. Resin matrix damage 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 127 of 229 Water and Aircraft Fluid Intrusion Intrusion of water and other fluids is especially a problem with honeycomb cores. It causes weight gain, contamination of bond joints, corrosion in aluminium honeycomb and disbonds if the water freezes and expands. Thermographic image showing moisture ingress Water intrusion is a very common problem with high-temperature repairs. The heat of curing causes the trapped water to turn to steam, disbonding face sheets around the repair. It is a progressive way to inadvertently convert a small area of damage into a large one. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 128 of 229 Water and aircraft fluid intrusion 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 129 of 229 Hole Damage Holes may occur from impact damage, from over-torqued fasteners or as a result of fastener pullthrough. Holes drilled in the wrong location, wrong size or wrong number can also be classified as hole damage. Holes caused by a lightning strike may burn off resins, leaving bare cloth. Tiny holes, known as pinholes, in the skin surface are not easily detected; however, they could lead to more extensive damage. If moisture is allowed to get into the core structure, along with the airflow over the part, it could cause a small delamination, which could grow into a very large delamination. Pin holes on skin surface could expand to cause large holes 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 130 of 229 Composite Repairs Composite Repairs The exact procedures for repair of various laminated composite structures depend partly on the type of damage incurred. The damage can range from a relatively simple surface scratch to damage extending completely through all internal plies and the core honeycomb material. There are four basic types of composite repairs: Bolted metal or cured composite patches Bonded metal or cured composite patches Resin injections New repair plies laminated to the damage. Bolted and bonded surface patches are not usually recommended due to the fact that these types of patches do not restore the strength characteristics of the original structure. A bolted or bonded patch that is attached to the surface also causes undesirable aerodynamic changes. Resin injection repairs fill holes or voids by injecting resin into the hole of a damaged area using a needle and syringe. This type of repair is usually done on non-structural parts. The injected resin does not restore the original strength and in some cases expands the delamination. The most desirable type of permanent repair to composite structure is lamination of new repair plies in the damaged area. This type of repair involves removing the damaged plies and laminating on new ones. Composite repair remove and relaminate 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 131 of 229 Composites Damage Assessment and Repair Process Below are the basic steps of damage assessment and repair: 1. Find the damage – clean damaged area and remove surface coatings. 2. Assess the extent of the damage – use visual and NDI techniques and then mark out the repair. 3. Define the repair procedure. 4. Fabricate the repair. 5. Prepare the repair site. 6. Lay-up install the repair. 7. Clean up the repair site. 8. Inspect the repair site for structural integrity – may use NDI again. 9. Document the repair. Composites damage assessment and repair process 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 132 of 229 Potted Repairs Potted repairs use a filler to complete the composite repair process. They provide an alternative to installing a core material plug but do not provide as much strength as a core material. Filling a hole with a resin-and-filler mixture adds weight and decreases the flexibility of the part. However, many structural repair manuals still list the potted repair as a viable repair for composite structures, Most potted repairs are appropriate for honeycomb core sandwich structures with damage up to 1 in. in diameter. In some cases, it is permissible to drill a small hole into a delaminated area and inject resin to strengthen the part. Potted repairs 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 133 of 229 Injection Repairs Sometimes internal delamination is minor enough to repair using a potting compound. It can be repaired by simply injecting resin into the cavity that was caused by the ply separation. If the delamination is severe enough, it must be removed and repaired or replaced; always check the manufacturer’s repair limitations. Injection repairs Minor edge delamination can sometimes be repaired by injecting resin into the delamination, clamping the edge and allowing the resin to cure. In addition, edges that have been damaged by crushing or puncture can be repaired by scarf-cutting and installing new plies. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 134 of 229 Delamination and injection repair Laminate Damage to One Surface Single-surface laminate damage repair calls for the removal and replacement of the damaged laminate plies. Fibre damage to one side of the surface that does not completely penetrate the part may be repaired as per diagram below. Laminate damage to one surface 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 135 of 229 Laminate Damage Through the Part Damage that runs through all of the laminate layers can be repaired in several ways depending on the number of plies, the location of the damage and the size of the damage. Check the manufacturer’s repair manual for specific limitations regarding each type of damage. The damage can be repaired using a step-cut that starts from one side of the part to the other, or, in the case of thicker laminate structures, by step-cutting from both sides and meeting in the middle. View A illustrates a step-cut repair that runs from one side of the part to the other. In addition, a surface patch and backing plate with a one-inch overlap are applied to both surfaces of the repair. View B illustrates a modified step-cut repair. The step-cuts are started from both sides and meet in the middle. This type of repair reduces the size of the patch when performed on thicker laminates. Using view B as an example, the modified two-sided step-cut repair results in a patch that is four inches in diameter. If the one sided step-cut repair was performed on this same five- layer laminate, the patch would be five inches in diameter. Laminate damage through the part 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 136 of 229 Sandwich Structure Repairs Sandwich structures are vulnerable to impact and puncture damage primarily because these types of structures usually incorporate relatively thin face sheets, so delaminations commonly occur at the point where the face sheet bonds to the core material. Puncture damage may be repaired in several different ways depending on the size, extent and location of the damage. Two of the more common types of sandwich structure repair are: Puncture repair Honeycomb core repairs. Sandwich structure repairs 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 137 of 229 Honeycomb Core Repairs As discussed previously, potted repairs may be made to areas of damage of up to 1 in. in diameter. If the damaged area is larger than an inch or in a more critical area, a balsa wood or composite honeycomb plug may be cut and bonded in place. If balsa is used, cut the plug so the grain is perpendicular to the skin. If honeycomb material is used, it should be the same density as the original. Honeycomb core repairs 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 138 of 229 Transparent Plastic Repair When a windshield on an unpressurised aircraft has a crack, it can be repaired by: Stop-drilling the ends of the crack to prevent it from growing Drilling a series of holes 13mm from the crack edges and about 13mm apart Lacing the holes with brass safety wire to hold the crack together. Transparent plastic repair Another temporary repair for a cracked windshield is: Stop-drilling the ends of the crack Drilling holes every inch throughout the crack Installing screws with nuts and washers. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 139 of 229 Temporary repair for cracked windshield 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 140 of 229 Sealants Sealants are manufactured to fulfil a multitude of applications and are generally available in two forms: One-part sealants are prepared by the manufacturer and are ready for application as packaged. Two-part sealants are compounds requiring separate packaging to prevent curing prior to application. The two parts are identified as the base sealing compound and the accelerator. Two-part sealants are generally mixed by combining equal portions (by weight) of the base and the accelerator compounds, and any deviation from the prescribed ratios can reduce the material’s quality. Aviation Australia - Dave Kingshott Sealant used inside an integral fuel tank Some of the applications include sealing fuel tanks and pressurised structures; weatherproofing skin joins; aerodynamically smoothing aircraft surfaces; gluing skins and structure together; and separating skins, structure and fasteners as a corrosion preventative. 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 141 of 229 Sealant in a pre-packaged disposable cartridge ready for mixing 2022-05-18 B-06a Materials and Hardware CASA Part 66 - Training Materials Only Page 142 of 229