Fatigue Crack Growth Rate Recommended in BS7910 and an Unique Crack Growth Rate Curve Under Different Load Ratios

Abstract

It is still difficult to select the proper constants of crack growth rate corresponding to the loading condition for R-ratio has significant effect on crack growth rate. An unique crack growth rate equation, taking equivalent stress intensity factor range (ESIFR) as the driving force, has been examined for steels and as welded joints. An empirical equation of β for structural steels has been presented. The crack growth rate data expressed by ESIFR instead of SIFR (stress intensity factor range) condensing the crack growth data under different R-ratios to the curve corresponding to R = 0. The most commonly tested crack growth rate constants under R = 0∼0.1 are sufficient in fatigue crack growth calculation under different loading condition. Furthermore, the crack growth rate constants for fatigue life calculation under different R-ratios can be obtained directly from the crack growth rate data tested under any given constant R-ratio test conditions according to the relationship between ESIFR and SIFR. This deduction can greatly reduced the requirement of quantity and cost of fatigue tests for determining the constants of crack growth rate. The gape between the curves corresponding to R<0.5 and R≥0.5 respectively in BS7910 can be filled by present model. For fatigue life prediction of as welded joints, the crack growth rate constants corresponding to R = 0 and R = 0.5 can be used.