Asymptotic stress field at the tip of an inclined crack terminating to an interface

This paper presents the numerical results for the asymptotic stress field and the fracture parameters at the tip of an inclined cracks terminating to a bi-material ceramic interface. The numerical analysis was carried out using FRANC2D/L fracture analysis code. A biaxial specimen was modeled for producing different mixed mode loads and two materials combinations of Al2O3 and ZrO2 were considered. The influence of the material combination and applied mixed mode load on the singularity orders, stress distributions and stress intensity factors is highlighted.This paper presents the numerical results for the asymptotic stress field and the fracture parameters at the tip of an inclined cracks terminating to a bi-material ceramic interface. The numerical analysis was carried out using FRANC2D/L fracture analysis code. A biaxial specimen was modeled for producing different mixed mode loads and two materials combinations of Al2O3 and ZrO2 were considered. The influence of the material combination and applied mixed mode load on the singularity orders, stress distributions and stress intensity factors is highlighted

Image Analyses of Two Crustacean Exoskeletons and Implications of the Exoskeletal Microstructure on the Mechanical Behavior

The microstructures of exoskeletons from Homarus americanus (American lobster) and Callinectes sapidus (Atlantic blue crab) were investigated to elucidate the mechanical behavior of such biological composites. Image analyses of the cross-sectioned exoskeletons showed that the two species each have three well-defined regions across the cuticle thickness where the two innermost regions (exocuticle and endocuticle) are load bearing. These regions consist of mineralized chitin fibers aligned in layers, where a gradual rotation of the fiber orientation of the layers results in repeating stacks. The exocuticle and endocuticle of the two species have similar morphology, but different thicknesses, number of layers, and number of stacks. Mechanics-based analyses showed that the morphology of the layered structure corresponds to a nearly isotropic structure. The cuticles are inter-stitched with pore canal fibers, running transversely to the layered structure. Mechanics-based analyses suggested that the pore canal fibers increase the interlaminar strength of the exoskeleton

Image Analyses of Two Crustacean Exoskeletons and Implications of the Exoskeletal Microstructure on the Mechanical Behavior

The microstructures of exoskeletons from Homarus americanus (American lobster) and Callinectes sapidus (Atlantic blue crab) were investigated to elucidate the mechanical behavior of such biological composites. Image analyses of the cross-sectioned exoskeletons showed that the two species each have three well-defined regions across the cuticle thickness where the two innermost regions (exocuticle and endocuticle) are load bearing. These regions consist of mineralized chitin fibers aligned in layers, where a gradual rotation of the fiber orientation of the layers results in repeating stacks. The exocuticle and endocuticle of the two species have similar morphology, but different thicknesses, number of layers, and number of stacks. Mechanics-based analyses showed that the morphology of the layered structure corresponds to a nearly isotropic structure. The cuticles are inter-stitched with pore canal fibers, running transversely to the layered structure. Mechanics-based analyses suggested that the pore canal fibers increase the interlaminar strength of the exoskeleton

Evolving fracture patterns: columnar joints, mud cracks, and polygonal terrain

When cracks form in a thin contracting layer, they sequentially break the layer into smaller and smaller pieces. A rectilinear crack pattern encodes information about the order of crack formation, as later cracks tend to intersect with earlier cracks at right angles. In a hexagonal pattern, in contrast, the angles between all cracks at a vertex are near 120$^\circ$. However, hexagonal crack patterns are typically only seen when a crack network opens and heals repeatedly, in a thin layer, or advances by many intermittent steps into a thick layer. Here it is shown how both types of pattern can arise from identical forces, and how a rectilinear crack pattern evolves towards a hexagonal one. Such an evolution is expected when cracks undergo many opening cycles, where the cracks in any cycle are guided by the positions of cracks in the previous cycle, but when they can slightly vary their position, and order of opening. The general features of this evolution are outlined, and compared to a review of the specific patterns of contraction cracks in dried mud, polygonal terrain, columnar joints, and eroding gypsum-sand cementsComment: 19 pages, 9 figures, accepted for publication in Phil. Trans. R. Soc. A; theme issue on Geophysical Pattern Formation (to appear 2013

