Abstract

Matrix/Inter-fibre failure is characterized by the appearance at the fiber-matrix interfaces of small debonds that can progress along them until reaching a certain extension, then, changing their orientation kinking towards the matrix and finally growing through it. The particular case of compressive loading is specially interesting, given the morphology of the interface cracks and the specific angle that the macrocracks form in the matrix.To date, the analysis of this problem at micromechanical level has been carried out mainly by means of Finite Element or Boundary Element models. In this work, the problem is approached from the experimental point of view, observing under the microscope specimens previously tested at different loading levels. Several aspects such as the identification of the phases of the faliure mechanism, the kinking angle, the extension of the interface cracks and the presence of damage as a function of the loading level are studied.

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.

Abstract

Nowadays, the use of composite materials in primary structures has considerably increased, in particular in what refers to primary structures and bonded joints. Thus, numerical tools and experimental tests have to be implemented in order to understand failure modes and critical loads in this kind of structures. Usually, bonded joints quality is tested based on the interlaminar fracture test DCB (G1c). Nevertheless, these tests have some limitations. The aim of the present study is to create a feasible and accurate numerical model which allows the quality of a bonded joint as function of the peeling load to be determined. The idea is then an actual alternative to offer in-situ test by means of a transportable and easy to use device. In this study, a research analyzing two tests able to identify the quality of a bonded joints, as an alternative to the interlaminar fracture test, based on a classic peeling test on carbon laminates, is presented. The numerical simulation is based on a Finite Element Analysis using the commercial software Abaqus®. Onset and propagation of adhesive damage is taken into account by means of a cohesive zone model. A virtual test of the peeling test is done and its behavior is compared with the experimental results from an extensive laboratory test campaign. Numerical results will allow us to verify the feasibility to compute the value of G1c from experimental results.