以數位影像關係法分析CuAlMn形狀記憶合金之超彈性疲勞現象

Abstract

本研究對單晶CuAlMn試片進行超彈性疲勞實驗後，再透過數位影像關係法(DIC)對試片表面應變進行分析，藉此了解超彈性循環過程中其應變場變化。實驗中使用的試片晶體方位分別為[ 0 2 5 ]、[ 1 1 4 ]與[ 6 0 5 ]，其中[ 0 2 5 ]與[ 1 1 4 ]方位試片表現出約10%的可拉伸應變量，而[ 6 0 5 ]方位試片則表現出約7%的可拉伸應變量。疲勞實驗結果顯示，[ 1 1 4 ]方位試片的相變應力衰退速度最快，且殘留應變累積速度也最快。DIC分析的結果顯示，殘留應變主要累積的位置會在應力誘發麻田散體開始出現的位置上，然後以此為中心，離此處越遠則累積的殘留應變越少。局部應變分析的結果顯示，以應力誘發麻田散體先出現的位置為中心，離中心越遠殘留應變累積越慢，則相變應力衰減越慢，此結果可得到殘留應變會使誘發麻田散體相變的應力下降這一結論。對能量耗散進行分析會發現在應力誘發麻田散體出現位置開始改變的時候，拉伸時的能量耗散會達到最大值，推測原因可能為此階段每次拉伸中同時誘發麻田散體相變的區域較多的緣故。比較三個試片會發現[ 1 1 4 ]方位試片的超彈性衰退過程與其他試片不同，[ 1 1 4 ]方位試片的相變應力會先大幅下降，然後才開始大量累積殘留應變，推測為拉伸前期局部會積累大量殘留應變，使試片整體相變應力大幅下降，由於僅局部積累大量殘留應變，因此在循環拉伸前期整體平均累積的殘留應變少，但是局部的相變應力已有明顯下降。

In this study, the surface strain distributions of the three CuAlMn single crystals were analyzed by the digital image correlation method (DIC) during cyclic superelastic tests. The purpose of this study was to reveal the evolutions of the strain field during superelastic cycles to understand the effects of residual strain on the decay of superelastic behavior. The tensile orientations of the studied samples were [ 0 2 5 ], [ 1 1 4 ], and [ 6 0 5 ], respectively. 10 % of strain was applied on the [ 0 2 5 ]- and [ 1 1 4 ]-orientated samples, and 7 % strain was applied on the [ 6 0 5 ] one. The fatigue test results showed that the [ 1 1 4 ]-orientated sample had the fastest decay rate of critical stress and the fastest accumulation rate of residual strain. The results of DIC analyses showed that the main accumulation position of residual strain was at the position where the stress-induced martensite nucleated. Furthermore, regions that experienced less martensitic transformation showed less residual martensite. The results of local strain analyses also indicated that residual strain accumulated the most at the position where the stress-induced martensite nucleated, which was associated with the fastest decay of critical stress. On the other hand, regions that underwent less martensitic transformation showed a more minor residual strain and slower decay of critical stress, indicating that the amount of martensitic transformation was directly related to the amount of residual strain and thus caused the decay of transformation stress. The analysis of energy dissipation showed that the energy dissipation increased with increasing cycle number and reached the maximum value at which the main stress-induced martensitic transformation band began to shift. The increase of energy dissipation with cycle number was considered caused by the simultaneous formations of martensite at various regions in the sample, resulting in more significant friction between martensite variants. Comparing these three samples showed that the superelasticity degradation process of the [ 1 1 4 ]-oriented sample was different from others. The critical stress of the [ 1 1 4 ]-oriented sample dropped sharply initially, but its residual strain accumulated later. The reason for this feature was that a large amount of residual strain accumulated in a local region at the first few cycles, causing a significant drop in the critical stress, while the average residual strain of the sample remained relatively small.