Experimental and numerical research on the nonlinear creep response of polymeric composites under humid environments

Abstract

The aim of this work is to study the nonlinear creep response of polymeric composites under humid environments. A three-dimensional orthotropic nonlinear creep constitutive model that considers the moisture effect is derived by introducing the moisture shift factor into Schapery’s nonlinear equation. Hygrothermal-mechanical coupling moisture absorption tests, as well as static mechanical property tests and short-term tensile creep-recovery tests with different moisture contents were performed to obtain the hygroscopic parameters and the moisture and stress dependent nonlinear creep parameters. A fully coupled numerical prediction model was developed in Abaqus with a user-defined material subroutine (UMAT) and user-defined field variable subroutine (USDFLD) to simulate the coupled diffusion-nonlinear creep behaviour. Good agreement was observed between the numerical and experimental results.

Introduction

Fibre reinforced polymer composites (FRPs) are becoming more common in the ships, aerospace, wind energy, civil engineering and anchor protection industries, due to their outstanding properties, including a high strength-to-weight ratio, good corrosion resistance, easy of design and low maintenance [1], [2], [3], [4]. However, the mechanical response of polymeric composites is time- and environmental-dependent due to the viscoelastic nature of the polymer matrix. The lack of techniques for predicting the long-term performance under the combined effects of mechanical loading and aggressive environment is currently compensated by applying large safety factors, which limits the widespread use of composites. Therefore, it is necessary to develop a method for predicting the long-term mechanical behaviour of FRPs after exposure to combined mechanical and environmental conditions.

Humidity and heat are the most common factors that have a substantial effect on the mechanical properties of composites. Moisture can not only reduce the quasi-static strength and Young's modulus, but it also substantially increases the creep rate of composites [5]. The moisture absorption of composites is a complex process, that can be affected by the temperature, the relative humidity, the stress–strain state and microdamage such as holes and cracks. Many studies have been carried out on the moisture absorption behaviour of composites under multifactor coupling conditions [6], [7], [8], [9], [10].

However, studying the moisture absorption behaviour of materials is not the ultimate goal, whereas the ultimate goal is to further study the effect of environmental factors on the mechanical properties. Mohan [11] carried out experimental research on the nonlinear creep-recovery behaviour of epoxy resin and its composites under different temperature and humidity conditions, which resulted in the conclusion that the moisture content and temperature have a similar influence on nonlinear creep. Additionally, Kontaxis [12] indicated that moisture can increase the bending creep strain of carbon/epoxy composites. Wang [13] developed an isotropic nonlinear viscoelastic constitutive equation to predict the long-term mechanical behaviour of adhesive joints under the coupling effect of temperature and humidity. Guedes et al. [14] predicted the long-term creep failure behaviour of glass fibre/epoxy composite pipes in aqueous environments. The results showed a strength reduction of approximately 60%, corresponding to a 50-year lifetime.

The long-term mechanical behaviour of composites under the combined effect of environmental conditions and mechanical loading is a complicated transient multi-physics coupling problem. It is expensive and inefficient to study the mechanical performance of actual structures during their service lifetime by experimental procedures. Therefore, numerical methods are increasingly used to predict the long-term performance of structures under the coupling of multi-physics problems. To date, research studies can be divided into two categories. One category is the sequence-coupled modelling category, that is, the moisture concentration is obtained through a mass diffusion analysis and then transferred to a stress analysis problem to investigate the mechanical behaviour of structures [15], [16], [17], [18], [19], [20]. The other category is the fully coupled modelling category, that is, moisture diffusion and stress analysis are completed in one step, so there is no need to transfer data and transform element type [21], [22], [23], [24].

However, coupled situations where moisture absorption and creep occur simultaneously in composites have not been studied. In the present work, prior to numerical modelling, a three-dimensional orthotropic nonlinear creep constitutive model considering the moisture effect is derived by introducing the moisture shift factor into Schapery’s nonlinear equation. Then, experimental studies were performed on composite laminates to obtain the hygroscopic parameters and the moisture- and stress-dependent nonlinear creep parameters. Based on the measured material data, a three-dimensional numerical model was then established to predict the fully coupled diffusion-nonlinear creep behaviour of composite laminates.