Investigation of the effects of material parameters on the relationship between crack tip constraint and CTOD fracture toughness

2020 ◽  
Vol 108 ◽  
pp. 102615 ◽  
Author(s):  
Yinhui Zhang ◽  
Jian Shuai ◽  
Zhiyang Lv ◽  
Kui Xu
Keyword(s):  
Fracture Toughness ◽  
Crack Tip ◽  
Material Parameters ◽  
Crack Tip Constraint ◽  
The Relationship

Investigation of Constraint Effects on Fracture Toughness for CC(T) Specimens

2004 ◽  
Author(s):  
Dieter Siegele ◽  
Igor Varfolomeyev ◽  
Kim Wallin ◽  
Gerhard Nagel
Keyword(s):  
Fracture Toughness ◽  
Crack Tip ◽  
Temperature Shift ◽  
Local Stress ◽  
Significant Loss ◽  
Reference Temperature ◽  
T Stress ◽  
Local Stress State ◽  
Constraint Effects ◽  
Crack Tip Constraint

Within the framework of the European research project VOCALIST, centre cracked tension, CC(T), specimens made of an RPV steel were tested and analysed to quantify the influence of local stress state on fracture toughness. The CC(T) specimens demonstrate a significant loss of crack tip constraint resulting in a considerable increase in fracture toughness as compared to standard fracture mechanics specimens. So, the master curve reference temperature, To, determined on the basis of CC(T) tests performed in this study is about 43°C lower than To obtained on standard C(T) specimens. Finite element analyses of the tests revealed that the above experimental finding is in a good agreement with the empirical correlations between the reference temperature shift and the crack tip constraint as characterised by the T-stress or Q parameter (Wallin, 2001; Wallin, 2004). The results of this work are consistent with a number of other tests performed within the VOCALIST project and contribute to the validation of engineering methods for the crack assessment in components taking account of constraint.

Prediction of Failure Behavior for Nuclear Piping Using Curved Wide-Plate Test

2004 ◽  
Vol 126 (4) ◽  
pp. 419-425 ◽  
Author(s):  
Nam-Su Huh ◽  
Yun-Jae Kim ◽  
Jae-Boong Choi ◽  
Young-Jin Kim ◽  
Chang-Ryul Pyo
Keyword(s):  
Fracture Toughness ◽  
Three Dimensional ◽  
Crack Tip ◽  
Full Scale ◽  
Failure Behavior ◽  
Resistance Curve ◽  
Testing Specimen ◽  
Plate Test ◽  
Crack Tip Constraint ◽  
Three Dimensional Finite Element

One important element of the Leak-Before-Break analysis of nuclear piping is how to determine relevant fracture toughness (or the J-resistance curve) for nonlinear fracture mechanics analysis. The practice to use fracture toughness from a standard C(T) specimen is known to often give conservative estimates of toughness. To improve the accuracy of predicting piping failure, this paper proposes a new method to determine fracture toughness using a nonstandard testing specimen, curved wide-plate in tension. To show validity of the proposed curved wide-plate test, the J-resistance curve from the full-scale pipe test is compared with that from the curved wide-plate test and that from C(T) specimen. It is shown that the J-resistance curve from the curved wide-plate tension test is similar to, but that from the C(T) specimen is lower than, the J-resistance curve from the full-scale pipe test. Further validation is performed by investigating crack-tip constraint conditions via detailed three-dimensional finite element analyses, which shows that the crack-tip constraint condition in the curved wide-plate tension specimen is indeed similar to that in the full-scale pipe under bending.

