Zoran Ren

Surface Fatigue of Gear Teeth Flanks

Mechanical behaviour of various machine elements, such as gears, brakes, clutches, rolling bearings, wheels, rails, and screw and riveted joints, are influenced by interaction between contact elements and surfaces. Surfaces in rolling and/or sliding contact are exposed to material contact fatigue. Contact fatigue can be defined as a kind of damage caused by changes in the material microstructure which results in crack initiation followed by crack propagation, under the influence of time-dependent rolling and/or sliding contact loads. The contact fatigue process can in general be divided into two main parts: initiation of micro-cracks due to local accumulation of dislocations, high stressesat local points, plastic deformation around inhomogeneous inclusions or other imperfections in or under the contact surface; crack propagation, which causes permanent damage to a mechanical element.

Simulation of surface pitting due to contact loading

A computational model for simulation of surface pitting of mechanical elements subjected to rolling and sliding contact conditions is presented. The two-dimensional computational model is restricted to modelling of high-precision mechanical components with fine surface finishing and good lubrication, where the cracks leading to pitting are initiated in the area of largest contact stresses at certain depth under the contacting surface. Hertz contact conditions with addition of friction forces are assumed and the position and magnitude of the maximum equivalent stress is determined by the finite element method. When the maximum equivalent stress exceeds the local material strength, it is assumed that the initial crack develops along the slip line in a single-crystal grain. The Virtual Crack Extension method in the framework of finite element analysis is then used for two-dimensional simulation of the fatigue crack propagation under contact loading from the initial crack up to the formation of the surface pit. The pit shapes and relationships between the stress intensity factor and crack length are determined for various combinations of contacting surface curvatures and loadings. The model is applied to simulation of surface pitting of two meshing gear teeth. Numerically predicted pit shapes in the face of gear teeth show a good agreement with the experimental observations.

Modelling of crack growth under cyclic contact loading

Abstract The paper describes a general computational model for modelling of subsurface fatigue crack growth under cyclic contact loading of mechanical elements. The model assumes that the initial fatigue crack develops along the slip line in a single crystal grain at the point of the maximum equivalent stress. The position and magnitude of the maximum equivalent stress are determined with the Finite Element Analysis of the equivalent contact model, which is based on the Hertzian contact conditions with the addition of frictional forces. The Virtual Crack Extension method is then used for simulation of the fatigue crack propagation from the initial to the critical crack length, when the surface material layer breaks away and a pit appears on the surface. The pit shapes and relationships between the stress intensity factor and the crack length are determined for various combinations of contacting surface curvatures and contact loadings. The computational results show that the model reliably simulates the subsurface fatigue crack growth under contact loading and can be used for computational predictions of surface pitting for various contacting mechanical elements.

Mechanical Properties of Advanced Pore Morphology Foam Elements

Advanced pore morphology (APM) foam, consisting of sphere-like metallic foam elements, exhibits some particular mechanical properties with unique application possibilities. The article presents the results of experimental and computational testing of APM foam elements to determine their mechanical behavior under quasi-static and dynamic compressive loading conditions. Additionally, an infrared thermal imaging camera has been used during experimental testing. Evaluated mechanical properties give better insight into the behavior of single APM foam elements under different types of loading and provide a good base for further studies of the topology and morphology influence on the global behavior of composite structures, based on APM foam elements.

Surface initiated crack growth simulation in moving lubricated contact

The paper describes a two-dimensional computational model for simulating surface initiated crack growth in the lubricated contact area that leads to surface pitting of mechanical components. The model assumes size and orientation of the initial crack which is subjected to contact loading conditions, accounting for the elasto-hydrodynamic-lubrication effects and tangential loading due to sliding. The influence of a lubricating fluid, driven into the crack by hydraulic mechanism, is also considered. The minimum strain energy density criterion is used to analyze crack propagation with the aim of the finite element analysis. The model is applied to a real pitting problem of a gear. The results for pit sizes correlate well with those observed in experimental testing.

