Ul o

Region No Propagat

Region No Propagat

Region III Rapid Crack Growth

A Kth

Log A K

Fig. C-13. Crack Propagation Curve for Fatigue Loading

Finally, in region III, the crack growth rate accelerates since the fracture toughness of the material is approached, and there is a local tensile overload failure.

C.5 Creep and Stress Rupture8 9

A metal subject to a constant tensile stress at a sufficient elevated temperature will creep, or undergo a time-dependent increase in length. Temperatures higher than about 1/2 the melting temperature, where diffusion mechanisms become active, are usually sufficient to make creep an important consideration. A creep test measures the dimensional changes that occur at elevated temperature, while a stress rupture test measures the effects of temperature on the long-term load-bearing characteristics.

To determine the creep curve for a metal, a constant load is applied to a tensile specimen maintained at the temperature of interest, and the strain (g), or extension, of the specimen is determined as a function of time. An idealized creep curve is shown in Fig. C.14. The slope of the curve is (g = dg/dt) called the creep rate. The three stages of creep are shown. Following the initial elongation of the specimen (go), the creep rate decreases with time during primary creep. During secondary creep, the creep rate reaches a steady state in which the creep rate changes little with time. During the third stage of creep (tertiary creep), the creep rate increases rapidly with time until fracture occurs. Creep tests are usually long time tests, with times between 2000 and 10 000 h typical, with total strains usually less than 0.5%. The purpose of the creep test is to precisely measure the creep rate and total creep strain. Creep tests are often terminated before final failure occurs.


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