Dieter Weichert

Critical evaluation of known bone material properties to realize anisotropic FE-simulation of the proximal femur.

PURPOSE In a meta-analysis of the literature we evaluated the present knowledge of the material properties of cortical and cancellous bone to answer the question whether the available data are sufficient to realize anisotropic finite element (FE)-models of the proximal femur. MATERIAL AND METHOD All studies that met the following criteria were analyzed: Youngs modulus, tensile, compressive and torsional strengths, Poissons ratio, the shear modulus and the viscoelastic properties had to be determined experimentally. The experiments had to be carried out in a moist environment and at room temperature with freshly removed and untreated human cadaverous femurs. All material properties had to be determined in defined load directions (axial, transverse) and should have been correlated to apparent density (g/cm(3)), reflecting the individually variable and age-dependent changes of bone material properties. RESULTS Differences in Youngs modulus of cortical [cancellous] bone at a rate of between 33% (58%) (at low apparent density) and 62% (80%) (at high apparent density), are higher in the axial than in the transverse load direction. Similar results have been seen for the compressive strength of femoral bone. For the tensile and torsional strengths, Poissons ratio and the shear modulus, only ultimate values have been found without a correlation to apparent density. For the viscoelastic behaviour of bone only data of cortical bone and in axial load direction have been described up to now. CONCLUSIONS Anisotropic FE-models of the femur could be realized for most part with the summarized material properties of bone if characterized by apparent density and load directions. Because several mechanical properties have not been correlated to these main criteria, further experimental investigations will be necessary in future.

An experimental investigation of hot spots in railway disc brakes

An experimental study of hot spots occurrence in railway disc brakes is reported on. The aim of this study was to better classify and to explain the thermal gradients appearance on the surface of the disc. Thermographic measurements with an infrared camera have been carried out on the rubbing surface of brake discs on a full-scale test bench. Based on thermography, a classification of hot spots observed in disc brakes is proposed. A detailed investigation of the most damaging thermal gradients, called macroscopic hot spots (MHS) is given. From these experimental researches, a scenario of hot spots occurrence is suggested step by step. Influence of parameter such as pad stiffness and pad contact length on hot spots developments is studied. Observations give new highlights on the conditions of hot spots appearance. Discussion of the theoretical approaches compared to experimental observations is proposed.

Shock wave-loaded plates

Abstract The present paper reports on modeling, numerical simulation, and experimental investigation of plates subjected to impulsive loading. The kinematical hypothesis used for the theoretical description of the transient response includes transverse shear deformations, rotary inertia, and geometrical nonlinear effects. The material modeling accounts for elastic–plastic behavior, isotropic and kinematical hardening, and strain rate sensitivity. The numerical simulation of the transient inelastic vibrations is performed using isoparametric finite elements. Both the Chaboche and the Bodner–Partom viscoplastic constitutive laws are used to trace the evolution of the material characteristics in the framework of a layered shell model. The theoretical and numerical developments are checked by experimental investigations of thin steel plates subjected to shock waves. These experiments are performed in a shock tube with various impact periods and loading histories. The topics addressed in this report include (a) the correlation of experimental and simulated transient inelastic response using the Chaboche and Bodner–Partom models, (b) the sensitivity of the predicted structural response to variations of the material parameters identified on the basis of uniaxial tension tests, (c) the effect of the transverse shear stress distribution on the local evolution of the material behavior and on the global dynamic response, (d) the evolution of deflections, stresses, and plastic zones under blast loading conditions.

