Crack closure can cause the underestimation or misdetection of fatigue cracks in ultrasonic testing (UT). Fatigue-crack closure due to an environmental factor, i.e., high temperature, was found in eddy current testing (ECT), which is used to inspect the vicinity of surfaces. However, its effect and countermeasures have yet to be examined in UT. In this study, we examined the fatigue-crack closure induced by heat processing using a surface-acoustic-wave phased array (SAW PA). SAW PA is a phased-array imaging method using Rayleigh waves, which can sensitively visualize defects in the vicinity of surfaces. As a result, the intensity of crack responses visualized by SAW PA markedly decreased after the heat processing of a fatigue-crack specimen. Furthermore, we demonstrated that the combination of SAW PA with a crack opening method, global preheating and local cooling (GPLC), and a load difference phased array (LDPA) is useful for the high-selectivity imaging of closed fatigue cracks. We also discussed a possible mechanism of the fatigue-crack closure induced by heat processing.

裂纹闭合会导致超声检测 (UT) 中对疲劳裂纹的低估或误检。在用于检查表面附近的涡流检测 (ECT) 中发现了由于环境因素（即高温）导致的疲劳裂纹闭合。然而，它的效果和对策还有待UT检验。在这项研究中，我们使用表面声波相控阵 (SAW PA) 检查了由热处理引起的疲劳裂纹闭合。SAW PA 是一种使用瑞利波的相控阵成像方法，可以灵敏地可视化表面附近的缺陷。结果，在对疲劳裂纹试样进行热处理后，SAW PA 可视化的裂纹响应强度显着降低。此外，我们证明了 SAW PA 与开裂方法的结合，全局预热和局部冷却 (GPLC) 以及负载差相控阵 (LDPA) 可用于闭合疲劳裂纹的高选择性成像。我们还讨论了由热处理引起的疲劳裂纹闭合的可能机制。