Georgios Savaidis

Transient dynamic analysis of a cracked functionally graded material by a BIEM

A hypersingular time-domain traction boundary integral equation method (BIEM) is presented for transient dynamic crack analysis in a functionally graded material (FGM). A finite crack in an infinite and linear elastic FGM subjected to an impact antiplane crack-face loading is investigated. The spatial variation of the materials constants is described by an exponential law. To solve the hypersingular time-domain traction BIE, a numerical solution procedure is developed. The numerical solution procedure uses a convolution quadrature formula for approximating the temporal convolution and a Galerkin method for the spatial discretization of the hypersingular time-domain traction BIE. Numerical examples are presented to show the effects of the materials gradients on the dynamic stress intensity factors.

Hot-spot stress evaluation of fatigue in welded structural connections supported by finite element analysis

Various welded joints from the floor structure of city buses have been examined numerically and experimentally under bending and tensional cyclic constant-amplitude loading. Hot-spot stresses at failure-critical locations were calculated by means of finite element analysis. The corresponding fatigue lives were determined experimentally. There was good overall agreement between calculated and experimentally determined critical locations. The recommendations within the IIW guideline concerning the Hot Spot Stress Approach were found to give accurate approximations for the probability of survival of fatigue-loaded welded joints. The restriction regarding the applicability of the Hot Spot Stress Approach to welds failing only from their transition zones could be relaxed, at least at least for welds similar to those investigated here.

FE fatigue analysis of notched elastic–plastic shaft under multiaxial loading consisting of constant and cyclic components

Abstract This paper presents a finite element analysis for a notched shaft subjected to nonproportional synchronous multiaxial loading. A combined tension/torsion is considered. For simplicity, one loading component is kept constant while the other is cyclic. The von Mises (von Mises R. Ges. Wiss. Gottingen, math. phys. K1 1913:582–93) yield criterion in conjunction with the kinematic hardening rule of Prager and Ziegler (Prager W. Proc. Inst. Mech. Engrs 1955;169:41–57; Ziegler H. Quart. Appl. Math. 1959;17:55–60) is applied to describe the elastic–plastic material behavior of the notched shaft. The boundary value problem is solved by using the commercial finite element program system ABAQUS (Theory manual, Version 4.9, 1991; Users manual, Version 4.9, 1991). A parametrical study is carried out and numerical results are presented to show the effects of the magnitude/amplitude ratio and the starting situation on the stabilized stress–strain hysteresis of the cyclic component and the stress–strain relation of the constant component.

Elastic–plastic FE analysis of a notched shaft under multiaxial nonproportional synchronous cyclic loading

Abstract An elastic–plastic finite element analysis is presented for a notched shaft subjected to multiaxial nonproportional synchronous cyclic tension/torsion loading. The elastic–plastic material property is described by the von Mises yield criterion and the kinematic hardening rule of Prager/Ziegler. The finite element program system ABAQUS is used to solve the boundary value problem. Special emphasis is given to explore the effects of the stress amplitude, the mean-stress, and the mutual interactions on the local stress–strain responses at the notch root.

On size and technological effects in fatigue analysis and prediction of engineering materials and components

Abstract Analytical approaches concerning size, stress gradient and technological effects such as surface roughness and residual stresses induced during manufacturing processes are presented and discussed in this paper. Their implementation into the Short-Crack-Model for fatigue-life (lifetime to initiation of cracks of a size of 0.5– 1 mm ) prediction of engineering components subjected to cyclic loading is explained in detail. The procedures to consider the aforementioned effects are demonstrated by using an example of a forged and tempered steering shaft made of low-alloyed steel subjected to variable amplitude bend loading. The corresponding experimental results are used to check the accuracy of the analytical fatigue-life prediction. The comparison between analytically calculated and experimentally determined fatigue-life values emphasises the significance of technological effects (surface roughness, residual stresses) on fatigue-life estimation and the usefulness of the Short-Crack-Model for fatigue-resistant design of engineering components.