Inelastic behaviour of structures under variable loads

1: Material Modelling for Cyclic Plasticity and Damage. The modified multisurface hardening model and its application to cyclic loading problems Z. Mroz, P. Rodzik. Structures consisting of two-phase materials under thermomechanical loads T. Siegmund, E. Werner, F.D. Fischer. Shakedown for systems of kinematic hardening materials E. Stein, Y.J. Huang. Related mathematical models for solids and structures deformation and failure processes under repeated loading D.A. Gokhfeld, O.S. Sadakov, O.F. Cherniasvsky. Nonlinear behaviour of low-plastic structures A. Cherniavsky. 2: General Theory of Accommodated Inelastic Structures under Variable Loads. Elastic-viscoplastic solids subjected to thermal and loading cycles G. Polizzotto. Some basic elements of the shakedown theory B. Nayroles. A variational deduction of upper and lower bound shakedown theorems by Markovs and Hills principles over a cycle G. de Saxce. Influence of cyclic creep on the upper bound to shakedown inelastic deflections S. Dorosz. On shakedown theorems in the presence of Signorini conditions and friction J.J. Telega. On theorems of adaptation of elastic-plastic structures J. Saczuk. Extrememum problems in shakedown theory J. Kamenjarzh. 3: Influence of Geometrical Changes on Cyclic Response of Structures. Geometric effects on shakedown and ratchetting of axisymmetric cylindrical shells subjected to variable thermal loading G. Maier, L.G. Pan, U. Perego. Shakedown of shells undergoing moderate rotations J. Gross-Weege, D. Weichert. Limit, shakedown, post-yield and inadaptation analyses of discrete plastic structures A. Siemaszko. Constitutive model and incremental shakedown analysis in finiteelastoplasticity H. Stumpf. Case studies on the influence of geometric effects on the shakedown of structures J.B. Tritsch, D. Weichert. Discussion of the chaotic behaviour of an elastic-plastic structure F. Bontempi, F. Casciati. 4: Numerical Techniques Applied to Industrial Problems, Damaged Structures. Inadaptation mechanisms in bellows subject to sustained pressure and cyclic axial loadings in terms of finite deformations B. Skoczen, J. Skrzypek. Min-max approach to shakedown and limit load analysis for elastic perfectly plastic and kinematic hardening materials J. Zwolinski. An indirect incremental method for a shakedown analysis based on the min-max approach S. Pycko. Slackened systems under variable loads A. Gawecki, P. Kruger. Plastic analysis and design of skeletal structures accounting for their sensitivity to ratchetting H.M. Bondok, M. Janas. Shakedown of rail corrugations W. Gambin. Steady cyclic state of a structure: methods of its direct determination D.A. Gokhfeld, O.S. Sadakov. An improved boundary element analysis for the bending of a thin plate with a crack O. Mahrenholtz, Pin Lu. Stability of pavement structures under long term repeated loading L. Raad, D. Weichert.

Numerical shakedown analysis of damaged structures

A numerical method is proposed to predict the failure of mechanical structures. It is based on the generalization of the classical static shakedown theorem to damaged inelastic structures. Ductile plastic damage is taken into account by using the models of Lemaitre and Shichun-Hua. The obtained results are compared in the special case of limit analysis to those obtained by incremental methods.

A THERMOMECHANICAL MODEL FOR THE ANALYSIS OF DISC BRAKE FRACTURE MECHANISMS

The aim of this article is to explain the damage mechanisms of disc brakes leading to macroscopic cracks on the friction surface. The work combines microanalysis of cracks and a thermomechanical modeling describing the thermal fatigue solicitation. The failure of disc brakes is discussed, giving indications of how to improve the design of this type of friction system in order to enhance its performances

Elastic-plastic shells under variable mechanical and thermal loads

Abstract A two-dimensional shakedown theory for shells under combined mechanical and thermal loading conditions is presented. Linear elastic-ideal plastic as well as linear elastic-limited linear hardening material behaviour is taken into account. Numerical examples of technical relevance illustrate the proposed method.

Application of shakedown theory to damaging inelastic material under mechanical and thermal loads

An extension of Melans shakedown theorem to a certain class of inelastic material with damage and kinematical hardening is presented. The developed numerical method is illustrated by two examples.

PROGRESSIVE WAVINESS DISTORTION: A NEW APPROACH OF HOT SPOTTING IN DISC BRAKES

ABSTRACT This paper deals with the mechanisms of the appearance of hot spots in railway brake disks. From infrared measurements, a classification of hot spots is presented. The most damaging thermal gradients are identified as macroscopic hot spots uniformly distributed on the friction surface. An explanation of the origin of these macroscopic hot spots is presented based on a scenario of progressive distortion of the disk. Three major effects have to be considered: the structural response of the disk and the pads; realistic boundary conditions, both geometric and loading; and the thermoelastoplastic behavior of the material. Finite element simulations based on the proposed model agree with experimental observations.

An extension of the static shakedown theorem to inelastic cracked structures

Abstract Here we have investigated under which conditions elastic-perfectly plastic, cracked bodies subjected to variable loads shake down. For this purpose, an extension of the static shakedown theorem (Melan’s theorem) is presented by using a crack analysis developed by Nguyen Quoc Son in the framework of the concept of generalized standard materials.