Levine Ross M, Michaels Jennifer E
IEEE Trans Ultrason Ferroelectr Freq Control. 2014 Jun;61(6):1006-15. doi: 10.1109/TUFFC.2014.2996.
A frequently investigated paradigm for monitoring the integrity of plate-like structures is a spatially-distributed array of piezoelectric transducers, with each array element capable of both transmitting and receiving ultrasonic guided waves. This configuration is relatively inexpensive and allows interrogation of defects from multiple directions over a relatively large area. Typically, full sets of pairwise transducer signals are acquired by exciting one transducer at a time in a round-robin fashion. Many algorithms that operate on such data use differential signals that are created by subtracting prerecorded baseline signals, leaving only signal differences introduced by scatterers. Analysis methods such as delay-and-sum imaging operate on these signals to detect and locate point-like defects, but such algorithms have limited performance and suffer when potential scatterers have high directionality or unknown phase-shifting behavior. Signal envelopes are commonly used to mitigate the effects of unknown phase shifts, but this further reduces performance. The blocksparse technique presented here uses a different principle to locate damage: each pixel is assumed to have a corresponding multidimensional linear scattering model, allowing any possible amplitude and phase shift for each transducer pair should a scatterer be present. By assuming that the differential signals are linear combinations of a sparse subset of these models, it is possible to split such signals into location-based components. Results are presented here for three experiments using aluminum and composite plates, each with a different type of scatterer. The scatterers in these images have smaller spot sizes than delay-and-sum imaging, and the images themselves have fewer artifacts. Although a propagation model is required, block-sparse imaging performs well even with a small number of transducers or without access to dispersion curves.
一种常用于监测板状结构完整性的典型方法是使用空间分布的压电换能器阵列，每个阵列元件都能够发射和接收超声导波。这种配置相对便宜，并且能够在相对较大的区域内从多个方向对缺陷进行检测。通常，通过以循环方式一次激励一个换能器来采集完整的成对换能器信号集。许多处理此类数据的算法使用差分信号，这些差分信号是通过减去预先记录的基线信号而创建的，只留下由散射体引入的信号差异。诸如延迟求和成像之类的分析方法对这些信号进行处理，以检测和定位点状缺陷，但此类算法性能有限，当潜在散射体具有高方向性或未知的相移行为时，性能会变差。信号包络通常用于减轻未知相移的影响，但这会进一步降低性能。这里提出的块稀疏技术使用不同的原理来定位损伤：假设每个像素都有一个相应的多维线性散射模型，这样如果存在散射体，每个换能器对都可能有任何可能的幅度和相移。通过假设差分信号是这些模型的稀疏子集的线性组合，可以将此类信号分解为基于位置 的分量。这里展示了使用铝板和复合板进行的三个实验的结果，每个实验都有不同类型的散射体。这些图像中的散射体光斑尺寸比延迟求和成像中的小，并且图像本身的伪像也更少。虽然需要一个传播模型，但即使换能器数量较少或无法获取频散曲线，块稀疏成像也能表现良好。
