Étude exploratoire sur le comportement en fatigue d’un composite à renfort tissé

Abstract

The growing trend in the aerospace industry to use composite material brings some interesting challenges. For example, the fatigue behaviours of composite that are exposed to the environmental conditions of an airplane are not well understood. This paper is part of a larger CRIAQ project on the flaw growth thresholds in a plain weave carbon fibre and thermoset epoxy composite with embedded internal flaw. Specifically, the present work as four objectives: designing, fabrication and validation of the tension-compression and fully reversed bending tests apparatus, determining tests protocols, quantify the effect of high temperature on the fatigue life in tension-tension and quantify at room temperature the fatigue life in fully reversed bending. Quasi-static tension and tension-tension fatigue tests were performed at high temperature. The results in quasi-static testing indicate that there is a small reduction of the maximum strength at high temperature. Furthermore, the specimen fracture doesn’t necessarily go through the imbedded flaw. The fatigue failure criterion was defined to be the start of propagation of the imbedded flaw. At high temperature, it was determined that the flaw starts to propagate when there’s an increase of 5% of deformation calculated after the initial deformation. There’s a fatigue life reduction in fatigue tension-tension at same applied load compared to room temperature. An equation was developped to interpolate specimen fatigue life for a temperature between room and high temperature. During priliminary tests, misalignment on the MTS test bench lead to specimen buckling. Thus, tension-compression test apparatus was not validated. The fully reversed bending test apparatus works as intended, but needed some modifications to reduce specimen installation time. Next, quasi-static bending and fully reversed fatigue bending tests were done at room temperature. In quasi-static bending, the specimen all broke at the loading nose. Surface microscopy suggests that cracks follow the plain weave pattern. In fatigue, the test frequency traget was not met because of the specimen flexibility and available test equipments. A much lower test frequency was used to complete the task. Failure criterion was not defined in fully reversed bending since the flaw did not propagate. The damage found, as delamination of the first plies, is larger than the loading nose contact area. At 55% of the bending strength, a surface crack and fibres breakage inside the crack was found. These observations suggest that the loading nose pressure is not the only factor causing the delamination.

Date	12 Feb 2015
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Anh Dung Ngô (Supervisor) & Martine Dubé (Co-supervisor)