In the traditional hole drilling method the in-plane STRESSes are studied in detail and the INTERLAMINAR STRESSes are not taken seriously. The INTERLAMINAR STRESSes on the free edge of composite materials are important and can cause matrix cracking and delamination. Inserting holes in composite laminates causes free edges and it can induce INTERLAMINAR shear and normal STRESSes. In this research, using a finite element method and the simulated hole drilling method for isotropic and orthotropic materials, the three dimensional state of STRESSes are studied. The number of steps for hole drilling, the uniform and non-uniform pre-STRESSes, and uniaxial and biaxial STRESSes are considered. The results show if the number of drilling steps changes from one to four, the ratio of the maximum three-dimensional STRESSes to the induced uniaxial STRESS is changed. The mechanical properties of each layer, number of layers, and the stacking sequence are important parameters for laminated composites. Therefore, unidirectional plies under uniaxial and biaxial state of STRESSes are considered. Then, the INTERLAMINAR STRESSes for the cross-ply, angle-ply and quasi-isotropic laminated are studied. The effects of INTERLAMINAR STRESSes on the residual STRESSes on the point of measurement of strains by strain gauges are studied in detail.

INTERLAMINAR thermal STRESSes and boundary layer effect in thin laminated composite cylinders which are subjected to temperature change are studied. To this aim a laminated cross-ply composite cylinder with finite length which is subjected to thermal loading is modeled. The displacement based layer-wise theory (LWT) is used for modeling the response of the composite cylinder in the thermal loading conditions. Using an appropriate displacement field and employing the LWT, the governing equations of the cylinder and the appropriate boundary conditions in the edges of the cylinder are derived with the principle of minimum total potential energy. An analytical solution is introduced for the governing equations and the solution is obtained for various boundary conditions. The numerical results are validated by comparison of the results of LWT with the predictions of the finite element method (FEM) and good agreements are seen. It is shown that the presented LWT solution is an efficient and accurate method for analysis of the edge effect and INTERLAMINAR STRESSes in composite cylinders. The INTERLAMINAR thermal STRESSes and in-plane STRESSes in the Glass/Epoxy composite cylinder which are subjected to thermal loading are investigated for various boundary (edge) conditions. Cylinders with symmetric and asymmetric layer staking and free, simple and clamped boundary conditions are studied in the numerical results.

The INTERLAMINAR fracture behaviour of unidirectional carbon/epoxy composites has been studied under flexural loading by using end-notched flexure (ENF) specimens. GIIc Values were calculated as total fracture toughness energy at the maximum load sustained by the materials as the delamination extended. The results showed that high temperature moulding systems (XHTM45) have the highest GIIc values well above 1000 J/m2. For medium temperature systems (MTM) GIIc have also increased significantly after post cure. For compression strength after impact (CSAI), the behaviour to a certain extent is related to that found for GIIc tests. Comparison of the GIIc values with CSAI also indicated a relationship between two test results. SEM Micrographs revealed their excellent delamination resistance as good crack stoppers with the evidence of strong fibre/matrix interface. Dynamic mechanical analysis (DMA) indicated the increased Tg and modulus retention of the LTM and MTM prepregs after post curing at elevated temperatures. The failure mechanisms seem to be different for different tough matrix materials and appear to be strongly dependent on the cure and post-curing conditions. This is particularly noticeable for curing at 135°C and 80°C. of medium and low temperature moulding systems.

Failures in composite materials occur mainly due to INTERLAMINAR fracture, also called delamination, between laminates. This indicates that characterizing INTERLAMINAR fracture toughness is the most effective factor in the fracture of composite materials. This study reports investigation on mixed-mode INTERLAMINAR fracture behaviour in woven carbon fibre/polyetherimide (CF/FEI) thermoplastic composite material based on experimental and numerical analyses.Experiments were conducted using the special test loading device. By varying the loading angle,α from 0º to 90º, pure mode-I, pure mode-II and a wide range of mixed-mode data were obtained experimentally. Using the finite-element results, geometrical factors were applied to the specimen. Based on experimentally measured critical loads, mixed-mode INTERLAMINAR fracture toughness for the composite under consideration determined. The fracture surfaces were examined by scanning electron microscopy to gain insight into the failure responses.

In this study, delamination of the corrugated composite plates made of unidirectional glass fibers and polyester resin has been investigated. The samples are fabricated by hand lay-up process based on the ASTM-D5528 standard. Using experimental tests, the strain energy release rate in mode I has been calculated for composite corrugated specimens by the prevalent method of INTERLAMINAR failure. Also, the samples were simulated as a double cantilever beam by Abaqus software, and the mechanical properties of unidirectional glass/polyester composite and the results of the numerical solution are also obtained. Fracture surfaces of experimental samples were analyzed by scanning electron microscope. Force-displacement curves obtained from experimental and numerical methods have been compared to find the material behavior and calculate the strain energy release rate for different samples with three pre-crack lengths. The results show that the corrugated composite plates have a higher INTERLAMINAR fracture toughness rather than flat samples and the four-wave sample with a crack length of 60 and 65 mm has the highest values of the strain energy rate released, equal to 963.77  J/m2 and 705.95  J/m2, respectively.