Imaging defects in austenitic welds presents a significant challenge for the ultrasonic non-destructive testing community. Due to the heating process during their manufacture, a dendritic structure develops, exhibiting large grains with locally anisotropic properties which cause the ultrasonic waves to scatter and refract. When basic imaging algorithms, which typically make constant wave speed assumptions, are applied to datasets arising from the inspection of these welds, the resulting defect reconstructions are often distorted and difficult to interpret correctly. However, knowledge of the underlying spatially varying material properties allows correction of the expected wave travel times and thus results in more reliable defect reconstructions. In this paper, an approximation to the underlying, locally anisotropic structure of the weld is constructed from ultrasonic time of flight data. A new forward model of wave front propagation in locally anisotropic media is presented and used within the reversible-jump Markov chain Monte Carlo method to invert for the map of effective grain orientations across different regions of the weld. This forward model and estimated map are then used as the basis for an advanced imaging algorithm and the resulting defect reconstructions exhibit a significant improvement across multiple flaw characterization metrics.

中文翻译：

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复杂和局部各向异性介质的有效晶粒取向映射，以改善超声无损检测中的成像

奥氏体焊缝中的成像缺陷对超声波无损检测界提出了重大挑战。由于制造过程中的加热过程，会形成树枝状结构，呈现出具有局部各向异性的大晶粒，导致超声波散射和折射。当基本成像算法（通常假设恒定波速）应用于检查这些焊缝所产生的数据集时，由此产生的缺陷重建通常会失真并且难以正确解释。然而，了解潜在的空间变化材料特性允许校正预期的波传播时间，从而导致更可靠的缺陷重建。在本文中，对底层的近似，焊缝的局部各向异性结构是根据超声波飞行时间数据构建的。提出了一种新的局部各向异性介质中波前传播的前向模型，并在可逆跳跃马尔可夫链蒙特卡罗方法中使用，以反转焊缝不同区域的有效晶粒取向图。然后将此前向模型和估计图用作高级成像算法的基础，由此产生的缺陷重建在多个缺陷表征指标上表现出显着改进。