scholarly journals Constraint Assessment for Specimens Tested Under Uniaxial and Biaxial Loading Conditions

2017 ◽  
Author(s):  
Yupeng Cao ◽  
Guian Qian ◽  
Yinbiao He ◽  
Yuh J. Chao ◽  
Markus Niffenegger
Keyword(s):  
Fracture Toughness ◽  
Material Property ◽  
Biaxial Loading ◽  
Compact Tension ◽  
Biaxial Stress ◽  
Integrity Assessment ◽  
Crack Tips ◽  
Shallow Crack ◽  
Crack Tip Constraint ◽  
Integrity Analysis

In the integrity analysis of a reactor pressure vessel (RPV), a postulated shallow crack is subjected to biaxial far-field stresses. However, the fracture toughness Kc or Jc, which is an important material property for the integrity assessment of a RPV, are usually tested with deeply-cracked compact tension [C(T)] or single-edged bending [SE(B)] specimens under uniaxial loading. Thus, the fracture toughness data do not reflect the biaxial loading state that the cracks in a RPV are subjected to. Cruciform bending specimen is therefore developed to simulate the biaxial stress state. In this paper, a series of finite element (FE) simulations of the cruciform specimens containing different crack geometries and of different material properties are conducted. The crack tip constraint is investigated using the J-A2 theory and the stress field near the crack tips is analyzed. The results show that the biaxial effect is material property dependent. This can contribute to the lifetime prediction of a RPV as well as better design of cruciform specimens.

scholarly journals Constraint Assessment for Cruciform Specimens With a Semi-Elliptical Crack

2018 ◽  
Author(s):  
Yupeng Cao ◽  
Guian Qian ◽  
Yinbiao He ◽  
Yuh J. Chao
Keyword(s):  
Fracture Toughness ◽  
Finite Element ◽  
Material Property ◽  
Structural Integrity ◽  
Biaxial Loading ◽  
Compact Tension ◽  
Biaxial Stress ◽  
Integrity Assessment ◽  
Shallow Crack ◽  
Crack Tip Constraint

A real crack to be assessed in a RPV is generally a shallow crack subjected to biaxial far-field stresses. However, the fracture toughness Kc or Jc, which is an important material property for the structural integrity assessment of RPV containing cracks, are usually tested on deep cracked compact tension [C(T)] or single-edged bending [SE(B)] specimens under uniaxial loading. The fracture toughness data do not reflect the realistic biaxial loading state that the cracks are subjected to. Cruciform bending [CR(B)] specimen is therefore developed to simulate the biaxial stress state. In this paper, a series of finite element (FE) simulations of the CR(B) specimens containing different semi-elliptical cracks are conducted. Stress-strain curves of materials of different yield strength and hardening behavior reflecting the variation in the mechanical properties of RPV steels due to aging or temperature change are implemented into the finite element models. The J-A2 theory is applied to analyze the crack tip constraint. The results show that the biaxial effect is material property dependent and affected by load levels.

scholarly journals Constraint Assessment for Specimens Tested Under Uniaxial and Biaxial Loading Conditions

2018 ◽  
Vol 140 (3) ◽  
Author(s):  
Yupeng Cao ◽  
Guian Qian ◽  
Yinbiao He ◽  
Markus Niffenegger ◽  
Yuh J. Chao
Keyword(s):  
Fracture Toughness ◽  
Material Property ◽  
Structural Integrity ◽  
Biaxial Loading ◽  
Compact Tension ◽  
Pressure Vessels ◽  
Test Method ◽  
Biaxial Stress ◽  
Operating Life ◽  
Integrity Assessment

In structural integrity analysis of reactor pressure vessels (RPVs), a postulated shallow crack is subjected to biaxial far-field stresses. However, the fracture toughness Kc or Jc, which is an important material property for the structural integrity assessment of RPVs, is usually obtained from testing deeply cracked compact tension (C(T)) or single-edged bending (SE(B)) specimens under uniaxial loading. Thus, the fracture toughness data do not reflect the biaxial loading state that cracks in a RPV are subjected to. Cruciform bending specimen was therefore developed to simulate the biaxial stress state. In this paper, a series of finite element (FE) simulations of the cruciform specimens containing different crack geometries and of different material properties are conducted. The crack tip stress fields are analyzed, and the constraint is investigated using the J–A2 theory. The results show that the biaxial effect is material property dependent which could be useful for the optimization of the test method and the better design of cruciform specimens. The trends about the biaxial loading effect revealed in this study would also be helpful in estimating the safe operating life of RPVs.

