Bailey Joseph, Long Nicholas, Hunze Arvid
Robinson Research Institute, Victoria University of Wellington, Lower Hutt 5010, New Zealand.
Sensors (Basel). 2017 Sep 28;17(10):2229. doi: 10.3390/s17102229.
This work investigates an eddy current-based non-destructive testing (NDT) method to characterize corrosion of pipes under thermal insulation, one of the leading failure mechanisms for insulated pipe infrastructure. Artificial defects were machined into the pipe surface to simulate the effect of corrosion wall loss. We show that by using a giant magnetoresistance (GMR) sensor array and a high current (300 A), single sinusoidal low frequency (5-200 Hz) pipe-encircling excitation scheme it is possible to quantify wall loss defects without removing the insulation or weather shield. An analysis of the magnetic field distribution and induced currents was undertaken using the finite element method (FEM) and analytical calculations. Simple algorithms to remove spurious measured field variations not associated with defects were developed and applied. The influence of an aluminium weather shield with discontinuities and dents was ascertained and found to be small for excitation frequency values below 40 Hz. The signal dependence on the defect dimensions was analysed in detail. The excitation frequency at which the maximum field amplitude change occurred increased linearly with the depth of the defect by about 3 Hz/mm defect depth. The change in magnetic field amplitude due to defects for sensors aligned in the azimuthal and radial directions were measured and found to be linearly dependent on the defect volume between 4400-30,800 mm³ with 1.2 × 10-1.6 × 10 µT/mm³. The results show that our approach is well suited for measuring wall loss defects similar to the defects from corrosion under insulation.
本研究探讨了一种基于涡流的无损检测（NDT）方法，用于表征保温管道的腐蚀情况，保温管道基础设施的主要失效机制之一。在管道表面加工人工缺陷以模拟腐蚀造成的壁厚损失效应。我们表明，通过使用巨磁电阻（GMR）传感器阵列和大电流（300 A）、单正弦低频（5 - 200 Hz）环绕管道的激励方案，可以在不拆除保温层或防雨罩的情况下对壁厚损失缺陷进行量化。使用有限元方法（FEM）和解析计算对磁场分布和感应电流进行了分析。开发并应用了简单算法来消除与缺陷无关的虚假测量场变化。确定了带有间断和凹痕的铝制防雨罩的影响，发现对于低于40 Hz的激励频率值，其影响较小。详细分析了信号对缺陷尺寸的依赖性。最大场振幅变化发生时的激励频率随缺陷深度线性增加，约为每毫米缺陷深度增加3 Hz。测量了沿方位角和径向排列的传感器因缺陷导致的磁场振幅变化，发现其在4400 - 30800 mm³的缺陷体积范围内与缺陷体积呈线性相关，为1.2×10 - 1.6×10 μT/mm³。结果表明，我们的方法非常适合测量类似于保温层下腐蚀产生的壁厚损失缺陷。
