Damage progression by the element-failure method (EFM) and strain invariant failure theory (SIFT)

A novel element-failure method (EFM) has been used with a recently proposed micromechanics-based failure criterion SIFT for the modeling of progressive damage in composite laminates. The element-failure concept is a potentially practical method for the modelling of damage, fracture and delamination in fiber-reinforced composite laminates. When the modeled damage is propagating within a finite element, the element is considered to have partially failed, but not removed from the computations. Consequently, only a fraction of the stresses that were computed before the damage entered the element contribute to the nodal forces of the element. The concept is especially useful when extended to composite structures because the nature of damage in composite laminates is generally diffused, characterized by multiple matrix cracks, fiber pullout, fiber breakage and delaminations. It is usually not possible to identify crack tips in the fashion of traditional fracture mechanics. Since parts of a damaged composite structure are often able to partially transmit load despite the presence of some damage, it is advantageous to model the damaged portions with partially failed elements. The damage may be efficiently modeled and tracked using element-failure concepts, with the application of appropriate failure criteria and damage evolution laws. The idea is to embody the effects of damage into the effective nodal forces of the finite element. Here, we report the novel use of element-failure concepts with a recently proposed composites failure criterion called the strain invariant failure theory (SIFT) in the prediction of damage progression and delamination in a composite laminate subjected to three-point bend. The predicted damage propagation patterns are in good agreement with experimental observation when the EFM is used with SIFT.