۱- Introduction

۲- Analysis: selective activation of intrinsic cohesive elements

۳- Results and discussion

۴- Conclusion

References

Abstract

In this paper, we studied the selective activation of intrinsic cohesive elements by using controllable multi-point constraints (MPCs) for fracture analyses of laminated composites. Cohesive elements inserted between bulk elements were deactivated by tying the cohesive nodes using MPC, which were selectively activated during analysis only for the failure region by releasing the constraints. A strategy for the systematic definition and release of MPCs that considers the composite failure modes was developed. When applied to the fracture analysis of laminated composites, the selective activation strategy was found to alleviate the artificial compliance problem of intrinsic cohesive elements while producing results that matched accurately those of conventional intrinsic cohesive elements.

Introduction

There has been continued interest in composite materials over the past several decades. Composite materials have excellent specific stiffness/strength properties compared to conventional materials. The application of composite materials has increased in advanced aerospace structures, and it is gradually expanding into structures in the general industrial sector. In laminated composites, failure occurs in a complex combination of diverse failure modes at the fiber, matrix and interface regions [1]. An accurate understanding of failure behavior is crucial for the design of efficient composite structures with structural integrity. A vast amount of experimental, analytical and numerical studies have been performed to investigate how failures, which include failure criteria and progressive failure models, initiate and propagate in composite materials. The failure behavior of laminated composites with stress concentrators, such as a hole and a notch, bears a particular importance. This problem has been extensively studied by a number of researchers (e.g., [2–۵]). When a tensile load is applied to both ends of a notched composite laminate, stress concentration develops at the notch tip, and matrix cracking and interlayer delamination start and propagate. These damages continuously interact with each other and grow until fiber breakage finally occurs, leading to ultimate failure. Numerical simulation is becoming a more efficient way to study the progressive fracture behavior of composite materials owing to the development of very fast computer hardware and reliable analysis software. Frequently used numerical methods include the virtual crack closure technique (VCCT) (e.g., [6,7]), continuum damage mechanics (CDM) (e.g., [8–۱۴]), cohesive zone modeling (CZM) (e.g., [15–۲۰]), and extended finite element method (XFEM) (e.g., [21,22]). VCCT is a linear elastic fracture mechanics (LEFM)-based method in which a crack is allowed to propagate when the calculated strain energy release rate (G) is larger than the material fracture energy (Gc). Although proven to be accurate in predicting crack propagation behavior, this method requires an initial crack and an a priori knowledge of the crack path. CDM, also known as progressive failure analysis, simulates failure behavior in such a way that the damage initiation is predicted by a failure criterion first, and then the corresponding material is softened by a property degradation model. This method has been developed quite well and is widely used; however, it is well-known to suffer from the mesh-dependence problem [23]. In CZM, material failure is explicitly described by interface elements inserted between bulk elements and by a constitutive relation called traction-separation law (TSL). CZM removes the initial crack requirement of the VCCT and provides much better mesh regularization characteristics compared to a CDM-based method. However, intrinsic CZM with the initially elastic TSL not only can alter the elastic bulk response but also can affect the fracture solution [24,25].