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 of 90 degrees unidirectional carbon fiber reinforced plastic (CFRP) laminates. The very high-cycle fatigue properties of the specimens were evaluated using ultrasonic and electromagnetic fatigue testing machines. Ultrasonic fatigue tests were conducted to obtain the fatigue properties under the giga-cycle regime. The specimen geometry was designed to resonate at 20 kHz, the specimen consisted of CFRP laminate and a metal tab to connect to the horn end of the testing machine. Additionally, the free volume of the matrix material, namely, epoxy resin, of the CFRP laminates was evaluated using the positron annihilation method. A slit was introduced in the specimen surface to identify the location of damage development, which facilitated the free volume measurement by positron microscopy. The obtained S-N curves reveal that failure did not occur at strain levels lower than epsilon(max) = 0.75% at the slit tip for all specimens up to N = 1.0 x 10(9) cycles. The free volume measurement for a specimen set above the threshold strain level revealed that the free volume increased in size and decreased in amount as the number of cycles increased. The test results revealed that the opposite trend existed below the threshold, which suggests that a fatigue limit may exist.