Macro- and micro-modeling of crack propagation in encapsulation-based self-healing materials : application of XFEM and cohesive surface techniques

Encapsulation-based materials are produced introducing some small healing fluid-filled capsules in a matrix. These materials can self-heal when internal cracks intercept and break the capsules. If the healing agent is released, the crack can be sealed. However, this is not always the case. These capsules need to be designed with the adequate shape and material to be properly broken. This paper presents two application models based on the combination of eXtended Finite Element Method (XFEM) elements and Cohesive Surfaces technique (CS) to predict crack propagation. Two types of encapsulated systems are considered: a concrete beam in a three-point bending test, and a micro-scale model of a representative volume element of a polymer subjected to a uniaxial tensile test. Despite both systems rely on different capsule shapes and different constituent materials, the models predict a similar non-linear response of the overall material strength governed by the coupled effect of the interface strength and the capsule radii-to-thickness ratio. Furthermore, even if an inadequate material and geometry combination is used, it is found that the mere presence of capsules might achieve, under certain conditions, an interesting overall reinforcement effect. This effect is discussed in terms of clustering and volume fraction of capsules

An efficient anisotropization technique to transform isotropic nonlinear materials into unidirectional and bidirectional composites

This work presents a procedure to transform an isotropic nonlinear and rate-dependent material model into orthotropic. This transformation relies on a fast computation modifying the material inelastic evolution along any desired direction. With this concept, the constitutive law of the pure isotropic, nonlinear and rate-dependent material is fully preserved. This paper shows a direct application to describe the nonlinear response of unidirectional (UD) or bidirectional (BD) fiber-reinforced polymer composites. Imposing one constraint leads to a UD composite, whilst imposing two orthogonal directions leads to a BD composite. The proposed methodology is implemented as user-defined material for Finite Element solvers. To prove the method, two different visco-plastic material models used to describe isotropic polymer resins are anisotropized to obtain their corresponding composite counterparts. The mechanical response of the simulations are compared qualitatively and quantitatively to experimental tests on a UD coupon under off-axis tensile test under different load directions. Consistency between the UD and BD anisotropization formulations is proved under tension, shear and different loading directions. Additionally, a complete study of computational performance is addressed to assess the anisotropization's feasibility to be applied in modeling and design of new laminated and multilayer-based structures exhibiting nonlinear material response