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In the past thirty years, the development of fiber reinforced composite materials has been substantial, leading to an increased use in industrial applications due to their high stiffness, strength-to-weight ratios and excellent fatigue properties. Composite materials consist of a matrix material with reinforcing fibers that are often woven into different patterns. They can be molded into complex shapes and their stiffness can be optimized in the loading directions by aligning fiber orientations. To facilitate the rapid development of composite materials, new numerical tools are needed to provide a cost-effective analysis of the mechanical behaviors, including the understanding of the micro-structural characteristics in general and the yarn-yarn/matrix interfacial behavior in particular. This constitutes the prime objective of this thesis. Upon loading, decohesion processes constitute important failure mechanisms in woven composites. The complex geometry of the yarn architecture makes it difficult to avoid mesh intersections between neighboring yarns in their geometrical description. This motivated recent works using a level set based methodology to generate smooth geometries without interpenetration of yarns. Such tools usually require the use of gaps between contacting yarns, which can lead to computationally costly discretizations for thin gaps. To avoid this, an implicit geometry description based on distance fields is used that supports the incorporation of cohesive zones, identifying the interface of yarn-yarn contacts and eliminating the gaps by manipulating distance fields of individual yarns. This enables the use of a conforming tetrahedral finite element discretizations with inserted cohesive elements in the contact areas. The methodology is shown to reduce the size of microstructural finite element models of a woven composite by over two orders of magnitude, thereby paving the way towards affordable damage simulations on such microstructural models. During, Doctorat en Sciences de l'ingénieur et technologie, info:eu-repo/semantics/nonPublished

The control and characterization of the fracture behavior of thin films and 2D materials, in the era of continuous device miniaturization, are of paramount importance for the design and integrity assessment of many coatings and microelectronics devices towards more reliable devices. However, the available test methods suffer from many limitations mainly when the film rests on a substrate due to different constraints that are difficult to deconvolute and or with the introduction of an initial sharp precrack in freestanding configurations. In this context, the objective of this research is to develop a robust on-chip fracture technique for freestanding thin films and 2D materials. This crack-on-chip method consists of actuator beams produced by lithography undergoing large internal stress attached to a notched specimen beam. Both actuators and specimens are deposited on top of a Si substrate. The etching of a part of this substrate induces the release of the test structure, with the actuators then contracting and pulling on the test specimen. If the applied displacement is large enough, a crack initiates at the notch root and propagates. The design has been selected such as to involve a decrease of the crack driving force after some amount of crack growth leading to crack arrest, avoiding the problem of notch opening of the precrack. The technique can also be used to investigate the environmentally-assisted cracking in thin-film materials. The fracture toughness of SiO2 (142 nm), SiN (50 nm), Al2O3 (190 nm), and CVD-grown monolayer graphene was extracted. It was found that Al2O3 films are more prone to subcritical aging compared to SiN, while simultaneously exhibiting larger resistance to environmental cracking than thermally-grown SiO2 films. For single-layer graphene, fracture strain, Young’s modulus, strength, and Weibull modulus were determined with, for the first time, statistically representative data., (FSA - Sciences de l'ingénieur) -- UCL, 2022

Cantor-type high entropy alloys form a new family of metallic alloys characterised by a combination of high strength and high fracture toughness. An experimental study on the CoCrNi alloy is first performed to determine the damage and fracture mechanisms under various stress states. A micromechanics-based ductile fracture model is identified and validated using these experimental data. The model corresponds to a hyperelastic finite strain multi-yield surface constitutive description coupled with multiple nonlocal variables. The yield surfaces consist of three distinct nonlocal solutions corresponding to three different modes of void expansion within an elastoplastic matrix: a void growth mode governed by a Gurson-based yield surface corrected for shear effects, an internal necking-driven coalescence mode governed by an extension of the Thomason yield surface based on the maximum principal stress, and a shear-driven coalescence mode governed by the maximum shear stress. This advanced formulation embedded in large strain finite element setup captures the effects not only of the stress triaxiality but also of the Lode variable. In particular, the analysis shows that a failure model accounting for these two invariants of the stress tensor captures the fracture in high-entropy alloys over a wide range of conditions.