Background

In polycrystal mechanics, determination of stress is associated with diffraction methods that measure (the inherently-related) elastic strain. Microscopic digital image correlation (DIC), while commanding much higher intragranular resolution, measures total strain, and its local accuracy is typically insufficient to evaluate elastic strain magnitudes.

Objective

In situ DIC measurements over a partial unload of the polycrystal, where strains are virtually elastic, are explored for grain-averaged elastic strains and then, through a posed formalism, the stresses at the point of unload. Grain averaging is functionally employed to improve the DIC accuracy. The large objective is to emulate in situ complementary diffraction.

Methods

Nickel with high elastic anisotropy is chosen. The utilized highly-automated instrument offers maximal resolution for DIC with optical microscopy over a gross grain field. Orientations are predetermined for the same grain layer via electron backscatter diffraction. High-accuracy grain masks are produced to isolate the strain fields of individual grains.

Results

Very promising results are shown over a number of grains with sensible apparent compliance and stress values as well as linear unload behavior. Grains with sane results are largely predicted by a posed objectivity test that relies on DIC repeated with multiple reference loads.

Conclusion

Though it will require extremely careful implementations of microscopic DIC with high intragranular resolution, the premise of measuring intergranular stress fields via partial unloads seems to be viable and worthy of further exploration and verification. This capability that is superposed over strain measurement offers a more stringent validation of high-fidelity crystal plasticity models.

中文翻译：

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使用部分卸载和原位显微图像相关测量晶粒平均应力的探索

背景

在多晶力学中，应力的确定与测量（固有相关的）弹性应变的衍射方法相关。显微数字图像相关 (DIC) 虽然具有更高的晶内分辨率，但可以测量总应变，但其局部精度通常不足以评估弹性应变大小。

客观的

对多晶的部分卸载进行原位DIC 测量，其中应变实际上是弹性的，探索晶粒平均弹性应变，然后通过提出的形式来探索卸载点的应力。颗粒平均在功能上用于提高 DIC 精度。主要目标是模拟原位互补衍射。

方法

选择具有高弹性各向异性的镍。所使用的高度自动化仪器通过光学显微镜在大面积粮田上提供 DIC 的最大分辨率。通过电子背散射衍射预先确定同一颗粒层的方向。生产高精度颗粒掩模来隔离单个颗粒的应变场。

结果

在许多具有明显顺应性和应力值以及线性卸载行为的颗粒上显示出非常有希望的结果。具有合理结果的颗粒很大程度上是通过所提出的客观性测试来预测的，该测试依赖于使用多个参考负载重复的 DIC。

结论

尽管需要非常仔细地实现具有高晶内分辨率的微观 DIC，但通过部分卸载测量晶间应力场的前提似乎是可行的，值得进一步探索和验证。这种叠加在应变测量上的功能可以对高保真晶体塑性模型进行更严格的验证。