An attempt to predict remanent life based on creep time law

本课题的目的是检验一种新的残余蠕变寿命预测方法的有效性，并明确基于θ (i=1-4)控制蠕变应变与时间关系的新方法的特点。首先，将新方法（θ -投影概念）应用于镍基高温合金（IN-738）。在这里，很明显，θ的应力依赖性包括由于蠕变机制和蠕变结构的变化而引起的阻力变化，这将发生在对应力进行大范围外推时。这一事实从多方面证明了新方法优于现有方法。因此，建议采用新方法积累蠕变数据。其次，对沉淀硬化铝合金（2024）的蠕变时间规律进行了研究。在此，根据短期蠕变数据预测了长期蠕变行为，并与实际的长期蠕变数据进行了比较。结果表明，预测蠕变曲线与实际蠕变曲线有较好的吻合。分析表明，在对应力进行log theta_i s时，theta_2的散点要比其他散点大得多。然而，值得注意的是，散点并没有严重影响外推的准确性。为了阐明为什么theta_2具有较大的散点，我们确定了误差图，即四个参数中两个参数变化时标准误差变化的图，另外两个参数保持不变。利用这些图，明确了参数之间的关系和产生大散点的原因。结果表明，这种误差图具有普遍适用性，对预测残余蠕变寿命有一定的帮助。

The purpose of the present project is to examine the validity of a new method for the remanent creep life prediction and to make clear the characteristics of the new method based on the theta (i=1-4 )controlling the relationship between creep strain and time. To begin with, the new method (theta - projection concept) was applied to Ni base superalloy (IN-738). Here, it was made clear that the stress dependence of theta includes the change in cesistance due to the change in creep mechanisms and creep structures, which would take place when extrapolation is made over a wide range on stress. This fact demonstrates that the new method is superior to the current methods from various views. Thus, the accumulation of creep data in the new method was recommended.Secondly, the studies on the creep time law of a precipitation hardened aluminum alloy(2024) was conducted. Here, long-termed creep behaviors were predicted from the short-termed creep data and they were compared with the actual long-termed creep data. It was found that there existed the fairly well coincidence between the predicted and actual creep curves. The present analysis showed that the scatter of theta_2 was much larger than those of others when log theta_i s were plotted against stress. However, it was to be noted that the scatter did not seriously affect the accuracy of extrapolation. In order to elucidate why theta_2 has larger scatter, the error map, namely , the map showing the change in standard error with the change of two parameters out of four, two others being kept constant, was determined. Using the maps, the relationship among parameters and the reason of the large scatter were made clear. It was shown that the error maps of this kind would be generally applicable and halpful for predicting the remanent creep life.