Effects of Matrix Toughness on Fatigue Response of Graphite Fiber Composite Laminates

To evaluate the possible advantages of using a tough resin, polyetheretherketone (PEEK), as a matrix in a composite material, the fatigue response of (0/+45/90/-45)_s4 AS4/PEEK graphite-epoxy specimens with drilled center-holes subjected to constant amplitude, fully reversed cyclic loading was investigated. The results were compared with those of a similar investigation using T300-5208 graphite-epoxy specimens to determine the effects of matrix toughness on fully reversed fatigue. Several load levels provided for lives between 10⁴ and 10⁷ cycles to establish baseline (R = -1) S-N curves for the two materials. Additionally, damage evaluation methods such as stiffness monitoring, penetrant-enhanced radiography, and residual strength measurements were used to identify damage mechanisms; to monitor damage growth process; and to establish relationships between damage and strength, stiffness, and life of the specimens. Damage initiated at the hole in all cases, but the damage modes, their subsequent growth and interaction, and their effects on fatigue response were dependent on cyclic load levels and material. Damage in the brittle 5208 matrix specimens consisted of matrix cracks followed by delamination. Over the range of fatigue lives studied, the residual tensile strengths of the 5208 matrix specimens were greater than the initial notched tensile strength. During loading the compressive strength degraded to values less than initial notched compressive strength, and the fatigue failure modes were compressive. Similarly, at the lower cyclic load levels, corresponding to lives between 10⁵ and 10⁷ cycles, the PEEK specimens also suffered matrix cracking, delamination, and an attendant compressive stiffness loss. The specimens failed under the compressive portion of the cyclic loading. However, at higher cyclic loads, 0° fiber damage in the PEEK specimens was observed in addition to matrix damage, and the tensile stiffness degraded. The failure modes were tensile in these cases. The results of this study show that matrix toughness influences the long-term behavior of graphite fiber composites. Although similar matrix damage modes were observed in the two material systems, the consequences of the damage, as measured by strength, life, and failure mode, were not similar.