Elastic-plastic FE analysis of a notched cylinder under multiaxial nonproportional fatigue loading with variable amplitudes

Abstract This paper presents an elastic–plastic finite element (FE) analysis of an axisymmetric circular cylinder with a circumferential notch subject to multiaxial nonproportional fatigue loading with variable amplitudes. The von Mises yield criterion and the linear kinematic hardening rule of Prager–Ziegler are applied to describe the elastic–plastic material behavior. Two different loading combinations are considered: (1) constant tension with variable amplitude torsion; (2) variable amplitude tension with variable amplitude torsion. Numerical results for the local stress–strain curves at the notch-root are presented and discussed.

FE simulation of vehicle leaf spring behavior under driving manoeuvres

Purpose – The purpose of this paper is to develop a FE based modeling procedure for describing the mechanical behavior of high‐performance leaf springs made of high‐strength steels under damaging driving manoeuvres.Design/methodology/approach – The type and number of finite elements over the thickness of leaves, as well as the definition of contact, friction and clamping conditions, have been investigated to describe the mechanical behavior in an accurate and time‐effective manner. The proposed modeling procedure is applied on a multi‐leaf spring providing complex geometry and kinematics during operation. The calculation accuracy is verified based on experimental stress results.Findings – A FE based modeling procedure is developed to describe the kinematics and mechanical behavior of high‐performance leaf springs subjected till up to extreme driving loads. Comparison of numerically determined stress distributions with corresponding experimental results for a serial front axle multi‐leaf spring providing c...

Advanced leaf spring design and analysis with respect to vehicle kinematics and durability

Purpose – The purpose of this paper is to provide sound understanding of the mutual interactions of the major leaf spring design parameters and their effects on both the stress behavior of the designed leaf and the steering behavior of the vehicle. Design/methodology/approach – Finite elements analyses have been performed referring to the design of a high performance monoleaf spring used for the suspension of the front axle of a serial heavy truck. Design parameters like eye type, eye lever, spring rate and arm rate difference have been parametrically examined regarding the stress performance and their influence on the wheel joint kinematics. The effect of each design parameter is exhibited both qualitatively and quantitatively. Findings – Eye lever and eye type affect significantly the wheel joint kinematics and therewith the steering behavior of the vehicle. Spring rate and arm rate difference affect solely the stress performance of the leaf spring. Practical implications – Design engineers may use the ...

Microstructural, surface and fatigue analysis of stress peened leaf springs

Purpose – The purpose of this paper is to investigate the fatigue and failure of commercial vehicle serial stress-peened leaf springs, emphasizing the technological impact of the material, the thermal treatment and the stress-peening process on the microstructure, the mechanical properties and the fatigue life. Theoretical fatigue analysis determines the influence of each individual technological parameter. Design engineers can assess the effectiveness of each manufacturing process step qualitatively and quantitatively, and derive conclusions regarding its improvement in terms of mechanical properties and fatigue life. Design/methodology/approach – Two different batches of 51CrV4 were examined to account for potential batch influences. Both specimen batches were subjected to the same heat treatment and stress-peening process. Investigations of their microstructure, hardness and residual stress state on the surface’ areas show the effect of the manufacturing process on the mechanical properties. Wohler cur...

Microstructure, Surface Characterization and Fatigue Assessment of 56SiCr7 Spring Steel

A high number of cycles sustained before failure, under cyclic loading application, is one of the key performance requirements of springs. Heat and surface treating of the spring steel can have a significant influence on its fatigue life. The main purpose of heat treating is to achieve a tempered-martensitic microstructure with appropriate surface hardness, in order to increase the fatigue limit of the spring. The heat treatment aims to an end martensitic formation from an initial ferritic/perlitic microstructure through the steps of heating, quenching and tempering. This study shows that the correct parameters chosen for each of these steps for 56SiCr7 steel can result to an appropriate microstructure, and therefore increase the steel surface hardness up to approximately 550-600 HV. With the aid of optical microscopy, the thickness of the decarburized surface layer is determined, in order to distinguish between the core and surface microstructure hardness of heat treated steel. Surface treatment, through shot-peening, induces compressive residual stresses on the surface of the steel, thus increasing the hardness by at least 204 HV, compared to the raw material, and doubling the number of cycles to failure. Vickers micro-hardness measurements conducted on a cross-section, at different depths from the surface of the steel, show the trend of hardness increasing towards its core, and verify the dependence of the surface hardness of the steel on heat treatment.