The Influence of Residual Stresses on Constant and Variable Amplitude Fatigue Behaviour of Steel

Subjected to dynamic loading, truck components are exposed to variable amplitude fatigue. The fatigue life of components is to a large degree controlled by the behaviour of the surface zones. Different mechanical surface treatments such as shot peening have been developed to introduce compressive stresses close to the surface, thereby delaying failure of components exposed to fatigue loading. During shot peening the specimen is bombarded by small hard shots leading to plastic deformation of a thin surface zone. The inhomogeneous plastic deformation leads to changes in interplanar spacing of the crystal lattice as well as the density of dislocations in the surface zone. We can express this as development of compressive residual stresses and a change in the work hardening state. The compressive residual stress is thought to improve the fatigue properties by increasing the surface resistance to crack initiation as well as reducing the crack propagation rate. Tie rod arm as a truck component is the subject of present study. This component may be made of two different materials: medium carbon microalloyed steel or quenched and tempered steel. The former is continuously cooled leading to essentially pearlitic microstructure and the latter has a tempered martensite microstructure. Although the pearlitic arms made of microalloyed steel were the main focus of this study, the tempered martensite microstructure was also investigated to a certain extent. Strain as well as stress control push-pull fatigue tests were performed on cylindrical specimens made from the actual components. The tests were conducted in constant amplitude, with occasional overloading as well as variable amplitude conditions. The results showed that the life time of specimens at higher strain amplitudes (higher plastic strain amplitudes) is not effected by shot peening. However at smaller plastic strain amplitudes the peened specimens exhibit a longer life time than unpeened ones. The residual stress relaxation and changes of work hardening state were also monitored at the surface and in depth for different strain and stress amplitudes as well as cycle numbers. The influence of plastic strain amplitude on relaxation of residual stresses was studied in detail as a function of fatigue cycle number. In parallel whole components were also tested in constant and variable amplitude loading fatigue, with and without shot peening. The fatigue behaviour in bending as well as relaxation of the residual stresses were also studied for the tempered martensitic steel with different degrees of decarburisation and subsequent shot peening. Also in this case a positive influence of shot peening is mainly seen at small strain amplitudes

The Influence of Residual Stresses on Fatigue Behaviour of Medium Carbon Microalloyed Steel

Subjected to the dynamic loading, truck components are exposed to fatigue loading. Hence the methods to improve the fatigue properties of the components are widely employed by manufacturers. One of the well known methods in this field is shot peening. Shot peening introduces compressive residual stresses in the surface zones and changes the work hardening state there. Compressive residual stresses provide higher resistance to crack initiation and may postpone the crack propagation. However the surface roughness is considerably increased by shot peening, which of course is not favourable for the fatigue durability.Steering arm as a truck component is the subject of present study. This component may be made of two different materials: medium carbon microalloyed steel or quenched and tempered steel. The former is continuously cooled leading to pearlitic-ferritic microstructure and the latter has a tempered martensite microstructure. Although the pearlitic-ferritic steering arm made of microalloyed steel is the main focus of this study, the tempered martensite microstructure was also investigated to a certain extent. Strain controlled, constant amplitude push-pull fatigue tests at different strain amplitudes were performed on cylindrical specimens. The specimens were machined out of the microalloyed steel steering arms. The specimens then were tested in two groups: one group in mirror finished condition and the other group in shot peened condition. The results showed that the life time of specimens at higher strain levels (higher plastic strain amplitudes) is not effected by shot peening. However at smaller plastic strain amplitudes the peened specimens exhibit a life time at least one order of magnitude longer than unpeened specimens. The residual stress relaxation and changes of work hardening state were also monitored at the surface and in depth for different strain amplitudes. The influence of plastic strain amplitude on relaxation of residual stresses was studied in detail as a function of fatigue cycle number. The fatigue behaviour in bending, including relaxation, was also studied for the tempered martensitic steel with different degrees of decarburisation and subsequent shot peening. Also in this case a positive influence of shot peening is mainly seen at small strain amplitudes. In parallel whole components were also tested in constant and variable amplitude loading fatigue, with and without shot peening. The results here also show a longer life time for the peened components

The Influence of Residual Stresses on Fatigue Behaviour of Medium Carbon Microalloyed Steel

Subjected to the dynamic loading, truck components are exposed to fatigue loading. Hence the methods to improve the fatigue properties of the components are widely employed by manufacturers. One of the well known methods in this field is shot peening. Shot peening introduces compressive residual stresses in the surface zones and changes the work hardening state there. Compressive residual stresses provide higher resistance to crack initiation and may postpone the crack propagation. However the surface roughness is considerably increased by shot peening, which of course is not favourable for the fatigue durability.Steering arm as a truck component is the subject of present study. This component may be made of two different materials: medium carbon microalloyed steel or quenched and tempered steel. The former is continuously cooled leading to pearlitic-ferritic microstructure and the latter has a tempered martensite microstructure. Although the pearlitic-ferritic steering arm made of microalloyed steel is the main focus of this study, the tempered martensite microstructure was also investigated to a certain extent. Strain controlled, constant amplitude push-pull fatigue tests at different strain amplitudes were performed on cylindrical specimens. The specimens were machined out of the microalloyed steel steering arms. The specimens then were tested in two groups: one group in mirror finished condition and the other group in shot peened condition. The results showed that the life time of specimens at higher strain levels (higher plastic strain amplitudes) is not effected by shot peening. However at smaller plastic strain amplitudes the peened specimens exhibit a life time at least one order of magnitude longer than unpeened specimens. The residual stress relaxation and changes of work hardening state were also monitored at the surface and in depth for different strain amplitudes. The influence of plastic strain amplitude on relaxation of residual stresses was studied in detail as a function of fatigue cycle number. The fatigue behaviour in bending, including relaxation, was also studied for the tempered martensitic steel with different degrees of decarburisation and subsequent shot peening. Also in this case a positive influence of shot peening is mainly seen at small strain amplitudes. In parallel whole components were also tested in constant and variable amplitude loading fatigue, with and without shot peening. The results here also show a longer life time for the peened components

