Simulation of anisotropic crack tip deformation processes and particle interactions in toughened polymers

This study develops a finite element-based simulation of submicrometer crack tip deformation processes in polymers to investigate local toughening effects. An initial study of how these processes interact with stiff inclusions is presented to enable further investigation of particulate toughening. Crack tip and process zone mechanisms, including polymer chain disentanglement, directional chain realignment with consequent anisotropy, and crack propagation, are considered in a dedicated user-defined material law. Such processes are generally homogenized on higher scale continuum levels analyses, but direct simulation can provide insight into toughening mechanisms that have been widely observed but not fully explained. The user material law herein was employed in a parametric study to investigate the relative importance of (1) the extent of local inelastic polymer chain realignment and (2) consequent anisotropic hardening of the realigned polymer chains. In order to explore the interaction of fracture processes with nanometer-scale inclusions, silica particles with varied spacing were also included in the simulations. The interaction between local stress concentration and energy dissipation mechanisms has been quantified. It is shown that in neat polymers, local yielding is the dominant toughening effect accounting for over 90% of the local energy absorption, whereas local stiffening alone would decrease toughness. Stiff inclusions were shown to generally decrease toughness, except in cases where local yielding greatly outweighs local stiffening effects. Roughly 45% increase in toughness was shown for a 250-nm particle spacing that balances the acceleration of elastic failure with the formation of a larger local yield zone size. This demonstrates the utility of employing dedicated material laws to microstructural scale analyses in providing design targets in material design.

Criticality in elastoplastic models of amorphous solids with stress-dependent yielding rates

Elastoplastic models are analyzed at the yielding transition. Universality and critical exponents are discussed. The flowcurve exponent happens to be sensitive to the local yielding rule. An alternative mean-field description of yielding is explained.

Researches on the Test of Web Local Failure Modes and Related FEA Study

In the analysis of structure steel beam member of mechanical module and piping support in nuclear power plant, beam web shall be analyzed complying with the ASME III NF requirements against web local failure including web local yielding, web local crippling, web sideway buckling and web compression buckling when the beam is subjected to concentrated loads. Referring to the H Type steel structure beam of China GB code; this paper researches the tests of carrying capacity related to the above mentioned local failure mechanism. The results could show that the evaluation method of web local failure mechanism in ASME III NF cover the China GB code steel structure member on one hand; And on the other hand, the results could show the rationalities of the sectional dimension design of the member and the rationality of the beam span. And based on the analysis of the test results, reasonable suggestions could made for the above mentioned member dimension design and beam span design.

The Influence of Long-Range Residual Stress on Plastic Collapse and Local Yielding of Internally Pressurized Pipes

The determination of plastic collapse for cracked pipes is important in structural engineering design and component integrity assessment. Long-range residual stresses are usually treated as primary stresses which contribute to plastic collapse of pipes subjected to internal pressure. This paper explores the differences between load and displacement controlled conditions applied to the ends of thin- and thick-walled pipes. Both load and displacement control can represent long range or fit-up residual stresses if they are considered as primary or secondary stresses respectively. Both global collapse and local yielding for pipes containing partially and fully circumferential cracks are examined. Detailed three-dimensional (3D) finite element (FE) analyses are used to simulate the pipe and crack geometry and the boundary conditions. The cracked pipes are assumed to be open ended. For a defect free pipe the FE results for global collapse agree with analytical solutions for both load and displacement controlled end conditions. For high tensile end loads and displacements lower collapse pressures are found for displacement conditions, while it is the converse for high compressive end loading. However, when a crack is introduced it becomes evident that tensile or compressive displacement control has little impact on global collapse and therefore longrange displacement controlled (or residual) stresses do not contribute to collapse. On the other hand local net section yielding is strongly affected by either load or displacement controlled end conditions.

An Analytical Approach to the Analysis of Inhomogeneous Pipes under External Pressure

Pipes for deep-water applications possess a diameter-to-thickness ratio in a region where failure is dominated by both instability and plastic collapse. This implies that prior to failure the compressive yield strength of the material must be exceeded, followed by ovalisation and further local yielding. This paper presents an investigation into the mechanics of this specific problem and develops an analytical approach that accounts for the effects of geometrical and material data on the collapse pressure of inhomogeneous rings under external hydrostatic pressure. The analytical expressions have been correlated to numerical and experimental test data, proving their accuracy.

Viscoelastic Analysis of Adhesively Bonded Double – Lap Joint

In this paper, stress distribution in a double lap joint subjected to a tensile load is investigated using the boundary element method. The adhesive used in this study is a commercial epoxy system which can be cured at room temperature. The adhesive is assumed to be linearly viscoelastic. The order of the singularity is obtained numerically for a given viscoelastic model. The numerical results show that interface stresses are large enough to initiate local yielding or edge cracks. Since the exceedingly large stresses cannot be borne by the adhesive layer, edge cracks can occur at the interface corner.

Plastification of Adhesive Micro-Spherical Contact During Jump-In Neck Formation

Approach between two deformable micro-spheres in the presence of adhesion is sometimes accompanied by discontinuous change of the surface profile at the narrow region near their summits (jump-in phenomenon or neck formation). The main goal of the current work is to show that neck formation can involve local plastification. The adhesive traction acting on the sphere’s surface is evaluated by the Lennard-Jones potential. The theoretical strength rather than the engineering yield strength of the material is used in the von Mises (VM) local yielding criterion. The results show that adhesion between deformable micro-spheres may cause elastic to plastic transition during the neck formation for specific combinations of geometrical and physical properties of the spheres.

An evaluation of differential susceptibility of barley varieties to lodging under varying environmental conditions

Information on the variation in susceptibility to lodging between barley varieties under different environments can be used for local prediction. It can also be used to identify varieties that are robust to environmental variation. Efficient methods to obtain analogous information on the local yielding capacity of crop varieties have recently been established. The present paper extends the use of these methods for the analysis of dichotomously measured performance characteristics, the susceptibility to lodging, in particular. The procedures examined were based on generalized linear models in which the expected conditional mean of the susceptibility to lodging was used as an explanatory variable to express the environmental effects of a given environment. Through the use of logistic transformation the approach extends previous methods developed for continuous response variables to binary response variables. Models were subsequently used to obtain measures of susceptibility to lodging of each barley variety in terms of environmental variation. While the emphasis is restricted to lodging, similar methods can also be applied to other performance characteristics.