amplitude S-N-curves is used with a modified slope k*. The modification of the slope k* depends on the interval of the maximum spectrum load and the constant amplitude fatigue limit described by the form parameter c. *k D D Sˆ S N N ÷ ø ö ç è æ = × ú ú ú û ù ê ê ê ë é - = × + 1 S Sˆ c k k 1 D * ( 2 ) SD: constant amplitude fatigue limit k: slope of constant amplitude S-N-curve Sˆ : spectrum maximum applied stress c: form parameter Setting the parameter to its extreme values the function describes the calculated lifetime curve of variable amplitude tests calculated by using the elementary or original Miner’s Rule. The influence of the spectrum length and the spectrum load distribution can be described by this function. The experimentally observed effect of a lifetime reduction in case of a load spectrum with a greater number of higher loads can be described as well as the effect spectrum length. 105 106 107 108 109 1010 1011 1012 0,9 1,0 1,0 1,2 1,6 2,0 load spectrum normal distribution n = 2 frequency distribution H 0 = 1 10 6 slope k = 5; N D = 2 106 relative maximum stress amplitude S / SD [-] number of cycles N [-] S-N-curve Miner mod. by Liu/Zenner Miner elementary Miner orginal parameter c c = 1 c = 0,05 c = 0,005 c = 0...¥ Figure 6: Capabilities of the Form Parameter c to Describe Variable Amplitude Lifetimes From the analysis of the published data [8] of variable amplitude tests tendencies for the parameter c can be concluded. The results of the quantitative analysis are shown in table 1. TABLE 1 INFLUENCE ON THE FORM PARAMETER C influence factors parameter indication form parameter c high - notch factor: Kt low ¯ 1 ¯ irregularity: I [11] 0 - -1 - stress ratio: R 0 ¯ steap ¯ slope of S-N-curve: k even -
RkJQdWJsaXNoZXIy MjM0NDE=