Material behaviour Tensile behavior of thermoplastic composites including temperature, moisture, and hygrothermal effects Mohammadreza Eftekhari, Ali Fatemi * Mechanical, Industrial and Manufacturing Engineering Department, The University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606, USA article info Article history: Received 29 January 2016 Accepted 8 March 2016 Available online 11 March 2016 Keywords: Tensile behavior Elevated temperature Temperature effect Moisture effect Hygrothermal effect abstract An experimental study was conducted to investigate the effect of temperature, moisture, and hygro- thermal aging on the tensile behavior of thermoplastic composites. Four different composites including talc filled and short glass fiber reinforced polypropylene, short glass fiber reinforced polyamide-6.6, and a blend of polyphenylene ether and polystyrene with short glass fibers were used for the study. Kinetics of water absorption and desorption were investigated for polyamide-6.6 composites and Fickian behavior was observed. The reductions in tensile strength and elastic modulus due to water absorption are rep- resented by mathematical relations as a function of moisture content. In addition to moisture content, aging time was also found to influence the tensile behavior. A parameter is introduced for correlation of normalized stiffness and strength with different aging times and temperatures. Higher strength and stiffness obtained for re-dried samples after aging was explained by an increase in crystallinity. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The use of thermoplastic composites in load-bearing applica- tions, including in the automotive industry, is constantly increasing due to a large number of advantages they present. These include ease of processing for complex geometries at high production rate, outstanding cost to performance ratio, ability to reprocess and corrosion resistance. Fillers such as talc and reinforcements such as glass are widely used to improve strength, elastic modulus and stability of neat thermoplastics. Components made of thermoplastic composites have applica- tions in harsh environmental conditions such as elevated temper- ature and moisture, as well as their combination which is known as hygrothermal effect. For example, most under the hood (bonnet) parts in an automobile are subjected to elevated temperature which can be as high as 130  C because of engine temperature and weather conditions, as well as moisture because of weather which can be 100% RH. Mechanical performance of all thermoplastic composites is highly temperature dependent, while the effect of moisture on their mechanical properties is dependent on matrix composition, fiber type and fiber/matrix interface. Tensile properties as the basic material properties are still widely used in design. A simple tension test provides valuable in- formation about stiffness, strength, toughness and ductility of a material. Fiber content, length, diameter and direction can affect tensile properties of polymer composites. These aspects have been studied in previous studies, such as in Refs. [1,2]. In a recent liter- ature survey by Eftekhari and Fatemi [3] a broad range of effects on tensile properties of short fiber reinforced polymer composites including temperature, moisture and hygrothermal effects was presented. Tensile strength and elastic modulus of thermoplastic compos- ites have been observed to decrease with increasing temperature, with a sudden drop near the glass transition temperature, T g [4,5]. Strain at fracture increases with increasing temperature, particu- larly at temperatures above T g, as molecular chain motion of poly- mers intensifies. This effect is much more accentuated in unreinforced materials than in reinforced materials [6]. The tem- perature dependency of tensile properties is generally more matrix dominant when loaded in the normal to the molding direction than when loaded in the molding direction. Tensile properties are more fiber dependent in the molding direction and temperature sensi- tivity is less [5]. Under tensile loading, fiber pull-out and matrix brittle fracture are the dominant failure mechanisms at low tem- peratures or high strain rates, while failure occurs via matrix crazing and crack propagation near the fiber ends at elevated temperatures or low strain rates [7]. Time-temperature * Corresponding author. E-mail addresses: mohammadreza.eftekhari@rockets.utoledo.edu (M. Eftekhari), afatemi@eng.utoledo.edu (A. Fatemi). Contents lists available at ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest http://dx.doi.org/10.1016/j.polymertesting.2016.03.011 0142-9418/© 2016 Elsevier Ltd. All rights reserved. Polymer Testing 51 (2016) 151e164