Recent Developments and Challenges of Cleavage Fracture Modelling in Steels: Aspects on Microstructural Mechanics and Local Approach Methods

2019 ◽  
Author(s):  
Quanxin Jiang ◽  
V. M. Bertolo ◽  
V. A. Popovich ◽  
Carey L. Walters
Keyword(s):  
Fracture Toughness ◽  
Cleavage Fracture ◽  
Structural Integrity ◽  
State Of The Art ◽  
Structural Steels ◽  
Ferritic Steels ◽  
Local Approach ◽  
Microstructural Parameters ◽  
Recent Developments ◽  
Size Scales

Abstract Offshore activity in low-temperature areas requires the use of analysis methods that are capable of reliably predicting cleavage (brittle) fracture of ferritic steels in order to guarantee the structural integrity during service. Cleavage fracture is controlled by physical events at different size scales and is influenced by the multiple microstructural parameters of the material. The prediction of fracture toughness of steels based on the microstructure has received great attention, and relevant techniques have been continuously developed. This paper is aimed at reviewing the recent development of cleavage fracture modelling in steels and identifying the existing challenges to inspire further research. The paper contains three parts aimed at explaining how methods are developed and utilized to predict fracture toughness of steel from its microstructures. (1) The complex multiparametric nature of the microstructures of ferritic steels and its influence on cleavage fracture is introduced. (2) A review is given on the main perspectives and models in micromechanisms of cleavage fracture in steels. (3) Discussion is contributed to the link between micromechanisms and the local approach in cleavage fracture modelling. As a result, the paper gives a state of the art on microstructural mechanics and local approach methods of cleavage fracture modelling in structural steels.

scholarly journals A Statistical Model of Cleavage Fracture Toughness of Ferritic Steel DIN 22NiMoCr37 at Different Temperatures

Materials ◽  
2019 ◽  
Vol 12 (6) ◽  
pp. 982 ◽  
Author(s):  
Guian Qian ◽  
Wei-Sheng Lei ◽  
Zhenfeng Tong ◽  
Zhishui Yu
Keyword(s):  
Fracture Toughness ◽  
Yield Strength ◽  
Statistical Model ◽  
Cleavage Fracture ◽  
Failure Probability ◽  
Ferritic Steel ◽  
Ferritic Steels ◽  
Local Approach ◽  
Weibull Statistics ◽  
Different Temperatures

It is a conventional practice to adopt Weibull statistics with a modulus of 4 for characterizing the statistical distribution of cleavage fracture toughness of ferritic steels, albeit based on a rather weak physical justification. In this study, a statistical model for cleavage fracture toughness of ferritic steels is proposed according to a new local approach model. The model suggests that there exists a unique correlation of the cumulative failure probability, fracture toughness and yield strength. This correlation is validated by the Euro fracture toughness dataset for 1CT specimens at four different temperatures, which deviates from the Weibull statistical model with a modulus of four.

Comparison of Global and Local Approaches to Predicting Warm Pre-Stress Effect on Cleavage Fracture of Ferritic Steels

2004 ◽  
Vol 261-263 ◽  
pp. 69-74 ◽  
Author(s):  
Saeid Hadidi-Moud ◽  
A. Mirzaee-Sisan ◽  
Christopher E. Truman ◽  
David John Smith
Keyword(s):  
Fracture Toughness ◽  
Cleavage Fracture ◽  
Type Model ◽  
Ferritic Steels ◽  
Stress Effect ◽  
Loading History ◽  
Global Approach ◽  
Local Approach ◽  
Global And Local ◽  
Weibull Stress

Potentially both global and local approaches may be used to predicting the effect of loading history on cleavage fracture toughness distribution of ferritic steels. In this paper the dramatic increase in the apparent lower shelf fracture toughness of A533B steel following warm pre-stressing (WPS) has been predicted using these approaches. Extensive experimental evidence suggesting significant enhancement in fracture toughness of ferritic steels within the lower shelf temperatures following WPS are used to verify and compare the applicability and the extent of validity of the models. The global approach is based on the distribution of toughness data described by Wallin statistical model in conjunction with the Chell model for WPS effect. The local approach on the other hand is a Beremin type model that uses the Weibull stress to predict the WPS effect. Weibull stresses would essentially reflect the WPS effect on redistribution of stress-state around the crack tip. Predictions for apparent toughness using the two approaches are discussed in the light of the suggestion that residual stresses are the main cause of the enhancement, at least for the material and geometry used in this study.

