Static, fatigue and impact behaviour of an autoclaved flax fibre reinforced composite for aerospace engineering

Abstract

This work describes the physical and mechanical characterization of unidirectional [0]₁₂ and crossply [(0/90)₃/$\bar{0}$]_S flax fibre reinforced composites fabricated in autoclave using a prepreg flax tape impregnated with fire retardant epoxy polymer. Tensile, bending and impact properties are evaluated along the longitudinal and transverse fibre directions. The tensile-tensile fatigue behaviour is characterised along the fibre direction. Physical and specific properties are also assessed to identify the potential characteristics of these bio-based composites for lightweight and secondary loadbearing applications. The robust manufacturing process described in this work, coupled with precision laser cutting, makes this type of composite a promising sustainable material for aircraft, transport and lightweight construction designs.

Introduction

Flax, also known as common flax or linseed, is a member of the genus Linum in the family Linaceae. It is a food and fibre crop cultivated in cooler regions of the world [1]. Flax fibre is extracted from the bast beneath the surface of the stem of the flax plant. Within eight weeks of sowing, the plant can reach 100–150 mm in height and grows several centimetres per day under its optimal growth conditions, reaching 700–800 mm within 50 days [2]. At the microscopic scale, each elementary fibre is itself made of concentric cell walls, which differ from each other in terms of thickness and arrangement of their constitutive components. At the centre of the elementary fibre, the concentric cylinders with a small open channel in the middle called the lumen, which contributes to water uptake. The outer cell wall designed as the primary cell wall is only 0.2 μm thick. On the outer side, the thin primary cell wall coats the thicker secondary cell wall which is responsible for the strength of the fibre and encloses the lumen. Each layer is composed of microfibrils of cellulose which run parallel one to another and form a micro fibrillar angle with the fibre direction; this angle is minimum in the secondary cell wall. This thickest cell wall contains numerous crystalline cellulose micro-fibrils and amorphous hemicellulose which are oriented at 10° with the fibre axis and give fibre its high tensile stiffness and strength [2,3].

There is a significant amount of work in the open literature regarding the use of plant fibres, and in particular flax as reinforcement of composite materials [4]. Most of them are in the preliminary research and manufacturing stage and still require research efforts to address semi-structural and multifunctional applications. Among this abundant literature, only a few works focus on structural applications. Regarding flax, as recently reported by Blanchard and Sobey [5], only a few studies out of the hundreds published in recent years study the structural scale [[6], [7], [8], [9]] and even less investigate the applicability of flax fibre-reinforced laminates in aerospace [10,11]. In fact, plant fibre composites are good candidates to be used for lightweight structural applications due to their high specific properties, however, there are still many technological and scientific barriers to break down to obtain fully optimised biocomposites for structural applications and high-added value products. It is mainly concerned with improving material durability, refining predictive models and developing robust design methods.

Several European projects (BRIGHT, NATEX, TEXFLAX, BIOBUILD, SSUCHY) have worked to manufacture aligned and continuous reinforcements from discontinuous technical plant fibres, particularly flax fibres [12]. Textile methods, involving fibre spinning and weaving of spun yarns have been shown to have several detrimental effects on composite properties. In addition to the high cost of these operations, it leads particularly to fibre misalignment and hinders resin impregnation, as well as requiring high energy consumption. To overcome these difficulties, some processes have been developed to produce tapes with perfectly aligned flax fibres [13] or fabrics made from low-twisted hemp rovings [14,15]. Currently, the only mature and commercialised unidirectional plant fibre continuous reinforcement is based on flax fibres and produced by the company Lineo-Ecotechnilin (FlaxTape™).

This work proposes an investigation into the mechanical performance of a flax/epoxy composite that meets the requirements of AC 25-853a standard [16] in terms of self-extinguishing. In addition to these fire-retardant properties and weight constraint, the main requirements to fulfil the specifications and certification rules for semi-structural parts in interiors of aircrafts are mechanical properties (static, fatigue and impact), vibroacoustic properties and environmental compliance (humidity, gas/vapour emission), i.e. all solicitations that play a critical role in the service life of the composite. This paper focuses on the mechanical behaviour, including static, fatigue and impact characterization of autoclave prepreg flax composites considering two stacking sequences: unidirectional [0]₁₂ and crossply [(0/90)₃/$\bar{0}$]_S.