Effect Of Damage On Delamination Tip-Field Of Lamintes And Analysis Of Intelaminar Toughening

Due to high strength, low weight and good durability, fiber reinforced composite material has been widely used in the field of aviation and space. Experiments show that the matrix micro-cracks in fiber reinforced laminates are the damage that occure first and it ieasily caused by the high stress and high strain in the stress concentration region.The matrix micro-crack damage results in the decrease of rigidity and other property of laminate. In addition, delamination is another main damage and hidden trouble of composite laminate which can lead to decrease of structural carrying capacity. So the studies on delamination crack tip stress field and interlaminar toughening are very important. This paper focus on two aspects:firstly investigation of the the influence of micro-crack damage and evolution on delamination tip field, energy release ratio and laminate failure behavior; secondly analytical research for the interlaminar chopped fiber toughening mechanism and the main parameters.Based on the continuum damage theory and existing experiment results, a multi-scalar damage model and damage evolutiont equations are presented. In the model, the damage depends on the strain in main direction of material, the strain softening characteristics of quasi-brittle materials are considered and four kinds of softening parameters are adapted to analyze the influence of strain softening.In order to study influence of the micro-crack on crack tip field, laminates with through-crack are considered under plane strain assumption. Using the principle of virtual work, the equations of equilibrium and their increment iteration form are deduced. Considering the geometry nonlinearity and physical nonlinearity caused by damage, a FORTRAN program is made to analyze the influence of damage on crack tip field, energy release ratio and expand of ultimate damage region. The computational results show that the damage of micro-crack can make the stress singularity decreased, damage and strain softening makes the crack tip energy release ratio no longer increase monotonously with the load, therefore it can not be used to describe the characteristic of delamimation growth.The expanding of ultimate damage region is influenced by softening parameters and fiber directions.To determine the influence of micro-crack damage on the laminate failure, a quasi-forming laminated plate element based on first order shear theory is used in FE analysis. Considering the rigidity decrease caused by micro-crack and using the Hoffman criterion, the numerical analysis of destroy course of laminates with a hole under tension and square laminated plates under uniform pressure are carried on. Compare of the numerical results with and without micro-crack damage indicates that damage evolution before destroy can chang stress transmission in laminates, release the stress concentration, delay the destroy of laminates, especially ones with0/90plies, and therefore ultimate carrying capability may be improved obviously.The commonly used interlaminar toughening methods are:improving matrix tougheness, interleaving, adding plastic particle to interlayer and stitiching etc. Although these methods can improve interlaminar tougheness, they are complex in technique and make the decrease of in-plane performances. But a new approach to toughening from interlaminar chopped-fibers may overcome the shortcomings. In this paper, a modeling for bridging of chopped fibers lying almost within the delamination plane is carried out by a analytical method. In the model, the spalling of matrix and pull-out of bridging fibers are considered, the effects of some parameters on tougheness increment are discussed including fiber elastic modulus, bridging fiber density and property of fiber matrix interface. Results show that the energy dissipation mainly comes from pull-out of bridging fibers, the high modulus Kevlar fibers and moderate interface friction force are more effective for improving delamination toughness, but the influence of interface strength is small. In addition, a part of chopped fibers in the middle of interlayer is not in bridging but they disturb pull-out of bridging fiber and they are not ignored.