Effects of Temperature and Crack Tip Constraint on Cleavage Fracture Toughness in the Weld Thermal Simulated X80 Pipeline Steels

2011 ◽  
Vol 197-198 ◽  
pp. 1595-1598 ◽  
Author(s):  
Jie Xu ◽  
Yu Fan
Keyword(s):  
Fracture Toughness ◽  
Cleavage Fracture ◽  
Crack Tip ◽  
Coarse Grained ◽  
Material Microstructure ◽  
Element Analysis ◽  
Pipeline Steels ◽  
Different Temperatures ◽  
Effects Of Temperature ◽  
Crack Tip Constraint

This paper studies the effects of temperature and crack tip constraint on cleavage fracture toughness of the weld thermal simulated X80 pipeline steels. A large number of fracture toughness (as denoted by CTOD) tests together with 3D finite element analysis are performed using single edge notched bending (SENB) and tension (SENT) specimens at different temperatures. Coarse-grained heat-affected zone (CGHAZ) is considered as the material microstructure in preparation of the weld thermal simulated fracture mechanics specimens.

Crack-Tip Fields of Short Cracks in Bending As Characterized by Two Parameter Criterion

1993 ◽  
Author(s):  
J. F. Zarzour ◽  
Y. Dah-Wei ◽  
M. J. Kleinosky
Keyword(s):  
Fracture Toughness ◽  
Large Scale ◽  
Stress Triaxiality ◽  
Crack Tip ◽  
Significant Loss ◽  
Integral Approach ◽  
Rigorous Framework ◽  
Crack Tip Constraint ◽  
Two Parameter ◽  
Crack Tip Stress

Abstract Single edge notched bars (SENB), in the bending mode, with a/W ratios ranging from 0.05 to 0.5 were examined for fracture toughness in terms of the J-integral approach. The results indicate that for a/W ratios less than 0.3, there is a significant loss of J-dominance. This loss is attributed to the effect of plastic deformation on the cracked face. For a/W ratios greater than 0.3, J-dominance is maintained into the large scale yielding regime. According to the recently developed two-parameter criterion (J,Q), compressive Q-stress was interpreted as an indication of low crack-tip stress triaxiality for shallow cracks, while positive Q-stress was associated with high crack-tip stress triaxiality for deep cracks. For the material properties and specimen geometries considered herein, a fracture toughness locus was constructed in terms of the (J,Q) parameters for each of the a/W ratios. The overall fracture data are in agreement with those predicted by other approaches and provide a rigorous framework for interpreting the effect of loss of crack-tip constraint in elastic-plastic fracture analyses.

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.

JEDI: A Code for the Calculation of J for Cracks Inserted in Initial Strain Fields and the Role of J and Q in the Prediction of Crack Extension and Fracture

2008 ◽  
Author(s):  
David W. Beardsmore
Keyword(s):  
Fracture Toughness ◽  
Strain Field ◽  
Crack Extension ◽  
Crack Tip ◽  
Initial Strain ◽  
Small Scale ◽  
Assessment Procedure ◽  
Crack Tip Fields ◽  
Crack Tip Constraint ◽  
Inelastic Strains

When crack tip constraint is high, the crack tip contour integral J characterises the asymptotic stress, strain and displacement fields of a stationary crack in an elastic-plastic material. In other cases, the crack tip fields can be related to J and a second parameter Q which governs the crack tip constraint. These observations are the basis of J-Q fracture mechanics assessments. In the most usual procedure J is compared to an effective, constraint-corrected fracture toughness Jc which is derived from Q and the fracture toughness of the material. The difference Jc – J is a measure of the margin of safety. The assessment procedure assumes there are no initial inelastic strains in the component or the fracture toughness specimen prior to introducing the crack and subsequent loading. However, plant components may contain inelastic strains prior to cracking arising from welding and other manufacturing or fit-up processes. This initial strain field can be established by a finite element analysis that simulates the welding and/or manufacture sequence. Weld residual stresses develop due to the accumulation of incompatible, inelastic strains, including thermal, plastic and transformation strains in the material. If a crack is inserted into an initial strain field, a procedure is required to calculate J by analysis of the resulting crack tip fields. Moreover, for the fracture assessment method to remain valid, it must be demonstrated that the values of J and Q continue to govern the onset of crack extension or fracture so that a meaningful comparison of J with Jc can be made. This paper describes a domain integral for calculating J when inelastic strains exist prior to cracking, and its implementation in the JEDI computer code. The code is used to determine J for a crack inserted into a three-point bend specimen containing an initial inelastic strain field representative of one that might develop during welding. The extent to which the crack tip stress field is characterised by J and Q is examined by comparing it to the field for high constraint, small-scale yielding conditions.

