Creep Rate Calculator
What is Creep Rate?
Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. It typically occurs as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.
The Creep Rate (denoted as ἐ) is the slope of the creep strain vs. time curve during the secondary creep stage (steady-state creep). Calculating this rate is critical for engineers designing components for high-temperature environments, such as jet engines, steam turbines, and nuclear power plant components.
How to Calculate Creep Rate
While theoretical models like the Norton-Bailey law relate creep rate to stress and temperature using material constants, experimental calculation is done by measuring deformation over time. The formula used in this calculator is based on the linear approximation of the secondary creep stage:
ἐ (Creep Rate) = (ε₂ – ε₁) / (t₂ – t₁)
Where:
- L₀: Initial gauge length of the specimen.
- ε₁: Strain measured at time t₁.
- ε₂: Strain measured at time t₂.
- t₁, t₂: Time points during the steady-state phase (usually in hours).
Understanding the Stages of Creep
- Primary Creep: The initial stage where the strain rate is relatively high but slows down over time due to work hardening.
- Secondary (Steady-State) Creep: The strain rate becomes relatively constant. This is the value calculated above and is the most important parameter for design life predictions.
- Tertiary Creep: The strain rate accelerates exponentially until material failure (rupture) occurs.
Example Calculation
Imagine a steel alloy specimen with an initial gauge length of 50mm tested at 600°C.
- At 100 hours (t₁), the length stretches to 50.12mm.
- At 200 hours (t₂), the length stretches to 50.15mm.
Step 1: Calculate Strains
ε₁ = (50.12 – 50) / 50 = 0.0024
ε₂ = (50.15 – 50) / 50 = 0.0030
Step 2: Calculate Rate
Δε = 0.0030 – 0.0024 = 0.0006
Δt = 200 – 100 = 100 hours
Creep Rate = 0.0006 / 100 = 0.000006 /hr (or 0.0006% /hr)