Behavior of composite advanced pore morphology foam

The mechanical characterization of advanced pore morphology (APM) foam, consisting of sphere-like metallic foam elements, is very limited since APM foam has been developed only recently. The purpose of this research was thus to determine the behavior of APM spheres and its composites when subjected to compressive loading. Single metallic APM spheres have been characterized with experimental testing and computational simulations, providing the basic properties and knowledge for an efficient composition of composite APM foam structures. Then, the APM foam elements were molded with epoxy matrix resulting in new composite structures. These composites have been adhered together with the epoxy resin achieving partial and syntactic morphology. The mechanical characterization of composite APM foam structures was based on experimental testing results with free and confined boundaries. The results of the performed research have shown valuable mechanical properties of the composite APM foam structures. Furthermore, they offer new possibilities for their use in general engineering applications.

Impact Behavior of Composite Hollow Sphere Structures

Porous composite materials constitute an innovative group of lightweight materials which combine high specific stiffness, good damping properties, and thermal insulation with the ability to absorb large amounts of energy at a low constant stress level. In the scope of this study, adhesively bonded metallic hollow sphere structures (MHSS) fully embedded within the adhesive matrix are considered with aim to determine their macroscopic behavior under uniaxial impact loading conditions by means of parametric computational simulations. The base material properties have been determined by quasi-static and dynamic experiments. Three topologies of syntactic hollow sphere structures of various dimensions are considered, namely the cubic primitive, the body centered cubic and the face centered cubic topology. Results of computational simulations show significant influence of topology and strain rate sensitivity on the composite structure behavior, while the influence of metallic hollow sphere wall thickness is less pronounced. Computational simulations show that it is possible to combine the MHSS topology, metallic hollow sphere wall thickness and strain rate sensitivity to achieve any desired dynamic response of MHSS adapted to a given engineering problem.

Evaluation of the service life of gears in regard to surface pitting

A computational model for determining the service life of gear teeth flanks in regard to surface pitting is presented. The model considers the material fatigue process leading to pitting, i.e. the conditions required for the short fatigue crack propagation originating from the initial crack in a single material grain. In view of small crack lengths observed in surface pitting, the simulation takes into account the short crack growth theory. The stress field in the contact area and the required functional relationship between the stress intensity factor and the crack length are determined by the finite element method. An equivalent model of two contacting cylinders is used for numerical simulations of crack propagation in the contact area. On the basis of numerical results, and with consideration of some particular material parameters, the probable service life period of contacting surfaces is estimated for surface curvatures and loadings that are most commonly encountered in engineering practice.

Behaviour of cellular structures with fluid fillers under impact loading

The paper describes experimental and computational testing of regular open-cell cellular structures behaviour under impact loading. Open-cell cellular specimens made of aluminium alloy and polymer were experimentally tested under quasi-static and dynamic compressive loading in order to evaluate the failure conditions and the strain rate sensitivity. Additionally, specimens with viscous fillers have been tested to determine the increase of the energy absorption due to filler effects. The tests have shown that brittle behaviour of the cellular structure due to sudden rupture of intercellular walls observed in quasi-static and dynamic tests is reduced by introduction of viscous filler, while at the same time the energy absorption is increased. The influence of fluid filler on open-cell cellular material behaviour under impact loading was further investigated with parametric computational simulations, where fully coupled interaction between the base material and the pore filler was considered. The explicit nonlinear finite element code ls-dyna was used for this purpose. Different failure criteria were evaluated to simulate the collapsing of intercellular walls and the failure mechanism of cellular structures in general. The new computational models and presented procedures enable determination of the optimal geometric and material parameters of cellular materials with viscous fillers for individual application demands. For example, the cellular structure stiffness and impact energy absorption through controlled deformation can be easily adapted to improve the efficiency of crash absorbers

Dynamic behaviour of regular closed-cell porous metals – computational study

The paper describes computational modelling of regular closed-cell cellular materials behaviour when subjected to impact loading conditions. Parametric computational simulations have been carried out to evaluate influences of the relative density, strain rate, pore gas and gas type on the macroscopic dynamic behaviour of cellular materials. The behaviour of the model under uniaxial impact loading conditions and large deformations has been analysed with the LS-DYNA code, which is based on the finite element method. This study helps to clarify which effects are indeed important and would have to be considered in developing new homogenised constitutive relationships for analysing impact problems with use of general computational codes. Additionally, the detailed computational models provide an insight into behaviour of cellular material accounting for pore filler and basic constitutive relations for further development of homogenised models under impact conditions and large deformations. Furthermore, they allow for determination of most appropriate geometrical and material parameters of cellular materials in regard to individual engineering application demands.