Fracture Toughness Evaluation in C(T) Specimens With Reduced Out-of-Plane Constraint

2012 ◽  
Author(s):  
R. S. Kulka ◽  
A. H. Sherry
Keyword(s):  
Fracture Toughness ◽  
Damage Model ◽  
Stress Triaxiality ◽  
Crack Tip ◽  
Damage Modelling ◽  
Out Of Plane ◽  
Toughness Evaluation ◽  
Toughness Testing ◽  
Crack Tip Constraint ◽  
Fracture Toughness Evaluation

During fracture toughness testing of C(T) specimens, an important assumption is that the test specimen is highly constrained. This is ensured by testing a deeply cracked specimen, with in-plane and out-of-plane dimensions that are sufficient to guarantee an appropriate level of crack tip stress triaxiality. This condition guarantees that high-constraint fracture toughness values are derived, conservative for use in standard fracture mechanics assessments. In reality, many components have small out-of-plane dimensions (small thicknesses). This often causes a reduction in crack tip constraint of a sufficient amount to increase the effective fracture toughness of the components. However, there is currently limited understanding as to the magnitude of the benefits that could be claimed from out-of-plane constraint loss. Finite element and damage modelling of thin C(T) specimens under different loading conditions has been undertaken, looking at the effects of loss of out-of-plane constraint, to help validate the results of an on-going testing programme. When available, data from testing of thin C(T) specimens could allow the parameters of the damage model, based upon a ductile criterion, to be calibrated. Material resistance to fracture under different situations has been determined, leading to a correlation of toughness to the constraint condition for a nominal set of material parameters.

Constraint Based Assessment of Postulated Nozzle Corner Cracks

2002 ◽  
Author(s):  
Dieter Siegele ◽  
Igor Varfolomeyev ◽  
Gerhard Nagel
Keyword(s):  
Fracture Toughness ◽  
Brittle Failure ◽  
Thermal Stresses ◽  
Crack Tip ◽  
Pressure Vessels ◽  
Significant Loss ◽  
Large Shift ◽  
Corner Crack ◽  
Crack Tip Constraint ◽  
Corner Cracks

Brittle failure for reactor pressure vessels (RPV) under loss of coolant accidents (LOCA) considering strip and plume cooling show the nozzle corner as leading region in load level (e.g. Siegele et al., 1999). For such transients postulated nozzle corner cracks are also leading in crack driving force compared to the fracture toughness of the material. On the other hand, the crack tip constraint and consequently the failure probability is reduced by yielding caused by high thermal stresses and in addition by the crack size being small relative to the bulk of the flange material. Under these conditions, the flaw assessment in the nozzle corner using fracture toughness data obtained on standard specimens with high constraint level is over-conservative. In this situation it is suitable to introduce the loss of crack tip constraint into the brittle failure analysis of the nozzle corner. This investigation focuses on the assessment of surface cracks in the nozzle corner of an RPV. Using the finite element method temperature and stress calculations are carried out for a postulated LOCA event. For crack postulates in the nozzle corner of various size and geometry in the nozzle corner, crack driving parameters (such as the J-integral and the stress intensity factor) are determined as functions of the crack tip temperature. To account for the crack tip constraint, the T-stress is then evaluated and used along with the master curve approach as suggested by Wallin (2001). Significant loss of constraint is found for the nozzle corner resulting in a large shift of the fracture toughness curve to lower temperatures, thus excluding initiation of the crack postulates.

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