Influence of shot peening on fatigue durability of normalized steel subjected to variable amplitude loading

The influence of shot peening on the fatigue durability of normalized carbon steels subjected to variable amplitude loading has been investigated. The relaxation of residual stresses was recorded during the fatigue life time. Strain amplitude spectra were extracted from real spectra recorded from components in service. The results were compared with data achieved from constant amplitude testing. In both types of tests parallel studies were made on both peened and unpeened specimens. Shot peening leads to pronounced increase in life time, especially for smaller amplitudes. For both variable and constant amplitude loading shot peened specimens exhibit longer life provided the residual stresses during fatigue loading do not relax more than to about 60% of their initial value. To get an improvement in life time of at least a factor two for peened specimens, the stress amplitude in constant amplitude loading or the maximum stress amplitude in variable amplitude history must not be more than 20% larger than the magnitude of the initial residual stresses. This limit corresponds to 1.2 times the yield strength of the unaffected material

Influence of overloading on fatigue durability and stability of the residual stresses in shot peened normalized steel

The present investigation deals with the effect of overloads on the stress relaxation and fatigue life time of shot-peened near-pearlitic steels. Single cycles with a total strain amplitude of 0.6 % with start in tension or compression were exerted either at the very beginning or after 1000 cycles and superimposed on the constant total strain amplitude test at 0.3 %. The results were compared with the constant amplitude test data. It was shown that such overloading can reduce the fatigue life time by 25%-60% of the life time obtained in constant amplitude tests. Maximum amount of reduction was obtained in overloading histories started with straining in tension. The stress amplitudes and corresponding mean stress development after overloading were also recorded and used to interpret the effects of overloads on the fatigue life times. In addition the residual stress relaxation throughout the whole lifetime was followed

Influence of overloading on fatigue durability and stability of the residual stresses in shot peened normalized steel

The present investigation deals with the effect of overloads on the stress relaxation and fatigue life time of shot-peened near-pearlitic steels. Single cycles with a total strain amplitude of 0.6 % with start in tension or compression were exerted either at the very beginning or after 1000 cycles and superimposed on the constant total strain amplitude test at 0.3 %. The results were compared with the constant amplitude test data. It was shown that such overloading can reduce the fatigue life time by 25%-60% of the life time obtained in constant amplitude tests. Maximum amount of reduction was obtained in overloading histories started with straining in tension. The stress amplitudes and corresponding mean stress development after overloading were also recorded and used to interpret the effects of overloads on the fatigue life times. In addition the residual stress relaxation throughout the whole lifetime was followed

Stability of Residual Stresses Created by Shot Peening in Monotonic Loading and at the Presence of Load Reversals - Experiments and Modeling

As a method for mechanical surface treatment, shot peening has been widely used to improve the fatigue strength of materials. However, the influence of residual stresses introduced by shot peening depends on their stability. The stability of residual stresses during fatigue may be studied in two stages: the first cycle and successive cyclic loading. In this study the stability and development of the residual stresses during the first cycle of strain controlled fatigue of normalized steel was investigated. The influence of total strain amplitude and the loading direction was studied. The residual stresses were obtained using the x-ray diffraction technique. It was shown that the stability and relaxation of the residual stresses depend both on the amount and the direction of the loading stresses. Finite element modeling (FEM) was used to rationalize the experimental data. Very good agreement between the experimental and FEM results were observe

Stability of shot peening induced residual stresses and their influence on fatigue lifetime

Mechanical surface treatment methods such as shot peening may improve the fatigue strength of materials. In this study, the effect of shot peening on strain controlled constant amplitude fatigue loading of a near pearlitic microalloyed steel was investigated. The stress amplitudes throughout the whole lifetime were followed, in addition to detailed recording of stress–strain hysteresis loops, particularly at small cycle numbers. The detailed relaxation of residual stresses and the changes in full width of half maximum (FWHM)of the X-ray peak at the surface and in depth as function of the number of cycles and plastic strain were recorded. By these techniques, the onset as well as the rate of relaxation of residual stresses could be followed at different strain amplitudes. Pronounced increase in lifetime of the shot peened specimens tested at total strain amplitude smaller than 0.3%(corresponding to 0.034% plastic strain amplitude)was achieved. This coincides with reasonably stable residual stresses at the surface and in depth

Stability of Residual stresses Created by Shot Peening of Pearlitic Steel and Their Influence on Fatigue Behaviour

In this study the effect of shot peening on the fatigue lifetime of a near pearlitic microalloyed steel was investigated. The fatigue tests were run in strain control with parallel recording of stress relaxation and recovery of the work hardened surface zone at different total strain amplitudes exerted to the test specimens. These relaxation processes were followed versus cycle number up to half of the fatigue life time (N=Nf/2). Provided that the global plastic strain amplitude is lower than about 0.08 % a noticeable increase in life time is seen. Lower plastic strain amplitude increases the life time. At small plastic strain amplitudes it was found that the fatigue life time could be increased more than tenfold by the shot peening process