Key Engineering Materials Vol. 627

Abstract: Fracture in heterogeneous materials under dynamic loading is modelled using a multi-scale method. Computational homogenization is considered, in which the overall properties at the global-scale are obtained by solving a boundary value problem for a representative volume element (RVE) assigned to each material point of the global-scale model. In order to overcome the problems with upscaling of localized deformations, a non-standard failure zone averaging scheme is used. Discontinuous cohesive macro-cracking is modelled using the XFEM and a gradient-enhanced damage model is used to model diffuse damage at the local-scale. A continuous-discontinuous computational homogenization method is employed to obtain the traction-separation law for macro-cracks using averaged properties calculated over the damaged zone in the RVE. In the multi-scale model, a dynamic analysis is performed for the global-scale model and the local-scale model is solved as a quasi-static problem. Dispersion effects are then captured by accounting for the inertia forces at the local-scale model via a so-called dispersion tensor which depends on the heterogeneity of the RVE. Numerical examples are presented and the multi-scale model results are compared to direct numerical simulation results. Objectivity of the multi-scale scheme with respect to the RVE size is examined.

Abstract: This paper deals with a description of the crack behaviour in the layered alumina-zirconia ceramic laminate. The main aim is to investigate the crack behaviour in the compressive layer. The crack propagation was investigated on the basis of linear elastic fracture mechanics. Two dimensional finite element models were developed in order to obtain a stress distribution around the crack tip. The stress intensity factors were computed numerically employing the direct method. The change in the crack propagation direction was estimated using criterion based on the strain energy density factor. Sharp crack deflection in the compressive layer was predicted by mentioned approach. The determined crack behaviour is qualitatively in a good agreement with experimental observations.

Abstract: Objectives: This paper presents a numerical fatigue life assessment of a self-expandable Nitinol stent. The analysis was performed using the ANSYS 11 software. Methods: Stent durability is an issue which must be addressed during the design of implants. Given the corrosive properties of blood and the cyclic loads that are applied on the stent (the cyclic variation of blood pressure), the determination of fracture parameters and fatigue characteristics of the implant is highly recommended. Breaking of the stent’s wire is particularly dangerous because it can cause the dislocation of a piece of stenotic plaque, which in turn can block a smaller artery, causing a heart attack. On the other hand, any discontinuity in stent structure acts as an accumulating place for stenosis particles, significantly shortening the life of the implant. The stent consists of a cylindrical tube 22.42 mm long, with a diameter of 8.3 millimeters. The wire section is square, 0.2x0.2 millimeters. The stent is only subjected to the pressure generated by the stenoted arterial wall. This evenly distributed pressure is defined at the outer surface of the stent and has a value of 2.5 MPa, corresponding to a 56% blood vessel stenosis. This way, the most severe loading conditions for the stent could be simulated. The stress distribution was then used to asses the fatigue life of the stent. Results and conclusions: The results showed that, in normal conditions (with the maximal internal pressure of 139 mm Hg = 18533 Pa), no damage appears on the stent after 10⁷ cycles.

Abstract: In this work, a new damage model for mixed-mode fracture in the scope of the discrete crack approach is introduced. An energy-based internal damage variable is adopted. In the model, deformation-driven loading surfaces are defined and the dual loading surfaces in the traction space are derived. Under proportional loading, it is found that the constitutive relationship is symmetric. Further enrichment of the energy-based variable on the traction field is also introduced, allowing for a better approximation of: i) the limit surface defined in traction space and ii) non-proportional loading. However, in this case symmetry of the constitutive tensor is lost.

Abstract: In this paper PG25 filter graphite is characterised using mechanical tests conducted over a range of specimen length-scales from the centimetre (three-point bending and Brazilian disc compression) to the micrometre (micro-scale cantilever bending in a FEI Helios dualbeam work station). However, high resolution 3D tomography has revealed that apart from the known millimetre range of pores, the matrix contains a large population of micro-scale porosity. This leads to two discrete distributions of pore sizes in this material, so that a reduction in mechanical test specimen size results in sampling different proportions of the milli-and micro-scale pores. As a consequence, the measured mechanical properties such as elastic modulus, tensile strength and flexural strength change as a function of specimen size. This paper explores the potential benefits, difficulties and value of small-scale mechanical tests for this particular application.

Abstract: Functionally graded materials (FGMs) have become helpful in our engineering applications. Analysis of functionally graded material (FGM) plate during debonding case with different boundary conditions is the main purpose of this investigation. Elastic modulus (E) of functionally graded (FG) plate is assumed to vary continuously throughout the height of the plate, according the volume fraction of the constituent materials based on a modified sigmoid function, but the value of Poisson coefficient is constant. In this research, the finite element method (FEM) is used in order to show the shape of a plate made of FGM during debonding case with different boundary conditions. In the present investigation, the displacement value applied to the FGM plate is changed in order to find the relationship between the maximum von Mises stress and the displacement. Also, the relationship between the maximum shear stress and the displacement is carried out in the present work. The material gradient indexes of the FGM plate are changed from 1 to 10. The stress distributions around the debonding zone with all the material gradient indexes of the FGM plate are investigated in this work.

Abstract: Von Karman’s Effective Width Theory is Constructed on the Assumption that Thin Plate of Elastic-Perfectly Plastic Solid is Subjected to Static Buckling. we Examined Whether Von Karman’s Effective Width Theory is Applicable to Design of Crush Absorption Structure. as a Result, Cross Section of the Member Becomes Effective against Axial Crush with Increasing Crush Speed. it is Found that the Design Based on the Conventional Theory is Provided Enough Evidence of Safety, Thus, Optimized Design Criteria can be Proposed.

Abstract: This work presents a set of experiments devoted to studying the crack initiation stage under different combined tension-compression and torsion loads. Two different load levels were applied, producing very different fatigue lives. Lower strains generated lives approximately 10 times longer than higher strains. Results allowed retardation and acceleration effects due to microstructure to be clearly visualised.

Abstract: This paper presents 25 new experimental results from gray cast iron notched specimens tested under torsion loading. V-notch (with an opening angle of 120°) is considered with a root radius ranging from 0.1 to 2.0 mm. Plots of torque loads versus twist angles are recorded varying the notch root radius. Such results can help in evaluating numerical and theoretical models of the fracture of notched components under mode III loading. The second part of the paper deals with a discussion on the experimental results. A non-conventional application of the strain energy density is carried out showing a good agreement between experimental results and theoretical fracture assessments and it is used to justify the link between nominal and local fracture approaches.

Abstract: In this paper a volume criterion based on a simple scalar quantity, the mean value of the strain energy (SED), has been used to assess the static strength of notched components made of Polymethylmethacrylate (PMMA). The local-strain-energy based approach has been applied to a well-documented set of experimental data recently reported in the literature. Data refer to blunt U-notched cylindrical specimens of commercial PMMA subjected to static loads and characterised by a large variability of notch tip radius (from 0.67 mm to 2.20 mm). Critical loads obtained experimentally have been compared with the theoretical ones, estimated by keeping constant the mean value of the strain energy in a well-defined small size volume. In addition, some new tests dealing with V-notched specimens with end holes have been carried out to investigate the effect of the notch opening angle.