Further Extension of the Unloading Compliance Method to Measure J-R Curves Based on CMOD Data

2008 ◽  
Author(s):  
Sebastian Cravero ◽  
Claudio Ruggieri
Keyword(s):  
Fracture Toughness ◽  
Crack Growth ◽  
Test Procedure ◽  
Crack Mouth Opening Displacement ◽  
Evaluation Procedure ◽  
Test Technique ◽  
Resistance Curves ◽  
Unloading Compliance ◽  
Resistance Data ◽  
Unloading Compliance Method

Laboratory testing of fracture specimens to measure resistance curves (J-Δa) have focused primarily on the unloading compliance method using a single specimen. Current estimation procedures (which form the basis of ASTM 1820 standard) employ load line displacement (LLD) records to measure fracture toughness resistance data incorporating a crack growth correction for J. An alternative method which potentially simplifies the test procedure involves the use of crack mouth opening displacement (CMOD) to determine both crack growth and J. This study provides further developments of the evaluation procedure for J in cracked bodies that experience ductile crack growth based upon the eta-method and CMOD data. The methodology broadens the applicability of current standards adopting the unloading compliance technique in laboratory measurements of fracture toughness resistance data (J resistance curves). The developed J evaluation formulation for growing cracks based on CMOD data provides a viable and yet simpler test technique to measure crack growth resistance data for ductile materials.

J-R Curve Testing of SE(T) Fracture Specimens Using Unloading Compliance and CMOD Data

2008 ◽  
Author(s):  
Sebastian Cravero ◽  
Claudio Ruggieri
Keyword(s):  
Fracture Toughness ◽  
Crack Growth ◽  
Test Procedure ◽  
Crack Mouth Opening Displacement ◽  
Evaluation Procedure ◽  
Test Technique ◽  
Resistance Curves ◽  
Unloading Compliance ◽  
Resistance Data ◽  
Fracture Specimens

Laboratory testing of fracture specimens to measure resistance curves (J - Δa) have focused primarily on the unloading compliance method using a single specimen. Current estimation procedures (which form the basis of ASTM E1820 standard) employ load line displacement (LLD) records to measure fracture toughness resistance data incorporating a crack growth correction for J. An alternative method which potentially simplifies the test procedure involves the use of crack mouth opening displacement (CMOD) to determine both crack growth and J. This study provides further developments of the evaluation procedure for J in cracked bodies that experience ductile crack growth based upon the eta-method and CMOD data. The methodology broadens the applicability of current standards adopting the unloading compliance technique in laboratory measurements of fracture toughness resistance data (J resistance curves). The developed J evaluation formulation for growing cracks based on CMOD data provides a viable and yet simpler test technique to measure crack growth resistance data for ductile materials.

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.

Determination of J Resistance Curves From CWP Specimens

2012 ◽  
Author(s):  
Fan Zhang ◽  
Honggang Zhou ◽  
Yong-Yi Wang ◽  
Ming Liu ◽  
Yaxin Song
Keyword(s):  
Fracture Toughness ◽  
Crack Growth ◽  
J Integral ◽  
Resistance Curve ◽  
Constraint Condition ◽  
Longitudinal Loading ◽  
Resistance Curves ◽  
Unloading Compliance ◽  
Girth Welds

A crack is highly constrained in traditional toughness tests, e.g., CVN and SE(B). However, a crack in the girth welds of pipelines under longitudinal loading is low constrained. Curved wide plate (CWP) test provides similar constraint condition as that of pipeline girth weld. CWP tests are being used recently for strain-based design. One of the desirable outcomes from those tests is fracture toughness resistance curves. The resistance curve consists of two components, the crack growth and the toughness measure, such as J-integral or CTOD. The paper describes the development of procedures for the determination of those two components. A normalized equation was developed to estimate the crack growth from the experimentally measured unloading compliance. The equation was verified by multiple FEA simulations with different pipe geometries and materials. The second set of equations was developed to evaluate the J-integral through an incremental frame based on the instantaneous crack growth and the load-CMOD record. The application of the resistance curve procedures was demonstrated through CWP tests of X80 and X100 welds.

scholarly journals Evaluation of ductile tearing for API-5L X70 pipeline grade steel using SENT specimens

2015 ◽  
Vol 6 (3) ◽  
pp. 8
Author(s):  
Nicholas Ohms ◽  
Diego Belato Rosado ◽  
Wim De Waele
Keyword(s):  
Crack Growth ◽  
Crack Extension ◽  
Crack Growth Resistance ◽  
Fracture Surfaces ◽  
Edge Notch ◽  
Resistance Curves ◽  
Unloading Compliance ◽  
Inner Diameter ◽  
Tearing Resistance ◽  
Hardness Map

Pipelines in harsh environments may be subjected to large deformations. Classic stress-based design needs to be complemented with strain-based design. An important parameter in the design is the crack growth resistance. SENT testing (Single Edge Notch Tension) allows to determine the so-called material’s tearing resistance curve. Very recently the first standard on SENT testing, BS 8571:2014, has been published. SENT testing is however still subject to extensive research and different approaches with respect to eg. notch placement, crack extension measurement and analysis exist. In this paper two methods for calculating crack extension based on the unloading compliance procedure are used and compared, proving that they show little difference. This is performed on an API-5L X70 steel grade and this for different configurations, namely an inner diameter notch and a through thickness notch. The results showed little difference between the different configurations, although the inner diameter showed higher crack growth resistance. Furthermore, the results are compared to visual observations of the fracture surfaces and a hardness map. The fracture surfaces corresponded to the obtained resistance curves. However, no real correlation between the hardness map and the other results could be seen.

