衍射波法测定裂纹高度时的最小可测高度和高度测试精度

Minimum Detective Height and Height Measurement Accuracy Use Diffraction Wave Method to Detect a Crack Height.

当用超声波法测定裂纹的高度时 ,总是存在一个最小可测高度。不同的研究者通过实验给出了不同的最小可测高度。笔者提出了两相邻信号的间隔为半个周期为可识别界线的观点。从这个观点出发 ,当采用5MHz窄脉冲探头时 ,最小可测高度可以不大于 1mm ,裂纹高度的测定误差亦不大于 1mm。当裂纹尺寸小于1mm时 ,如果裂纹上下端点衍射波的相位相反时 ,信号间隔将会不小于半个周期 ,可明显分辨。如果两者的相位相同时则不能分辨。所以 ,得出可测高度小于 1mm的试验结果仅仅是一种巧合 ,并不是普遍规律。

There is a minimum detective height when we measure the height of a crack. the different investigator give the different minimum defective height by experiment . The author provide a standpoint that it's distinguishable when the interval of two adjacent indications is 1/2 cycle。 According to this standpoint, when we use a 5 MHz narrow pulse probe, the minimum detective height shall not be more than 1mm, the error of a crack height shall not be more than 1mm either. When the dimension of a crack is less than 1mm, the indications interval will not be less than 1/2 cycle so that the crack is clearly distinguishable if the diffraction wave phases from up and down end points is reverse, and the crack is indistinguishable if the diffraction wave phases is the same. It's just a coincidence that the detective height is less than 1mm by experiment, it is mot a common regular.