Pseudo-ductile effects in ±45∘ angle-ply CFRP laminates under uniaxial loading: Compression and cyclic tensile test

Abstract

The capacity of symmetric ±45∘ angle-ply laminates of carbon fibre reinforced polymer to hold large strains before final failure is studied in uniaxial tensile and compressive loading. In particular, [±45]2S and [±45]4S laminates are tested under tension and compression respectively. Both present a three-staged stress–strain response with a first linear evolution followed by a plateau and a final strain hardening. Thanks to the Loading–Unloading–Reloading tensile test, the pseudo-ductile process is analysed in terms of the strain energy. In the first two stages the percentage of the total energy recovered in each cycle follows a similar correlation than the relation of the apparent tensile stiffness with the initial one. Therefore, assuming a similar pattern in both tensile and compressive loading cases, the mechanical response during the linear and plateau stages undergoes a progressive damage with a stiffness reduction related to the dissipated energy. However, a damage model that includes the influence of the fibre’s reorientation would not predict the re-stiffening of the third stage. In this work the last stage of strain hardening is assumed to follow a different pattern because of the microstructural changes of the matrix during the plateau stage. These are promoted by strain localisation processes both under tension and compression, with dimensional changes in perpendicular to the loading direction that are observed thanks to the strain fields obtained by Digital Image Correlation. Finally, the main differences between both loading cases are evidenced with the help of the final failure modes.