Research on Nondestructive Evaluation of a Crack Existing at Weld by Means of D-C Potential Drop Technique

研究了一种基于直流电位降技术的焊缝裂纹无损评价方法。主要研究结果总结如下：提出了一种无限大带焊件后壁二维裂纹无损检测方法。有焊件的带钢的裂纹几何形状是通过结合无焊件的带钢裂纹和无裂纹的带钢焊接的简单问题确定的。提出了一种利用直流电势法无损、高精度地评价三维裂纹形状的新方法。对某板后壁上的椭圆型预埋裂纹和半椭圆型表面裂纹进行了评价，重点考虑了裂纹的位置、长度和倾斜度。此外，开发了一种无损方法来评估焊接板中裂纹的三维形状，其中考虑了焊件的电气和几何不均匀性，并对其效果进行了校准。用不锈钢进行了实验，验证了所提方法的有效性。闭合对裂纹尺寸评估值的影响进行了实验研究，其中评估采用直流电势法进行。结果表明，当施加的荷载为自由荷载或拉伸荷载时，势法能够对闭合裂纹给出有效的评价。提出了一种应用电位滴法对焊管内表面三维裂纹进行无损检测的简化方法。该方法基于二维理论分析，并考虑了电流的实际三维特性。

The research has been performed to develop a method based on d-c potential drop technique for evaluating nondestructively a crack at weld. The main results are summarized as follows:1. A method was presented for the nondestructive evaluation of a 2-D crack on the back wall of an infinite strip having weldment. The crack geometry in the strip having the weldment was determined by combining simple problems of a cracked strip without the weldment and a welded strip without the crack.2. A new procedure was proposed to evaluate a 3-D crack shape nondestructively with high accuracy by utilizing the d-c potential method. An elliptical embedded crack and a semielliptical surface crack on the back wall in a plate were evaluated especially with respect to location , length and inclination of the crack. Furthermore a nondestructive procedure was developed to evaluate a 3-D shape of a crack in a welded plate, where the electrical and geometrical inhomogeneity of the weldment was considered, and the effect was calibrated. The availability of the presented procedures was verified by experiments stainless steels.3. Effects of closure on an evaluated value of crack size were examined experimentally, where the evaluation was carried out by using d-c electrical potential method. It was found that the potential method is able to give valid evaluation for close cracks, provided that the applied load is free or tensile.4. A simplified method was developed for applying potential drop technique to nondestructieve evaluation of a 3-D crack on the inner surface of a welded pipe. The method was based on 2-D theoretical analysis with modification for considering the actual 3-D nature of current flow.