Deformation that progresses over time under a constant load. It becomes significant above roughly 0.4 times the melting point (in absolute temperature) and passes through primary, secondary (steady-state) and tertiary stages before rupture.
Japanese name: クリープ
Creep appears in 13 practice questions across 4 topics, including High-temperature materials and creep, Polymers and composites, Mechanical properties and fracture.
A strain–time curve under constant load passes through primary creep, in which the strain rate gradually falls; secondary (steady-state) creep, in which work hardening and recovery balance and the strain rate is nearly constant; and tertiary creep, in which void formation or necking accelerates the rate, ending in rupture. In design, the minimum strain rate, the steady-state creep rate, and the time to rupture (the creep rupture time) are important. The empirical Monkman–Grant relation, in which the product of the two is nearly constant, is also used to estimate life.
The steady-state creep rate is often expressed in the form ε̇ = Aσⁿexp(−Q/RT). In dislocation creep, by dislocation climb and glide, the stress exponent n is about 3 to 8; at low stress and high temperature, diffusion creep (n ≈ 1), in which grains elongate by atomic diffusion, dominates. Diffusion creep includes Nabarro–Herring creep, with diffusion through the grains, and Coble creep, with diffusion along grain boundaries, whose rates are inversely proportional to the square or cube of the grain size. The activation energy Q is often close to that for self-diffusion.
Creep becomes significant at roughly 0.3 to 0.5 times the melting point (in kelvin), so in low-melting materials it occurs even at room temperature. For tin-based solders with melting points of about 490 K, room temperature is about 0.6 of the melting point, and creep of solder joints is a major reliability issue. Polymers, lead and ice also creep near room temperature. Moreover, fine grains that raise strength at room temperature speed up diffusion creep and grain boundary sliding at high temperature. This is why jet engine turbine blades use single crystals with no grain boundaries.
Service lives of real equipment reach tens or hundreds of thousands of hours, so long-term lives are estimated from short tests at higher temperatures or stresses. A typical tool is the Larson–Miller parameter P = T(C + log t_r), where T is absolute temperature, t_r is the rupture time (h), and C of about 20 is commonly used for steels. For example, with C = 20, a condition equivalent to 1000 hours at 600 °C corresponds to about 57 hours at 650 °C. Note, however, that extrapolation errors grow for materials whose microstructure changes over long times.
Answer: A. About 100 °C
This rule of thumb works in absolute temperature, so with Tm = 660 + 273 = 933 K, 0.4Tm ≈ 373 K, about 100 °C. About 264 °C is the mistake of computing 0.4 × 660 in °C, and about 373 °C mistakes 373 K for °C. This ratio (the homologous temperature) varies between materials, roughly 0.3–0.5, but for aluminum alloys creep and loss of strength must be considered from about 100 °C upward, and low-melting solders creep even at room temperature. Creep proceeds through three stages, primary, secondary (steady-state) and tertiary, before rupture, and the steady-state creep rate is the key quantity in design.
Answer: B. In transient creep the strain rate gradually decreases, in steady-state creep work hardening and recovery balance so the rate is nearly constant, and in accelerating creep the rate rises until rupture
After the instantaneous strain on loading, in transient creep work hardening from multiplying and tangling dislocations outweighs recovery, and the strain rate gradually falls. In steady-state creep, work hardening and recovery by processes such as dislocation climb balance, so the strain rate is at its minimum and nearly constant. This stage occupies most of the life, so design uses the steady-state (minimum) creep rate. In accelerating creep, the nucleation and coalescence of voids on grain boundaries, the loss of cross section by necking and microstructural degradation raise the strain rate until rupture. The description with the rate trends reversed confuses the order of the stages. Steady state means the strain rate is constant, not that the strain stops.
Creep is the continued increase of strain under a constant load (stress), while stress relaxation is the gradual drop in stress while the strain is held constant. The loss of clamping force in a bolt at high temperature over time is an example of stress relaxation.
It is the strain rate in the secondary stage of the creep curve, where the rate is nearly constant, also called the minimum creep rate. In design it is the basis for estimating strain over the service life and predicting rupture time.
Yes. The guideline is the ratio to the melting point in kelvin, so for materials with low melting points or glass transition temperatures, such as solders, lead, polymers and ice, creep must be considered even at room temperature.
This ratio (the homologous temperature) varies between materials, roughly 0.3–0.5, but for aluminum alloys creep and loss of strength must be considered from about 100 °C upward, and low-melting solders creep even at room temperature.
Take about 0.4 times the melting point (in absolute tempera…After the instantaneous strain on loading, in transient creep work hardening from multiplying and tangling dislocations outweighs recovery, and the strain rate gradually falls.
Which statement about the three stages of a creep curve (st…The activation energy for high-temperature creep of pure metals is often close to that for self-diffusion, showing that creep is controlled by diffusion.
An alloy has a steady-state creep rate that follows ε̇ = Aσ…What the Voigt model represents is delayed elasticity (creep), in which strain approaches a constant value over time under constant stress.
Which statement about mechanical models of viscoelasticity …About 4.3 MPa is 5.0 × (1 − exp(−2)), confusing relaxation with an expression that grows over time, like creep in the Voigt model.
A constant strain is applied to a polymer that can be repre…Dislocation Nickel-based superalloy Power-law creep Diffusion creep Viscoelasticity Work hardening Recovery Larson–Miller parameter Hot isostatic pressing Sintering