Tension behavior of unidirectional glass/epoxy composites under different strain rates

https://doi.org/10.1016/j.compstruct.2008.06.012Get rights and content

Abstract

By considering wide applications of composite materials, having a proper knowledge of them under dynamic loading is necessary. In order to study the effects of strain rates on the behavior of the materials, special testing machines are needed. Most of the research in this field is focused on applying real loading and gripping boundary conditions on the testing specimens. In this study, behavior of unidirectional glass fiber reinforced polymeric composites under uni-axial loading is determined at quasi-static and intermediate strain rates of 0.001–100 s−1. The tests were performed using a servo-hydraulic testing apparatus equipped with a strain rate increase mechanism. For performing the tests, a jig and a fixture are designed and manufactured. The performance of the test jig was evaluated and found to be adequate for testing of composites. Dynamic tests results are compared with the results of static tensile tests carried out on specimens with identical geometry. Experimental results show a significant increase of the tensile strength by increasing the strain rate. The tensile modulus and strain to failure are also observed to increase slightly by increasing the strain rate.

Introduction

Composite materials can provide significant functional and economic benefits, ranging from increased strength and durability features to weight reduction. However, the mechanical responses of fiber-reinforced polymeric composites are sensitive to the rate at which they are loading. In many technological applications, under dynamic loading conditions, the response of a structure designed with static properties might be too conservative. The main reason is that mechanical properties of composites vary significantly with changing the strain rate. Unlike metals, which have been studied extensively over a wide range of strain rates, only limited amount of information is available on the effects of strain rate on the response of fibrous composites.

The above argument results the need for dynamic characterization of composite materials to understand the strain rate effects on their mechanical properties. Davies and Magee [1], [2] studied the effect of strain rates from 10−3 to 103 s−1 on the ultimate tensile strength of glass/polyester composites. They reported the glass/polyester composites to be rate sensitive with the magnitude of the ultimate tensile strength increasing by 55% over the strain rate change. Rotem and Lifshitz [3] investigated the tensile behavior of unidirectional glass fiber/epoxy composites over a wide range of strain rates from 10−6 to 30 s−1 and found that the dynamic strength is three times higher than the static strength and the dynamic modulus is 50% higher than the static modulus. However, while investigating angle ply glass/epoxy laminates, Lifshitz [4] found that the elastic modulus and failure strain were independent on the strain rate and the dynamic failure stress was only 20–30% higher than the static failure stress. The dependence of the transverse tensile properties on strain rate of a high performance carbon fiber/epoxy composite loaded in transverse tension was investigated by Melin and Asp [5]. Dog-bone shape specimens were tested under quasi-static and dynamic loading conditions (10−3–103 s−1). The average transverse modulus was observed to be independent of strain rate while the initial transverse modulus was found to decrease slightly with increased strain rate. The strain and stress at failure were found to increase slightly with increased strain rate. Thus, when loaded in the transverse direction it was concluded that the carbon/epoxy composite exhibited a weak dependence on strain rate.

Tensile tests were performed on a glass/epoxy laminate at different rates (1.7 × 10−2–2000 mm/s) by Okoli and Smith [6], [7] to determine the effects of strain rate on Poisson’s ratio of the material. Poisson’s ratio was found to be rate insensitive. It was suggested that the rate insensitivity of Poisson’s ratio of the laminates tested is due to the presence of fiber in the composites. In other studies the effects of rate sensitive in the range of speeds from 0.008 mm/s to 4 m/s on the tensile, shear, and flexural properties of glass/epoxy laminate were investigated by Okoli and Smith [6], [8]. The above observation was in agreement with the results of the investigation conducted by Armenakas and Sciamarella [9] at various rates of strain (0.0265–30,000 min−1), that suggested a linear variation of the tensile modulus of elasticity of unidirectional glass/epoxy composites with the log of strain rate. However, the ultimate tensile strain and stress of the composite decreased with the increase in strain rate. An increase in tensile, shear, and flexural energy of 17%, 5.9%, and 8.5%, respectively, per decade of increase in the log of strain rate was observed [8]. The study indicated that there is a change in failure modes as strain rate is increased.

