Effect of cryogenic conditions on mechanical properties of polypropylene composite (GF50-PP) produced by thermocompression process

The thermocompression molding process allows composites to be made with longer fibers compared to injection molding and extrusion. However, during thermo-compression molding fiber-reinforced composites are exposed to complex effects, which can lead to specific fiber orientation/distribution and therefore affect the mechanical properties. The aim of this work is to study the effect of fiber orientation on the mechanical properties of the long-glass-fiber-reinforced polypropylene (GF50-PP) composite produced by the thermocompression process. Moreover, due to the application of this composite in cold liquid gas storage, it is interesting to study this effect under cryogenic conditions. In this study, composite plates of GF50-PP were cut to analyze the effect of fiber orientations: 0°, 45°, and 90° with respect to the Mold Flow Direction (MFD). After microstructural observations by SEM microscopy and physico-chemical analysis, the mechanical behavior of GF50-PP samples was examined at monotonic tensile loading and tension-tension fatigue at two loading temperatures of 20°C and -70 °C. The results of the tensile tests revealed that the values of stress at break were almost similar at 20°C and -70°C for different fiber orientations of 0°, 45° and 90° and that the temperature effect for the two test conditions is almost insignificant. The tensile-tension fatigue tests at 20°C show the significant effect of the orientation of the fibers when the amplitude of deformation is important. On the other hand, when the amplitude is small, this effect is negligible. In this case, all samples exhibit almost the same behavior. However, during fatigue tests at -70 ° C (cryogenic temperature), the material shows almost uniform linear behavior, but still sensitive to the orientation of the fibers.