Numerical fatigue life analysis of a high frequency mechanical impact treated industrial component based on damage mechanics models

Abstract

After post weld treatment with high frequency mechanical impact (HFMI) treatment of welds, a significant increase of fatigue life (up to a factor of 10) can be achieved. During the last years numerous experimental tests of welded joints with simple geometry under constant amplitude loading have been performed to quantify the positive effect of high frequency mechanical impact treatment. Due to the lack of methods for the prediction of the high frequency mechanical impact benefits, a widespread use of this process is not the case yet. Furthermore, it is still not clear if the results of these fatigue tests can be transferred to complex geometries and complex loading conditions such as in industrial applications. Therefore, an approach to assess the fatigue life of complex welded structures under variable amplitude loading was developed. For this purpose, high frequency mechanical impact treatment and fatigue load of simple welded specimen made of S690QL steel were simulated with finite element analysis (FEA) firstly. Then, the needed damage parameters for the fatigue life correlation were evaluated from the finite element postprocessing. The calculated life time to crack initiation was in good agreement with the experimental fatigue test results. In the next step, this procedure was implemented on a welded arm of an evacuator of type EW180B of the company Volvo Construction Equipment made of S700MC. The variable amplitude load measured under real service condition was transferred to single constant amplitude load cycles using a rainflow-counting algorithm. By simulation and damage mechanics evaluation of each load cycle the total damage sum could be calculated and compared with the experimental results from Volvo Construction Equipment.Keywords: Fatigue / welding / residual stresses / high frequency mechanical impact (HFMI) / finite element analysis (FEA) Nach einer Schweißnahtnachbehandlung mittels Hochfrequenzhämmern (engl. high frequency mechanical impact treatment (HFMI)) kann eine signifikante Lebensdauerverlängerung des Bauteils (bis zu einem 10-fachen) festgestellt werden. Während der letzten Jahre wurde dieser positive Effekt vielfach experimentell mit einfachen Proben-

Experimental and analytical study of residual stresses relaxation in nitrided 42CrMo4 parts

“…For the high cycle fatigue domain, the lifetime of the nitrided components was much better than that of the untreated state, this was closely related to the high level of relaxed residual stress, which delays the initiation of cracks in fatigue, Figures 3,4. In addition, the level of loading has no significant influence on the relaxation of the residual stresses since the relaxation rate is relatively low, i. e. of the order of 14.5 %; it goes from À 850 MPa, without any load up to À 660 MPa for a loading level of the order of 760 MPa (σ a < σ D ), Table 4.…”

“…The modeling of cyclic relaxation of residual stresses has been studied by several researchers, many empirical equations have been proposed on the basis of experimental results. Several studies have produced estimates of residual stress relaxation phenomenon but there is still insufficient data for 42CrMo4 nitrided steel [2][3][4][12][13][14]. For this study three analytical models have been proposed in this paper in order to determine the residual stress values after cyclic loading.…”

“…Fatigue life prediction of components is an important issue in engineering applications and could be extended by the near-surface macroscopic compressive residual stresses that retard fatigue crack initiation and crack growth. Major fatigue life prediction methods modify the fatigue limits to account for the presence of residual stresses [1][2][3][4][5][6][7][8][9][10]. The compressive residual stress resulting from the different surface treatments such as shot peening, carburizing, nitriding, have a favorable effect on the resistance of fatigue crack initiation [3,7,[10][11][12].…”

“…The residual stresses are decreased and redistributed through a process called relaxation. The residual stresses are the key factor for fatigue strength improvement; however, mechanical load redistributes and relaxes the beneficial compressive residual stresses [3,4,6,10,11].…”

“…Considerable amounts of experimental data showed that relaxation begins rapidly within the first cycles and slows with subsequent cycling. Additionally, a large relaxation of the compressive residual stress occurring on the first fatigue cycle has been observed by many researchers [1,3,4,[12][13][14][15]. Therefore, an accurate calculation of residual stresses and its relaxation becomes essential for fatigue life prediction.…”