Shallow Flaws Under Biaxial Loading Conditions—Part I: The Effect of Specimen Size on Fracture Toughness Values Obtained From Large-Scale Cruciform Specimens1

2000 ◽  
Vol 123 (1) ◽  
pp. 10-24 ◽  
Author(s):  
Wallace J. McAfee ◽  
B. Richard Bass ◽  
Paul T. Williams
Keyword(s):  
Fracture Toughness ◽  
Transition Temperature ◽  
Temperature Region ◽  
Large Scale ◽  
Biaxial Loading ◽  
Crack Tip ◽  
Specimen Size ◽  
Biaxial Stress ◽  
Lower Transition ◽  
Crack Tip Constraint

A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed. This technology is for application to the safety assessment of RPVs containing postulated shallow-surface flaws. It has been shown that relaxation of crack-tip constraint causes shallow-flaw fracture toughness of RPV material to have a higher mean value than that for deep flaws in the lower transition temperature region. Cruciform beam specimens developed at Oak Ridge National Laboratory (ORNL) introduce far-field, out-of-plane biaxial stress components in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock (PTS) loading of an RPV. The biaxial stress component has been shown to increase stress triaxiality (constraint) at the crack tip, and thereby reduce the shallow-flaw fracture toughness enhancement. The cruciform specimen permits controlled application of biaxial loading ratios, resulting in controlled variation of crack-tip constraint. An extensive matrix of intermediate-scale cruciform specimens with a uniform depth surface flaw was previously tested and demonstrated a continued decrease in shallow-flaw fracture toughness with increasing biaxial loading. This paper describes the test results for a series of large-scale cruciform specimens with a uniform depth surface flaw. These specimens were all of the same size with the same depth flaw and were tested at the same temperature and biaxial load ratio (1:1). The configuration is the same as the previous set of intermediate-scale tests, but has been scaled upward in size by 150 percent. These tests demonstrated the effect of biaxial loading and specimen size on shallow-flaw fracture toughness in the lower transition temperature region for RPV materials. For specimens tested under full biaxial (1:1) loading at test temperatures in the range of 23°F (−5°C) to 34°F (1°C), toughness was reduced by approximately 15 percent for a 150-percent increase in specimen size. This decrease was slightly greater than the predicted reduction for this increase in specimen size. The size corrections for 1/2T C(T) specimens did not predict the experimentally determined mean toughness values for larger size shallow-flaw specimens tested under biaxial (1:1) loading in the lower transition temperature region.

Evaluation of Fracture Toughness Behavior of SA508 Steel Considering Biaxial Loading Condition

2012 ◽  
Author(s):  
Jong-Min Kim ◽  
Ki-Hyoung Lee ◽  
Ho-Jin Lee ◽  
Bong-Sang Lee
Keyword(s):  
Fracture Toughness ◽  
Finite Element ◽  
Loading Condition ◽  
Biaxial Loading ◽  
Small Scale ◽  
Constraint Effect ◽  
Integrity Assessment ◽  
Cruciform Specimen ◽  
Elastic Plastic ◽  
Deep Crack

The crack-tip stress field and fracture mechanics assessment parameters, such as the elastic stress intensity factor and the elastic-plastic J-integral, for a surface crack can be significantly affected by the loading condition and crack geometry. Current guidance considers that the ductile-to-brittle transition is defined using uniaxially loaded specimens with a deep crack even when the reactor pressure vessel is under biaxial loading and the existence of deep crack is not probable through periodic in-service-inspection. Thus, such a constraint effect caused by differences between standard specimens and a real structure can overestimate the fracture toughness and affects the results of the structural integrity assessment. The present paper investigates the constraint effect by evaluating the Mater Curve T0 reference temperature of PCVN (Pre-cracked Charpy V-Notch) and small scale cruciform specimens of SA508 Gr. 3 low alloy steel through the fracture toughness tests of theses specimens and 3-dimensional elastic-plastic finite element analyses. Based on the finite element results, the fracture toughness values of a small-scale cruciform specimen were estimated, and the geometry-dependent factors of the cruciform specimen considered in the present study were determined and then the transferability of the test results of these specimens were discussed.

