Keywords: voids, porosities, composite, multiscale, finite element method.

Despite significant improvements in the production technology for fibre reinforced composite materials, porosities can still only be avoided at a significant increase in manufacturing costs. Replacing experimentally obtained knock-down values by accurate numerical predictions of the influence of porosities can lead to a more economic design of composite structures.

Void inclusions can be categorized into interlaminar and intralaminar voids. While the former primarily lead to a reduction of the delamination resistance, the latter significantly influence the inplane properties of unidirectional laminae.

Several analytical approaches exist to determine the reduction of elastic properties due to porosity. However, these analytical methods do not account for the spatial distribution and the geometry of the voids. Size, shape, location and geometric distribution of the voids significantly influences the effect of porosity defects on material properties. In the case of intralaminar voids, fibers are being pushed aside during the forming of the voids, and hence, fiber undulations occur in the surrounding regime. The interaction between these deviations from the desired fiber angle and the porosity itself is not captured by analytical methods and calls for a more thorough investigation. In addition, analytical methods are generally restricted to the prediction of elastic properties.

In the work presented, a continuum damage mechanics approach is chosen to model porosity defects within a multi-scale framework. Elastic-plastic material models for epoxy resin and fiber bundles have been developed. An invariant based quadratic failure criterion is used to model damage propagation. These material models are applied to study the failure modes induced by porosity defects and to obtain reduced homogenized material parameters for the macro scale.

Both intralaminar and interlaminar voids are being investigated for UD-laminates. Different failure modes, such as kink-band failure can be observed and give valuable insight into the micromechanical behavior of imperfect structures. The stochastic distribution of porosities is accounted for by investigating a statistically representative volume element for interlaminar voids. A parametric study shows a strong dependence of interlaminar shear strength on interlaminar void content. It is also shown that the maximum fiber misalignment angle has a significant influence on the strength reduction under compressive loading.

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