Revised η-Factors for 3P SE(B) Fracture Specimens Incorporating 3-D Effects

2014 ◽  
Author(s):  
Rodolfo F. de Souza ◽  
Claudio Ruggieri
Keyword(s):  
Fracture Toughness ◽  
Mouth Opening ◽  
Estimation Procedure ◽  
Crack Mouth Opening Displacement ◽  
Evaluation Procedure ◽  
Specimen Thickness ◽  
Crack Mouth ◽  
Opening Displacement ◽  
Wide Range ◽  
Fracture Specimens

Standardized procedures to measure cleavage fracture toughness of ferritic steels in the DBT region most commonly employ three-point bend fracture specimens, conventionally termed SE(B) or SENB specimens. The evaluation protocol of fracture toughness for these crack configurations builds upon laboratory records of load and crack mouth opening displacement (CMOD) to relate plastic work with J (or, equivalently, CTOD). The experimental approach employs a plastic η-factor to relate the macroscale crack driving force to the area under the load versus crack mouth opening displacement for cracked configurations. This work provides revised η-factors derived from CMOD records applicable to estimate the J-integral and CTOD in SE(B) specimens with varying crack size and specimen configuration. Non-linear finite element analyses for plane-strain and 3-D models provide the evolution of load with increased CMOD which is required for the estimation procedure. The analysis matrix considers SE(B) specimens with W = 2B and W = B configurations with and without side grooves covering a wide range of specimen thickness, including precracked Charpy (PCVN) specimens. Overall, the present results provide further validation of the J and CTOD evaluation procedure currently adopted by ASTM 1820 while, at the same time, giving improved estimation equations for J incorporating 3-D effects which enter directly into more accurate testing protocols for experimental measurements of fracture toughness values using 3P SE(B) specimens.

scholarly journals Implementation of an unloading compliance procedure for measurement of crack growth in pipeline steel

2011 ◽  
Vol 2 (3) ◽  
pp. 397-406
Author(s):  
Koen Van Minnebruggen ◽  
Dries Van Puyvelde ◽  
Wim De Waele ◽  
Matthias Verstraete ◽  
Stijn Hertelé ◽  
...  
Keyword(s):  
Crack Growth ◽  
Fossil Fuels ◽  
Standardized Test ◽  
Pipeline Steel ◽  
Test Procedure ◽  
Ductile Failure ◽  
Single Edge ◽  
Single Edge Notch ◽  
Edge Notch ◽  
Unloading Compliance

As the demand for fossil fuels increases, pipelines are constructed in inhospitable areas. Underthese conditions, not only the strength but also the deformability of the pipelines becomes crucial. A strainbased design (SBD) procedure needs to be established. Traditional stress based approaches to evaluatedefect tolerance lead to conservative predictions. There is a need to accurately define the fracturetoughness of the pipeline steel and assess the criticality of weld defects under strain based conditions. Thispaper focuses on the implementation of the unloading compliance method to determine stable crackgrowth. The standardized test procedure described in ASTM E1820 is applied. This method is a handy toolto obtain the J-resistance curves which can forecast ductile failure in pipeline girth welds. Preliminaryexperiments have been performed on Single Edge Notch Bend (SENB) specimens of plain pipe metal.Using the implemented procedure, it was possible to obtain a good fit between calculated and measuredcrack size. The most important result is the smoothness of the calculated crack growth and the rathermonotonic increase of crack size. Since testing on SENB is well known to provide conservativemeasurements, Single Edge Notch Tension (SENT) specimens will be evaluated in future work

Improved Incremental J-Integral Equations for Determining Crack Growth Resistance Curves

2012 ◽  
Vol 134 (5) ◽  
Author(s):  
Xian-Kui Zhu
Keyword(s):  
Fracture Mechanics ◽  
Crack Growth ◽  
Integral Equations ◽  
Structural Integrity ◽  
Test Procedure ◽  
Fracture Toughness Test ◽  
J Integral ◽  
Integrity Assessment ◽  
Growth Increments ◽  
Unloading Compliance

The J-integral resistance curve is the most important material properties in fracture mechanics that is often used for structural integrity assessment. ASTM E1820 is a commonly accepted fracture toughness test standard for measuring the critical value of J-integral at the onset of ductile fracture and J-R curve during ductile crack tearing. The recommended test procedure is the elastic unloading compliance method. For a stationary crack, the J-integral is simply calculated from the area under the load-displacement record using the η-factor equation. For a growing crack, the J-integral is calculated using the incremental equation proposed by Ernst et al. (1981, “Estimations on J-integral and Tearing Modulus T From a Single Specimen Test Record,” Fracture Mechanics: Thirteenth Conference, ASTM STP 743, pp. 476–502) to consider the crack growth correction. For the purpose of obtaining accurate J-integral values, ASTM E1820 requires small and uniform crack growth increments in a J-R curve test. In order to allow larger crack growth increments in an unloading compliance test, an improved J-integral estimation is needed. Based on the numerical integration techniques of forward rectangular, backward rectangular, and trapezoidal rules, three incremental J-integral equations are developed. It demonstrates that the current ASTM E1820 procedure is similar to the forward rectangular result, and the existing Garwood equation is similar to the backward rectangular result. The trapezoidal result has a higher accuracy than the other two, and thus it is proposed as a new formula to increase the accuracy of a J-R curve when a larger crack growth increment is used in testing. An analytic approach is then developed and used to evaluate the accuracy of the proposed incremental equations using single-edge bending and compact tension specimens for different hardening materials. It is followed by an experimental evaluation using actual fracture test data for HY80 steel. The results show that the proposed incremental J-integral equations can obtain much improved results of J-R curves for larger crack growth increments and are more accurate than the present ASTM E1820 equation.

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