Mode I delamination fatigue properties of interlayer-toughened CF/epoxy laminates

Mode I delamination fatigue crack growth behavior was investigated in carbon fiber (CF)/epoxy laminates with two kinds of interlayer/interleaf. One was with heterogeneous interlayer with fine polyamide particles, T800H/3900-2, and the other was with interleaf of new types of thermoplastic resin, ionomer, UT500/111/ionomer. Tests were carried out using double cantilever beam specimens. For T800H/3900-2 laminates, the crack path shifted from the heterogeneous interlayer region (Stage I) to the interlayer/base lamina interface (Stage II) with the increase of the crack length. The delamination fatigue crack growth resistance of the T800H/3900-2 laminates was about 3 times higher than that of the reference CFRP for Stage I. This ratio decreased to 1.5 times for Stage II. For UT500/111/ionomer laminates, the high level of the fatigue crack growth resistance was kept without respect to the crack length. The threshold value of UT500/111/ionomer laminates was about 3 times higher than base UT500/111 laminates. The mechanism of the difference of the toughening mechanism for these interlayer/interleaf laminates was discussed on the bases of fractrographic observation and mechanism consideration.