Elsevier

Composites Part B: Engineering

Volume 72, April 2015, Pages 116-129
Composites Part B: Engineering

Effects of fiber orientation and anisotropy on tensile strength and elastic modulus of short fiber reinforced polymer composites

https://doi.org/10.1016/j.compositesb.2014.11.041Get rights and content

Abstract

An experimental study was conducted to investigate anisotropy effects on tensile properties of two short glass fiber reinforced thermoplastics. Tensile tests were performed in various mold flow directions and with two thicknesses. A shell–core morphology resulting from orientation distribution of fibers influenced the degree of anisotropy. Tensile strength and elastic modulus nonlinearly decreased with specimen angle and Tsai–Hill criterion was found to correlate variation of these properties with the fiber orientation. Variation of tensile toughness with fiber orientation and strain rate was evaluated and mechanisms of failure were identified based on fracture surface microscopic analysis and crack propagation paths. Fiber length, diameter, and orientation distribution mathematical models were also used along with analytical approaches to predict tensile strength and elastic modulus form tensile properties of constituent materials. Laminate analogy and modified Tsai–Hill criteria provided satisfactory predictions of elastic modulus and tensile strength, respectively.

Introduction

Applications of short fiber reinforced polymer composites (SFRPCs) have been rapidly growing, particularly in the automotive industry. They are easily produced materials with the capability of being manufactured in complex geometries. A high production rate by manufacturing techniques such as injection molding and lower cost are significant advantages of SFRPCs over continuous fiber composites.

Mechanical performance of SFRPCs strongly depends on the number of fibers aligned with the loading direction. The geometry of part, process variables such as position and number of injection gates, pressure and temperature of mold, and filling time are some of the factors controlling orientation of fibers in injection molding process. Even in a simple plate-type geometry, the orientation distribution of fibers is not uniform across the thickness and a layered morphology has been reported, consisting of two near the wall shell layers and a core layer [1].

Microscopic techniques such as X-ray radiography, electron microscopy and optical diffraction can be used to investigate orientation distribution of fibers. However, performing tensile tests on specimens prepared in various mold flow directions is a common and simple approach to evaluate anisotropy of SFRPCs. In this technique, local distribution of fibers and non-homogeneity are being averaged throughout the specimen volume [2].

Wang et al. [3] studied tensile properties of short glass fiber reinforced polyamide-6 with specimens machined parallel and perpendicular to the mold flow direction and tested at different strain rates and temperatures. Tensile strength and elastic modulus in the mold flow direction were a factor of two higher than those in the direction perpendicular to the flow direction. Zhou and Mallick [4] conducted an experimental study on tensile behavior of short glass fiber reinforced polyamide-6.6 with specimen axis parallel and perpendicular to the injection molding direction. Tensile strength and elastic modulus decreased about 35% and 43%, respectively, as specimen axis increased from in-flow direction to perpendicular-to-flow direction.

De Monte et al. [2] studied effect of mold flow direction on tensile behavior of short glass fiber polyamide-6.6 using 1, 2, and 3 mm thickness specimens with their axes at 0°, 10°, 20°, 30°, 45°, 60°, and 90° directions with respect to the mold flow direction. Tensile strength and elastic modulus decreased as specimen axis increased. Variation of elastic modulus with specimen axis was less pronounced for thicker specimens. This effect was attributed to formation of shell–core morphology across the specimen thickness which was confirmed by SEM and mold flow simulation. In shell layers fibers were aligned in the mold flow direction and in the core layer fibers were disoriented. The ratio of the core layer thickness to the specimen thickness was 0.2 and 0.33 for 1 mm and 3 mm thickness, respectively.

Micro- and macro-mechanical methods were applied to calculate tensile properties of SFRPCs in [2]. In the micro-mechanical technique, unidirectional tensile properties of the composite were computed by using tensile properties of fibers and matrix. Orthotropic tensile properties of elements were then computed by weighting average of unidirectional properties by an orientation distribution function. In the macro-mechanical method, the material was assumed to be transversely isotropic, neglecting real orientation distribution of fibers and the presence of core layer. Theories of linear elasticity for orthotropic materials were then applied to estimate the variation of tensile properties with specimen angle.

Fiber length variation and its effect on mechanical properties of short fiber polymer composites has been investigated in several studies. Heim et al. [5] introduced a method to determine the critical fiber length in a short carbon fiber composite and the transition of failure from fiber pull-out to fracture was correlated with the critical fiber length. In a study by Fu et al. [6] the effect of thermoplastics ratio on mean fiber length and mechanical properties of rubber-toughened polyamide-6.6/polypropylene blends was studied. With increasing PA66/PP ratio, the composite strength increased, while the elastic modulus decreased. This effect was explained by variation of interfacial adhesion as well as fiber length with thermoplastics ratio.

A number of models have been developed which relate elastic modulus of short fiber composites to mechanical properties of the constituent materials. Cox [7] established the first mathematical model for elastic modulus of fibrous materials, named Cox’s shear-lag model, based on axial equilibrium of a single fiber. Halpin–Tsai [8] developed a model based on the self-consistent approach [9], [10], where a single fiber is embedded in an infinite matrix which has the average mechanical properties of the composite [11]. This approach is originally credited to Hill [12] and Budiansky [13] which was extended to short fiber composites by Law and McLaughlin [14] and Chou et al. [15]. Halpin–Tsai equations were provided to estimate the longitudinal and transverse elastic moduli of unidirectional short fiber composites. Halpin–Tsai equations were modified by considering maximum volumetric packing fraction of fibers which correct for volume fraction of fibers due to contact between fillers [16], [17]. A comparison between Halpin–Tsai model and experimental elastic modulus of unidirectional discontinuous nylon fiber/rubber composite was done and correlations between the theory and experiment were obtained [8].

