The interlaminar shear stress plays a very important role in the damage of composite laminates. With higher interlaminar shear stress, delamination can easily occur on the composite interface. In order to calculate the interlaminar shear stress, a laminate theory, which accounts for both the interlaminar shear stress continuity and the transverse shear deformation, was presented in this study. Verification of the theory was performed by comparing the present theory with Pagano’s elasticity analysis. It was found that the present theory was able to give excellent results for both stresses and displacements. More importantly, the interlaminar shear stress can be presented directly from the constitutive equations instead of being recovered from the equilibrium equations.

In the conventional analysis for laminated composite materials, the composite interface is always assumed to be rigidly bonded. However, due to the low shear modulus and poor bonding, the composite interface can be nonrigid. Based on the previously developed inter laminar shear stress continuity theory and a linear shear slip law, this study presents a so-called interlayer shear slip theory to investigate the effect of the interfacial bonding on the behavior of cross-ply laminates. Closed-form solutions for the cases of the cylindrical bending of long composite strips with [0/0] and [0/90] sequences are obtained. Numerical results for the laminates with different length-to-thickness ratios are presented. They reveal that the interlayer shear slip theory is valid for cross-ply laminates with rigid and nonrigid interfaces. It is also concluded from this study that at some special locations, namely, singular points, the transverse shear stress or in-plane normal stress remains insensitive to the condition of interfacial bonding.

SUMMARYDelamination is a major damage mode in laminated composites since it can cause severe structural degradation. Based on an interlaminar shear stress continuity theory and a linear shear slip theory, a so-called Interlayer Shear Slip Theory was presented in a previous study. This theory was verified to be feasible for shearing-mode delamination analysis. However, in order to account for opeming-mode delamination in laminated composites, the continuity of interlaminar normal stress and the modelling of normal separation on the composite interface should also be considered. The present study gives a complete discussion on the Interlaminar Bonding Theory. The effects of interlaminar bonding condition on the laminate deformation and stress distribution are also presented. It is concluded from numerical results that the present theory is suitable for analysis of composite laminates with imperfect interfaces.

SUMMARYA finite element method for stress and vibration analysis of laminated composite beams was investigated. The analysis was based on a multilayered theory presented by Lu and Liu. This theory accounts for the continuity of interlaminar shear stress. The principle of minimum potential energy was used in the finite element formulation. The interlaminar shear stress was obtained directly from the constitutive equations. It was verified that the present technique was able to give excellent results for displacements, stresses and vibration frequencies for both thin and thick composite beams. The effects of the number of layers and the number of elements on the convergence were also discussed.

This study discusses the repairability of impact-induced damage in an SMC composite which is made of chopped glass fibers and polyester matrix. The impact resistance and notch sensitivity of the composite material are characterized by tensile and flexural tests. Equivalent hole sizes for impact-induced damage are identified through the comparisons between the residual strengths of composites with impact-induced damage and artificial circular cutouts. It is concluded that no damaged portion should be removed from the impacted composites; otherwise more damage could be introduced to the com posites. The damaged composites are then repaired with a technique combining resin in jection and reinforcing patches. The resin injection is performed to seal the matrix cracks which would otherwise result in high stress concentrations due to geometrical discon tinuity. The reinforcing patches are employed to compensate for the strength reduction due to fiber breakage. It is revealed from the experiments that the repair technique is very effi cient for restoring the tensile and flexural strengths of the composite after impact-induced damage.