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In this study, a model for the three-dimensional effective compliance of composite laminates with transverse cracks is developed based on continuum damage mechanics. Three-dimensional laminate theory is used to reproduce all the thermoelastic properties of the damaged laminate. The damage variable, which describes the degree of stiffness reduction caused by transverse cracking, is formulated based on a three-dimensional micromechanical model, with a loose boundary condition and assuming parabolic crack opening. These assumptions contribute to the analytical accuracy of the stiffness reduction model, while simplifying the damage variable expression. The effective thermomechanical properties of various composite laminates are predicted using the proposed model and compared with finite element analysis (FEA) and experimental results. We found that the proposed model with derived damage variable successfully reproduces the FEA and experimental results of stiffness degradation of damaged composite laminates. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ceramic matrix composites (CMCs) are currently being considered for applications in the hot-section components of aviation gas turbines owing to their favorable characteristics. Herein, a micromechanical modeling is presented for orthogonal 3 D woven CMCs under in-plane loading. The three-dimensional effective compliance of the 3 D woven composite was derived using three-dimensional laminate theory and continuum damage mechanics. The damage variables were used to describe the stiffness reduction due to the transverse and matrix cracking in each fiber bundle. The calculation method for the transverse and matrix cracking evolutions under in-plane loading was established by introducing mixed-mode damage criteria. The stress redistribution among the fiber bundles of 3 D woven CMCs due to the fiber/matrix interfacial debonding around matrix cracking was considered to capture the interaction between the matrix and transverse crack evolutions. Additionally, a mesomechanical model comprising finite element analysis and damage mechanics was established to evaluate the stress perturbation due to the geometry of the woven structure. The edge face of the 3 D woven CMC was experimentally observed to measure the transverse and matrix cracks that occurred in each fiber bundle. The transverse and matrix crack densities predicted by the micromechanical and mesomechanical models reasonably agreed with the experimental results up to crack saturation. Furthermore, the micromechanical model reproduced the nonlinear stress–strain response under tensile and shear loading using mixed-mode damage criteria. [ABSTRACT FROM AUTHOR]

In this study, the continuum damage mechanics model for predicting the stiffness reduction of composite laminates including transverse cracks is formulated as a function of crack density. To formulate the model, first the damage variable in the direction normal to the fiber of a ply including transverse cracks is derived. The damage variable is derived by the model assuming a plane strain field in the isotropic plane and using the Gudmundson–Zang model for comparison. The effective compliance based on the strain equivalent principle proposed by Murakami et al. and classical laminate theory are then used to formulate the elastic moduli of laminates of arbitrary lay-up configurations as a function of the damage variable. Finally, the results obtained from this model are compared to the finite-element analysis reported in previous studies. The model proposed in this paper can predict the stiffness of laminates containing damage due to transverse cracks (or surface crack) from just the mechanical properties of a ply and the lay-up configurations. Furthermore, this model can precisely predict the finite-element analysis results and experiment results for the elastic moduli of the laminate of arbitrary lay-up configuration, such as cross-ply, angle ply, and quasi-isotropic, including transverse cracks. This model only considers the damage of the transverse crack; it does not consider damage such as delamination. However, this model seems to be effective in the early stage of damage formation when transverse cracking mainly occurs. The model assuming plane strain field in the isotropic plane which is proposed in this paper can calculate the local stress distribution in a ply including transverse cracks as a function of crack density. The damage evolution of transverse cracks can thus be simulated by determining the fracture criterion. [ABSTRACT FROM AUTHOR]