Halpin and Pagano [18] used the laminate analogy theory to estimate elastic modulus of randomly oriented short fiber composites. In this theory, several laminae of unidirectional short fiber composites in different directions were considered. Manera [19] utilized a larger number of laminae with different directions to predict the elastic modulus of random short fiber composites. Fu and Lauke [20] employed the laminate analogy approach along with probability density functions for distribution of fibers to estimate elastic modulus of partially aligned short fiber composites. Elastic modulus of polyester and polypropylene polymers reinforced with short glass fibers was estimated with the laminate analogy approach and good correlations were found between the theory and experimental results [19], [20].

Christensen and Waals [21], [22] predicted elastic modulus of two and three dimensional random short fiber composites based on equivalent stored energy of a homogenous material. Fukuda and Kawata [23] and Jayaraman and Kortschot [24] determined the total force in short fiber composites by attaining a total force each fiber can endure. The Fukuda and Kawata theory was in agreement with experimental elastic modulus results of discontinuous glass fiber reinforced epoxy composite [23]. Pan [25] developed a method based on the relation between fiber volume fraction and area fraction to calculate elastic modulus of short fiber composites. Liang [26] correlated experimental elastic modulus of short inorganic fiber reinforced polymer composites with a micromechanical model, by introducing an interfacial strength factor.

Mathematical equations have also been developed to predict tensile strength of SFRPCs from tensile properties of matrix and fillers. Kelly and Tyson [27] developed a tensile strength prediction model from equilibrium of a single fiber. Piggott [28] modified the Kelly and Tyson model by considering that some of the transferred stress from matrix to fiber is through the plastic deformation of matrix. Chen [29] established a method based on the distortion energy criterion to predict the strength of aligned short fiber composites. Failure mechanisms including fiber failure and matrix failure in either shear or plane strain were also considered to calculate tensile strength of random SFRPCs. Agreements were found between the theory and the experimental elastic modulus of randomly oriented glass epoxy composites. Kuriger et al. [30] used the modified Tsai–Hill criterion with consideration of fiber density functions and reasonably estimated tensile strength of a partially aligned short carbon fiber reinforced polypropylene.

Fukuda and Chou [31] utilized a critical zone concept to calculate the number of fibers which contribute to strength. According to this concept, fibers bridging an arbitrary zone contribute to strength of the composite. Fu and Lauke [32] considered the total force required to break fibers crossing an arbitrary section to calculate tensile strength of short fiber composites in terms of a modification of linear rule of mixture. Tensile strength calculation was close to experimental results of nylon, polypropylene and polybutylene resins reinforced with short glass fibers [32].

In this paper effects of anisotropy and fiber orientation on tensile behavior of two short glass fiber reinforced thermoplastics are investigated. Microscopic studies were conducted to evaluate fiber length, diameter and orientation distribution effects. Variations of thickness and its effect on tensile properties are also investigated. Mechanisms of failure and fracture paths in various mold flow directions were identified. A comparison is also made between tensile and compressive properties in different mold flow directions. A number of analytical and semi-analytical models are applied to predict tensile strength and elastic modulus from properties of matrix and fillers. Effects of temperature and strain rate on tensile behavior are discussed in [33].

Section snippets

Material

Two short glass fiber reinforced thermoplastics were considered for the experimental study, polybutylene terephthalate with 30 wt% glass fibers (here referred to as PBT), and polyamide-6 containing 35 wt% glass fibers and about 10 wt% rubber impact modifier (here referred to as PA6). State of fibers in matrix depends on injection molding condition, geometry of mold, volume fraction of fibers, initial length of fibers, and physical and viscoelastic properties of matrix. Fig. 1(a) shows the

Thickness and edge effects

Duplicate tensile tests on both 3 and 3.8 mm thickness samples were performed at 23 °C under displacement rate of 1 mm/min. Fig. 3 shows the effect of thickness on stress–strain behavior of 0° and 90° samples of PBT and PA6. The maximum strain in these plots corresponds to fracture strain. More significant effect of thickness is observed in 90° direction samples, as compared to the 0° direction samples and the effect is larger for PA6. This effect is due to formation of core and shell layers

Predictions of elastic modulus and tensile strength

Due to presence of rubber in PA6, tensile strength and elastic modulus of matrix were modified to a smaller value than the polyamide-6 values reported in Table 1 by using the linear rule of mixture. Therefore, tensile strength and elastic modulus of matrix in PA6 were used as 1.92 GPa and 52.7 MPa, respectively. Several prediction models described earlier are used to predict elastic modulus and tensile strength of the two considered short glass fiber composites based on the constituent material

Conclusions

Effects of fiber orientation and anisotropy on tensile properties of two short glass fiber reinforced thermoplastics were evaluated. Based on the observed experimental behavior and the analysis performed, the following conclusions can be made:

  • (1)

    A layered core–shell structure was observed across the thickness. The ratio of core to shell thickness can explain the effect of anisotropy in tensile behavior due to specimen thickness and its location in the injection molded plaques.

  • (2)

    Specimens machined in

Acknowledgements

This project was financially supported by General Motors. Technical assistance of Dr. A.K. Khosrovaneh and Mr. Charles Buehler at GM is appreciated.

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