DIRECT-CURRENT ELECTRIC POTENTIAL (D-C EP) TECHNIQUE VALIDATION THROUGH AN EXPERIMENTAL AND COMPUTATIONAL STUDY ON PIPE WITH SURFACE CRACK

摘要

The direct-current electric potential (d-c EP) technique (also known as Electrical Potential Drop, EPD) was developed by researchers in the 1960s and applied to cracked geometries. In this investigation, measurement of the d-c EP signature from a circumferential surface-crack profile in a pipe was attempted to characterize the flaw shape with higher resolution using state-of-the-art digital equipment. A part-circular profile of crack was inserted using an EDM technique in a small diameter (4-inch diameter Schedule 160) TP304 (Type304) stainless steel pipe. Experimentally, different magnitudes of electric-current were applied to obtain the d-c EP across the length of the crack (from the shallowest to the deepest point). Finite Element Analysis (FEA) was performed to calculate the variation of the d-c EP across the length of the crack. A sensitivity study was done for various distances between the d-c EP probe locations near the crack. A comparison of the d-c EP values obtained from the part-circular crack front and a semi elliptical crack FEA (more realistically seen/assumed in service crack cases and used in the ASME Section XI calculations) were made. The study also investigated the variation of the d-c EP for various crack depths through the thickness for the applied constant amplitude direct-current. The sensitivity on d-c EP probe location distance from the surface flaw and d-c EP probe location along the length of the surface flaw (from deepest or center of the surface flaw to the shallowest point or corner of the surface flaw) were investigated. The scatter in the acquired d-c EP data across the two sides of the crack was investigated and accuracy of crack depth characterization was characterized in detail. This was done to investigate the limits of d-c EP calibration curves used for crack growth predictions. The d-c EP calibration curves are useful in determining the crack growth that occurs without destructively opening the specimen and also measuring the in-situ crack depth measurements real time during a pipe or other surface flawed component/fitting experiments.