Background: Fiber Reinforced Polymer (FRP) laminates have properties, which are highly dependent on the ply fiber orientations and which can be designed for optimum laminate performance. The purpose of this study is to investigate the effect of matrix cracking on the mechanical properties of FRP laminates with various off-axis angles, and to provide a critical test for an analytical solution using variational stress analysis. Methods: Carbon and glass fiber reinforced polymer laminates (CFRP and GFRP) are tested. FRP prepregs are cured by using autoclave method to form laminates with layups [θ_m/ φ_n]_s. The laminates are then loaded monotonically and cyclically to obtain their mechanical properties and the effect of matrix cracks on the properties. Some of the effects include reduction of laminates’ stiffness and residual strains after unloading. In order to obtain higher crack densities in specimens, artificial cracks method was introduced in this study, where notches were made at the edges of some specimens before tested in tension. Cracks observation for CFRP laminates is done by using the X-ray technique, while for GFRP laminates a DSLR camera is used. Results: The measured stiffness reduction as a function of the crack density is compared to an analytical prediction for cracked angle-ply laminates based on a variational stress analysis. The experimental results for stiffness reduction agree well with the analytical results. Conclusion: Understanding the behavior of damaged laminates with simple configurations, as performed in this study, is of high importance for prediction of damage effects on laminates with more complex configuration, e.g. with quasi-isotropic layups.

Carbon fiber-reinforced plastics (CFRP) laminate structures manufactured using prepregs usually contain fiber discontinuities that result from the prepreg cutting for design and cost-efficiency. In this study, the sampling moiré method was used to measure the microscopic strain of unidirectional CFRP laminates with fiber discontinuities to investigate the deformation behavior that causes damage. In addition to the strain distributions of the laminate before and after cracking, the strain results due to thermal residual stress relaxation and residual plastic deformation during cracking were also obtained. The experimental results of this study were in good agreement with the finite element method results. Our results revealed that the sampling moiré method is an accurate method of measuring microscopic strain that can be used to evaluate the mechanical properties, damage behavior, residual strains in laminates with fiber discontinuities, and other different materials and structures.

The transverse cracking behavior of a carbon-fiber-reinforced plastic (CFRP) cross-ply laminate is investigated using a fatigue test and an entropy-based failure criterion in this study. The results of fatigue experiments show that the crack accumulation behavior depends on the cyclic number level and frequency, in which two obvious transverse cracks are observed after 10⁴ cyclic loads and 37 transverse cracks occur after 10⁵ cycles. The final numbers of transverse cracks decrease from 29 to 11 when the load frequency increases from 5 Hz to 10 Hz. An entropy-based failure criterion is proposed to predict the long-term lifetime of laminates under cyclic loadings. The transverse strength of 90° ply is approximated by the Weibull distribution for a realistic simulation. Progressive damage and transverse cracking behavior in CFRP ply can be reproduced due to entropy generation and strength degradation. The effects of stress level and load frequency on the transverse cracking behavior are investigated. It is discovered that, at the edge, the stress σ₂₂ + σ₃₃ that is a dominant factor for matrix tensile failure mode is greater than the interior at the first cycle load, and as stress levels rise, a transverse initial crack forms sooner. However, the initial transverse crack initiation is delayed as load frequencies increase. In addition, transverse crack density increases quickly after initial crack formation and then increases slowly with the number of load cycles. The proposed method’s results agree well with those of the existing experimental method qualitatively. In addition, the proposed entropy-based failure criterion can account for the effect of load frequency on transverse crack growth rate, which cannot be addressed by the well-known Paris law.

Digital Image Correlation (DIC) is a non-contact method to analyse the deformation of materials that can measure displacement or strain behaviour during tensile loading. In this study, DIC is used to measure the deformation of cross-ply carbon fibre reinforced plastics (CFRP) laminates and for the detection of damages in them. The objective of this study is to measure the deformation around the damages in cross-ply CFRP laminates by using the DIC system from both width and thickness directions. For this purpose, thick CFRP [0₄/90₂₄]_s and thinner [0/90₆]_s laminates were tested. In cross-ply laminates, usually a straight transverse crack will initially occur in 90 degree ply followed by the following damages such as delamination, fiber fracture, and etc. To start, straight transverse cracks are induced in the laminates by using an artificial crack method. Then, X-ray radiography is used to detect the location of the straight cracks in order to be used for DIC observation to examine the strain distribution around the existed cracks. The DIC observation from the surface of the laminate around the cracks area clearly showed how strain is distributed from one crack to another adjacent crack. From the result, secondary mode damages of matrix cracking such as oblique and curved cracks can also be observed and are being discussed in this paper.

This study presents a comprehensive quantitative investigation into the material properties and damage behavior of unidirectional carbon fiber reinforced plastic laminates with ply discontinuities. Laminates with varying resin gap lengths were evaluated. In situ edge observations and X-ray computed tomography were employed to assess the damage processes in the area of the ply discontinuity. These methods examined initially occurring cracks, secondary cracks, and delamination. Some unexpected cracking behavior was observed in this study. For instance, initial cracks were observed to form not at the interface but at a distance from the resin–ply interface, likely due to a constraint condition caused by the chemical shrinkage of the resin during the curing process. Furthermore, all cracks in the resin region curved toward the center, with successively formed cracks having higher curvatures. This behavior can be attributed to the redistribution of principal stresses within the resin pocket following the initial cracks. The matrix cracking was followed by delamination that started from the tips of the matrix cracks and progressed toward the ends of the specimen. Additionally, the number and locations of cracks in the resin pocket were influenced by the length of the resin gap and the uniformity of the thickness and width of the specimen. In addition to the experimental investigation, a stress-based variational analysis was employed to predict crack onset stresses in the laminates. The analysis successfully calculated the energy release rate for laminates with varying resin pocket lengths and provided predictions for crack onset stresses. The analytical predictions demonstrated good agreement with the experimental observations, enhancing the understanding of resin pocket behavior in CFRP laminates.