Influence of Ramberg-Osgood fitting on the determination of plastic displacement rates in creep crack growth testing

This paper investigates the effect of the Ramberg-Osgood (R-O) fitting procedures on plastic displacement rate estimates in creep crack growth testing, via detailed two-dimensional and three-dimensional finite-element analyses of the standard compact tension specimen. Four different R-O fitting procedures are considered: (i) fitting the entire true stress-strain data up to the ultimate tensile strength, (ii) fitting the true stress-strain data from 0.1% strain to 0.8 of the true ultimate strain, (iii) fitting the true stress-strain data only up to 5%; strain and (iv) fitting the engineering stress-strain data. It is found that the first two fitting procedures can produce significant errors in plastic displacement rate estimates. The last two procedures, on the other hand, provide reasonably accurate plastic displacement rates and thus should be recommended in creep crack growth testing. Several advantages of fitting the engineering stress-strain data over fitting the true stress-strain data only up to 5% strain are discussed.