Question

The time to rupture of an S-590 alloy exposed to a tensile stress of 140 MPa at 800C is 9.7 days. Compute the rupture lifetime of this alloy when subjected to a tensile stress of 140 MPa at 650C.

Please, explain in details. Thanks!

Answer 1

-rupture data is Larson-Miller Time –temperature parameters. This parameter can be derived from the stress and temperature dependence of the creep rate or time to rupture. The rate equation generally can be written in the form of the Arrhenius equation and expressed as PLM= T [21.577+ log (tr)].

plm =800+273k(21.577+log(9.7))

plm =22.56

hence

rupture lifetime is equal to

22.56 = 600+273 ( 21.577 +log(tr))

22.56 =18836.721 +873log(tr)

log(tr) = -18814.161 /873

tr = 2.81 x10^-22

rupture, creep, which occurs as a linear function of time, is heavily dependent on temperature and stress. Since creep is a thermally activated process, the minimum secondary creep rate can be described by a fundamental Arrhenius equation of the form:

ε= dε/dt = Kσn exp(- Q/RT); for T=const, dε/dt = Kσn

Where:
T - temperature [K];
tr - time to rupture, [h]
K - creep constant
n - creep exponent (varying from metal to metal n=3÷8; most common value is n=5; determined experimentally determined by plotting the strain rate as a function of stress)
σ - applied stress/nominal stress
Q - activation energy for creep
R - gas constant (8,314 J/mol K)

The critical temperature for creep is 40% of the melting temperature in Kelvin:

If T>0.40Tm then creep is likely.

For metallic materials most creep tests are conducted in uni-axial tensile mode. However, uni-axial compression tests are also used for brittle materials to avoid stress amplification and corresponding crack propagation. For most materials creep properties are independent of loading direction.

The minimum creep rate is the most important design parameter derived from the creep curve. This particular engineering design parameter is primarily considered for long-life applications, e.g. nuclear power plant components. Conversely, for short-life components (e.g. turbine blades, rocket motor nozzles), time to rupture or rupture lifetime is the dominant factor in design. Minimum creep rate is attained using the stress rupture test which tests the material to the point of failure. This is basically similar to a creep test except that the test is always carried out at higher loads. In a creep test total strain is often less than 0.5%, while in stress-rupture tests it is normally around 50%.