Experience in Using Direct Current Electric Potential to Monitor Crack Growth in Ductile Metals

2009 ◽  
pp. 163-163-15 ◽  
Author(s):  
MP Landow ◽  
CW Marschall
Keyword(s):  
Crack Growth ◽  
Direct Current ◽  
Electric Potential ◽  
Ductile Metals

Determination of Crack-Initiation in Fracture Toughness Testing Using an Experimental Key-Curve Methodology

2021 ◽  
Author(s):  
S. Pothana ◽  
G. Wilkowski ◽  
S. Kalyanam ◽  
J. K. Hong ◽  
C. J. Sallaberry
Keyword(s):  
Fracture Toughness ◽  
Crack Initiation ◽  
Crack Growth ◽  
Power Law ◽  
Direct Current ◽  
Electric Potential ◽  
Crack Mouth Opening Displacement ◽  
Ductile Tearing ◽  
Curve Method ◽  
Unloading Compliance

Abstract A new approach was implemented to confirm the start of ductile tearing relative to assessments by other methods such as direct-current Electric Potential (d-c EP) method in coupon specimens. This approach was developed on the Key-Curve methodology by Ernst/Joyce and is similar to the ASTM E-1820 Load Normalization procedure used to determine J-R curves directly from load versus Load-Line Displacement (LLD) record of the test specimen. It is consistent with Deformation Plasticity relationships for fully plastic behavior. Using this Experimental Key-Curve method, crack initiation can be determined directly from load versus LLD data or load versus Crack-Mouth Opening Displacement (CMOD) obtained from a fracture test without the need for additional instrumentation required for crack initiation detection. It is based on the fact that plastic deformation of homogeneous metals at the crack tip follows a power-law function until the crack tearing initiates. Crack tearing initiation is determined at the point where the power-law fit to the load versus plastic part of CMOD or LLD curve deviates from the total experimental load versus plastic-CMOD or LLD curve. The procedure for fitting of the data requires some care to be exercised such that the fitted data is beyond the elastic region and early small-scale plastic region of the Load-CMOD or Load-LLD curve but include data before crack initiation. An iterative regression analysis was done to achieve this, which is shown in this paper. The iterative fitting in this region typically results with a coefficient of determination (R2) values that are greater than 0.990. This method can be either used in conjunction with other methods such as direct-current Electric Potential (d-c EP) or unloading-compliance methods as a secondary (or primary) confirmation of crack tearing initiation (and even for crack growth); or can be used alone when other methods cannot be used. Furthermore, when using instrumentation methods for determining crack-initiation such as d-c EP method in a fracture toughness test, it is good to have a secondary confirmation of the initiation point in case of instrumentation malfunction or high signal to noise ratio in the measured d-c EP signals. In addition, the Experimental Key-Curve procedure provides relatively smooth data for the fitting procedure, while unloading-compliance data when used to get small crack growth values frequently has significant variability, which is part of the reason that JIC by ASTM E1820 is determined using an offset with some growth past the very start of ductile tearing. In this work, the Experimental Key-Curve method had been successfully used to determine crack tearing initiation and demonstrate the applicability for different fracture toughness specimen geometries such as SEN(T), and C(T) specimens. In all the cases analyzed, the Experimental Key-Curve method gave consistent results that were in good agreement with other crack tearing initiation measuring method such as d-c EP but seemed to result in less scatter.

Direct-Current Electric Potential (D-C EP) Technique Validation Through an Experimental and Computational Study on Pipe With Surface Crack

2019 ◽  
Author(s):  
Y. Hioe ◽  
S. Kalyanam ◽  
G. Wilkowski ◽  
F. W. Brust ◽  
E. Punch
Keyword(s):  
Crack Growth ◽  
Direct Current ◽  
Electric Potential ◽  
Surface Crack ◽  
Computational Study ◽  
Crack Depth ◽  
Electrical Potential ◽  
Surface Flaw ◽  
Calibration Curves ◽  
Probe Location

Abstract 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.

ELECTRIC-POTENTIAL TECHNIQUE FOR DETERMINING SLOW CRACK GROWTH

1963 ◽  
Author(s):  
Albert A. Anctil ◽  
Eric B. Kula ◽  
Eugene DiCesare
Keyword(s):  
Crack Growth ◽  
Electric Potential ◽  
Slow Crack Growth
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