Strain Invariant Failure Theory Research Articles

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.

Application of a First Invariant Strain Criterion for Matrix Failure in Composite Materials

This paper reports an application of the strain invariant failure theory (SIFT) proposed by Gosse to through-thickness failure of composite structures [Gosse, J.H. and Christensen, S. (2001). Strain Invariant Failure Criteria for Polymers in Composite Materials, AIAA-2001-1184]. It includes global 3D finite element analysis on a ply-by-ply basis and the determination of amplification factors for strains in the intrafiber resin and the calculation of thermal residual strain due to manufacturing for intrafiber resin using micromechanical models. The new procedure has been applied to matrix failures for one type of curved beam, two types of I-beam and two types of T-cleat structures. The resulting predictions of the failure loads for both the I-beams and the T-cleats follow experimental trends. The limiting value of the first invariant strain for failure of the resin in the unidirectional tape is 17,000–20,000 ", and in the bidirectional fabric is 26,000–27,000 ".