This study concerns a commonly-used procedure for evaluating the steady state creep stress exponent, n , from indentation data. The procedure involves monitoring the indenter displacement history under constant load and making the assumption that, once its velocity has stabilised, the system is in a quasi-steady state, with stage II creep dominating the behaviour. The stress and strain fields under the indenter are represented by "equivalent stress" and "equivalent strain rate" values. The estimate of n is then obtained as the gradient of a plot of the logarithm of the equivalent strain rate against the logarithm of the equivalent stress. Concerns have, however, been expressed about the reliability of this procedure, and indeed it has already been shown to be fundamentally flawed. In the present paper, it is demonstrated, using a very simple analysis, that, for a genuinely stable velocity, the procedure always leads to the same, constant value for n (either 1.0 or 0.5, depending on whether the tip shape is spherical or self-similar). This occurs irrespective of the value of the measured velocity, or indeed of any creep characteristic of the material. It is now clear that previously-measured values of n , obtained using this procedure, have varied in a more or less random fashion, depending on the functional form chosen to represent the displacement-time history and the experimental variables (tip shape and size, penetration depth, etc.), with little or no sensitivity to the true value of n .

中文翻译：

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从恒定载荷压痕蠕变测试中获得蠕变应力指数的“稳定压头速度”方法的极限情况分析。

这项研究涉及从压痕数据评估稳态蠕变应力指数n的常用程序。该过程包括在恒定负载下监视压头位移历史，并假设一旦其速度稳定，系统将处于准稳态，第二阶段蠕变将主导该行为。压头下的应力和应变场由“等效应力”和“等效应变率”值表示。然后，将n的估计值作为等效应变率的对数与等效应力的对数的关系图的梯度来获得。但是，人们对该程序的可靠性表示了担忧，实际上，它已经显示出从根本上来说是有缺陷的。在本文中，它进行了演示，使用非常简单的分析，即对于真正稳定的速度，该过程始终会导致n的常数恒定（1.0或0.5，取决于尖端形状是球形还是自相似）。不管所测量的速度值，还是实际上材料的任何蠕变特性，都会发生这种情况。现在很明显，根据选择用来表示位移时间历史的函数形式和实验变量（尖端形状和大小，穿透深度等），对n的真实值几乎没有或没有敏感度。取决于笔尖的形状是球形还是自相似）。不管所测量的速度值，还是实际上材料的任何蠕变特性，都会发生这种情况。现在很明显，根据选择用来表示位移时间历史的函数形式和实验变量（尖端形状和大小，穿透深度等），对n的真实值几乎没有或没有敏感度。取决于笔尖的形状是球形还是自相似）。不管所测量的速度值，还是实际上材料的任何蠕变特性，都会发生这种情况。现在清楚的是，根据选择用来表示位移时间历史的函数形式和实验变量（尖端形状和大小，穿透深度等），对n的真实值几乎没有或没有敏感度。