Mesoscale analysis of damage growth in woven composites

Abstract

The characteristic damage mechanisms of a four-layer plain weave glass fiber/epoxy matrix composite are analyzed by means of optical microscope observations on the edge of a rectangular specimen under tensile loading. Digital image correlation is used to determine the size of transverse yarn cracks and yarn-yarn debonding around the crack tips. An algorithm based on Finite Fracture Mechanics is proposed to model crack and debond initiation and propagation in the composite, which takes into account possible couplings between the cracks. The predicted debond and crack densities are in good agreement with the experimental observations.

Introduction

Woven composites are increasingly used for lightweight aeronautical and automotive applications. The large variety in the choice of the reinforcing fabric allows the number of assembly operations to be reduced, which results in fewer weak points in a structure and lower production costs.

The behavior of woven composites may be optimized using design tools able to describe the change in their mechanical behavior from damage initiation to failure. Currently, design methods are based on macroscopic models predicting damage growth and failure in 2D [1], [2] and 3D [3], [4], [5], [6], [7] woven composites. These models require time consuming and expensive experimental tests in order to calibrate their parameters. Moreover, the model parameters must be recalibrated each time the constituents or the fiber architecture change. The number of tests may be reduced if some of the parameters can be determined by simulation of virtual tests with explicit modeling of damage initiation and propagation at the mesoscopic scale, at which the architecture of the fiber reinforcement architecture is defined.

Several strategies have been introduced to model damage at the mesoscopic scale. Gao et al. [8] proposed an analytical damage model using a mosaic laminate representation of the composite. This model allows for the calculation of the Young’s modulus of the damaged composite. However, yarn undulation, which has a major influence on damage location [9], [10], is neglected. Damage in woven composites at the mesoscopic scale is generally modeled using Finite Element (FE) approaches based on Continuum Damage Mechanics (CDM) [9], [10], [11], [12]. One drawback of such models is an erroneous prediction of damage propagation directions [12], [13]. Moreover, these models require regularization methods in order to avoid damage pattern dependence on the FE mesh [14], [15], and very fine meshes in order to obtain a good representation of localized cracks observed experimentally [16], [17], [18], [19]. Localized cracks can be modeled more accurately using discrete damage models [19], [20], [21], [22], which consist in directly inserting cracks in the FE mesh.

The modeling of damage growth requires tools that are able to determine damage initiation and propagation within the composite. Damage initiation in woven composites is generally modeled using a stress-based criterion [9], [10], [11], [12], [19], [23], which gives a good estimate of the crack locations [19], [23], [24]. However, the damage initiation strain is underestimated if only a stress criterion is used and a better estimate is obtained [24] if energetic considerations based on Finite Fracture Mechanics (FFM) [31] are also taken into account, as proposed by Leguillon [32], for instance. In a previous work [24], it has been shown that, in the studied composite, the energy criterion is dominant and thus Finite Fracture Mechanics can be used to accurately model the initiation of yarn cracks. Moreover, it also allows for the determination of the crack initiation configuration (e.g., length, orientation). The propagation of existing cracks in quasi-brittle materials can also be modeled using Fracture Mechanics. Yet, to the authors’ knowledge, damage propagation in woven composites has mainly been studied using models based on CDM [9], [10], [11], [12].

In this work, Finite Fracture Mechanics is followed to model damage growth in a composite consisting of four layers of glass fiber plain weave and epoxy matrix. In Section 2, optical micrographs of yarn cracks and debonding are presented. The crack and debond lengths are determined by means of digital image correlation (DIC). The algorithm developed to assess damage propagation is summarized in Section 3. It is used in Section 4 to determine the propagation of multiple cracks taking into account the possible coupling between several cracks. A comparison of the predicted damage growth to the experimental observations is presented in Section 5.