L-shaped composites are fundamental parts of complex-shaped structures and it is well-known that shape distortion (i.e., spring-in deformation) arises after curing due to the orthotropic nature of composites. This residual deformation significantly increases the manufacturing costs, attracting significant attention to the mechanisms of spring-in deformation. Recently developed aerospace-grade composites include inter-laminar toughened layers composed of thermosetting resin and thermoplastic particles to enhance inter-laminar fracture toughness and improve impact resistance. Even though inter-laminar toughened layers can affect process-induced strain and deformation, their effects have not yet been studied in detail. Therefore, fiber-optic-based internal strain measurements were performed and residual deformation of L-shaped parts with inter-laminar toughened layers was investigated in this study. The through-thickness cure shrinkage strain at the corner part was relaxed and the spring-in angle decreased by holding the parts at the cure temperature, indicating that the viscoelasticity of the thermoplastic particles is important. Viscoelastic finite element analysis supported this finding and indicated that the effect of the toughened layers on the residual deformation should be considered to optimize curing processes.

X-ray computed tomography was used to obtain cross-sectional images of a unidirectional carbon fiber reinforced plastic, where fiber locations in each cross-sectional image were identified. The three-dimensional model with fiber waviness was developed by connecting the fiber locations along the fiber direction. Numerical simulation for the initiation and formation of a kink-band during axial compression was performed using the three-dimensional finite element model. The load was increased almost linearly until it reached the compressive strength, after which both load and displacement were decreased, showing snap-back behavior. The matrix yielded locally with the increased axial compression, and fibers started to fall due to insufficient support by the yielded matrix. A kink-band was formed with an increase in the yielded area, and thus, the initiation of a kink-band was defined as the local yielding of the matrix. It was also shown that the kink-band was formed at the longitudinal location at which the average of local fiber misalignment angles in the cross-section was relatively large.

In this study, flexural tests using both thermoset and thermoplastic carbon fiber reinforced plastics (CFRP) laminates with varied ply thicknesses were conducted in order to evaluate the effect of ply thickness and matrix resin on the flexural properties of thin ply composites. Results confirmed that the flexural properties were improved by decreasing the ply thickness in both thermoset and thermoplastic laminates. Furthermore, in-situ observation of specimens during the test revealed that short delamination occurred in the carbon fiber carbon fiber reinforced thermoplastics (CFRTP) thin ply laminate, while long delamination was observed in case of carbon fiber reinforced thermosets (CFRTS). This indicates that damage progressions of the laminates were influenced by the toughness of the matrix resin. On comparing the in-situ strength in the first ply failure calculated from the classical laminate theory in the thin and thick ply laminates, it was confirmed that high damage resistance was caused by higher in-situ strength in each layer in the thin ply laminate.

The effects of specimen geometry on the Poisson’s ratio of a quasi-isotropic carbon fiber reinforced polymer laminate under compression tests were examined experimentally. In particular, the divergence of the apparent Poisson ratio from the theoretical value was measured experimentally for a decreasing specimen width/thickness ratio (b/t). The observed tendency was then confirmed by simulations using finite element analyses (FEAs). FEAs using a 3D model of the specimens were performed to investigate the strain and stress distribution. Moreover, it was confirmed that the width-direction strain in evaluating the apparent Poisson’s ratio varies with the condition of a low b/t specimen. It was confirmed from the FEA results that the cause of an increasing or reducing width strain was sectional deformation under compressive loading, resulting from differences in the layer angles and stiffness between the outermost and inner layers.