According to the periodical inspection manual for civil infrastructures in Japan, the visual inspection plays a very important role to detect deterioration such as fatigue cracking. Since the steel bridges are covered with paint coating, a fatigue crack is often detected as a crack of the paint. Conventionally, when a paint crack is detected, the paint coating is removed and magnetic particle test is applied. However, it is not always true that there is a fatigue crack under the paint crack. This process is time-consuming, especially when no fatigue crack is detected. The objective of this study is to detect fatigue crack without removing the paint coating by nondestructive technique. Among nondestructive techniques, ultrasonic test was applied, because an internal crack can be detected. This study focused on flange gusset welded joint; lateral connection plates were welded to the base plate from both sides. After being welded, the specimens were painted by either fluorine resin paint or epoxy resin paint. Fluorine and epoxy paints are mainly applied for highway road and railway bridge, respectively. Fatigue tests were carried out for the specimens. During the fatigue test, when strain to monitor crack initiation was changed, ultrasonic test by creeping wave was conducted. The creeping wave is a longitudinal wave that travels very close to the test surface. A fatigue crack of 1.1 by 1.6 mm could be detected by the ultrasonic test.

根据日本民用基础设施的定期检查手册，目视检查在检测疲劳裂纹等劣化方面起着非常重要的作用。由于钢桥上覆盖有油漆涂层，疲劳裂纹通常被检测为油漆的裂纹。通常，当检测到油漆裂纹时，去除油漆涂层并应用磁粉测试。然而，并不总是在油漆裂纹下方存在疲劳裂纹。这个过程非常耗时，特别是当没有检测到疲劳裂纹时。本研究的目的是通过无损技术在不去除油漆涂层的情况下检测疲劳裂纹。在无损技术中，应用了超声波测试，因为可以检测到内部裂纹。本研究主要针对法兰角撑板焊接接头；横向连接板从两侧焊接到基板上。焊接后的试样涂刷氟树脂漆或环氧树脂漆。氟漆和环氧漆分别主要用于公路公路和铁路桥梁。对试样进行了疲劳试验。在疲劳试验期间，当监测裂纹萌生的应变发生变化时，通过蠕变波进行超声波试验。爬行波是一种非常靠近测试表面行进的纵波。通过超声波测试可以检测到 1.1 x 1.6 毫米的疲劳裂纹。分别。对试样进行了疲劳试验。在疲劳试验期间，当监测裂纹萌生的应变发生变化时，通过蠕变波进行超声波试验。爬行波是一种非常靠近测试表面行进的纵波。通过超声波测试可以检测到 1.1 x 1.6 毫米的疲劳裂纹。分别。对试样进行了疲劳试验。在疲劳试验期间，当监测裂纹萌生的应变发生变化时，通过蠕变波进行超声波试验。爬行波是一种非常靠近测试表面行进的纵波。通过超声波测试可以检测到 1.1 x 1.6 毫米的疲劳裂纹。