Thermoplastic carbon fiber-reinforced plastics (CFRPs) are increasingly utilized in the aerospace industry owing to their beneficial properties and enhanced formability relative to thermoset CFRPs. Despite the extensive use of these materials, studies focusing on compression after impact (CAI) tests with impact energies exceeding 20 J for thermoplastic CFRPs remain scarce. This study examines CAI tests on quasi-isotropic laminates of thermoplastic CFRP, subjected to a low-velocity impact energy of 27.04 J. For comparison, quasi-isotropic laminates of thermoset CFRP were subjected to a low-velocity impact energy of 36.5 J. These tests reveal that the CAI strength of both materials is comparable, notwithstanding the lower fiber volume fraction in the thermoplastic CFRP. Further, this research incorporates a finite element (FE) analysis to investigate the damage mechanisms in thermoplastic CFRP. The FE model, integrating interlaminar damage observed during the impact tests, accurately predicted the relationship between compressive stress and strain, correlating closely with the experimental outcomes. It was observed that both interlaminar and intralaminar damage propagation were constrained until the point of maximum compressive stress. Prior to reaching this maximum, a region of elevated compressive stress in the fiber direction was noted in the 0° layer near the non-impacted side. These findings indicate that the compressive stress in the fiber direction in the 0° layer adjacent to the non-impacted side is pivotal in dictating the final failure, which determines the CAI strength of thermoplastic CFRP laminates.

This study proposes a new damage identification method based on topology optimization, combined with visualized ultrasonic wave propagation. Although a moving diagram of traveling waves aids in damage detection, it is difficult to acquire quantitative information about the damage, for which topology optimization is suitable. In this approach, a damage parameter, varying Young’s modulus, represents the state of the damage in a finite element model. The feature of ultrasonic wave propagation (e.g., the maximum amplitude map in this study) is inversely reproduced in the model by optimizing the distribution of the damage parameters. The actual state of the damage was successfully estimated with high accuracy in numerical examples. The sensitivity of the objective function, as well as the appropriate penalization exponent for Young’s modulus, was discussed. Moreover, the proposed method was applied to experimentally measured wave propagation in an aluminum plate with an artificial crack, and the estimated damage state and the sensitivity of the objective function had the same tendency as the numerical example. These results demonstrate the feasibility of the proposed method.

Lamb wave propagation must be understood comprehensively for simply evaluating delamination during ultrasonic testing. However, the difference between wave propagation, visualized using laser Doppler vibrometer and pulsed-laser scanners, has not been sufficiently investigated, and knowledge of optimal conditions for evaluating delamination is limited. Thus, in this study, the mode conversion and multiple reflections of Lamb waves propagating in a delaminated cross-ply laminate were visualized using different laser scanners, delamination depths, and wave incident angles. Delamination was characterized using maximum-amplitude map postprocessing under specific conditions. Further numerical analysis revealed that owing to multiple reflections of the antisymmetric mode in incident and mode-converted waves, standing waves were generated in the delaminated sublaminate. Dispersion curve and flexural stiffness calculations confirmed the conditions required for high-amplitude standing waves, thereby providing guidelines for simply and reliably evaluating delamination during inspections.