Creep Creep is slow plastic extension (strain) at elevated temperatures (T ~> 0.3 melting T) and stress. Typical strain vs time at elevated temperature and stress is: rupture X stage I: primary creep: rapid initial deformation stage II: secondary creep: a.k.a. steady state creep: constant strain rate designated "minimum strain rate" stage III: tertiary creep: reduction in cross section eventually leading to fracture T2 III strain ε II minimum creep rate dε/dt I T1 < < T2 time Typical creep data is usually shown as stress vs time to rupture, 0.5% strain, and 0.2% strain for a given temperature. A set of curves derived manually from NACA TN 4112 Generalized Master Curves for Creep and Rupture, Heimerl and McEvily Oct 1957 (Aero Library) for Aluminum 2024-T3 alloy plate is shown: NACA TN 4112 data creep data shown as stress (ksi) vs time (hours) parameterized by temperature - includes rupture, 0.5% strain and 0.2% strain Creep for Aluminum 2024-T3 alloy sheet 100 stress (ksi) T = 500 F rupture T = 500 F 0.5 % strain T = 500 F 0.2 % strain T = 350 F rupture T = 350 F 0.5 % strain T = 350 F 0.2 % strain 10 0.1 1 10 100 3 1 .10 time (hours) 11/28/2005 1 4 1 .10 5 1 .10 6 1 .10 creep data shown as stress (ksi) vs temperature parameterized by time (hours) - rupture only shown - gets too busy Creep for Aluminum 2024-T3 alloy sheet 100 stress (ksi) (log) time to rupture = 1000 hours time to rupture = 5000 hours time to rupture = 10000 hours 10 200 250 300 350 400 450 500 temperature deg F These curves are difficult to use so researchers proposed a parameter that collapses data empirically. E.g.these data can be represented can be represented on a Master Curve for Creep using the Larson-Miller (or another similar parameter). The plots lack smoothness due to manual extraction of data. "Master Creep Curve" Al 2024-T3 50 rupture 0.5% strain 0.2% strain stress (ksi) 40 30 20 10 13 14 15 16 17 Larson-Miller TR(17+log(time) (*10^-3) 11/28/2005 2 18 19 The Larson Miller parameter combines temperature and times (rupture, strain) as follows: ( ( )) LM = TR⋅ C1 + log tr C1 = constant_based_on_material C1_al_2024 := 17 C1_carbon_molybdenum_steel := 20 TR = temperature_deg_R = deg_F + 460 tr = time_to_rupture_hours above curves similar parameter for t ε_0.5 tε_0.5 = time_to_point_5%_strain_hours tε_0.2 = time_to_point_2%_strain_hours Master Creep Curves - rupture stress (ksi) 100 10 Carbon Molybdenum steel Aluminum 2024-T3 alloy plate 1 10 15 20 25 30 35 Larson Miller TR(C1+log(tr)) (*10^-3) 11/28/2005 3 40 45