ABSTRACT

Inductive thermography can be excellently used to detect surface cracks in metals. A short induction heating pulse (0.1-1s) induces eddy currents in the sample and an infrared camera records the surface temperature distribution. As cracks disturb the eddy current distribution and the heat diffusion, they become visible in the infrared images. In this paper it is investigated, how different parameters influence the surface pattern around short cracks (0.5-12mm length). The main emphasis is on finite element simulations, but some experimental results are presented, too. The influence of crack geometry, as crack depth, length, inclination angle and crack shape below the surface are investigated for ferro-magnetic and austenitic steel. Around the crack tips high temperature ‘hot spots’ can be observed, which intensity increases with the crack depth. But this intensity is strongly affected by the crack shape, whether it is rectangular, trapezoid or half-penny shape. For longer cracks (6-8mm length) simulation results show, that in the middle of the crack the phase distribution can be used to estimate the crack depth. Furthermore, the effect of experimental parameters, as excitation frequency, heating pulse duration and the angle between crack line and induction coil are investigated in order to optimize an experimental setup.

摘要

感应热成像可以很好地用于检测金属的表面裂纹。短感应加热脉冲（0.1-1s）在样品中感应出涡流，红外摄像机记录表面温度分布。由于裂纹干扰涡流分布和热扩散，它们在红外图像中变得可见。本文研究了不同参数如何影响短裂纹（0.5-12mm 长度）周围的表面图案。主要重点是有限元模拟，但也提供了一些实验结果。研究了裂纹几何形状对铁磁钢和奥氏体钢的影响，如裂纹深度、长度、倾角和裂纹形状。在裂纹尖端周围可以观察到高温“热点”，其强度随着裂纹深度的增加而增加。但这种强度受裂缝形状的影响很大，无论是矩形、梯形还是半便士形状。对于较长的裂纹（6-8mm 长度）模拟结果表明，在裂纹中间的相分布可以用来估计裂纹深度。此外，为了优化实验装置，研究了实验参数的影响，如激励频率、加热脉冲持续时间和裂纹线与感应线圈之间的角度。