At room temperature, structural materials develop the full strain they will exhibit as soon as a load is applied. This is not necessarily the case at high temperatures (for example, stainless steel above 1000°F or zircaloy above 500°F). At elevated temperatures and constant stress or load, many materials continue to deform at a slow rate. This behavior is called creep. At a constant stress and temperature, the rate of creep is approximately constant for a long period of time. After this period of time and after a certain amount of deformation, the rate of creep increases, and fracture soon follows. This is illustrated in Figure 2.

Initially, primary or transient creep occurs in Stage I. The creep rate, (the slope of the curve) is high at first, but it soon decreases. This is followed by secondary (or steady-state) creep in Stage II, when the creep rate is small and the strain increases very slowly with time. Eventually, in Stage III (tertiary or accelerating creep), the creep rate increases more rapidly and the strain may become so large that it results in failure.

Figure 2 Successive Stages of Creep with Increasing Time

The rate of creep is highly dependent on both stress and temperature. With most of the engineering alloys used in construction at room temperature or lower, creep strain is so small at working loads that it can safely be ignored. It does not become significant until the stress intensity is approaching the fracture failure strength. However, as temperature rises creep becomes progressively more important and eventually supersedes fatigue as the likely criterion for failure. The temperature at which creep becomes important will vary with the material.

For safe operation, the total deformation due to creep must be well below the strain at which failure occurs. This can be done by staying well below the creep limit, which is defined as the stress to which a material can be subjected without the creep exceeding a specified amount after a given time at the operating temperature (for example, a creep rate of 0.01 in 100,000 hours at operating temperature). At the temperature at which high-pressure vessels and piping operate, the creep limit generally does not pose a limitation. On the other hand, it may be a drawback in connection with fuel element cladding. Zircaloy has a low creep limit, and zircaloy creep is a major consideration in fuel element design. For example, the zircaloy cladding of fuel elements in PWRs has suffered partial collapse caused by creep under the influence of high temperature and a high pressure load. Similarly, creep is a consideration at the temperatures that stainless-steel cladding encounters in gas-cooled reactors and fast reactors where the stainless­steel cladding temperature may exceed 540°C.

Summary

The important information in this chapter is summarized below.

Plant Material Problems Summary

Fatigue Failure

Thermal fatigue is the fatigue type of most concern. Thermal fatigue results from thermal stresses produced by cyclic changes in temperature.

Fundamental requirements during design and manufacturing are used to avoid fatigue failure.

Plant operations are performed in a controlled manner to mitigate cyclic stress. Heatup and cooldown limitations, pressure limitations, and pump operating curves are also used to minimize cyclic stress.

Work Hardening

Work hardening has the effect of reducing ductility, which increases the chances of brittle fracture.

Prior work hardening causes the treated material to have an apparently higher yield stress; therefore, the metal is strengthened.

Creep

Creep is the result of materials deforming when undergoing elevated temperatures and constant stress. Creep becomes a problem when the stress intensity is approaching the fracture failure strength. If the creep rate increases rapidly, the strain becomes so large that it could result in failure. The creep rate is controlled by minimizing the stress and temperature of a material.

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