Hygrothermal viscoelastic material characterisation of unidirectional continuous carbon-fibre reinforced polyamide 6

Abstract

This paper presents results of material characterisation experiments on the hygrothermal viscoelastic behaviour of unidirectional laminates of continuous carbon-fibre reinforced polyamide 6. The material behaviour when subjected to the automotive painting process is of interest. Coefficients of thermal- and -moisture expansion were determined from dilatometer experiments and micrometer measurements together with weighing, respectively. Diffusion coefficients were generated from thermogravimetric analysis and fitted with the Arrhenius equation. Dynamic mechanical analysis and digital image correlation of quasi-static tensile tests were performed to obtain a relaxation curve and a major Poisson's ratio, respectively. The Williams-Landel-Ferry equation was fitted to the time shift factors.

Introduction

The application of composite materials in the body structure of passenger vehicles has seen a gradual increase over the past few years due to the efforts made in the automotive industry to restrain the trend of increasing kerb weight [[1], [2], [3]]. Structural composite parts in the automotive industry most often feature a thermoset matrix due to lower raw materials costs in comparison to a thermoplastic equivalent [4]. However, legislative changes to the requirements on recyclability of newly produced vehicles have led to the interest in utilising polymers with a thermoplastic nature because of the possibility to recycle [5]. Contrarily, composites made of thermoset matrix material cannot be remelted because of cross-linking [6,7]. The matrix- and fibre material of a continuous fibre-reinforced thermoplastic can be separated by means of melting since a melting temperature exists for thermoplastics. Nevertheless, recyclable composites are often still landfilled due to the difficulties in physically recovering them due to strong connections to other parts [8].

In addition to providing recyclability advantages, thermoplastic composites benefit from rapid processing, improved fracture toughness and joining possibilities through welding [[9], [10], [11], [12], [13]]. It must be kept in mind that adapting the conventional automotive process chain of existing vehicle models to optimise the manufacturing process for thermoplastic matrix materials is not sensible from an economical perspective. It must therefore be noted that composite parts are connected to the body-in-white (BIW) in the body shop, which occurs prior to entering the paint shop. The consequence is that structural parts with a thermoplastic matrix, intended for existing vehicle models, are subjected to the automotive paint shop. The BIW is passed through a series of drying ovens during the automotive paint shop in which a maximum temperature up to approximately 190 °C is encountered [14,15]. This temperature surpasses the glass transition temperature of economically sensible thermoplastic matrix materials and the deformation behaviour must be investigated to be able to safeguard the intended geometry and functioning of these structural parts.

The deformation of polymer composites is governed by thermal- and hygroscopic expansion in combination with clamped boundary conditions [16,17]. The automotive painting process is of special interest due to the deformation that is caused by the imposed thermal loads. The material characterisation techniques in this study were carried out with polyamide 6 (PA6) as matrix material and were aimed at identifying all necessary material parameters to accurately model the deformation behaviour of thermoplastic composite structural parts during the automotive painting process. It is noted that PA6 is a hydrophilic polymer and absorbs moisture by diffusion [[18], [19], [20]]. Hence, composite structural parts with PA6 as matrix material display an increase in moisture concentration as a result of the pre-treatment and cathodic dip painting process. It follows that these parts enter the dryers with a non-zero moisture content. The moisture diffusion of various polyamides is often conform to Fickian behaviour and can be modelled accordingly [[21], [22], [23]].

Additionally, thermoplastic materials show some polymer-specific phenomena. Recrystallisation occurs during the application of the thermal load and results in a higher degree of crystallinity that increases the density of the polymer and yields post-shrinkage [[24], [25], [26]]. Specifically to PA6, the application of a thermal load also causes a transition of the polymer crystals with a γ-form to an α-form, leading to an increase in density as well [27,28]. The effects of recrystallisation and change in crystal form can be modelled by utilising temperature-dependent coefficients of thermal expansion (CTE), analogous to the approach taken by Heinle and Drummer [29]. Baran et al. also report the importance of including shrinkage due to crystallisation in the modelling of composite manufacturing processes [30]. The determination of a CTE is often carried out by means of a dilatometer, as demonstrated by Gabrion et al. [31]. Characterising the viscoelastic properties of a thermoplastic composite is often performed through dynamic mechanical analysis (DMA) [[31], [32], [33], [34], [35], [36]]. Ma et al. proposed a method to measure hygroscopic expansion by means of DMA [37]. Mechanical models to describe the long-term stress relaxation behaviour of structural members are presented by Ascione et al. [38] and Berardi and Mancusi [39,40]. The long-term creep behaviour of glass-fibre-reinforced polymers (GFRP) was studied by Berardi et al. [41] and successfully modelled by means of Burger's model. It was concluded that long-term creep effects are relevant in GFRP laminates manufactured with an epoxy matrix. Long-term stress relaxation behaviour of thermorheologically simple materials can also be described by experimental data obtained during short-term testing through the time-temperature superposition principle [42].

This paper presents the results of several material characterisation methods that have been carried out with specimens taken from unidirectional (UD) laminates of continuous carbon-fibre reinforced PA6. The objective of the study is to identify the hygrothermal- and mechanical response of these laminates. This information will be used later as input for a numerical model to simulate the deformation behaviour of multi-directional laminates, manufactured with identical material, subjected to the automotive painting process.