F

220° F ¥ 250 I i 1

Carbon/Epoxy 0.6% Moisture

Fig. C-21. Effects of Temperature and Moisture on Strength of Carbon/Epoxy1

temperatures, but the affects are not as severe as the effects of elevated temperature on the matrix-dominated properties. As shown in Fig. C.21, the design parameters for carbon/epoxy are cold-dry tension and hot-wet compression.

The amount of absorbed moisture (Fig. C.22) is dependent on the matrix material and the relative humidity. Elevated temperature speeds the rate of

Time = 0 Time > 0 Time = Saturation
Fig. C-22. Absorption of Moisture for Polymer Composites13

moisture absorption. Absorbed moisture reduces the matrix-dominated mechanical properties. Absorbed moisture also causes the matrix to swell. This swelling relieves locked-in thermal strains from elevated temperature curing. These strains can be large and large panels, fixed at their edges, can buckle due to the swelling strains. During freeze-thaw cycles, the absorbed moisture expands during freezing and can crack the matrix. During thermal spikes, absorbed moisture can turn to steam. When the internal steam pressure exceeds the flatwise tensile strength of the composite, the laminate will delaminate.

Composites are also susceptible to delaminations (ply separations) during fabrication, assembly, and in-service. During fabrication, foreign materials, such as prepreg backing paper, can be inadvertently left in the lay-up. During assembly, improper part handling or incorrectly installed fasteners can cause delaminations.

When in-service, low velocity impact damage (LVID) from dropped tools, or fork lifts running into aircraft, can cause damage. The damage may appear as only a small indentation on the surface but can propagate through the laminate, forming a complex network of delaminations and matrix cracks, as depicted in Fig. C.23. Depending on the size of the delamination, it can reduce the static and fatigue strength, and the compression buckling strength. If it is large enough, it can grow under fatigue loading.

Typically, damage tolerance is a resin-dominated property. The selection of a toughened resin can significantly improve the resistance to impact damage. During the design phase, it is important to recognize the potential for delaminations and use conservative enough design strains so that damaged structure can be repaired.

Recommended Reading

[1] Dieter, G.E., Mechanical Metallurgy, 3rd Edition, McGraw-Hill Book Co., 1986.

[2] Courtney, T.H., Mechanical Behavior of Materials, 2nd Edition, McGraw-Hill Book Co., 2000.

[3] Myers, A.M., Chawla, K.K., Mechanical Metallurgy - Principles and Applications, Prentice-Hall, Inc., 1984.

[4] Askeland, D.R., The Science and Engineering of Materials, 2nd Edition, PWS-KENT Publishing Co., 1989.

References

[1] Askeland, D.R., "Mechanical Testing and Properties", in The Science and Engineering of Materials, 2nd edition, PWS-KENT Publishing Co., 1989, pp. 145-181.

[2] Callister, W.D., "Failure", in Fundamentals of Materials Science and Engineering, 5th edition, John Wiley & Sons, Inc., 2001, pp. 234-280.

[4] Dieter, G.E., "Brittle Fracture and Impact Testing", in Mechanical Metallurgy, 3rd edition, McGraw-Hill Book Co., 1986, pp. 471-500.

[5] Courtney, T.H., "Fracture Mechanics", in Mechanical Behavior of Materials, 2nd edition, McGraw-Hill Book Co., 2000, pp. 404-453.

[6] Dieter, G.E., "Fatigue of Metals", in Mechanical Metallurgy, 3rd edition, McGraw-Hill Book Co., 1986, pp. 375-431.

[7] Courtney, T.H., "Fatigue of Engineering Materials", in Mechanical Behavior of Materials, 2nd edition, McGraw-Hill Book Co., 2000, pp. 566-629.

[8] Dieter, G.E., "Creep and Stress Rupture", in Mechanical Metallurgy, 3rd edition, McGraw-Hill Book Co., 1986, pp. 432-470.

[9] Courtney, T.H., "High Temperature Deformation of Crystalline Materials", in Mechanical Behavior of Materials, 2nd edition, McGraw-Hill Book Co., 2000, pp. 293-353.

[10] Askeland, D.R., "Corrosion and Wear", in The Science and Engineering of Materials, 2nd edition, PWS-KENT Publishing Co., 1989, pp. 777-804.

[11] Courtney, T.H., "Embrittlement", in Mechanical Behavior of Materials, 2nd edition, McGraw-Hill Book Co., 2000, pp. 630-685.

[12] Smith, W.F., "Nickel and Cobalt Alloys", in Structure and Properties of Engineering Alloys, 2nd edition, McGraw-Hill, Inc., 1993, pp. 487-536.

[13] Campbell, F.C., Manufacturing Processes for Advanced Composites, Elsevier Ltd, 2004, p. 30-33.

This Page is Intentionally Left Blank

Was this article helpful?

0 0

Post a comment