Element-Failure: An Alternative to Material Property Degradation Method for Progressive Damage in Composite Structures

In order to develop more rational design and damage tolerance approaches, there is a need for more accurate, realistic, and practical modeling of damage progression in composite structures at the component level, and not just at the laboratory specimen level. This presents a considerable challenge because experimental and analytical results at the specimen level do not always translate to observations of damage at the component or structural levels. In this article, a simple but novel finite element-based method for modeling progressive damage in fiber-reinforced composites is proposed. The element-failure method (EFM) is based on the idea that the nodal forces of an element of a damaged composite material can be modified to reflect the general state of damage and loading. The EFM, when employed with suitable micromechanics-based failure criteria, may be a practical method for mapping damage initiation and propagation in composite structures. This concept is especially useful because the nature of damage in composite laminates is in general complex and diffused, characterized by multiple matrix cracks, fiber pullout, fiber breakage, and delaminations. It is therefore not practical or even possible to identify and model the multitude of cracks in the fashion of traditional fracture mechanics. Currently, progressive damage in composites is most commonly modeled using material property degradation methods (MPDM). Unfortunately, MPDM often employs rather arbitrary and restrictive degradation schemes that, in some cases may result in computational problems. In contrast, computational convergence and stability is always assured in EFM because the stiffness matrix is never altered. Furthermore, no contact algorithm to prevent nonphysical solutions involving interpenetration of delamination and crack surfaces is necessary, because the failed elements are not removed from the model. It is shown in this article that the EFM is a more general method than the MPDM. A comparison study of damage progression and delamination in a composite laminate subjected to three-point bend using a recently proposed failure criterion called the strain invariant failure theory (SIFT) is presented. The results show that the EFM is able to predict the damage pattern more accurately than the MPDM.