Interlaminar shear fatigue damage evolution of 2-D carbon-carbon composites

Low-cycle and high-cycle bending fatigue experiments were conducted on 2-D woven carbon-carbon composites at room temperature to evaluate the possible evolution of damage within their complex microstructure. Since carbon-carbons are known to be weak in interlaminar shear, a 3-point bending geometry was chosen to produce high interlaminar shear stresses. This paper extends previous work by focusing on interlaminar shear fatigue, and by using a specially developed technique for the in-situ examination of distributed damage. For the low-cycle tests the load was cycled between 10% and 90% of the material's monotonic failure load; the range was 8% to 82% for the high-cycle experiments. A load-frame mounted optical microscope allowed the collection of digital images of the surface microstructure at various intervals up to 100,000 cycles for the low-cycle and up to 20 X 10(6) cycles for the high-cycle experiments. Microcrack lengths and orientations were extracted from these images, which permitted defining a discrete damage function as a measure of the distributed microcrack damage. Within the uncertainty of the measurements, distributed fatigue damage evolution was not discovered. A slight increase in residual static strength was detected with no change in stiffness.