Answers - Composite


Fill in the Blanks

1. high strength; relatively low weight; corrosion resistance
2. fairings, control surfaces, landing gear doors, leading and trailing edge panels, interior components, floor beams and boards, stabilizer primary structure on new generation large aircraft, turbine engine fan blades, propellers.
3. fiber and matrix
4. wing spoilers; fairings; ailerons; flaps; nacelles; floor boards; rudders
5. insulating properties; energy absorption and/or redirection; smooth cell walls; moisture and chemical resistance; environmentally compatible; aesthetically pleasing; relatively low cost.
6. high compressive strength; resistance to water penetration.
7. delamination; resin starved areas; resin rich areas; blisters/air bubbles; wrinkles; voids; thermal decomposition
8. improper cure or processing; improper machining; mishandling; improper drilling; tool drops; contamination; improper sanding; substandard material; inadequate tooling; mislocation of holes or details.
9. interlaminar shear ; compression strength
10. scorches, stains, dents, penetration, abrasion, or chips
11. resin starvation, resin richness, wrinkles, ply bridging, discoloration (due to overheating, lightning strike, etc.), impact damage, foreign matter, blisters, disbonding
12. calibration; damage
13. dust; oil; vapors; smoke; moisture
14. secondary bonding
15. nylon; polyester
16. ability to slow or stop fatigue crack growth; replace lost structural area due to corrosion grind‐outs; structurally enhance areas with small and negative margins.
17. vertical; controlled; no bleed out; horizontal (or edge)
18. strength requirements
19. potted
20. freeze
21. more
22. lightly loaded; small
23. fiberglass
24. wet lay up, prepreg, DVD, wet lay up
25. special storage, handling, curing
26. honeycomb sandwich, bearing
27. Hi loks, lockbolts
28. diamond coated, solid carbide, high speed steel
29. flush type
30. higher, lower

True or False

1. true
2. true
3. false; fiberglass yarns are twisted, Kevlar is not
4. false; Unidirectional Tape Fibers are held in place by the resin and have a higher strength than woven fabrics.
5. true
6. true
7. true
8. false; Carbon Fibers have a high potential for causing galvanic corrosion with metallic fasteners and structures.
9. true
10. true
11. true
12. true
13. true
14. true
15. true
16. false
17. false; when using the through transmission ultrasonic inspection method, areas with a greater loss than the reference standard indicate a defective area.
18. false; the handling life is shorter
19. true
20. true
21. false; in a resin starved repair
22. true
23. true
24. false
25. true
26. false
27. true
28. false
29. true
30. false

Knowledge Application

1. A homogeneous material has a uniform composition throughout and has no internal physical boundaries.

2. An anisotropic material has mechanical and/or physical properties that vary with direction relative to natural reference axes inherent in the material.

3. Quasi‐isotropic material has approximately the same properties as isotropic. An example is a composite laminate with the fibers orientated in the 0°, 90°, +45°, and ‐45° direction to simulate isotropic properties.

4. More flexibility for lay‐up of complex shapes.

5. Save weight; Minimize resin void size; Maintain fiber orientation during the fabrication process.

6. nickel‐coated graphite cloth; metal meshes; aluminum fiberglass; conductive paints.

7. low smoke and flammability characteristics.

8. Three – A, B & C stage

9. In the frozen state the resin of the prepreg material will stay in the B stage of curing. The curing will start when the material is removed from the freezer and heated up again.

10. Panels with complex curves

11. Small core cells provide better support for sandwich face sheets. Higher density core is stronger and stiffer than lower density core.

12. too much resin is used. For non‐structural applications this is not necessarily bad but it will add weight.

13. too much resin is bled off during the curing process or if not enough resin is applied during the wet lay‐up process.

14. The method is accomplished by tapping the inspection area with a solid round disk or lightweight hammer‐like device and listening to the response of the structure to the hammer.

15. A clear, sharp, ringing sound

16. A dull or thud‐like sound

17. Changes within the internal elements of the structure might produce pitch changes that might be interpreted as defects, when in fact they are present by design.

18. Through transmission ultrasonic inspection uses two transducers, one on each side of the area to be inspected. The ultrasonic signal is transmitted from one transducer to the other transducer. The instrument then measures the loss of signal strength. The instrument shows the loss as a percent of the original signal strength or the loss in decibels. The signal loss is compared to a reference standard. Areas with a greater loss than the reference standard indicate a defective area.

19. Cover the unprotected sides of the prepreg material with parting film and clean the area being repaired immediately before laying up the repair plies.

20. two parts are simultaneously cured. A typical application is the simultaneous cure of a stiffener and a skin.

21. A permanent repair restores the required strength and durability, an interim repair only restores the required strength and has a different inspection interval.

22. A potting repair is heavier than the original control and this could affect the control balance and introduce flutter.

23. Water will boil at cure temperatures and could blow the facesheets of the core.

24. Two holes are drilled on the outside of the delamination area and a low‐viscosity resin is injected in one hole till it flows out the other hole.

25. thick structures or structures made of carbon fiber or Kevlar would block the radar signals.

26. Place impregnated fabric in the debulking chamber and apply a vacuum bag over the lay up. Apply a vacuum to the chamber (box). Because of a vacuum their will not be any pressure applied to the lay up but volatiles will be removed. This will reduce voids in the lay up. Heat will be applied to lower the viscosity of the resin to help volatile removal. After debulking the vacuum in the outer chamber is removed and the lay up will be compacted by the atmospheric pressure. The patch will be staged at temperature. After the staging the patch will be removed and bonded to the aircraft with a past adhesive and cured with a heater blanket.


Multiple Choices

1. b
Aircraft structure must not only be strong, but rigid as well. Very thin aluminum skins normally provide adequate strength, but lack the rigidity needed. A structural material that provides both the strength and rigidity with light weight is the honeycomb structure.

2. a
A change in sound when performing a ring (coin-tap) test may indicate an area of delamination. The change in sound could also be due to the transition to a different internal structure. More sophisticated testing would be necessary to determine the extent of the delamination, if present.

3. a
X-ray inspection allows you to "see" moisture trapped in the structure. Acoustic emission testing picks up the "noise" of defects and displays it on an oscilloscope. This type of testing detects entrapped water, cracks, delamination, and other subsurface flaws. Backlighting does not detect entrapped water.

4. a
After damaged honeycomb core has been removed, either a balsa wood or honeycomb replacement plug is installed. If balsa is used, the plug should be cut so the grain is perpendicular to the skin.

5. b
A doubler should be cut from a piece of aluminum the same thickness as the original face or one and one-half times as thick, and it should be tapered at a ratio of about 100:l. Always refer to the manufacturer's guidelines for repair specifications.

6. b
Mix enough extra resin in each batch to make an identical lay-up on a piece of scrap aluminum. This test sample will give a good indication of the cure time needed, as well as the physical characteristics of the lay-up.

7. a
Acoustical emission monitoring is a system using a very sensitive microphone and an amplifier. With it you can pick up the sound of the bubbles associated with active corrosion.

8. a
When metal is used as a facing for laminated structure, it must be protected from corrosion by wing a corrosion inhibiting primer and a finish that will completely seal the atmosphere away from the repaired area.

9. a
If there is surface indication of damage to a fiberglass honeycomb or fiberglass laminated sheet, remove the paint from both sides of the sheet in the suspect area and hold a strong light so it will shine through.

10. a
If honeycomb structure is faced with metal, the way to find delamination is by tapping on the surface with a coin. A good bond will produce a clear ringing sound. A delamination will produce a sound which is dull and lifeless.

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