This study investigates the effect of voids as manufacturing defects on the matrix-dominated strengths of carbon fiber reinforced plastic (CFRP) by coupon level testing. Specimens were cut from CFRP laminates prepared with different void volume fractions, V_v, by intentionally changing the curing conditions during their fabrication. The transverse tensile strength and the inter-laminar shear strength were evaluated by transverse-tensile testing and short-beam shear testing, respectively, using unidirectional flat specimens. The inter-laminar tensile strength was evaluated by performing four-point bending tests using unidirectional L-shaped curved specimens. The test results showed that these strengths decreased with increase in V_v and the ratio of decrease in strength to the increase in V_v varied according to the loading direction. The X-ray computed micro-tomography of the samples taken from the laminates used for the strength tests revealed the microscopic features of the shapes and the distributions of the voids in the CFRP laminates. The effect of voids on the strengths of CFRP laminates considering the stress concentrations around the voids is discussed based on the microscopic features of the voids and the internal loading directions in the critical regions.

Stitched CFRP, reinforced through the thickness by z-fibers like kevlar and carbon fiber, is expected to be used in the automobile industry. In the previous study, the interlaminar toughness properties evaluations of the stitched CFRP have been made. But, many of those evaluates were in a single mode, and there are few studies about mixed mode. In this study, the mixed-mode interlaminar fracture properties of the stitched CFRP laminates were evaluated, and it is intended to clarify the mode ratio dependence of fracture toughness. The I+II mixed mode interlaminar fracture toughness testing, Mixed mode bending (MMB) testing were performed for the stitched CFRP laminates specimen (reinforced by polyarylate z-fibers) and unstitched specimen. For these MMB testings, the specimens, test apparatus and crack observation devices were designed and produced. The crack growth observations were performed in the mode ratioG_I/G_II=0.55, 1.0 and 1.8. As a result, the fracture criterion of the stitched CFRP was revealed that was applicable ellipse law, G_I is in the major axis and G_II is in the minor one, and stitching is not contributed to Mode II fracture suppression.

This study proposes an ultrasonic inspection technique for detecting disbonds in adhesively bonded CFRP/aluminum joints using pulsed laser scanning. A specimen with artificially induced square disbonds was scanned by a pulsed laser for ultrasound generation, and the propagated waves were received by a transducer placed on its surface. A series of images of the traveling waves were obtained by processing the received signals. An initial, quick inspection was performed using the low-frequency Lamb wave. Changes in the propagation of the Lamb wave were observed at the regions of the disbonds, and disbonds larger than 5×5 mm were successfully detected. The second, detailed inspection of the detected region of disbond was performed using high-frequency through-transmission ultrasonic method. The shape of the disbond was precisely imaged, and the evaluated size of the disbond matched well with the actual size. We demonstrated the efficiency and the feasibility of the proposed technique for the inspections of adhesively bonded CFRP/aluminum joints.