Compressive Failure of NCF Composites

Abstract: The necessity to reduce environmental impact promotes transportation industry to reduce energy consumption of vehicles. One possible way to improve vehicles' structural efficiency is to utilize modern composite materials that offer excellent mechanical performance-to-weight ratio. Mass production of composite parts requires cost effective manufacturing methods. One potential rational method is to use dry textile preforms and liquid moulding methods, e.g vacuum infusion or resin transfer moulding. Among different types of textile preforms, non-crimp fabrics (NCFs) are most attractive for load bearing applications as they offer considerably higher in-plane mechanical properties compare to other textiles such as wovens or random mats. Composites manufactured with NCF fabrics are characterised by distinct fibre bundles separated by resin rich areas. These bundles are not perfectly straight but have a small yet significant waviness, both in-plane and out-of-plane. The waviness will influence the performance of NCF composites and especially the compressive properties. Design of structural parts made of NCF composites requires both a thorough understanding of the compressive failure process and effective failure prediction models. This is particularly relevant for the critical compressive loaded parts, such as bolted joints.The present work is concerned with the compressive failure of NCF composites and focuses on two major goals. First is to experimentally characterise the compressive failure process of various NCF composites and identify relevant damage modes and mechanisms. Secondly is to develop and propose suitable failure prediction models for reliable design of NCF composite parts with special emphasis on cost-effective methods relevant for industrial design processes.In the present work, a combination of experimental studies, modelling methods development and implementation of advanced state-of-the-art failure criteria have been performed. Optical methods were used to characterise the damage mechanisms in the material at different stress levels. This allowed both identification of the critical damage mechanisms and the whole damage progression sequence. Engineering models were developed to predict the compressive failure of NCF composites. In the models, the fibre bundles' waviness was dealt with in a cost-effective way. The models utilise a state-of-the-art failure criteria that predict both intra-laminar and inter-laminar damage. The proposed models demonstrated good accuracy in the predictions of both compressive and bearing failures. In addition, a cost-effective high-fidelity meso-scale modelling methodology was developed for bearing failure prediction of NCF composites. The methodology demonstrated a potential for cost-effective and highly detailed analysis of the bearing failure process and possible method for parameter studies of mechanical properties and their relation to the reinforcement architecture.