A systematic study of the strain rate effects on the mechanical behavior of glass/epoxy angle ply laminates was done by Staab and Gilat [10], [11] using a direction tension split Hopkinson bar apparatus for the high strain rate tests (approximately 103 s−1) and a servo hydraulic testing machine for the quasi-static tests (approximately 10−5 s−1). The tensile tests at higher strain rates (in the order of 1000 s−1) showed a marked increase in the maximum normal stress and strain when compared to the values obtained in the quasi-static tests. Although both fibers and matrix are strain rate sensitive, the fibers were thought to influence laminate rate sensitivity more than the matrix. Harding and Welsh validated a dynamic tensile technique by performing tests (over the range 10−4 to 1000 s−1) on carbon/epoxy, glass/epoxy, glass/polyester, carbon/polyester, and Kevlar/polyester composites [12], [13]. The modulus, failure stress, and failure mode of the carbon/epoxy composite were found to be strain rate insensitive. The dynamic modulus and strength for the glass/epoxy composite were about twice the static value. Daniel et al. [14] investigated the dynamic tension response of unidirectional carbon/epoxy composites at high strain rates using an internal pressure pulse produced explosively through a liquid medium. In the test method used for dynamic testing of thin laminates in tension, a carbon/epoxy laminate was characterized under longitudinal, transverse, and in-plane shear loading at strain rates up to 500 s−1. In the longitudinal direction the modulus increased moderately with strain rate (up to 20% over the static value) but the strength and ultimate strain did not vary significantly. The modulus and strength increased sharply over static values in the transverse direction but the ultimate strain only increased slightly. There was also a 30% increase in the in-plane shear modulus and strength.

The tensile mechanical behavior of a short carbon fiber filled liquid crystalline polymer (LCP) composites (30% fiber weight), Vectra A230, was examined under static loading (10−2 s−1) and dynamic loading (400 s−1) by Shim et al. [15]. A pendulum-type tensile split Hopkinson bar device was used to apply dynamic tension. The fracture strain and Young’s modulus of the composite were found to be noticeably influenced by changes in the strain rate. Experimental studies on the effects of strain rate from 3 × 10−5 to 8 × 10−3 s−1 on the tensile properties of glass bead/HDPE (high density polyethylene) composites were conducted by Bai et al. [16]. Both Young’s modulus and the tensile strength of the glass bead/HDPE composite were found to increase with strain rate.

Hayes and Adams [17] constructed a specialized pendulum impactor to investigate the strain rate effects on the tensile properties of unidirectional glass/epoxy and carbon/epoxy composites. The modulus and strength of the glass/epoxy composites were concluded to be rate insensitive at impact speeds in the range of 2.7–4.9 m/s. However, the modulus and strength of the graphite/epoxy composites decreased with increasing impact speeds. Daniel and Liber [18], [19] attempted to characterize the effect of strain rate on the mechanical properties of unidirectional boron/epoxy, S-glass/epoxy, carbon/epoxy, and Kevlar/epoxy composites. While the Kevlar/epoxy composite showed a 20% increase in tensile modulus and failure strength in the fiber direction with increasing tensile strain rate from 10−4 to 27 s−1, the tensile modulus and failure strength of the boron/epoxy, S-glass/epoxy, and carbon/epoxy composites were found be rate insensitive. The increase in modulus and failure strength of the Kevlar/epoxy composite was 40% and 60%, respectively, during transverse and shear (off-axis) loadings.

Daniel et al. [20] and Chamis and Smith [21] studied the mechanical behavior of unidirectional carbon/epoxy laminates at strain rates up to 500 s−1. The tensile strength in the fiber direction was the same in static and dynamic cases, confirming previous results of Daniel and Liber [18], [19]. The results also showed an increase in the transverse tensile properties and shear properties with increasing loading rate.

Kawata et al. [22], [23] studied a large range of composite materials under tension from 10−3 to 2000 s−1. The materials included glass/polyester, glass/epoxy, graphite/epoxy, and graphite short fiber-reinforced nylon 6,6. For the glass/epoxy and glass/polyester composites the strength clearly increased with the strain rate which was not the case for the graphite/epoxy and graphite/nylon 6,6 and the injection-molded specimens. This study led to the conclusion that glass fiber-reinforced plastic has a higher impact absorption capacity than carbon fiber-reinforced plastics. Chiem and Liu [24] investigated the dynamic behavior of woven carbon/epoxy composites under tensile and shear impact loadings in the orthogonal direction using the tensile and torsional split Hopkinson bars at various strain rates, ranging from 500 to 3000 s−1. The experimental results indicated an increase in both the tensile and the shear strengths with increasing strain rate.

