Larson-Miller Parameter

Such parameter, indicated by m, is expressed as:

                                                             m = T(In t - C).............. Eq(7)

                                                             In t = ((m/T) + C) ........ Eq(8)

                                                             In s = k1 m  + c1........ Eq(9)

In these equations, T is temperature in K, t is time for rupture or a particular deformation in hours, σ is stress in N/mm² or MPa. C, k1 and c1 are constants that are found from experiment. The benefits of the parameter is that via performing rapid rupture test or creep test on high temperature and high stress Eq. (9) can be established. After that this equation can be extrapolated for any of combination of σ, t; σ, T or T, t whether t can be large, σ can be small and T can be large or small. Such parameter can find out T for a known combination of σ and t or σ for a specified combination of T and t or t for a specified combination of σ and T. This parametric technique is a good design tool.

Illustration .9 What is stress relaxation? Provide illustration.

Solution:-

Stress relaxation is creep concerned phenomenon. This is loss of stress in a part beneath constant strain over a specified period of time at estimated temperature.

For illustrations bolts are tightened to a fixed stress and strain to maintain the cover of a pressure vessel leak proof. If the pressure vessel is employed at high temperature the stress in bolts will decreases whereas the bolts will not lose their extension.