Đang chuẩn bị liên kết để tải về tài liệu:
Thumb for Mechanical Engineers 2011 Part 12

Tham khảo tài liệu 'thumb for mechanical engineers 2011 part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 320 Rules of Thumb for Mechanical Engineers CREEP RUPTURE Creep is plastic deformation which increases over time under sustained loading at generally elevated temperatures. Stress rupture is the continuation of creep to the point where failure takes place. Metallic and nonmetallic materials vary in their susceptibility to creep but most common structural materials exhibit creep at stress levels below the proportional limit at elevated temperatures which exceed one-third to one-half of the melting temperature. A few metals such as lead and tin will creep at ordinary temperatures. The typical sttain-time diagram in Figure 39 for a material subject to creep illustrates the three stages of creep behavior. After the initial elastic deformation the material exhibits a relatively short period of primary creep stage 1 where the plastic strain rises rapidly at first. Then the sttain versus time curve flattens out. The flatter portion of the curve is referred to as the secondary or steady state creep stage 2 . This is the stage of most importance to the en- gineer in the design process. The final or tertiary creep stage stage 3 is characterized by an acceleration of the creep rate which leads to rupture in a relatively short period of time. High stresses and high temperatures have comparable effects. Quantitatively as a function of temperature a logarithmic relationship exists between stress and the creep rate. A number of empirical procedures are available to correlate stress temperature and time for creep in commercial alloys. The Larson-Miller parameter Klm is an example of one of these procedures. The general form of the Larson-Miller equation is Klm 0.001 T 460 log Ct 20 where T temperature in F. t time hours c empirical parameter relating test specimens to design component Creep strength is specified as the stress corresponding to a given amount of creep deformation over a defined period of time at a specified temperature i.e 0.5 creep in 10 000 hours at l 200 F. The .