Late-stage fatigue damage in a 3D orthogonal non-crimp woven composite: An experimental and numerical study

Abstract

Late-stage fatigue damage of an E-glass/epoxy 3D orthogonal non-crimp textile composite loaded in the warp direction has been investigated using a combination of mechanical testing, X-ray micro computed tomography (μCT), optical microscopy and finite element modelling. Stiffness reduction and energy dissipated per cycle were found to be complementary measurements of damage accumulation, occurring in three stages: a first stage characterised by rapid changes, a more quiescent second stage, followed by a third stage where the (decreasing) stiffness and (increasing) energy dissipation change irregularly and then rapidly, to failure. Microscopy of specimens cycled into the transition between the second and third stages showed macroscopic accumulations of fibre fractures in sections of warp tows which lying adjacent to the surface weft tows which are crowned-over by the Z-tows. At these locations, the warp tow fibres are subjected to stress concentrations both from transverse weft tow matrix cracks and resin pocket cracks.

Introduction

Laminated composite materials based on 2D woven fabric plies having fibre reinforcement in two orthogonal directions have found ready applications in complex engineering structures. However, with only the polymer holding adjacent plies together, delamination is a common cause of failure, and a number of remedies have been proposed to enhance through-thickness properties, such as stitching and z-pinning [1], [2]. As reviewed in [3], a further option is to use 3D reinforced fabric preforms which have the advantage of incorporating through-thickness (Z-direction) reinforcing yarns that provide, in a controlled fashion, an extraordinary resistance to delamination unmatched by any other translaminar method of reinforcement. Three variations of 3D warp-interlock woven preforms are available at present: orthogonal, through-thickness angle interlock and layer-to-layer angle interlock. Of these variations, the orthogonal preforms (where through-thickness yarns pass through the entire preform thickness along a nearly vertical path) generally yield the highest volume fraction for the composite leading to the best in-plane mechanical properties. One particular preform type was termed “non-crimp 3D orthogonal weave” in [3], [4], [5], [6] and “3D non-crimp orthogonal weave” in a more recent publication [7]; we will use the latter term, and its acronym 3DNCOW, here. This type of preform has already found numerous civilian and military applications, and many more are envisioned in the future, in areas such as ballistic and blast protection (e.g. in personnel and vehicle armour shields), marine structures, wind energy (wind turbine spar caps), aircraft structures (from small components to primary load-bearing structures) and in civil engineering (decks, beams, columns).

To date, however, there has been limited progress both with understanding the mechanical behaviour of 3DNCOW composite materials and with computational modelling due to their geometrical complexity. Nevertheless, a number of studies have contributed to an increasing understanding of their internal geometry and behaviour for quasi-static loading cases (see, for example [4], [5], [6], [7], [8], [9]), with a growing number of studies on fatigue loading of 3DNCOWcomposites and related 3D structures [10], [11], [12], [13], [14], [15], [16]. Several studies of textile composites have used sectioning and microscopy of sacrificial specimens to investigate damage development, with an increasing use of X-ray micro computed tomography (μCT) to characterise the actual microstructure and damage accumulation behaviour so that comparisons can be made with modelling predictions [17], [18], [19], [20].

With regard to modelling, a wide range of strategies for predicting different mechanical properties of 3D woven composites (e.g. elastic properties; impact behaviour) were reviewed in [21]. Fewer publications are available on numerical simulations of progressive failure and strength predictions of 3D woven composites under quasi-static loading [9], [22]. Of relevance to the current paper, to the best of the authors’ knowledge, there are no published works available providing a theoretical basis for understanding progressive damage development in 3D woven composites under fatigue loading.

This work is concerned with contributing to the understanding of the fatigue failure of 3DNCOW composites by focussing on the development of late-stage fatigue damage; interpretation of the experimental results is assisted by a 3D finite-element analysis.