Coupling aspects in the simulation of hydrogen-induced stress-corrosion cracking

AbstractModelling of hydrogen-induced stress-corrosion cracking (HISCC) has to consider coupling effects between the mechanical and the diffusion field quantities. Four main topics are addressed: i) surface kinetics, ii) diffusion, iii) deformation and iv) crack growth. Surface kinetics is realised by a chemisorptions model, hydrogen diffusion is formulated by an enhanced diffusion equation including effects of plastic deformation, deformation rate and hydrostatic pressure, deformation is described by von Mises plasticity, and crack growth is simulated by a cohesive model, where both yield and cohesive strength depend on the hydrogen concentration. The effect of atomic hydrogen on the local yield strength is modelled by the so-called HELP (Hydrogen-Enhanced Localised Plasticity) approach, and the influence on the cohesive strength is taken into account by the so-called HEDE (Hydrogen-Enhanced DEcohesion) model. As the two models predict contrary effects of atomic hydrogen on the material behaviour, namely a decrease of the local yield strength resulting in larger plastic deformations and a reduction of the cohesive strength and energy inducing lower ductility, respectively, the coupling phenomena are studied in detail. The model is verified by comparing experimentally measured and numerically simulated CTOD R-curves of C(T) specimens

Constitutive and fracture behavior of ultra-strong supercrystalline nanocomposites

Supercrystalline nanocomposites are a new class of hybrid and nanostructured materials that can reach exceptional mechanical strength and can be fabricated at low temperatures. Hierarchically arranged, they bridge the gap from the nano- to the macro-scale. Even though their mechanical properties are starting to be characterized, their constitutive behavior is still largely unexplored. Here, the mechanical behavior of supercrystalline nanocomposites of iron oxide nanoparticles, surface-functionalized with oleic acid and oleyl phosphate ligands, is investigated in both bending and compression, with loading-unloading tests. A new bar geometry is implemented to better detect deformation prior to unstable crack propagation, and notched bending bars are tested to evaluate fracture toughness. Micro-mechanical tests result in the values of strength and elastic modulus that are extremely high for supercrystals, reaching record-high numbers in the oleic acid-based nanocomposites, which also show a significant tension-compression asymmetry. The constitutive behavior of both materials is predominantly linear elastic, with some more marked nonlinearities arising in the oleyl phosphate-based nanocomposites. The fracture toughness of both types of nanocomposites, ∼0.3 MPa√m, suggests that extrinsic toughening, associated with both material composition and nanostructure, plays an important role. Fractographic observations reveal analogies with shear and cleavage in atomic crystals. The influence of material composition, nanostructure, and processing method on the mechanical behavior of the nanocomposites is analyzed.</p

Anisotropic constitutive model incorporating multiple damage mechanisms for multiscale simulation of dental enamel

An anisotropic constitutive model is proposed in the framework of finite deformation to capture several damage mechanisms occurring in the microstructure of dental enamel, a hierarchical bio-composite. It provides the basis for a homogenization approach for an efficient multiscale (in this case: multiple hierarchy levels) investigation of the deformation and damage behavior. The influence of tension-compression asymmetry and fiber-matrix interaction on the nonlinear deformation behavior of dental enamel is studied by 3D micromechanical simulations under different loading conditions and fiber lengths. The complex deformation behavior and the characteristics and interaction of three damage mechanisms in the damage process of enamel are well captured. The proposed constitutive model incorporating anisotropic damage is applied to the first hierarchical level of dental enamel and validated by experimental results. The effect of the fiber orientation on the damage behavior and compressive strength is studied by comparing micro-pillar experiments of dental enamel at the first hierarchical level in multiple directions of fiber orientation. A very good agreement between computational and experimental results is found for the damage evolution process of dental enamel

Guidelines for Applying Cohesive Models to the Damage Behaviour of Engineering Materials and Structures

This brief provides guidance for the application of cohesive models to determine damage and fracture in materials and structural components. This can be done for configurations with or without a pre-existing crack. Although the brief addresses structural behaviour, the methods described herein may also be applied to any deformation induced material damage and failure, e.g. those occurring during manufacturing processes. The methods described are applicable to the behaviour of ductile metallic materials and structural components made thereof. Hints are also given for applying the cohesive model to other materials

Guidelines for Applying Cohesive Models to the Damage Behaviour of Engineering Materials and Structures

XII, 89 p. 70 illus., 5 illus. in color.online re

Ultra-thin and ultra-strong organic interphase in nanocomposites with supercrystalline particle arrangement : mechanical behavior identification via multiscale numerical modeling

© 2020 The Authors. A key challenge in the development of inorganic-organic nanocomposites is the mechanical behavior identification of the organic phase. For supercrystalline materials, in which the organic phase ranges down to sub-nm areas, the identification of the organic materials' mechanical properties is however experimentally inaccessible. The supercrystalline nanocomposites investigated here are 3D superlattices of self-assembled iron oxide nanoparticles, surface-functionalized with crosslinked oleic acid ligands. They exhibit the highest reported values of Young's modulus, nanohardness and strength for inorganic-organic nanocomposites. A multiscale numerical modeling approach is developed to identify these properties using supercrystalline representative volume elements, in which the nanoparticles are arranged in a face-centered cubic superlattice and the organic phase is modeled as a thin layer interfacing each particle. A Drucker-Prager-type elastoplastic constitutive law with perfectly plastic yielding is identified as being able to describe the supercrystals' response in nanoindentation accurately. As the nanoparticles behave in a purely elastic manner with very high stiffness, the underlying constitutive law of the organic phase is also identified to be Drucker-Prager-type elastoplastic, with a Young's modulus of 13 GPa and a uniaxial tensile yield stress of 900 MPa, remarkably high values for an organic material, and matching well with experimental and DFT-based estimations. Furthermore, a sensitivity study indicates that small configurational changes within the supercrystalline lattice do not significantly alter the overall stiffness behavior. Multiscale numerical modeling is thus proven to be able to identify the nanomechanical properties of supercrystals, and can ultimately be used to tailor these materials' mechanical behavior starting from superlattice considerations.Deutsche Forschungsgemeinschaft (DFG)Ministry of National Education of the Republic of Turke