Fracture Toughness Characterization of Vessel Forging With Hydrogen Flaked Specimens

2016 ◽  
Author(s):  
Rachid Chaouadi ◽  
Robert Gérard ◽  
Eric van Walle
Keyword(s):  
Fracture Toughness ◽  
Nuclear Power ◽  
Structural Integrity ◽  
Compact Tension ◽  
Test Program ◽  
Innovative Methodology ◽  
Safety Case ◽  
Component Manufacturing ◽  
Integrity Analysis

The 2012 in-service vessel inspections performed at Doel-III and Tihange-II Nuclear Power Plant revealed a large number of ultrasonic indications in the forged core shells suggesting nearly-laminar defects attributed to hydrogen flakes induced during the component manufacturing process. These observations have initiated a very large experimental test program not only for reliably and accurately characterizing the ultrasonic indications but also to characterize the mechanical properties in presence of hydrogen flakes. A key point of the Safety Case was the justification of the material properties to be used in the structural integrity analysis, in particular the local fracture toughness of the material ahead of hydrogen flakes. An innovative methodology was used to manufacture fracture toughness specimens, including both compact tension and Charpy size specimens, with a hydrogen flake substituting the standard fatigue pre-crack. As a result, it is possible to measure the fracture toughness directly at the tip of real hydrogen flakes. This paper describes the procedure and the results that were obtained in comparison to standard fatigue precracked specimens.

Size Effects on Fracture in Uncontained Plasticity Conditions in 304(L) Stainless Steel

2011 ◽  
Author(s):  
Andrew P. Wasylyk ◽  
Andrew H. Sherry
Keyword(s):  
Fracture Toughness ◽  
Plastic Zone ◽  
Crack Growth ◽  
Structural Integrity ◽  
Plastic Zone Size ◽  
Compact Tension ◽  
Specimen Size ◽  
Image Correlation ◽  
Local Approach ◽  
Integrity Assessment

In the structural integrity assessment of structures containing defects, ductile tearing and plastic collapse are treated as competing failure mechanisms. The validity of fracture toughness measurements in test specimens is limited by the development of plasticity ahead of the crack tip. Compact Tension (CT) specimens are commonly used to characterise the ductile fracture toughness. Three sizes of CT specimens (thickness 25, 15 and 10mm) were tested using the unloading compliance technique and the J-Resistance curve characterised. Concurrently, the development of the plastic zone was monitored on the surface of specimens using digital image correlation. This enabled the plastic zone size to be correlated with the evolution of crack growth. It was found that in all specimens no crack growth occurred prior to plastic yielding of the un-cracked ligament on the specimen surface. Furthermore, a reduction in initiation and tearing toughness was observed with reduction in specimen size. The Rice and Tracey local approach was developed to predict the specimen size effect.

Evolution of Plasticity in Relation to Ductile Tearing in 304(L) Stainless Steel

2010 ◽  
Author(s):  
Andrew P. Wasylyk ◽  
Andrew H. Sherry
Keyword(s):  
Fracture Toughness ◽  
Plastic Zone ◽  
Crack Growth ◽  
Structural Integrity ◽  
Plastic Zone Size ◽  
Compact Tension ◽  
Image Correlation ◽  
Ductile Tearing ◽  
Integrity Assessment ◽  
Unloading Compliance

In the structural integrity assessment of structures containing defects, ductile tearing and plastic collapse are treated as competing failure mechanisms. The validity of fracture toughness measurements in test specimens is limited by the development of plasticity ahead of the crack tip. Compact Tension (CT) specimens are commonly used to characterise the ductile fracture toughness. Two sizes of CT specimens (thickness 25 and 15mm) were tested using the unloading compliance technique and the J-Resistance curve characterised. Concurrently, the development of the plastic zone was monitored on the surface of specimens using digital image correlation. This enabled the plastic zone size to be correlated with the evolution of crack growth. It was found that in both specimens no crack growth had occurred prior to plastic yielding of the un-cracked ligament on the specimen surface.

