Elsevier

Composites Part B: Engineering

Volume 66, November 2014, Pages 388-399
Composites Part B: Engineering

A DIC-based study of in-plane mechanical response and fracture of orthotropic carbon fiber reinforced composite

https://doi.org/10.1016/j.compositesb.2014.05.022Get rights and content

Abstract

The in-plane elastic properties and quasi-static fracture response of an orthotropically woven carbon fiber reinforced composite are investigated using 3D digital image correlation. The elastic properties are determined as a function of fiber orientation, and the effect of fiber angles on the crack extension direction is investigated. The modified maximum hoop stress criterion is used to predict the crack extension angle of the examined material. The predicted angles are in very good agreement with those found experimentally. Optical observations and local strain data from the quasi-static fracture studies reveal that crack initiation occurs well before the maximum far-field load is reached.

Introduction

The in-plane elastic properties and quasi-static fracture response of an orthotropically woven, carbon fiber reinforced composite are investigated using 3D digital image correlation. Of particular interest in this study are graphite/epoxy composites, which are especially suitable for many applications because of their exceptional thermo-mechanical attributes [1]. While this material may possess many desirable characteristics, a complete set of mechanical properties, including fracture parameters, is requisite for proper engineering use.

Failure and damage as a result of fracture is a major concern with the use of any composite material provoking substantial research in the area [2]. Crack propagation along the laminar interfaces, including the effect of strain rate, has been most thoroughly studied [3], [4], [5], [6]. In-plane fracture in composite has been also documented in the literature [7], [8], [9], [10], [11]. Most early experimental work relied on conventional material testing techniques to obtain mechanical and fracture properties of composites, point measurements, load data and specimen geometry. These experimental techniques become more robust with the advent of the full field measurement technique, digital image correlation (DIC). Most recently Pollock et al. [12] utilized such full-field measurements to extract elastic properties of woven glass/epoxy composites through tensile loading of specimens extracted at different angles relative to the primary fiber direction.

The advent of DIC has also proven beneficial to the study of fracture mechanics, as seen in pioneering work by McNeill et al. [13], in which DIC data is used to extract stress intensity factors. Pataky et al. [14] utilized full-field measurements in conjunction with a least squares algorithm to investigate mixed mode stress intensity factors in an anisotropic, single crystal stainless steel. Such work demonstrates the applicability and accuracy that full-field measurements offer to fracture studies.

However, the examination of criterion that predicts the direction of crack propagation in woven composites was missing from previous work. Early work by Erdogan and Sih [15] that considered crack propagation plane loading of a brittle material was one of the first to describe such a methodology, suggesting that the crack will initiate along a path perpendicular to the maximum tensile direction at the crack tip. Kidane et al. [16], [17] extended the method by employing the minimum energy density criteria to study crack propagation in functionally graded materials subjected to thermo-mechanical loading. Saouma et al. [18] extended the maximum circumferential tensile stress theory presented by Erdogan and Sih [15] to predict the direction of crack propagation in a homogeneous, anisotropic specimen subjected to mixed mode loading, documenting the substantial influence that material anisotropy has on the direction of crack propagation. Most recently, Cahill et al. [9] presented the use of the extended finite element method (XFEM) in conjunction with a modified maximum hoop stress criterion to describe the direction of crack propagation in a unidirectional fiber reinforced composite that has a high degree of anisotropy. Although this model appears applicable in unidirectional composites that exhibit extreme degrees of anisotropy, the use of this criterion has not been documented in woven composites, which can often exhibit nearly orthotropic properties.

The present study focuses on the development of a general and robust methodology for extraction of mechanical properties and fracture parameters for orthogonally woven composites using full-field measurements. To our knowledge, this is the first detailed study of woven composites where the elastic properties, stress intensity factor and crack direction are investigated as a function of fiber orientation using full field measurement technique. The elastic properties of the examined composite are first determined experimentally with the help of 3D DIC, based on which the elastic stress fields are extracted. Stress fields extracted from fracture experiments, in conjunction with the maximum modified hoop stress criterion, are then utilized to predict the angle of crack propagation as a function of fiber orientation angle.

Section snippets

Material and specimen geometry

The material examined in the present work is a three-layer orthogonally woven carbon fiber-reinforced epoxy resin matrix composite with a nominal density of 1384 kg/m3. The plain weave structure of the laminate consisted of two mutually orthogonal directions (warp and weft) with an approximate volume fraction of 70% for the reinforcing carbon fibers. The weave structure can be seen in a magnified image of the material surface in Fig. 1. Rectangular 175 × 25 (mm2) coupon specimens were cut from

Elastic properties

Elastic stress-strain relation of an orthotropic lamina under plane-stress conditions can be written in matrix form as [12]:εxxεyyεxy=s11(=1/E1)s12(=-υ12/E1)0s21(=-υ12/E1)s22(=1/E2)000s66(=1/G12)σxxσyyσxywhere εij and σij are the components of the strain and stress tensors, respectively. In order to determine the in-plane mechanical properties of the woven composite as a function of fiber orientation, off-axis tensile specimens were tested. The tensile load, however, was applied along the

Elastic properties

Following the experimental procedure described earlier, the elastic properties of the examined materials were determined as a function of fiber orientation angles. Typical stress–strain curves for the composite as a function of fiber orientations angle are shown in Fig. 4. The elastic portion of the stress–strain relation has been magnified and shown in Fig. 4. As clearly seen in Fig. 4, the stress–strain relations are consistent with previous studies [12] and show different trends for

Conclusions

A detailed and robust digital image correlation-based metrology is used to determine the elastic and fracture properties of woven composites. In-plane mechanical parameters are quantified and the fracture response of an orthotropically woven carbon fiber reinforced composite are determined using DIC measurements. The stress intensity factor at the onset of crack extension is calculated based on both the far-field load and also the displacement fields obtained from 3D-DIC. A detailed study of

Acknowledgement

The financial support of NASA through EPSCOR under Grant No. 21-NE-USC_Kidane-RGP, the College of Engineering and Computing and the Department of Mechanical Engineering at the University of South Carolina is gratefully acknowledged.

References (24)

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