The influence of strain rate from 0.1 to 10 s−1 on the tensile properties of glass/phenolic resin and glass/polyester resin composites was studied by Barre et al. [25]. The elastic modulus and strength were found to increase with strain rate. Peterson et al. [26] studied the tensile response of chopped glass fiber-reinforced styrene/maleic anhydride (S/MA) materials in the range of 10−3 to 10 s−1 and observed a 50–70% increase in the elastic modulus and strength with increase in strain rate.

The tensile behavior of carbon/epoxy composites was investigated by Gilat et al. [27], using a hydraulic testing machine for the quasi-static and intermediate tests and a tension split Hopkinson bar apparatus for the high strain rate tests. Tensile tests were performed at strain rates ranging from 10−5 to 650 s−1 for fiber orientations of 90°, 10°, 45° and [±45°]s. In all of the configurations tested, a significant increase in the stiffness was observed with increased strain rate. A slightly increase in the maximum stress with increased strain rate was observed in the tests with layups of 90° and 10° while a more significant effect of the strain rate on the maximum stress was found in the tests with layups of 45° and [±45°]s. Also, the maximum strain at all strain rates in the tests with the [±45°]s layups is much larger than in all the other types of configurations.

The aim of this research is to investigate the behavior of unidirectional glass/epoxy composite materials at quasi-static and dynamic strain rates. Dynamic properties of lamina are extracted from the results of tensile tests on tabbed rectangular section specimens under low and high-speed loadings. A servo-hydraulic testing apparatus is used to develop the quasi-static strain rate of approximately 0.001 s−1 and intermediate strain rates ranging from 1 to 100 s−1. Specimens with identical geometry are used in all the tests. The results of the dynamic tests were subsequently compared with quasi-static tests. Effects of the strain rates on the magnitudes of tensile strength have been observed. Also, the experimental results show that the tensile modulus and strain to failure of the composites slightly increase with increase of strain rate.

Section snippets

Materials and preparation of test specimens

Unidirectional glass fiber-reinforced epoxy was studied in this paper. For this reason, thin laminate composed of five plies of reinforcement with epoxy resin ML-506 were fabricated, giving a laminate approximately 1 mm in thickness. Unidirectional tensile specimens were cut in the fiber direction from laminates. Woven glass/epoxy tabs 2.5 mm thick and 35 mm long with tapered ends were locally bonded on each side of the specimens. These tabs allow a smooth load transfer from the grip to the

Stress–strain relation

Tensile test were conducted on five cross-head stroke rates of 0.0216, 12.7, 127, 635 and 1270 mm/s on glass/epoxy composite specimens. These stroke rates applied nominal strain rates of 0.0017, 1, 10, 50 and 100 s−1 on the specimens. The nominal strain rates are calculated by dividing of the stroke rate of the cross-head of the machine by the gauge length of the specimen. The corresponding true strain rates are measured by using strain gauges directly mounted on the specimens. It is obvious that

Conclusions

Tensile failure properties unidirectional glass/epoxy composites were studied at various strain rates from 0.001 to 100 s−1 using a high-speed servo-hydraulic testing apparatus. In this study, it was shown that this testing machine can be suitable to describe the overall composites dynamic behavior under intermediate strain rates up to 160 s−1. The evaluation of the test jig performance was also shown that it was suitable for dynamic testing for the polymer composites.

The extracted longitudinal

References (29)

  • A.E. Armenakas et al.

    Response of glass-fibers-reinforced epoxy specimens to high rates of tensile loading

    Exp Mech

    (1973)
  • G.H. Staab et al.

    High strain rate response of angle-ply glass/epoxy laminates

    J Compos Mater

    (1995)
  • Staab GH, Gilat A. High strain rate characterization of angle-ply glass/epoxy laminates. In: Proceedings of the 9th...
  • J. Harding et al.

    A tensile testing technique for fibre-reinforced composites at impact rates of stain

    J Mater Sci

    (1983)
  • Cited by (207)

    View all citing articles on Scopus
    View full text