tailieunhanh - Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) WW Part 13

Tham khảo tài liệu 'encyclopedia of smart materials (vols 1 and 2) - m. schwartz (2002) ww part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | dffr dT is often considered a material constant directly derived from a Clausius-Clapeyron equation. Such descriptions should be considered very elementary simplifications. The results in Fig. 5 show clearly that the stress rate decreases during constrained heating. It has been also found that the stress is affected by many other parameters including the thermomechanical history and the prestrain 69 86 . Depending on the magnitude of the prestrain either a plastic upper limit or an elastic upper limit to ơr exists. At lower prestrains the stress increases during heating until the reverse transformation is completed. The upper stress limit in this case is given by the strain divided by Young s modulus. When the prestrain is sufficiently high the stress increases during heating until plastic yield occurs at a temperature Md. So the upper stress limit in this case is the plastic yield stress ơy. Evidently in cyclic actuation the maximum temperature should be kept below Md. In all cases discussed a constraint prevents the SMA element from returning to the hot shape when heated. Thus a more specific name would be hot shape recovery stress. It has been found in trained Cu-based SMA-elements that stresses can also be generated when the TWME is impeded during cooling 69 83 . Because the constraint in this case prevents the sample from returning to the cold shape when cooled the generated stress was called cold shape recovery stress to contrast with hot shape recovery stress. Practical applications have not been reported so far. Quantitative comprehension of recovery stress generation presented in the literature is far below the comprehension of the other functional properties of shape-memory alloys. Therefore recovery stress generation was discussed a bit more extensively than other functional properties. Considering the substantial research efforts in developing hybrid composites that have embedded shape-memory elements substantial progress in quantitatively understanding