Very high-cycle fatigue properties of 90° unidirectional CFRP laminates and evaluation of fatigue limits by free volume measurement using positron microscopy

The objective of this study was to experimentally evaluate the fatigue limit 90° unidirectional carbon fiber reinforced plastic (CFRP) laminates. very high-cycle properties specimens were evaluated using ultrasonic and electromagnetic testing machines. Ultrasonic tests conducted obtain under giga-cycle regime. specimen geometry designed resonate at 20 kHz, consisted CFRP laminate a metal tab connect horn end machine. Additionally, free volume matrix material, namely, epoxy resin, laminates positron annihilation method. A slit introduced in surface identify location damage development, which facilitated measurement by microscopy. obtained S-N curves reveal that failure did not occur strain levels lower than εmax = 0.75% tip for all up N 1.0 × 109 cycles. set above threshold level revealed increased size decreased amount as number cycles increased. test results opposite trend existed below threshold, suggests may exist.