Behaviour and material properties of composites for crash modelling

Licentiatavhandling, 2016

The transport industry must find solutions to reduce its impact on climate change. A promising way to reduce the weight of vehicles and therefore to reduce the CO2 emissions is to introduce components made of lightweight composite materials, in particular laminated carbon fibre reinforced polymers (CFRPs). Aside from the new design possibilities for lighter vehicle structures, CFRPs can also potentially offer large improvements in terms of energy absorption capabilities in comparison to traditional crash components made of metals. During crushing of composites, a large amount of energy is absorbed through stable progressive failure of the material. The crushing process is a complex phenomenon because it is driven by the combination of many failure mechanisms and frictional effects. A limited amount of research has been performed on crushing of composites, mainly because crashworthiness is not a critical requirement in the aerospace industry (the predominant market for advanced composites today). As a result, there is today no reliable numerical tool to predict the crashworthiness of CFRP structures, which is a hindrance to the introduction of composite materials in mass-produced automobiles. Joint research efforts from both numerical and experimental perspectives are needed to fill this void and reach the goal of reliable crash predictions of composite vehicles. The focus of this thesis is on a material characterisation strategy for crash modelling of composites. An experimental methodology is developed to provide relevant and accurate input to a physically-based material model for crash, currently being developed in parallel to this thesis. The material selected for this research is a CFRP with non-crimp fabric (NCF) reinforcements. The first step in the material characterisation is to extract the different strengths and stiffnesses of the material, which requires dedicated tests because of the orthotropic nature of NCF composites. In a second step, more specific inputs to ply damage models for progressive failure are extracted from experiments. Those parameters are (1) damage evolution laws, identified from Iosipescu shear tests, and (2) the longitudinal and the transverse crushing behaviour of unidirectional laminates, extracted from relatively simple crush tests on flat specimens.