Numerical simulation of grain-size effects on creep crack growth by means of grain elements

The effect of grain size on creep crack growth is investigated by means of a numerical technique in which the actual crack growth process is simulated in a discrete manner by grain elements and grain boundary elements. The grain elements account for the creep deformation of individual grains, while grain boundary cavitation and sliding are accounted for by grain boundary elements between the grains. This grain-element technique allows for an independent study of multiple grain size effects: a (direct) size effect related to the specimen size/grain size ratio or an (indirect) effect related to the effect of grain size on nucleation rate and creep resistance. Preliminary numerical results are presented concerning the direct effect of grain size, which predict that the crack growth rate and brittleness increase with grain size.

Microstructural modelling of creep crack growth from a blunted crack

The effect of crack tip blunting on the initial stages of creep crack growth is investigated by means of a planar microstructural model in which grains are represented discretely. The actual linking-up process of discrete microcracks with the macroscopic crack is simulated, with full account of the underlying physical mechanisms such as the nucleation, growth and coalescence of grain boundary cavities accompanied by grain boundary sliding. Results are presented for C*-controlled mode I crack growth under small-scale damage conditions. Particular attention is focused on creep constrained vs. unconstrained growth. Also the effect of grain boundary shear stresses on linking-up is investigated through shear-modified nucleation and growth models. The computations show a general trend that while an initially sharp crack tends to propagate away from the original crack plane, crack tip blunting reduces the crack growth direction. Under unconstrained conditions this can be partly rationalized by the strain rate and facet stress distribution corresponding to steady-state creep.

Micromechanics of creep fracture: simulation of intergranular crack growth

A computational model is presented to analyze intergranular creep crack growth in a polycrystalline aggregate in a discrete manner and based directly on the underlying physical micromechanisms. A crack tip process zone is used in which grains and their grain boundaries are represented discretely, while the surrounding undamaged material is described as a continuum. The constitutive description of the grain boundaries accounts for the relevant physical mechanisms, i.e. viscous grain boundary sliding, the nucleation and growth of grain boundary cavities, and microcracking by the coalescence of cavities. Discrete propagation of the main crack occurs by linking up of neighbouring facet microcracks. Assuming small-scale damage conditions, the model is used to simulate the initial stages of crack growth under C* controlled, model I loading conditions. Initially sharp or blunted cracks are considered. The emphasis in this study is on the effect of the grain microstructure on crack growth.

Anisotropic plastic deformation by viscous flow in ion tracks

A model describing the origin of ion beam-induced anisotropic plastic deformation is derived and discussed. It is based on a viscoelastic thermal spike model for viscous flow in single ion tracks derived by Trinkaus and Ryazanov. Deviatoric (shear) stresses, brought about by the rapid thermal expansion of the thermal spike, relax at ion track temperatures beyond a certain flow temperature. Shear stress relaxation is accompanied by the generation of viscous strains. The model introduces differential equations describing the time evolution of the radial and axial stresses, enabling an exact derivation of the viscous strains for any ion track temperature history T(t). It is shown that the viscous strains effectively freeze in for large track cooling rates, whereas reverse viscous flow reduces the net viscous strains in the ion track for smaller cooling rates. The model is extended to include finite-size effects that occur for ion tracks close to the sample edge, enabling a comparison with experimental results for systems with small size. The "effective flow temperature approach" that was earlier introduced by Trinkaus and Ryazanov by making use of Eshelby's theory of elastic inclusions, follows directly from the viscoelastic model as a limiting case. We show that the viscous strains in single ion tracks are the origin of the macroscopic anisotropic deformation process. The macroscopic deformation rate can be directly found by superposing the effects of single ion impacts. By taking realistic materials parameters, model calculations are performed for experimentally studied cases. Qualitative agreement is observed

Three-dimensional cross-linked F-actin networks:Relation between network architecture and mechanical behavior

Numerical simulations are reported for the response of three-dimensional cross-linked F-actin networks when subjected to large deformations. In addition to the physiological parameters such as actin and cross-linker concentration, the model explicitly accounts for filament properties and network architecture. Complementary to two-dimensional studies, we find that the strain-stiffening characteristics depend on network architecture through the local topology around cross-links

The origin of stiffening in cross-linked semiflexible networks

Strain stiffening of protein networks is explored by means of a finite strain analysis of a two-dimensional network model of cross-linked semiflexible filaments. The results show that stiffening is caused by non-affine network rearrangements that govern a transition from a bending dominated response at small strains to a stretching dominated response at large strains. Thermally-induced filament undulations only have a minor effect; they merely postpone the transition.Comment: 5 pages, 5 figure

A New Model for Void Coalescence by Internal Necking

A micromechanical model for predicting the strain increment required to bring a damaged material element from the onset of void coalescence up to final fracture is developed based on simple kinematics arguments. This strain increment controls the unloading slope and the energy dissipated during the final step of material failure. Proper prediction of the final drop of the load carrying capacity is an important ingredient of any ductile fracture model, especially at high stress triaxiality. The model has been motivated and verified by comparison to a large set of finite element void cell calculations.