PSI - Issue 46

Przemysław Strzelecki et al. / Procedia Structural Integrity 46 (2023) 99– 104 Przemys ł aw Strzelecki / Structural Integrity Procedia 00 (2021) 000–000

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characteristics, e.g. ISO-12107, (2012), ASTM E-739-91, (2015), while ensuring the reliability of the test result. Unfortunately, a significant amount of samples is required. According to the standard ISO-12107, (2012), it is 30 samples for reliability & design purposes and 10 for preliminary tests. P. Gope, (1999) has analysed a number of specimens for particular stress level. He estimated that 10 specimens are required for a 10 % probability of failure and 90 % confidential level. Lewis & Sadhasivini, (2004) have gotten 7 specimens for two-parameter Weibull distribution. Then P. C. Gope, (2012) estimated minimum specimens to estimate the S-N curve and he has gotten 13 samples. The estimation of the minimum number of specimens was presented by Soh Fotsing et al., (2010), where they got 7 for 50 % of probability of failure. The author has not found any methods of increasing the accuracy of the fatigue characteristic by taking information from the tensile test. The work aims to develop a method for determining the S-N fatigue characteristics with a low probability of damage (e.g. 5%) based on several samples, e.g. 8 and the results of the monotonic tensile test. An idea of how to estimate such characteristics is depicted in Figure 1. The mean of fatigue strength is estimated from fatigue tests for a small number of specimens and the standard deviation is estimated from tensile test.

Nomenclature N

number of cycles stress amplitude ultimate tensile stress

S a S u S y m σ R σ S b

yield stress

intercept coefficient of S-N curve slope coefficient of S-N curve standard deviation of the yield stress standard deviation of the fatigue strength

indexes i i -th number of sample ഥ mean value

Experimental yield strength

S-N curve– log( S a ) =m log( N )+ b

Stress amplitude S a (log)

Mean value from S-N curve

Experimental fatigue life

Strength distribution- f ( S a ) with standard deviation from tensile tests acc. eq. (4)

Number of cycles, N (log)

Fig. 1. Scheme of S-N curve obtained by the proposed method