tailieunhanh - Material Science_ Vol 2 of 2 - US DOE (1993) WW part 12

Tham khảo tài liệu 'material science_ vol 2 of 2 - us doe (1993) ww part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 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 1000 F or zircaloy above 500 F . 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 PLANT MA TERIAL PROBLEMS DOE-HDBK-1017 2-93 Plant Materials Figure 2 Successive Stages of Creep with Increasing Time The rate of creep is highly dependent on both stress and temperature. With most of the engineering alloys used in construction at room temperature or lower creep strain is so small at working loads that it can safely be ignored. It does not become significant until the stress intensity is approaching the fracture failure strength. However as temperature rises creep becomes progressively more important and eventually supersedes fatigue as the likely criterion for failure.