A self-adaptive cohesive zone model for interfacial delamination

Interfacial failure in the form of delamination, often results in malfunction or failure of laminated structures. Different numerical approaches have been proposed for the simulation of this process. Due to the appealing feature of predicting both the delamination onset and its growth, cohesive zone models have been widely used to simulate delamination as a result of a gradual degradation of the adhesion between two materials when they become separated. Application of cohesive zone models for the modelling of delamination in brittle interfaces in a quasi-static finite element framework suffers froman intrinsic discretization sensitivity. A large number of interface elements are needed for the discretization of the process zone of a cohesive crack. Otherwise, a sudden release of energy in large cohesive zone elements results in a sequence of snap-through or snap-back points to appear in the global load-displacement response of the system which compromises the numerical efficiency. While computationally expensive path-following techniques can be used to follow the oscillatory path, the efficiency and robustness of brittle cohesive zone models can be significantly increased by reducing the oscillations observed in the global load-displacement behaviour without a further mesh refinement. In line with this purpose, the separation approximation in the process zone is enriched with an adaptive hierarchical extension. The linear separation approximation throughout the cohesive zone element is enriched with a bi-linear function, where the enrichment peak position and the magnitude of the enrichment are regarded as additional degrees of freedom obtained by minimization of the total potential of the global system. The mobility of the peak of the enrichment function within individual cohesive zone elements locally adapts the discretization to the physics governing the problem. Important numerical aspects of the proposed enrichment strategy such as its mobility and uniqueness have been thoroughly investigated while its limitations are addressed. The efficiency and robustness of the enrichment are shown through numerical examples which prove the general applicability of the methodology. In fact, application of the elaborated enrichment eliminates the need for a further mesh refinement while keeping the standard Newton-Raphson approach applicable in the case of a relatively coarse mesh which saves considerable computational costs. Extension of the proposed enrichment scheme to delamination in a threedimensional finite element framework has been carried out as well. Planar interix face elements have been enriched along all edges by bi-linear functions with mobile peaks. The effect of the proposed methodology on reducing discretization-induced oscillations is quantitatively evaluated. To deal with planar crack growth where the crack front is oblique with respect to element edges, a non-hierarchical enrichment strategy is also developed and its performance is compared with its hierarchical counterpart. The self-adaptive finite element formulation is extended to a framework suitable for large deformations and is applied to interfaces in microelectronics under realistic mixed-mode loading conditions. In particular, the material/interface systems used in miniaturized mixed-mode bending tests, which are conducted for a wide range of mode angles, are modelled to make a direct comparison with experimental results. The interface constitutive lawthat is used takes the dependence of fracture toughness on mode-mixity into account. Thereby, the enhanced cohesive zone model can be used for the simulation of the behaviour of brittle interfaces in an accurate, effective, and efficient manner

Scattering of SH wave by surface depression or convex in density inhomogeneous media

L'abstract è presente nell'allegato / the abstract is in the attachmen

Micromechanics-based phase field fracture modelling of CNT composites

L. Quinteros acknowledges financial support from the National Agency for Research and Development (ANID) , Chile/Scholarship Pro-gram/DOCTORADO BECAS CHILE/2020-72210161. E. Martinez-Paneda was supported by an UKRI Future Leaders Fellowship, UK (grant MR/V024124/1) .We present a novel micromechanics-based phase field approach to model crack initiation and propagation in carbon nanotube (CNT) based composites. The constitutive mechanical and fracture properties of the nanocomposites are first estimated by a mean-field homogenisation approach. Inhomogeneous dispersion of CNTs is accounted for by means of equivalent inclusions representing agglomerated CNTs. Detailed parametric analyses are presented to assess the effect of the main micromechanical properties upon the fracture behaviour of CNT-based composites. The second step of the proposed approach incorporates the previously estimated constitutive properties into a phase field fracture model to simulate crack initiation and growth in CNT-based composites. The modelling capabilities of the framework presented is demonstrated through three paradigmatic case studies involving mode I and mixed mode fracture conditions.National Agency for Research and Development CHILE/2020-72210161UK Research & Innovation (UKRI) MR/V024124/

Increasing Safety and Reducing Environmental Damage Risk from Aging High-Level Radioactive Waste Tanks

Cracks of various shapes and sizes exist in large high-level waste (HLW) tanks at several DOE sites. There is justifiable concern that these cracks could grow to become unstable causing a substantial release of liquid contaminants to the environment. Accurate prediction of crack growth behavior in the tanks, especially during accident scenarios, is not possible with existing analysis methodologies. This research project responds to this problem by developing an improved ability to predict crack growth in material structure combinations that are ductile (Fig. 1). This new model not only addresses the problem for these tanks, but also has applicability to any crack in any ductile structure