The Weibull Stress Model for Predicting Cleavage Fracture in the Ductile-to-Brittle Transition Region

2008 ◽  
Author(s):  
Xiaosheng Gao ◽  
Jason P. Petti ◽  
Robert H. Dodds
Keyword(s):  
Fracture Toughness ◽  
Transition Region ◽  
Cleavage Fracture ◽  
Model Parameters ◽  
Ferritic Steels ◽  
Stress Model ◽  
Brittle Transition ◽  
Ductile To Brittle Transition ◽  
Constraint Effects ◽  
Weibull Stress

Transgranular cleavage fracture in the ductile-to-brittle transition region of ferritic steels often leads to spectacular and catastrophic failures of engineering structures. Due to the strongly stochastic effects of metallurgical scale inhomogenieties together with the nonlinear mechanical response from plastic deformation, the measured fracture toughness data exhibit a large degree of scatter and a strong dependence on constraint. This has stimulated an increasing amount of research over the past two decades, among which the Weibull stress model originally proposed by the Beremin group has gained much popularity. This model is based on weakest link statistics and provides a framework to quantify the relationship between macro and microscale driving forces for cleavage fracture. It has been successfully applied to predict constraint effects on cleavage fracture and on the scatter of macroscopic fracture toughness values. This paper provides a brief review of the research conducted by the authors in recent years to extend the engineering applicability of the Weibull stress model to predict cleavage fracture in ferritic steels. These recent efforts have introduced a threshold value in the Weibull stress model, introduced more robust calibration methods for determination of model parameters, predicted experimentally observed constraint effects, demonstrated temperature and loading rate effects on the model parameters, and expanded the original Beremin model to include the effects of microcrack nucleation.

Fracture Toughness Transferability Study Between the Master Curve Method and a Pressure Vessel Nozzle Using Local Approach

2003 ◽  
Author(s):  
Anssi Laukkanen ◽  
Pekka Nevasmaa ◽  
Heikki Keina¨nen ◽  
Kim Wallin
Keyword(s):  
Fracture Toughness ◽  
Pressure Vessel ◽  
Master Curve ◽  
Structural Integrity ◽  
Reference Temperature ◽  
Cleavage Crack ◽  
Local Approach ◽  
Master Curve Method ◽  
Curve Method ◽  
Constitutive Parameters

Local approach methods are to greater extent used in structural integrity evaluation, in particular with respect to initiation of an unstable cleavage crack. However, local approach methods have had a tendency to be considered as methodologies with ‘qualitative’ potential, rather than quantitative usage in realistic analyses where lengthy and in some cases ambiguous calibration of local approach parameters is not feasible. As such, studies need to be conducted to illustrate the usability of local approach methods in structural integrity analyses and improve upon the transferability of their intrinsic, material like, constitutive parameters. Improvements of this kind can be attained by constructing improved models utilizing state of the art numerical simulation methods and presenting consistent calibration methodologies for the constitutive parameters. The current study investigates the performance of a modified Beremin model by comparing integrity evaluation results of the local approach model to those attained by using the constraint corrected Master Curve methodology. Current investigation applies the Master Curve method in conjunction with the T-stress correction of the reference temperature and a modified Beremin model to an assessment of a three-dimensional pressure vessel nozzle in a spherical vessel end. The material information for the study is extracted from the ‘Euro-Curve’ ductile to brittle transition region fracture toughness round robin test program. The experimental results are used to determine the Master Curve reference temperature and calibrate local approach parameters. The values are then used to determine the cumulative failure probability of cleavage crack initiation in the model structure. The results illustrate that the Master Curve results with the constraint correction are to some extent more conservative than the results attained using local approach. The used methodologies support each other and indicate that with the applied local approach and Master Curve procedures reliable estimates of structural integrity can be attained for complex material behavior and structural geometries.

Implications of Cleavage Fracture Criteria for Structural Integrity Assessment of RPV Steels

2010 ◽  
Author(s):  
S. J. Lewis ◽  
C. E. Truman ◽  
D. J. Smith
Keyword(s):  
Cleavage Fracture ◽  
Structural Integrity ◽  
Strain History ◽  
Assessment Procedure ◽  
Load History ◽  
Safe Operation ◽  
Local Approach ◽  
Integrity Assessment ◽  
Rpv Steel ◽  
Current Assessment

To accurately assess the safe operation of structures containing defects, it is necessary to consider the influences of previous load cycles on crack propagation. A number of current assessment codes contain advice to account for strain history and residual stress, but are generally known to be highly conservative which may potentially result in the unnecessary and expensive repair or replacement of infrastructure. This paper considers the results of previous investigations into cleavage fracture of an A533b RPV steel to determine the accuracy of the widely used R6 structural integrity assessment procedure for fracture following significant load history. The levels of conservatism associated with a number of assessment methods are discussed and compared with experimental data. The general trends suggested an improvement in assessment accuracy may be obtained by using local approach methods, compared to crack tip failure parameters. It is noted however that all the methods used produced some unsafe estimates of failure load, which is felt to be related to an over estimate of the characteristic material toughness.