An Investigation of Cladding Effects on Shallow-Flaw Fracture Toughness of Reactor Pressure Vessel Steel Under Prototypic Biaxial Loading

1999 ◽  
Vol 121 (3) ◽  
pp. 257-268 ◽  
Author(s):  
B. R. Bass ◽  
W. J. McAfee ◽  
J. W. Bryson ◽  
W. E. Pennell
Keyword(s):  
Fracture Toughness ◽  
Pressure Vessel ◽  
Biaxial Loading ◽  
Reactor Pressure Vessel ◽  
Biaxial Stress ◽  
Pressure Vessel Steel ◽  
Test Results ◽  
Vessel Steel ◽  
Nonlinear Stress ◽  
Reactor Pressure

Potential structural-integrity benefits or liabilities of the stainless steel cladding on the inner surface of a reactor pressure vessel (RPV) are important considerations in the effort to refine or improve safety assessment procedures applied to RPVs. Clad-beam tests were carried out to investigate and quantify effects of the clad structure on fracture initiation toughness of through-clad shallow surface flaws in RPV material. A cruciform beam specimen was developed at ORNL to introduce a prototypic, far-field, out-of-plane biaxial stress component that provides a linear approximation of the nonlinear stress distribution generated by thermo-mechanical loading transients in an RPV. The cruciform specimens (102-mm-thick test section) were fabricated from RPV shell segments available from a canceled pressurized-water reactor plant. The specimens were tested under biaxial load ratios ranging from 0.0 (uniaxial) to 1.0 (full biaxial), the ratio being defined as the total load applied to the transverse beam arms divided by that applied to the longitudinal arms. The test results imply that biaxial loading is effective in reducing the shallow-flaw fracture toughness of the clad/heat-affected zone/structural-weld region of the RPV shell below that determined from uniaxial loading conditions. The lowest toughness value from the clad cruciform specimens tested under biaxial loading is only slightly above the ASME Section XI KIc curve. For all biaxiality ratios, the test results imply that shallow-flaw fracture toughness data from the RPV structural weld material are significantly lower than that obtained from a high-yield strength plate material.

Crack Tip Constraint Under Biaxial Loading in Elastic-Plastic Materials

2010 ◽  
Author(s):  
Z. X. Wang ◽  
Jian-ye Huang ◽  
Y. J. Chao ◽  
P. S. Lam
Keyword(s):  
Driving Force ◽  
Enhancement Factor ◽  
Structural Integrity ◽  
Biaxial Loading ◽  
Crack Tip ◽  
Element Analysis ◽  
Crack Driving Force ◽  
Integrity Assessment ◽  
Elastic Plastic ◽  
Crack Tip Constraint

Crack tip constraint is known to affect the fracture resistance of materials. The effect of biaxial loading on a center crack in an X100 steel plate has been investigated. The crack driving force and the constraint parameter are estimated based on the two-parameter J-A2 theory in elastic-plastic fracture mechanics with the aid of finite element analysis. The center-cracked plate is subject to various degrees of biaxiality (defined as the ratio of the transverse stress parallel to the crack and the opening stress normal to the crack). Using the constraint parameter (A2) in uniaxial loading condition as a reference value, a Constraint Enhancement Factor is introduced to facilitate the investigation of crack tip constraint under biaxial loading. The analysis carried out in this paper has established a relationship between the Constraint Enhancement Factor and the biaxiality. With the J-A2 fracture model, the critical applied load and the critical crack driving force can be expressed as functions of biaxial loading ratio. The methodology and analysis results can be used in structural integrity assessment of a pressure vessel or piping which contains a crack under biaxial loading.

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