M. R. Johnson

Bio: M. R. Johnson is an academic researcher from IIT Research Institute. The author has contributed to research in topics: Automatic summarization & Residual stress. The author has an hindex of 2, co-authored 2 publications receiving 24 citations.

Analysis of Thermal Stresses and Residual Stress Changes in Railroad Wheels Caused by Severe Drag Braking

Abstract: A finite-element computer program, which takes into consideration nonlinear material behaviour after the yield point has been exceeded, has used to analyze the thermal stresses in railroad freight car wheels subjected to severe drag brake heating. The analysis has been used with typical wheel material properties and wheel configurations to determine the thermal stress field and the extent of regions in the wheel where the yield point is exceeded. The resulting changes in the residual stress field after the wheel has cooled to ambient temperature have also been calculated. It is shown that severe drag braking can lead to the development of residual circumferential tensile stresses in the rim and radial compressive stresses in the plate near both the hub and rim fillets.

A comparison of analytical and numerical methods for the calculation of temperatures in wheel/rail contact

Abstract: The maximum surface temperature during rolling contact of railway wheels with sliding friction can be estimated using Blok’s flash temperature formula. For a more detailed investigation, semi-analytical and numerical methods are available. A survey of various methods is given and an efficient approach is proposed for Hertzian contact. The actual contact temperature is confined to a very thin surface layer. Due to continuous frictional heating, the bulk temperature of the wheel increases with time. For the long-term behaviour of the wheel temperature, not only the convection at the free wheel surfaces but also the heat conduction from the wheel into the colder rail has to be considered. Practical consequences of the theoretical results are discussed.

Crack propagation life of detail fractures in rails. final report

Abstract: The results of a comprehensive study of the crack propagation behavior of detail fractures in railroad rails are presented. The study includes full-scale crack growth experiments in a test track under simulated heavy freight train service, similar field tests and observations on revenue tracks, and static tests to determine the breaking strengths of rails containing detail fractures. A fracture mechanics model of the detail fracture is presented, and the results of laboratory tests to determine the basic crack growth rate properties of rail steel are reviewed. Correlation of most of the experimental results by the model is demonstrated. The model is used to illustrate the sensitivity of safe crack growth life to nine railroad environmental factors. The most influential factors are found to be thermal stress in continuous welded rail, the curve high rail position (relative to tangent track), and residual stress in the rail head. The experimental and analytical results indicate that further investigation is required into the influence on residual stress of: track curvature, rail steel tensile strength, roller-straightening of rails, and increases in axle loads above the maximum currently permitted by U.S. freight railroad interchange rules.

Thermal stresses and shakedown in wheel/rail contact

Abstract: Sliding friction between railway wheels and rails results in considerable contact temperatures and gives rise to severe thermal stresses at the surfaces of the wheels and rails. An approximate analytical solution is presented for a line contact model. The increased bulk temperature of the wheel after a long period of constant operating conditions is also taken into account. The thermal stresses have to be superimposed on the mechanical contact stresses. They reduce the elastic limit of the wheel and rail, and yielding begins at lower mechanical loads. When residual stresses build up during the initial cycles of plastic deformation, the structure can carry higher loads with a purely elastic response in subsequent load cycles. This phenomenon is referred to as shakedown. Due to the distribution of temperature, the rail surface is generally subjected to higher stresses than the wheel surface. This can cause structural changes in the rail material and hence rail damage.

Shakedown limit of rail surfaces including material hardening and thermal stresses

Abstract: Sliding friction between railway wheels and rails results in elevated contact temperatures and gives rise to severe thermal stresses at the wheel and rail surfaces. The thermal stresses have to be superimposed on the mechanical contact stresses. Due to the distribution of stresses, the rail surface is generally subjected to higher stresses than the wheel surface. The elastic limit is reduced and yield begins at lower mechanical loads. During the first cycles of plastic deformation, the material hardens and residual stresses build up. The residual stresses provide the structure to shake down to pure elastic behaviour in subsequent load cycles up to a shakedown limit. The kind of hardening observed for rail steel has a considerable influence on the shakedown limit. The shakedown limit is dropped to lower mechanical loads due to the thermal stresses in the rail surface as well. This might cause structural changes in the rail material and rail damage.

Structural changes and viscoplastic behavior of a generic embedded-atom model metal in steady shear flow

Abstract: We study equilibrium and nonequilibrium properties of a simple ``generic embedded-atom model'' (GEAM) for metals. The model allows to derive simple analytical expressions for several zero-temperature constitutive properties---in overall agreement with real metals. The model metal is then subjected to shear deformation and strong flow via nonequilibrium molecular dynamics simulation in order to discuss the origins of some qualitative properties observed using more specific embedded-atom potentials. The ``common neighbor analysis,'' based on planar graphs is used to obtain information about the transient structures accompanying viscoplastic behavior on an atomic level. In particular, pressure tensor components and plastic yield are investigated and correlated with underlying structural changes. A simple analytical expression for the isotropic pressure at finite temperatures is proposed. A nonequilibrium phase diagram is obtained by semianalytic calculation.