A Model of Pre-Strain Effects on Fracture Toughness

2001 ◽  
Vol 123 (4) ◽  
pp. 182-190 ◽  
Author(s):  
Andrew Cosham
Keyword(s):  
Fracture Toughness ◽  
Stress State ◽  
Ductile Fracture ◽  
Cleavage Fracture ◽  
Critical Strain ◽  
Experimental Studies ◽  
Local Approach ◽  
Stress Strain ◽  
Critical Fracture ◽  
Strained Material

A simple theoretical model for predicting the effect of tensile pre-strain on fracture toughness has been developed using the local approach. The HRR singularity is assumed to describe the stress-strain field around the crack tip. A stress-modified critical strain-controlled model is assumed to describe ductile fracture (and a critical stress-controlled model for cleavage fracture). The Rice and Tracey void growth model is used to characterize the variation of the critical strain with the stress state. The model further assumes that the fracture process does not change with increasing pre-strain. The effect of pre-strain is expressed in terms of an equation for the ratio of the fracture toughness of the pre-strained material to that of the virgin material. The model indicates that the effect of tensile pre-strain on fracture resistance can be characterized in terms of the effect of pre-strain on the stress-strain characteristics of the material, the critical fracture strain for a stress state corresponding to that during pre-strain, and several parameters that relate to the conditions for ductile fracture (or cleavage fracture). Previous experimental studies of the effect of pre-strain on toughness are summarized and compared with the predictions of the model.

scholarly journals Critical Distance Default Values for Structural Steels and a Simple Formulation to Estimate the Apparent Fracture Toughness in U-Notched Conditions

Metals ◽  
2018 ◽  
Vol 8 (11) ◽  
pp. 871 ◽  
Author(s):  
Sergio Cicero ◽  
Juan Fuentes ◽  
Isabela Procopio ◽  
Virginia Madrazo ◽  
Pablo González
Keyword(s):  
Fracture Toughness ◽  
Structural Integrity ◽  
Critical Distance ◽  
Structural Steels ◽  
Simple Formulation ◽  
Integrity Assessment ◽  
Apparent Fracture Toughness ◽  
Linear Behavior ◽  
Macro Scale ◽  
Failure Loads

The structural integrity assessment of components containing notch-type defects has been the subject of extensive research in the last few decades. The assumption that notches behave as cracks is generally too conservative, making it necessary to develop assessment methodologies that consider the specific nature of notches, providing accurate safe predictions of failure loads or defect sizes. Among the different theories or models that have been developed to address this issue the Theory of Critical Distances (TCD) is one of the most widely applied and extended. This theory is actually a group of methodologies that have in common the use of the material toughness and a length parameter that depends on the material (the critical distance; L). This length parameter requires calibration in those situations where there is a certain non-linear behavior on the micro or the macro scale. This calibration process constitutes the main practical barrier for an extensive use of the TCD in structural steels. The main purpose of this paper is to provide, through a set of proposed default values, a simple methodology to accurately estimate both the critical distance of structural steels and the corresponding apparent fracture toughness predictions derived from the TCD.

scholarly journals Assessment of Local Approach Methods for Predicting End-of-Life Toughness of RPV Steels

2011 ◽  
Author(s):  
Andrey P. Jivkov ◽  
David P. G. Lidbury ◽  
Peter James
Keyword(s):  
Fracture Toughness ◽  
End Of Life ◽  
Cleavage Fracture ◽  
Failure Probability ◽  
Sufficient Accuracy ◽  
Model Parameters ◽  
Local Approach ◽  
Component Failure ◽  
Weakest Link ◽  
Future Work

Local approach methods are becoming increasingly popular as practical tools for cleavage fracture toughness prediction. Their application involves two distinct elements: calculation of ‘individual’ probabilities of failure, dictated by the local mechanical fields; and summation of these failure probabilities to predict the probability of component failure. In this work, we demonstrate that development of the local approach methods to date has been essentially focused on improving the criterion for predicting local failure as a function of the local mechanical fields. Yet, the existing methods fail to predict with sufficient accuracy the effects of irradiation and defect geometry on fracture toughness when the calculations are based on a common set of model parameters. A possible reason for this, common to all methods, is found in the calculation of the cumulative failure probability, which is based on the weakest-link argument. We discuss the implications of the weakest-link assumption, identify those situations where it needs to be reconsidered, and propose future work that will increase our understanding for improving the calculation of global failure probability.

Sign in / Sign up

Export Citation Format

Share Document