Damage in 3D Textile Composites under Static Loading

There is a growing trend of using 3D textile composites to replace conventional laminated composites in structural components where through-the-thickness loadings are substantial or better impact resistance is required. With its increasing popularity, there has been a steady stream of investigations regarding the damage behaviour of 3D textile composites. In this section, damage mechanisms and methods of damage modelling for 3D textile composites are reviewed.

2.4.1 Damage Mechanisms

There are a large number of experimental investigations reported in the literature for characterising damage in various types of 3D textile composites. Among them, most are focused on the damage in 3D orthogonal woven composite.

Tan et al. [154] conducted static tensile tests in the warp and weft directions of a 3D orthogonal woven carbon fibre composite material. Although the resulting stress-strain curves were linear, debonding of the z-fibre tows, tow pull-out and tow breakage were discovered in the tested specimens.

Kuo et al. [155] studied the compressive response of orthogonal 3-axis woven carbon fibre composites. They observed progressive compressive damage and kink bands associated with stuffer rods and stuffer tows. Later, Kuo et al. [156] investigated the effect of varying surface tow patterns on the compressive response of 3-axis orthogonal composites.

Leong et al. [157] reported extensive longitudinal tow splitting in a 3D orthogonal woven carbon fibre composite during tensile testing. They deduced that such a

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damage mode should be caused by the extensive longitudinal matrix cracking as a result of the Poisson’s ratio mismatch between the matrix and longitudinal tows. A detailed experimental methodology for characterising damage in 3D textile composites was suggested by Lomov et al. [158]. They recommended the use of acoustic emission for identifying strain levels of interest, the use of full-field strain measurement for locating strain concentrations, and the use of computerised axial tomography scan and optical microscopy for identifying local damage modes. Bogdanovich et al. [159] and Lomov et al. [160] applied this experimental methodology for identifying damage events during tensile loading of 3D orthogonal woven carbon composites. The damage events discovered were cracking of boundary tows, intra-tow transverse cracks and tow/matrix debonding. Apart from 3D orthogonal woven composites, studies regarding the damage in 3D interlock woven composites are also available in the literature.

Normal-layered interlock and offset-layered interlock glass fibre composites were tested in tension by Callus et al. [52]. It was found that crimped tow straightening may have contributed to substantial nonlinearity in the stress-strain responses obtained. John et al.[161] also reported damage in these two kinds of 3D woven composites under tensile loading. They concluded that intra-tow cracking and debonding of warp tows were the major damage modes observed.

Damage in a 3D angle interlock carbon fibre woven composite was studied by Cox et al. [162]. They conducted tensile, compressive and bending tests for the material. It was found kink band formation and debonding were the major damage modes caused by compressive loading while tow rupture and tow pull-out were the major damage modes observed under tensile loading. A combination of these

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damage modes were discovered when the material was subjected to bending. Later, Cox et al. [163,164] identified warp tow straightening as the primary damage mechanism for causing softening in the tensile stress-strain responses of layer-to-layer and through-the-thickness interlock 3D woven carbon fibre composites.

Tensile, compressive and in-plane shear tests were carried out by Warren et al. [165] for ply-to-ply angle interlock 3D woven composites reinforced by IM7 carbon fibre tows. Based on the test result, it was concluded that crimped warp tows contributed to the reduced strengths and the non-linear stress-strain behaviours in the warp direction when compared with those in the weft direction. For in-plane shear, the non-linear stress-strain response observed was similar to that of plain woven laminates with the same fibre volume fraction.

Based on the experimental result available in the literature, it is found for most 3D textile composites, damage initiates in the form of intra-tow transverse cracks. With further loading, multiplication of intra-tow cracks occurs until crack saturation state is reached. This is then followed by the formation and propagation of inter-tow cracks which are normally found around crimping tows. Depending on the textile reinforcement architecture, tow straightening may occur after the formation of inter-tow cracks. Final failure modes of 3D textile composites are usually tow rupture under tensile loading and extensive brooming or kink band type of tow failure under compression.

Moreover, it has been observed that the textile reinforcement architecture and the type of loading affect many aspects of the damage process in 3D textile composites. These aspects include the point of damage initiation, non-linearity induced by damage, crack density, damage modes and so on.

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2.4.2 Damage Modelling

In contrast to damage modelling for laminates where most models are only two- dimensional, for 3D textile composites, three-dimensional models capturing detailed geometry of textile reinforcement may become a necessity. The reason for this is that the damage process in 3D textile composites is highly influenced by the internal architectures of these materials [166].

However, a few simplified two-dimensional damage analysis methods for 3D textile composites were developed. Most of these can be classified as the equivalent laminate method, where the 3D textile architectures were approximated as laminates comprised of UD laminae. For example, Pickett et al.[167] and Fouinneteau et al. [168] used a laminate representation for analysing damage in carbon fibre and glass fibre braided composites, where the continuum damage model by Ladeveze [169] was incorporated for damage prediction. The same approach was used by Greve et al. [170] for predicting damage in carbon fibre non-crimp fabric composites. Despite the simplicity of two dimensional analysis methods, they are unable to capture the effect of tow crimp, which is known to cause damage like inter-tow cracks, fibre rupture and tow micro-buckling [166]. In contrast to two-dimensional damage analysis methods, three-dimensional finite element analysis models, capturing tow shape and tow path explicitly, can provide the maximum of geometrical details for the textile reinforcements inside the 3D textile composites. Thanks to this, these finite element models are normally capable of predicting local damage associated with textile geometry [171]. In these models, due to the periodicity of 3D textile reinforcement, the representation

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of 3D textile composites is normally in the form of a unit cell. The unit cell is further discretised into element volumes of tows and matrix material so that tow shape and tow path are explicitly modelled. Different material constitutive models are assigned to tow elements and matrix elements as the former is normally treated as a transversely-isotropic material and the latter as an isotropic material [171].

Among the finite element analysis models developed for damage prediction in 3D textile composites, there appeared to be two major approaches adopted for introducing damage into the models: the continuum damage mechanics (CDM) approach and the cohesive element approach [172].

In the CDM approach, damage variables are introduced into the constitutive relationships of tow and matrix materials. Since tows are normally regarded as UD composites, well-established UD composite failure criteria as reviewed in Section 2.2.2 and the CDM models for UD composite as mentioned in Section 2.3.2.2 can be used for tow damage modelling. However, for simplicity, many researchers used element discount method such that once damage initiation is detected, damage variables would jump to the maximum value and result in a sudden complete loss of stiffness. Based on finite element modelling, these researchers used CDM approach combined with element discount method to predict the damage in woven [173,174], braided [175], and non-crimp fabric composites [176]. On the other hand, CDM approach combined with gradual damage evolution laws were applied by others for the damage analysis of 3D braided composites [177] and 3D woven composites [178].

As an alternative to the CDM approach, the cohesive element approach models cracks as discontinuities, where damageable surfaces represented by cohesive

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elements are introduced into the finite element model. However, it requires the definition of crack orientation and location prior to the analysis which is not always possible. McLendon and Whitcomb [179] used this approach to predict tow-matrix interfacial damage inside textile composites, where they found that the choice of stiffness degradation law affects greatly the accuracy of the prediction since stress redistribution is sensitive to the stiffness degradation law formulated.