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.

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.

Three-Dimensional Identification of Semi-Elliptical Surface Crack by Means of Direct-Current Electrical Potential Difference Method With Multiple-Probe Sensor

2006 ◽  
Vol 129 (3) ◽  
pp. 441-448 ◽  
Author(s):  
Naoya Tada ◽  
Masayoshi Okada ◽  
Jun Iwamoto
Keyword(s):  
Direct Current ◽  
Surface Crack ◽  
Difference Method ◽  
Electrical Potential ◽  
Three Dimensional ◽  
Potential Difference ◽  
Electrical Potential Difference ◽  
Crack Plane ◽  
Multiple Probe ◽  
Probe Sensor

A method of three-dimensional identification of a semi-elliptical surface crack by direct-current electrical potential difference method with a multiple-probe sensor was proposed and its validity was numerically examined. The condition of the surface crack embedded in a conductive plate was specified by the two-dimensional location of the crack center, length, and depth of the crack, and the surface and inward angles of the crack plane. Identification was carried out based on the distribution of the electrical potential difference around the crack measured on the surface of the plate with the “multiple-probe sensor” which is composed of many probes aligned in two orthogonal directions. The location and surface angle were evaluated using the point symmetry of the potential difference distribution. The inward angle was determined by the magnitude of symmetry of potential difference distribution with reference to the evaluated crack line. Finally, length and depth of the crack were determined using the exact solution of potential difference for an inclined inner elliptical crack which yields similar potential difference to that of the inclined semi-elliptical surface crack. The validity of the method was numerically confirmed by carrying out the evaluation based on the result obtained by finite element analysis.

Fracture Toughness Variation With Flaw Depth in Various Specimen Geometries and Role of Constraint in Material Fracture Resistance

2017 ◽  
Author(s):  
Y. Hioe ◽  
S. Kalyanam ◽  
G. Wilkowski ◽  
S. Pothana ◽  
J. Martin
Keyword(s):  
Fracture Toughness ◽  
Crack Initiation ◽  
Crack Growth ◽  
Surface Crack ◽  
Crack Depth ◽  
Limit Load ◽  
Surface Cracks ◽  
Maximum Moment ◽  
Toughness Variation ◽  
Axial Surface

A series of pipe tests with circumferential surface cracks has been conducted along with fracture toughness tests using single-edge notch tension (SENT) specimens having similar crack depths and crack orientations as the surface-cracked pipes. This paper presents observation of measured fracture toughness variation due to the crack depth and discusses the effect of constraint on the material resistance to fracture. Crack-tip-opening displacement (CTOD) measurements were obtained with the use of a dual clip-gauge mounted on both the SENT specimens and center of the surface-cracks in the pipes. CTOD was obtained at both the crack initiation and during the crack growth through the ligament. CTOD is a direct measure of the material toughness in the pipe and SENT tests. CTOD at crack initiation and during crack growth can also be related to the material J-Resistance (J-R) curve. Commonly, the material resistance is assumed to be the same for all circumferential surface-crack geometries in a surface-cracked pipe fracture mechanics analyses. However, based on experimental observations on a series of recently conducted surface-cracked pipe tests, the CTOD at the center of the surface crack at the start of ductile tearing and maximum moment changed with the depth of the surface crack. This is believed to be a constraint effect on plasticity in the ligament which depends on crack depth. The CTOD values at crack initiation were decreasing linearly with crack depth. This linear decrease in CTOD trend with flaw depth was also observed in SENT tests. More importantly, the decrease in CTOD with surface crack depth was significant enough that the failure mode changed from being limit-load to elastic-plastic fracture even in relatively small-diameter TP304 stainless steel pipe tests. This toughness drop explains why the Net-Section-Collapse (limit-load) analysis overpredicted the maximum moment for some crack geometries, and why the deeper surface cracks tore through the pipe thickness at moments below that predicted by the NSC analysis for a through-wall crack of the same circumferential length. An “Apparent NSC Analysis” was developed in a companion paper to account for the changing toughness with crack depth [1]. Finally, this same trend in decreasing toughness with flaw depth is apparent in surface-cracked flat plates [2] and axial surface flaws in pipes [3]. The leak-before-break behavior for axial surface cracks is also not explained by numerical calculations of the crack-driving force when assuming the toughness is constant for all surface cracks and the through-wall cracks, but the change in toughness with surface flaw depth explains this behavior. Previously, axial flaw empirical limit-load solution was developed by Maxey and Kiefner [4], and is consistent with the observations from this paper.

Experimental and Finite Element Study on the Fatigue Growth of a Semi-Elliptical Surface Crack in a X80 Pipeline Steel Specimen

2014 ◽  
Vol 580-583 ◽  
pp. 3026-3029 ◽  
Author(s):  
Qiao Jin ◽  
Ze Yu Sun ◽  
Wan Nan Guo
Keyword(s):  
Finite Element ◽  
Crack Growth ◽  
Test Data ◽  
Surface Crack ◽  
Evolution Equations ◽  
Pipeline Steel ◽  
Crack Depth ◽  
Steel Specimen ◽  
X80 Pipeline Steel ◽  
Depth Direction

In this paper, a fatigue test of a X80 pipeline steel specimen with a semi-elliptical defect was performed to study the fatigue propagation stages of the Surface cracks. Based on the obtained test data, a three dimentional finite element procedure was developed for the crack growth estimation of the surface crack in the specimen. In the numerical analysis of crack growth, both the crack depth direction and the surface direction were investigated by using two different evolution equations. The stress intensity factors along the crack front were determined by applying the virtual crack closure technique. The predictions of crack growth were compared with the test data.

