Creepcurves

Creepcurves, or creep curves, are plots used to characterize the time-dependent deformation of materials subjected to sustained loads at elevated temperatures. In a typical constant-load creep test, strain is plotted against time at fixed stress and temperature, producing a curve that reflects the material’s creep response.

The curve commonly exhibits three stages: primary or transient creep, where the creep rate decreases with time; secondary or steady-state creep, where the creep rate is approximately constant; and tertiary creep, where the rate accelerates until failure or rupture occurs. Key metrics derived from creep curves include the minimum creep rate (steady-state rate), time to reach secondary creep, time to rupture, and the total strain at failure.

Factors that shape creep curves include the applied stress, temperature, material composition and microstructure (such as grain size and precipitates), and the environment (oxidation or humidity). Different materials exhibit different dominant creep mechanisms, such as dislocation climb, diffusion, or grain boundary sliding, which influence the curve’s shape and the duration of each stage.

Modeling and analysis: creep data can be described by empirical or physically based models. The Norton-Bailey law expresses a steady-state creep rate as ε̇ = A σ^n exp(-Q/(RT)). The Larson-Miller parameter P_LM = T (C + log t_r) is used for extrapolating life to service conditions. Creep life assessment combines experimental curves with these relationships.

Applications: creep curves are essential in designing and evaluating high-temperature components such as turbines, boilers, and reactor parts, where long-term deformation and failure risk must be anticipated. Limitations include reliance on accelerated testing and potential differences between laboratory and in-service environments.