Extension of the Theta Projection Concept in Multi-Axial and Stress-Change Creep Behavior

该项目的目的是在应力变化状态下或在多轴应力下，借助θ投影概念和应变硬化定律来估计蠕变行为。（1）首先对奥氏体不锈钢AISI 304在923 K下进行了一系列恒载荷蠕变试验，用非线性最小二乘法计算了每一条蠕变曲线的θ投影概念中的四个参数。由于这些参数可以很好地表示为施加应力的指数函数，因此即使在未知应力下也可以很容易地绘制蠕变曲线。（2）在90 ~ 140 MPa的应力范围内进行了变应力蠕变试验。试样在初始应力为90 MPa的条件下蠕变50、100、200、300和400 h后，施加的应力增加到140 MPa。蠕变试验中断的时间分别对应于第一次、第二次和第三次蠕变（前、中、后）。（3）利用该方法计算了外加应力变化后的蠕变行为。将估算曲线与实验结果进行了比较。当中断点位于蠕变第三阶段的前半部分时，它们彼此吻合得很好。但在第三阶中期的范围内，计算值偏低。因此，如果同时采用应变硬化定律，则θ投影概念的应用应限于第三蠕变的初始阶段。（4）本文所报道的实验是在一定的蠕变变形后应力增加时的结果。但这种方法是否也适用于应力降低后的变应力蠕变试验，还有待进一步研究。

The purpose of this project is to estimate creep behavior at the stress-change state or under multi-axial stresses with a help of both the theta projection concept and the strain hardening law.(1)Initially a series of constant-load creep tests were conducted on austenitic stainless steel AISI type 304 at 923K.Four parameters in the theta projection concept were computed for each creep curve using the non-linear least square method. Since these parameters were well expressed as an exponential function of the applied stress, creep curves even under unknown stresses could be easily drawn.(2)Stress-change creep tests were performed between the stresses of 90MPa and 140MPa. After the specimens were crept at the initial stress of 90MPa for 50, 100, 200, 300 and 400 hours, the applied stress was increased to 140MPa. These hours when the creep tests were interrupted corresponded to the primary, secondary, and (former, middle, and latter part of) tertiary creep respectively.(3)Creep behavior after the change of the applied stress was calculated using the proposed method. The estimation curves were compared with the experiments. When the interrupted points were places within the former part of the tertiary stage of creep, they well agreed each other. However the calculation were underestimated over the range of the middle part of the tertiary stage. Then we concluded that the application of the theta projection concept should be limited within the initial part of the tertiary creep if the strain hardening law as employed together.(4)The experiments reported here were the results when the stress was increased after some creep deformation. However we should investigate whether this method can also be applied to the stress-change creep test after the applied stress was decreased.