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Work  (Strain)  Hardening
Figure 2   Successive Stages of Creep with Increasing Time

Material Science Volume 2 of 2
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Plant Materials DOE-HDBK-1017/2-93 PLANT MATERIAL PROBLEMS Figure 1   Nominal Stress-Strain Curve    vs True Stress-Strain Curve Work hardening can also be used to treat material.  Prior work hardening (cold working) causes the   treated   material   to   have   an   apparently   higher   yield   stress. Therefore,   the   metal   is strengthened. Creep At room temperature, structural materials develop the full strain they will exhibit as soon as a load is applied.  This is not necessarily the case at high temperatures (for example, stainless steel above 1000F or zircaloy above 500F).   At elevated temperatures and constant stress or load, many materials continue to deform at a slow rate.   This behavior is called creep.   At a constant stress  and  temperature,  the  rate  of  creep  is  approximately  constant for  a  long  period  of  time. After this period of time and after a certain amount of deformation, the rate of creep increases, and fracture soon follows.   This is illustrated in Figure 2. Initially, primary or transient creep occurs in Stage I.   The creep rate, (the slope of the curve) is high at first, but it soon decreases.   This is followed by secondary (or steady-state) creep in Stage II,   when   the   creep   rate   is   small   and   the   strain   increases   very   slowly   with   time. Eventually, in Stage III (tertiary or accelerating creep), the creep rate increases more rapidly and the strain may become so large that it results in failure. Rev. 0 Page 29 MS-05







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