| Resumo : |
The need for advanced damage tolerant design of components in the aerospace industry promotes a demand for highly efficient materials with known damage evolution properties, and consequently accurate property characterization procedures. An investigation of the translaminar fracture behaviour of a composite laminate is presented in this work regarding the assessment of resistance properties to crack propagation under static and fatigue loading and how the parameters obtained can be interpreted looking through an application perspective. The laminates constituted of carbon fibre-epoxy plain weave were manufactured by resin infusion under flexible tooling (RIFT). Pre-cracked compact tension (CT) specimens were used to perform both quasi-static and fatigue tests. Different data reduction techniques for fracture toughness calculation were used and compared to each other in terms of accuracy and reliability. The standard fracture toughness evaluation method ASTM E399 developed for isotropic materials was modified to account for the material orthotropy and specimen geometry effects using correction functions, based on both experimental and numerical evaluations of the strain energy release rate. The proposed modifications show good agreement against each other and its application was validated for reduction of fatigue data. Fatigue testing showed that failure in undesired modes is likely to occur prior to translaminar fracture for the used specimen configuration, and that the achievement of fatigue crack propagation itself is very challenging. Which was attributed to a much higher fatigue threshold associated with crack propagation than the fatigue threshold related to ordinary shear stresses. A strategy for achieving adequate crack growth was developed and consisted of maintaining the severity of the crack damage while decreasing other types of damage severity by controlling the initial notch length. The experimental results were compiled and represented by different approaches of fatigue crack growth resistance evaluation, namely, Paris and Forman approaches, and their particularities were discussed. A fractographic analysis was carried out to define damage patterns and its evolution process, as well as to correlate the composite microstructure influence on the propagation behaviour observed in the tests. |