Abstract

Abstract

High performance composites can be designed, and their properties tailored to enhance specific mechanical (or multifunctional) properties of the material. Fibre hybridization, which consists of combining multiple types of fibres in the same composite material, is a strategy that can lead to improved composite properties and performance, as it not only changes the material properties but also  the damage propagation mechanisms leading to final failure. In this work, two numerical frameworks are proposed to analyse the mechanics of failure of hybrid polymer composites. Firstly, a FEM framework is used to analyse the mechanisms that lead to longitudinal failure of composite materials. The effects of superimposed hydrostatic pressure on longitudinal tensile failure is analysed and a generalized reduction in strength with increasing pressure is predicted. Secondly, a dynamic Spring Element Model is proposed and used to analyse the dynamics of the formation of clusters of broken fibres and the effects of fibre dispersion in hybrid composites, concluding that higher fibre dispersion leads to improved material performance. Additionally, an experimental campaign on two different hybrid materials (inter and intratow) is performed to understand the effect of hybridization on fibre dominated properties, namely longitudinal tensile strength and translaminar fracture toughness. The interply hybrid material is obtained by stacking alternating T800 and HR40 thin-ply layers, while the intratow is obtained by combining the two types of fibres in the same layer using spread tow technology. A change in the damage mechanisms through hybridization is observed, which can lead to improved material performance.

Abstract

Nowadays, manufacturing of composite parts using out-of-Autoclave (OoA) processes is gaining importance because it allows a reduction of initial investments and recurring costs. Among these technologies, the consolidation of out-of-autoclave prepregs using vacuum bag only technologies (VBO) is proven to provide high-quality composite structural parts without the need of autoclaves.  OoA prepregs are designed to be semi-impregnated creating engineered channels within the tows to evacuate internal air or volatiles generated during the cure cycle. The aim of this work is to understand and explain the void formation mechanisms, their spatial distribution and transport mechanisms during the consolidation of OoA prepregs using vacuum bag only methods. This work includes parallel studies carried out using X-ray computed tomography (XCT) to determine the most important factors for air removal process in laminated composites produced by hand lay-up (HLU) and automated fiber placement (AFP). A big challenge is the accurate monitoring of the evolution of porosity during composite curing, which has not been done so far and even less with a 3D visual technique. This combination of monitoring the dynamics of pore evolution by laboratory XCT and synchrotron tomography (SXCT) at different resolutions during a cure process will provide a profound understanding of the pore evacuation process which is strategic for the further development of OoA materials.