In this study, the prediction of the progression of transverse cracking in laminates, including 90° plies, was discussed. A refined stress field (RSF) model was formulated that takes into account the thermal residual strain for plies, including transverse cracks, and the energy release rate associated with transverse cracking was calculated using this RSF model. For comparison, the energy release rate based on a continuum damage mechanics (CDM) model was also formulated. Next, the prediction for the progression of transverse cracking in carbon fiber-reinforced plastic (CFRP) cross-ply laminates, including 90° plies, based on both stress and energy criteria was implemented using Monte Carlo methods. The results showed that the RSF and CDM models proposed in this paper can predict the experiment results for the relationship between the transverse crack density and ply strain in 90° plies. The models presented in this paper can potentially be applied to any arbitrary laminate that includes 90° plies.

Carbon-fiber-reinforced thermoplastics (CFRTP) are highly expected to find application in mass-production vehicles due to their high-productivity, thermal bonding characteristics, and recycle performance. In addition, discontinuous CFRTP adopts a ‘variable thickness structure’, which enables a wide range of practical applications including large structural parts. In this study, the optimum variable thickness of a simple supported beam was theoretically obtained by solving the Euler–Lagrange equation. In addition, the robustness of the variable thickness structure was evaluated through parametric study. Due to the low shear modulus of thermoplastic resin, the ratio of in-plane Young’s modulus (E₁) to out-of-plane shear modulus (G₁₃) of CFRTP is larger than that of other materials. To examine its effect, Timoshenko assumption was applied and was compared with the Euler–Bernoulli assumption. Consequently, it was shown that the Timoshenko-based variable thickness structure required minimum thickness on the edge, which was affected by the length of the beam and the E₁-to-G₁₃ ratio. The theoretical weight reduction rate was obtained by applying both material substitution and optimum variable thickness structure. Those values of carbon-fiber-tape-reinforced thermoplastics (CTT) and carbon-fiber-mat-reinforced thermoplastics (CMT) are 70.1% and 67.9%, respectively.

Generally, polymeric materials exhibit strong time–temperature dependence and viscoelastic behavior, and the time–temperature superposition principle is typically used to estimate their long-term viscoelastic behavior. In addition, Mises and Tresca criteria have been proposed to estimate the yield and failure of isotropic materials exposed to multiaxial stress. The Christensen failure criterion, on the other hand, can be applied for isotropic materials with different tensile and compressive strengths. In this study, using a molecular dynamics method, uniaxial and multidirectional tensile and compression test simulations were performed for polypropylene using various strain rates (2.16×10¹⁰, 2.16×10⁹, and 2.16×10⁸s⁻¹) and temperatures (200, 250, and 300K). The resulting stress–strain curves showed non-linear behavior, and the compressive stresses were higher than the tensile stresses. In addition, the fracture stress was increased at a low temperature or high strain rate, and these failure stresses are in good agreement with the Christensen failure criterion curves. Furthermore, time–temperature superposition principle was applied for the relationship between failure stress and failure time. Consequently, long-term viscoelastic behavior could be estimated from the short-term viscoelastic behavior at different temperatures.

In this study, a numerical analytical process was proposed and validated to define the material model and its parameters for randomly-oriented chopped carbon fiber tape-reinforced thermoplastic (CTT), a type of carbon fiber-reinforced plastic (CFRP). The model assumed that CTT is isotropic and homogeneous in its in-plane direction because of the random orientation of chopped long fibers. In its out-of-plane direction, on the other hand, CTT has much lower mechanical properties due to the rarely fiber-reinforced configuration. Therefore, when failure criteria and damage functions were applied to the CTT model, the out-of-plane pure shear-strain relationship was employed as the main factor for damage development and stiffness degradation, especially for the three-point bending behavior of CTT. Material input parameters for the numerical analytical model were mostly obtained by experimental test methods, including out-of-plane pure shear testing. All material functions and parameters obtained through the proposed processes were used in the finite element model for the numerical simulation of three-point bending tests. The resulting flexural behavior of CTT as predicted by the numerical simulation was in good agreement with the experimentally obtained results, in particular for damage development and stiffness degradation. These results indicate that the flexural behavior of CTT is dominated by its out-of-plane non-linear properties.