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

Model to accurately predict out-of-plane shear stiffness reduction in general cracked laminates

The practically important problem of the modification of laminate out-of-plane shear stiffness by ply cracks is hardly investigated in the literature. In this paper, out-of-plane shear stiffness reduction of laminates containing uniform and non-uniform distributions of ply cracks is studied. A novel variational model is developed to determine accurately stress transfer mechanisms and consequently out-of-plane shear stiffness reduction of general cracked laminates under applied out-of-plane shear loads. It is shown that the presence of ply cracks in a laminate under out-of-plane shear loads, perturbs the uniform distribution of shear stresses and induces high gradients of in-plane stresses leading to large shear stiffness reductions. The results are compared with those of the finite element method (FEM) implementing periodic boundary conditions. It is shown that there is excellent accordance between the results obtained from these approaches. The outcome of the paper provides necessary information for determination of damage-based constitutive laws for composites

Modeling of self-healing in concrete and polymeric materials

We present a modeling methodology to assess and improve the encapsulation-based self-healing strategies that are currently being applied in polymeric and cementitious-based materials. We combine numerical techniques based on the eXtended Finite Element Method, Cohesive Surface techniques and Computational Fluid Dynamics. As an example, we show some cases that illustrate how numerical and experimental results can be properly correlated, as well as the important role of the collaboration between the groups involved

Effect of intra-ply voids on the homogenized behavior of a ply in multidirectional laminates

This work focuses on the effect of intra-ply voids on the homogenized nonlinear behavior of a ply in multidirectional composites. Voids were modeled explicitly on the fiber scale and linked to the ply-scale by the recently developed two-scale framework which couples Classical Laminate Theory on the macro-scale with Finite Element analysis on the micro-scale. Laminates [+/- 45](2s) and [+/- 67.5](2s) were used as validation cases. The computed homogenized behavior of plies with and without voids for each laminate were compared against existing experimental data on manufactured plates. The nonlinearity of the homogenized stress-strain curves of all models is in a good agreement with experiments up to 1% of applied deformation for a laminate [+/- 45](2s) and up to 0.4% for a laminate [+/- 67.5](2s). The effect of voids was assessed only virtually and it is shown that 4% of void content decreases the ply strength by 30%, transversal Young's and shear moduli by around 10% and 8% respectively, whereas longitudinal stiffness is only slightly affected by the presence of voids. This work is the first step towards automatization of the virtual identification of the complete set of damage-plasticity parameters for the LMT-Cachan damage model accounting for the presence of intra-ply voids

Mesoscale finite element analysis of cracked composite laminates under out-of-plane loads using 3D periodic boundary conditions

The behavior of composite materials under out-of-plane loads is strongly affected by the presence of transverse ply cracks. The cracks perturb the distribution of stresses leading to large out-of-plane shear stiffness reductions. It is crucial to include these effects in the damage material models to improve their accuracy. Therefore, the stress transfer and stiffness reduction in cracked laminates have been studied with a mesoscale finite element model (FEM) under general in-plane, out-of-plane normal and shear loads. A symmetric laminate containing ply cracks in a single orientation has been considered under the hypothesis of periodicity using a novel relaxed three-dimensional formulation of Periodic Boundary Conditions (PBCs). The local stresses have been verified versus different analytical and numerical methods. In addition, the degraded effective thermo-elastic constants involving out-of-plane properties have been calculated as a function of crack density. Both uniform and non-uniform distributions of cracks have been considered for different lay-ups including angle-ply and unbalanced laminates. The effect of contact between the crack surfaces has been studied for specific loading conditions. It is shown that a single formulation based on three-dimensional periodic boundary conditions is sufficient to determine the interfacial stresses and the complete thermo-elastic constants under in-plane and out-of-plane loads accurately

Stress-strain synchronization for high strain rate tests on brittle composites

Nowadays, many researchers develop rate-dependent composite material models for application in dynamic simulations. Ideally, full stress-strain curves at a wide range of strain rates are available for identification of the different parameters of these models. Dynamic tensile tests are needed to produce the experimental input data. However, especially for brittle materials, the data acquisition during these tests becomes critical. The effect of synchronization on the test results is investigated by conducting a series of dynamic tensile tests on three different brittle continuous-fibre composite laminates. It is demonstrated that synchronization errors of the order of 1 microsecond already have a significant effect on the test outcome at high rates. With the aid of a finite-element model, the limiting factors on the maximum attainable strain rate are quantified