Energy-Based Mechanical Model for Mixed Mode Failure of Laminated Composites

A nonlinear elastic foundation model has been developed to predict the mixed-mode (modes I and II) delamination failure of laminated composites. Two types of nonlinear elastic foundations were used to represent pure mode I and pure mode II components of the material failure ahead of the crack tip. One is a tension spring foundation for mode I component and the other is a shear spring foundation to represent the mode II component. Each spring foundation was characterized based on the linear elastic two-dimensional asymptotic solution of the strain field ahead of the crack tip and appropriate nonlinear constitutive laws. To predict the onset of crack propagation, the model employed an energy criterion as a failure condition derived from experimental trends of mixed-mode failure of laminated composites. Mode I, mode II, and mixed-mode fracture tests were performed to investigate the validity of the current model. A finite element was developed and incorporated into a computer code to simulate laboratory fracture tests, utilizing Timoshenko's first-order shear beam theory and a nonlinear constitutive law. The current model predicted the experimental results of load vs displacement curves closely. It also exhibited a satisfactory mode separation capability and was able to predict mode mixture results available in the literature.