Creep is defined as the tendency of a solid material to slowly and permanently deform when subjected to a constant mechanical stress over an extended period. This time-dependent process causes strain to accumulate progressively, even under stresses well below the material’s yield point. Creep is distinctly different from the instantaneous plastic deformation that occurs when a material’s yield strength is exceeded.
The rate of creep is highly sensitive to temperature, making it a primary concern in high-temperature applications. High-strength alloys typically begin to creep at temperatures above 30 to 40% of their absolute melting temperature. This occurs because the movement of atoms and defects is thermally activated, allowing the material’s microstructure to slowly rearrange under the applied load.
The Stages of Creep Deformation
When a material is subjected to a constant load at an elevated temperature, its deformation over time reveals three distinct phases on a creep curve.
Primary Creep (transient creep) has a high initial strain rate that continuously decreases. This reduction is due to strain hardening, where the material resists stress through internal microstructural changes.
Secondary Creep (steady-state creep) is important for engineering design. The strain rate becomes nearly constant and reaches its minimum value as strain hardening and thermal recovery achieve a dynamic balance. This minimum creep rate is used to predict the long-term service life of a component.
Tertiary Creep follows the steady state, where the strain rate rapidly accelerates until the material fails (stress rupture). This acceleration is caused by internal damage, such as micro-cracks and voids, which reduce the load-bearing cross-sectional area. The increasing true stress drives the rapid deformation, leading to structural instability.
Atomic and Microstructural Mechanisms
Creep involves the thermally activated movement of defects within the crystal lattice, allowing the material to flow slowly.
Dislocation Climb is a common mechanism in metals where dislocations bypass obstacles by climbing to a different slip plane. This movement is assisted by vacancy diffusion and dominates at higher stress levels and elevated temperatures.
At lower stresses and high temperatures, diffusional creep mechanisms become prominent, relying on the movement of atoms to change the grain shape. Nabarro-Herring Creep involves the bulk diffusion of atoms through the crystal lattice. Coble Creep is a similar process but occurs through the faster diffusion of atoms along the grain boundaries.
Coble creep is active in fine-grained materials because grain boundaries provide more pathways for atomic movement. Nabarro-Herring creep dominates in materials with larger grains. Grain boundary sliding, where adjacent grains slide past one another, also contributes to deformation. The dominant mechanism depends on the operating temperature, applied stress, and microstructure.
Influencing Variables and Material Selection
The rate of creep is sensitive to two primary external factors: temperature and applied stress. A small increase in operating temperature can dramatically increase the creep rate because the diffusion of atoms and dislocation movement are highly dependent on thermal energy. This sensitivity is often understood using Homologous Temperature, the ratio of the operating temperature to the material’s absolute melting temperature.
Engineers select creep-resistant materials by focusing on properties that inhibit microstructural movements. The first consideration is selecting materials with a high melting point to keep the homologous temperature low. High-performance alloys resist creep using microstructural design, such as larger grain sizes or single-crystal components, to minimize high-diffusivity grain boundaries.
Alloying elements are introduced to create fine, stable precipitates that anchor dislocations, impeding their movement and promoting resistance to dislocation climb. Superalloys are designed with specific microstructures to withstand the high temperatures and stresses found in jet engines and power generation turbines. This ensures the steady-state creep rate remains low throughout the component’s service life.
Practical Consequences and Real-World Examples
Creep represents a failure mechanism in industries relying on components operating under sustained high temperatures and loads. Creep-induced failure, or creep rupture, results from long-term dimensional instability, which can lead to interference between moving parts. Engineers must account for the accumulation of strain over years of service to maintain structural integrity.
A prominent example occurs in the turbine blades of jet engines, which operate under high centrifugal force and gas temperatures exceeding \(1,000^{\circ}\text{C}\). The gradual lengthening of these blades due to creep can cause them to contact the casing, leading to failure. In power plants, boiler tubes and superheater pipes carrying high-pressure steam are susceptible to creep, manifesting as tube bulging and eventual stress rupture.
Creep is a concern in high-pressure chemical vessels, nuclear reactors, concrete structures, and solder joints in electronics. Design philosophy dictates that components must be retired or replaced before entering the tertiary creep stage. This final acceleration is an irreversible precursor to failure, requiring extensive testing to predict time-to-rupture.