A SIMPLE PROBABILISTIC APPROACH TO THE DERIVATION OF MINIMUM YIELD STRENGTH VALUES OF STEELS AT ELEVATED-TEMPERATURES

For the derivation of minimum yield strength values of steels at eleva ted temperatures, the formula R-p0.2/T/min = R-p0.2/20/min (1.1.f(av/T )-0.1) is proposed, where f(av/T) is the interpolated average trend va lue at a specified temperature, obtained by-polynomial interpolation o f the individual ratio values f(T)= Rp(0.2/T)/R-p0.2/20 at all the tes t temperatures. The values of f,vir characterize the trend of the R-p0 .2 -mean value line in function of the temperature, They are dependent on the material and on the temperature. The factor 1.1 and the reduct ion value 0.1 are interpreted in terms of a simple probability model. The model is based on the assumption that the total scatter of the tes t results is composed of two parts. One of these two parts is the corr elation induced scatter due to the correlation between Rp(0.2/T)- and R-p0.2/20-values. The other one is the residual scatter. By evaluation of a number of data groups it is demonstrated that, for engineering p urposes, the standard deviation of the residual scatter may be taken a s about 5% of R-p0.2/20/min independent of the temperature. The validi ty of the formula is confirmed for ferritic and austenitic steels with minimum yield strength values at room temperature between 200 and 800 MPa and for f(av/T)-values between 1 and 0.3. The connections with th e ISO-method set down in ENV 22605-1 and ENV 22605-2, with the trend c urve method set down in ENV 22605-3 and with the recently proposed mod ified trend curve method are explained.