Annex R
(normative)

Interpolation and extrapolation procedures for creep strengths

 

 

R.1 General

 

 

Interpolation of creep material strength parameters Rp1, T, t or Rm, T, t, for times T and temperatures t shall be in accordance with the procedures outlined in R.2 through R.4. The determination of times to creep rupture or to 1% creep strain for temperatures t and stresses s shall be in accordance with the procedure outlined in R.5. Extrapolation of creep strengths for times larger than 100 000 hours, up to 200 000 hours, shall be in accordance with the procedure outlined in R.6.

 

 

R.2 Interpolation of creep strengths in temperature

 

If the considered time T is a time for which in the material standard creep strength values are given, then the required interpolation in temperature shall be a linear one:

With RMt, tA, denoting the value of the creep strength parameter specified in the material standard for a temperature tA greater than t, and RM T, tB the one specified in the material standard for a temperature tB less than t, the approximation for the required creep strength parameter RMT, t, for the temperature t given by

 

                                                                    (R.2-1)

shall be used.

 

 

R.3 Interpolation of creep strengths in time

 

If the considered temperature t is a temperature for which in the material standard creep strength values are given, the then required interpolation in creep strength time shall follow the power law given by equation (R.3-1).

 

With RMA,t denoting the value of the creep strength parameter specified in the material standard for a time TA less than T, and RMTB, t the one specified ion the material standard for a time TB larger than T, the approximation for the required creep strength parameter RMT,t for time T given by

 

                                                                                    (R.3-1)

 

                                                                                    (R.3-2)

shall be used.

 

 

R.4 Interpolation of creep strengths in temperature and time

 

If neither the considered time nor the considered temperature agree with the ones for which creep strength values are specified in the material standard, then linear interpolation for the considered temperature at two pivot times TA and TB, according to B.9.2, shall be performed first, and then the power law interpolation for the considered time, according to B.9.3, next.

 

 

R.5 Determination of time to creep rupture or creep strain

 

For the determination of the time to rupture, or to attainment of a specified strain, for a specific stress s and a specific temperature t, equation (R.4-1) shall be used.

Denoting by s1 and s2 the closest larger and closest smaller stress to s for which the relevant lives T1 and T2 for the temperature t are known, the required life T is

 

                                                                                                             (R.5-1)

where

 

.                                                                                (R.5-2)

 

If required values s1, T1, s2, T2  for the specific temperature t are not specified in the material standard, they shall be determined by interpolation according to R.2.

 

 

R.6 Extrapolation of creep strengths

 

If the considered time T is between 1000 000 hours and (including) 200 000 hours, and if in the material standard the maximum time for which creep strength values are specified is 100 000 hours, the creep strength value for 100 000 hours divided by 0,8 + 0,2 T/100 000 may be used for the corresponding creep strength for T hours.