Creep, also known as cold flow, is the permanent deformation that increases with time under constant load or stress. It results from long-time exposure to large external mechanical stress within the limit of yielding and is more severe in materials subjected to heat for a long time. The rate of deformation is a function of the material’s properties, exposure time, exposure temperature, and the applied structural load. Creep is very important if we use materials at high temperatures. Creep is very important in the power industry and is of the highest importance in designing jet engines. Time to rupture is the dominant design consideration for many relatively short-life creep situations (e.g., turbine blades in military aircraft). Of course, for its determination, creep tests must be conducted to the point of failure, termed creep rupture tests.
Creep becomes a problem when the stress intensity approaches the fracture failure strength. If the creep rate increases rapidly, the strain becomes so large that it could fail. The creep rate is controlled by minimizing the stress and temperature of a material. Creep is more severe in materials subjected to heat for long periods and generally increases as they near their melting point. It is observed in all materials types; for metals, it becomes important only for temperatures greater than 0.4Tm, where Tm is the absolute melting temperature.
For example, creep-caused failure is an important failure mode for turbine blades of an aircraft engine. In this case, a turbine blade will cause the blade to contact the casing, failing the blade and the engine.
Stages of Creep
As can be seen from the figure, creep is time-dependent, and it goes through several stages:
- Primary Creep. The strain rate is relatively high in the initial stage, or primary creep, or transient creep. Still, it decreases with increasing time and strain because the material is experiencing an increase in creep resistance or strain hardening. This is followed by secondary (or steady-state) creep in Stage II when the creep rate is small, and the strain increases slowly with time.
- Secondary Creep. For secondary creep, sometimes termed steady-state creep, the rate is constant—that is, the plot becomes nearly linear. The strain rate diminishes to a minimum and becomes near constant as the second stage begins. This is due to the balance between work hardening and annealing (thermal softening). This stage of creep is the most understood. The steady-state creep is often the stage of creep that is of the longest duration. No material strength is lost during these first two stages of creep. The most important parameter from a creep test in materials engineering is the slope of the second portion of the creep curve (ΔP/Δt). It is the engineering design parameter that is considered for long-life applications. This parameter is often called the minimum or steady-state creep rate.
- Tertiary Creep. In tertiary creep, there is an acceleration of the rate and possibly ultimate failure. The strain rate exponentially increases with stress because necking phenomena or internal cracks, cavities, or voids decrease the effective area of the specimen. These all lead to a decrease in the effective cross-sectional area and an increase in strain rate. Strength is quickly lost in this stage while the material’s shape is permanently changed. The acceleration of creep deformation in the tertiary stage eventually leads to failure, frequently termed rupture resulting from microstructural and/or metallurgical changes.
Creep prevention is based on the proper choice of material is also crucial. The creep resistance of materials can be influenced by many factors such as diffusivity, precipitate, and grain size. In general, there are three general ways to prevent creep in metal. One way is to use higher melting point metals, the second is to use materials with greater grain size, and the third is to use alloying. Body-centered cubic (BCC) metals are less creep resistant in high temperatures. Therefore, superalloys (typically face-centered cubic austenitic alloys) based on Co, Ni, and Fe can be engineered to be highly resistant to creep and have thus arisen as an ideal material in high-temperature environments.
Operating the device within limits is of the highest importance for devices of selected material, especially regarding maximum service temperature and stress. The rate of creep is highly dependent on both stress and temperature. With most engineering alloys used in construction at room temperature or lower, creep strain is so small at working loads that it can safely be ignored. 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.