Cumulative fatigue damage evaluations on spot‐welded joints using 590 MPa‐class automobile steel

Abstract

A B S T R A C T This study focused on local strain behaviour near the slit edge of spot-welded joints, where the fatigue crack initiated, and investigated methods of evaluating cumulative fatigue damage. A method of evaluating local strain amplitude by following modified Goodman's law gave almost the same result as an evaluation approach based on the external force and provided reasonable result on general strength design. An approach based on Smith-Watson-Topper's equation was easy to evaluate cumulative fatigue damage compared with the method based on modified Goodman's law and gave a good agreement with a criterion of the modified Miner's rule.Keywords fatigue damage; lap joints; mean stress effects; random loading; spot weld.cumulative fatigue damage value f = frequency [Hz] f 1 = lower value of the frequency [Hz] f u = upper value of the frequency [Hz] M = division number on the power spectrum n = actual cyclic number [cycles] N = estimated fatigue life [cycles] N f = number of cycles to failure [cycles] P a = force amplitude [kN] P B = tensile strength of the spot welded joints [kN] P Goodman,a = equivalent force amplitude evaluated following the modified Goodman's law [kN] P max = maximum force [kN] P mean = mean force [kN] P min = minimum force [kN]R p = force ratio S = power spectrum density function T = time x(t) = extreme values simulated following the stationary Gaussian random process at a certain time t. δ = residual of ε estimation,a and ε Goodman,a [με] Δω = division width of the angular frequency ε a = strain amplitude [με] ε B = fracture ductility [με] ε estimation,a = strain amplitude estimated using the regression curve of fatigue test results [με] ε Goodman,a = equivalent strain amplitude evaluated following the modified Goodman's law [με] ε max = maximum strain [με] Correspondence: A. Sugeta.

Cumulative fatigue damage of stress below the fatigue limit in weldment steel under block loading

“…In this study, the cumulative fatigue damage value obtained by the modified Miner's rule is much lower than unity; thus, the calculated outcomes of cumulative fatigue damage using Haibach's rule are not shown in this study. According to Table , one can conclude that the two above‐mentioned rules lose their accuracy for evaluating cumulative fatigue damage for the current utilized material under variable‐amplitude‐loading conditions, including cycles below the fatigue limit, although they retain accuracy for predicting fatigue damage or life under the above‐mentioned condition for friction stir spot welding and spot welded steel sheets, respectively. In summary, neither the modified Miner's nor Haibach's rule gives accurate results.…”

“…However, for a specific material, the coefficient β cannot be determined without experiments. It has been demonstrated that the modified Miner's approach can precisely predict fatigue damage under block loading with a force below the fatigue limit for spot welding‐joined steel sheets, whereas Haibach's approach yields accurate evaluation of cumulative fatigue damage in friction stir spot welding‐joined steel sheets . Furthermore, the real damage values of welded steels range from 0.25 to 0.5 when applying the modified Miner's or Haibach's rule has been proposed …”

A new frequency domain method for random fatigue life estimation in a wide‐band stationary Gaussian random process

“…According to Kihl and Sarkani, the Goodman equation [28][29][30] can provide a reasonable and conservative prediction of the fatigue life, whether the mean stress is tensile or compressive. According to Kihl and Sarkani, the Goodman equation [28][29][30] can provide a reasonable and conservative prediction of the fatigue life, whether the mean stress is tensile or compressive.…”

Effect of jig constraint state during welding process on fatigue properties of Al/CFRP dissimilar welds and fatigue life evaluation based on singular stress

“…This, in turn, means that a unified evaluation of the joint strength, independent of welding conditions, needs to be conducted to more precisely predict joint fatigue life. In previous studies, there are many cases of fatigue life predictions where parameters such as stress, 19,20 strain 21,22 and stress intensity factor 23–27 have been considered. However, most of those studies aimed at other welding technologies, and few reports involving heat welding technology for dissimilar material welding are available.…”