Evaluation of Growth of Subsurface Crack by Fatigue: Simulation of Penetration of Internal Flaw to Surface of the Structure

2013 ◽  
Author(s):  
Masanori Kikuchi ◽  
Shinya Yamada ◽  
Ryousuke Serizawa ◽  
Yulong Li
Keyword(s):  
Free Surface ◽  
Power Plant ◽  
Nuclear Power Plant ◽  
Crack Growth ◽  
Surface Crack ◽  
Nuclear Power ◽  
Crack Depth ◽  
Subsurface Crack ◽  
Loading Direction ◽  
Tension Loading

In nuclear power plant, there is a proximity rule to evaluate subsurface crack, which exists near free surface of the structure. If the distance between this subsurface crack to free surface exceeds some limit, this subsurface crack is transformed to surface crack, and residual fatigue life is evaluated. Subsurface crack has many parameters, such as, crack length, crack depth, distance between crack front to free surface, and aspect ratio of subsurface crack. As a result, proximity rule is proposed by many organizations, and each rule is different from each other largely. It is necessary to verify which is more preferable, but to determine it experimentally is very difficult due to existence of many parameters. Numerical simulation is needed for this purpose. This problem is simulated using S-version FEM. Using S-FEM, subsurface is modeled independently from global structure, and crack growth is easily simulated. In maintenance code of nuclear power plant, initial defects are modeled as elliptical cracks in a normal plane to tension loading direction, and growth rate is estimated in this plane. But by using S-FEM, real defect shape is modeled realistically, and crack growth by fatigue is simulated. Usually, such small defects are subjected to multi-axial loading, and crack growth behaviors are very complicated. Finally, detect shape becomes elliptical or circular crack in a plane normal to tension loading direction in the structure. Fatigue cycles for these growing processes are calculated, and conservativeness of this maintenance code is discussed. Then subsurface crack growth is simulated. Inner subsurface crack grows toward free surface, penetrate to free surface and grows as a surface crack. These processes are simulated smoothly by S-FEM. Parametric studies are conducted for this problem, and proximity rules are verified with numerical results.

Measurement of Crack Growth Rates By use of the SEM

1980 ◽  
Vol 38 ◽  
pp. 138-139
Author(s):  
R. Brown
Keyword(s):  
Crack Growth ◽  
Surface Crack ◽  
Fatigue Testing ◽  
Electrical Potential ◽  
Potential Drop ◽  
In Situ Observation ◽  
Growth Data ◽  
Short Cracks ◽  
Optical Examination ◽  
Number Of Cycles

Often, measurement of the growth of short cracks with the number of cycles is required during fatigue testing. Optical examination is limited to useful magnification of ×1000 and the distinction between slip and a short crack is extremely difficult to make. Electrical potential drop measurement is not sufficiently sensitive to determine the start, on a microscopic level, of a short single crack. In situ observation of fatigue in the SEM presents enviromental problems. However plastic replicas taken from a smooth specimen surface during fatigue can be imaged in the SEM to produce extremely accurate short surface crack measurements. The initiation of a crack can be very accurately determined and surface crack lengths as short as 2μm can be measured. The technique allows a comparison between macroscopic crack growth data and data produced from short cracks, 2 to 300μm in length. As example of the process has been carried out for Ti-6A1-4V.

New Electrical Potential Method for Measuring Crack Growth in Nonconductive Materials

2003 ◽  
Author(s):  
H. Nayeb-Hashemi ◽  
D. Swet ◽  
A. Vaziri
Keyword(s):  
Thin Layer ◽  
Crack Length ◽  
Crack Growth ◽  
Electric Potential ◽  
Electrical Resistance ◽  
Electrical Potential ◽  
Bonded Joints ◽  
Adhesively Bonded ◽  
Adhesively Bonded Joints ◽  
Adhesively Bonded Joint

D.C. electric potential technique has been used to monitor crack growth in conductive materials. A constant DC current is ppased through thesse materials and the crack length is measured through the changes in the electrical voltage at the crack mouth. However, this method is not applicable in crack growth measurement in nonconductive materials or adhesively bonded joints. For these materials, a new method is developed and is shown to provide a very accurate method for measuring the crack length. The surface of these materials is coated with a thin layer of carbon paint and the crack lenght is measured through the changes in the electrical resistance of the carbon paint, as the crack grows both in the base material and the thin layer carbon paint. In contrast to the D.C. electric potential technique where the position of the probes for measuring the crack length is very important for an accurate measurement of the crack length, the new technique is little sensitive to the probe location. Crack growth is measured in adhesively bonded joints subjected to creep loadings. A modified Compact tension specimen is cut in two pieces across its notch area. The pieces are then glued jusing an adhensive. The surface of the specimen is painted with a thin layer of carbon paint and the changes in its electrical resistance are monitored. It is shown that the carbon paint method provides a quiet sensitive method for monitoring the crack growth. The creep crack growth rate in the adhesively bonded joint is related to Mode I energy release rate, G1. It is shown that the crack grows in the middle of the adhesive layer rather than at the interface of the joint. Micromechanisms of the crack growth are studied using a scanning electron microscope. The damage consists of numerous crazed regions at the crack tip. Crack grows by the linkage of the crazed region.

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