Journal of Multiscale Modelling

An integrated crystal viscoplastic modeling process has been developed to account for the effect of microstructure in the mechanical response of polycrystalline materials. Grain distributions, including size, shape and orientation, are generated automatically based on probability theories using VGRAIN software. For each set of control parameters (average, maximum and minimum grain size) used in the micro-film simulations, six grain orientation patterns were generated randomly for a micro-film based on a gamma distribution; a large number of analyses have been carried out to account for statistical variations in the spatial pattern of grain orientations. The simulations are used to investigate the effects of grain size and orientation on necking and flow stress in stainless steel under uniaxial tension, and to quantify the extent that variability in the spatial distribution of orientations affects the predictions. Based on the numerical studies, a map was generated indicating under what circumstances macro-mechanics theory can be used and when Crystal Plasticity (CP) theory must be used to ensure the accuracy of the analysis; if the theories are not used appropriately, huge errors can be expected.

A nonclassical multiscale "morphological approach" devoted to highly-filled particulate composites was proposed by Nadot et al.¹ It is extended by developing complementary ingredients in order to describe the evolution of damage by decohesion of grain/matrix interfaces. A defect nucleation criterion allows the evolution of the number of defects to be followed, and a closure criterion monitors the respective proportions of open and closed defects. The nucleation criterion is associated with a critical value of the current distance between two points located on both sides of the interface considered, whereas the closure criterion is based on the normal component of the displacement jump across the interface. The "morphological approach" is tested at macroscopic and local scales for a random particulate microstructure under loading paths involving tension and compression. The corresponding results are a specific contribution of this work. At global scale, the numerical results presented show the effects of damage induced anisotropy and recovery of mechanical properties by defects closure. Thanks to explicit schematization of the microstructure, the discrete model provides, at microscale, the characteristics of damage (chronology of events, position and morphology of defects) as well as its effects on local strains.

Free-Cutting Steels (FCS), also referred to as Free Machining Steels, are characterised by their excellent chip formation and low cutting force during machining. However, the microstructure features which provide the advantageous machining properties have been linked to poor formability during secondary processing such as those occurring during hot rolling of bar products (Y. Liu et al., On micro-damage in hot metal working Part 1: Experimental investigation, Eng. Trans.54 (2006) 271–287). In this paper physically-based modelling equations are formulated from microstructure and flow stress observations, specifically for the high temperature deformation of FCS. The isotropic, viscoplastic-damage and stress state dependent material model is determined for a typical low-carbon FCS. The model is integrated with the commercial FE code ABAQUS Explicit through user routine VUMAT. Results are validated with the recently introduced conical splay test.

Deterioration of the materials, particularly metals under environmental conditions such as high temperature, alternating loading and chemical aggression is an example of processes which happen on the microstructure levels but project themselves upon macroscopically observed behaviour of materials and structures. This connection between both levels of observation was obvious to many researches, even if they aimed at macroscopic description on the level of continuum mechanics. The wisdom of micro- and macro-coupling was induced by a very complex nature of microstructural processes which demonstrated themselves as transgranular or intergranular failures, just to mention two typical modes of creep failure. The gap between micro- and macro-world was a challenge to both material science and mechanics societies throughout the second half of 20th century. A proposed method to cover this gap for polycrystalline materials is based on Cellular Automata (CA) technique well suited to be used on the microscopic level and giving responses relevant to macroscopic observations. It allows for microstructure modelling to distinguish grains and grain boundaries. Once it is done, a cellular automaton can be attributed to the Representative Volume Element (RVE) and failure mechanism described on the basis of appropriate transition rules. Examples of transgranular and intergranular creep damage growth are demonstrated. This procedure can be extended over the feedback from micro-level to macro-level leading to the formation of so-called CAFE technique.

This paper presents the results of experimental measurements of transverse thermal diffusivity for six different industrial Ceramic Matrix Composite materials under the action of in-plane uni-axial mechanical loading. Measurements have also been taken using a one-dimensional Laser Flash technique on 8 mm diameter disc specimens without mechanical load. These results have been used to benchmark data obtained using a three-dimensional Laser Flash rig developed to operate on large mechanical test specimens whilst being loaded in a test machine. The latter facility has been used to measure the degradation of transverse thermal diffusivity with uni-axial strain. For five of the composites tested the degradation of transverse thermal diffusivity was small; and the degradation of transverse thermal diffusivity was linear with strain. One composite showed evidence of fibre pullout, and a decrease of transverse thermal diffusivity to a low value. The different behaviours observed were believed to be associated with the weave type/composite lay-up, and the selection of composite constituent materials, although insufficient data is available to make general conclusions.

Ceramic matrix composites (CMCs) are candidates for pressurized tubes which operate under conditions of severe cyclic thermal loading. Simple models describing the properties of CMCs are used to estimate the behaviour of a pressurized tube subjected to cyclic thermal loading and to establish shakedown and failure conditions. Analytical procedures are described which evaluate the component response in the cyclic state. The approach is illustrated by analysing the classical Bree problem assuming material properties which are representative of a SiC/SiC composite. Performance diagrams are presented which identify safe operating conditions and the extent of damage in the component.

Some fundamental principles and advanced applications of thermo-mechanical modelling techniques used for industrial welding processes are described. The paper covers a range of modelling procedures comprising welding simulation and residual stress analysis of multi-pass, thick-walled ferritic steel pipes; welding simulation and distortion analysis of nickel-based superalloy thin plates; and inertia friction welding of dual alloy drive shafts. A number of pertinent mechanical and metallurgical concepts are discussed, including material behaviour models and material properties, microstructural evolution, solid state phase transformation and post-weld heat treatment. The paper emphasises the general methodologies of thermal modelling procedures and their role in the holistic process of the life assessment and design improvement of power plant piping systems and aeroengine casings and shaft components, operating under high temperature creep and fatigue service conditions.

Creep damage due to constant loading at elevated temperatures and damage due to plastic deformation at room temperature are assessed using destructive and nondestructive methods in steels (40HNMA and P91). In the destructive methods the standard tension tests were carried out after prestraining and variations of the selected tension parameters were taken into account for damage identification. In order to assess damage development during the creep and plastic deformation the tests for both steels were interrupted for a range of selected strain magnitudes. Ultrasonic and magnetic techniques were used as the nondestructive methods for damage evaluation. The last step of the experimental programme contained microscopic observations. The results show that ultrasonic and magnetic parameters can be good indicators of material degradation and can help to locate the regions where material properties are changed due to prestraining. A good correlation of mechanical and selected nondestructive parameters identifying damage was achieved for both tested steels.