Abstract

In the formulation of the Stiffness-Force method (SFM) are taken into account the equilibrium equations and the compatibility conditions. These are obtained from the principles of virtual work and complementary virtual work. The structure of the governing equation of this method is similar to that of stiffness-method (SM) as the principal unknows are the displacements. This method is equivalent to IFMD formulated by Patnaik and to the hybrid methods formulated by Pian.For the Stiffness-Force method finite element analysis, a user subroutine UEL for a four node quadrilateral element has been developed. This subroutine has been implemented in the commercial program Abaqus. The procedure followed for the analysis of the model has been:1. Analysis of the model using the program Abaqus that employs the SM in its formulation.2. Analysis of the same model using the user subroutine UEL employing the SFM.3. Comparison of the results of both analysis, using varying degrees of discretization, to those obtained by an analytical approach.This procedure has been used in the analysis of the open-hole tensile strength test of a unidirectional composite.

Abstract

The study of the matrix/inter-fibre failure at micromechanical level by means of numerical methods predicts the appearance of differents stages in the development of this mechanism of damage; these numerical studies have also allowed the main features of each stage (such as the debond length at the interface and the kinking angle) to be identified. The development of experimental studies aiming to check the relevance of the aforementioned numerical results, the validity of the hypotheses assumed, as well as the identification of additonal parameters, is crucial for the advance in the knowledge of this mechanism of damage. Based on this, the research shown in this work focused on the tensile test (under different loading levels) of specimens manufactured from carbon-epoxy cross-ply symmetrical laminates. The microscopical observation of the 90º laminae of the tested samples, first results in the analysis of the appearance of the transverse cracks as a function of the load. Second, the study concentrates in the identification of the previously numerically predicted stages of the mechanism of damage and the measurement of key  parameters such as the debond length, the kinking path and the influence of nearby fibres.