Local Criterion For Cleavage Fracture Research Articles

Temperature and Loading-Rate Dependence on a Local Criterion for Cleavage Fracture in the Transition Region of the Tempered Martensitic Steel Eurofer97.

Finite element calculations of stationary cracks were performed in the ductile to brittle transition region of the reduced activation tempered martensitic steel Eurofer97, for which a large fracture toughness database was already available. Two models were run in this work: i) compact tension specimens simulated in 3D, and ii) small scale yielding boundary layer model in 2D plane strain conditions. The analysis was focused on the unusual low fracture toughness of Eurofer97 on the lower shelf, with respect to other "ferritic" steels, as well as in the lower transition. The analysis of the near tip stress field suggested that the minimum stress intensity factor on the lower shelf represents the minimum loading condition to have the local fracture stress act over a distance of the order of the prior austenite grain size. The effect of loading rate on the tip stress fields were also investigated by considering a strain rate dependence of the flow stress explicitly in the finite element calculations. These effects were shown to be non-negligible and to depend on the ratio between the peak stress in the fracture process zone and the local fracture stress.

Study of cleavage initiation under thermal shock by tests on cracked rings and thermomechanical calculations

This article describes a new type of test that easily creates a thermal shock combined with mechanical loading by using standard laboratory equipments. The method consists of rapidly heating a ring initially at −175 °C by circulating hot water through judiciously placed holes. It allows to obtain, at minimal cost, data on crack initiation under thermal shock in the transition zone of steel 16MND5 used for pressure vessels, these data being necessary for work on fracture criteria. Details of two tests are given, one with preloading in which cleavage fracture was observed and one without preloading in which cleavage initiation did not occur. The thermomechanical calculations necessary for the interpretation of both tests are also given. Finally, the global approach to fracture is used involving comparison of the loading paths J( T) with the master curves [ASTM E1921-97, 1997. Standard test method for determination of reference temperature to for ferritic steels in the transition range, ASTM standards], as well as the local approach proposed by Beremin [Beremin, F.M., 1983. A local criterion for cleavage fracture of a nuclear pressure vessel steel. Metall. Trans. A 14A, 2777–2287] for estimating the probability of fracture of such rings. It is shown that, in both approaches, the calculated probabilities are consistent with the experimental observations.

Thermomechanical analysis of thermal shock fracture in the brittle/ductile transition zone—Part II: Numerical calculations and interpretation of the test results

The integrity of PWR pressure vessels is assured by keeping the crack tip stress intensity factor below the toughness of the material under monotonic isothermal loading. To study the effects of sudden cooling associated with a thermal gradient, a specially modified compact specimen has been developed. This has been used to carry out tests in the transition zone with different loading-temperature sequences liable to call the conventional criteria into question. The test is described in detail in Part I of this article [Chapuliot S, et al. Thermomechanical analysis of thermal shock fracture in the brittle/ductile transition zone. Part I: Description of the tests. Engng Fract Mech, 72, 2005, 661–73]. The second part describes numerical investigations to estimate the local mechanical fields at the crack tip and the overall parameters of the fracture mechanics. Finite element thermomechanical calculations are used to interpret the results of these new thermal shock tests using the master curve concept [ASTM E 1921–1997. Standard test method for determination of reference temperature To for ferritic steels in the transition range, 1997] and the Beremin statistical model [Beremin FM. A local criterion for cleavage fracture of a nuclear pressure vessel steel. Metall Trans A, 14A, November 1983, 2287–777].

A new statistical local criterion for cleavage fracture in steel. Part II: application to an offshore structural steel

In the first part of this paper, a new model for cleavage fracture in steel was presented, based on a new statistical local criterion, which expresses the necessity of simultaneously fulfilling the conditions for both cleavage microcrack nucleation and propagation. In this second part, the assumptions and predictive capabilities of the new model are assessed using a modern offshore structural steel plate (Grade 450EMZ). It is shown that the model assumptions are consistent with the cleavage fracture behaviour of the steel and that the new model has the potential of correctly quantifying the effects of size, constraint, temperature and strain rate on cleavage fracture risk.

A new statistical local criterion for cleavage fracture in steel. Part I: model presentation

Since it was first proposed in the early eighties, the local approach to cleavage fracture, or Beremin model, has been applied to a wide range of engineering problems. However, several results have cast doubts on both the predictive capabilities and fundamental assumptions of the Beremin model. This three-part paper shows that most problems encountered with the original Beremin model result from an unnecessarily oversimplified description of the local cleavage event. A new statistical local criterion for cleavage is proposed, which expresses the necessity of maintaining a critical dynamic connectivity between microcrack nucleation and unstable propagation in order for cleavage fracture to occur.

A new statistical local criterion for cleavage fracture in steel. Part I: model presentation

Since it was first proposed in the early eighties, the local approach to cleavage fracture, or Beremin model, has been applied to a wide range of engineering problems. However, several results have cast doubts on both the predictive capabilities and fundamental assumptions of the Beremin model. This three-part paper shows that most problems encountered with the original Beremin model result from an unnecessarily oversimplified description of the local cleavage event. A new statistical local criterion for cleavage is proposed, which expresses the necessity of maintaining a critical dynamic connectivity between microcrack nucleation and unstable propagation in order for cleavage fracture to occur.

A new statistical local criterion for cleavage fracture in steel. Part II: application to an offshore structural steel

In the first part of this paper, a new model for cleavage fracture in steel was presented, based on a new statistical local criterion, which expresses the necessity of simultaneously fulfilling the conditions for both cleavage microcrack nucleation and propagation. In this second part, the assumptions and predictive capabilities of the new model are assessed using a modern offshore structural steel plate (Grade 450EMZ). It is shown that the model assumptions are consistent with the cleavage fracture behaviour of the steel and that the new model has the potential of correctly quantifying the effects of size, constraint, temperature and strain rate on cleavage fracture risk.

Probabilistic model for fracture toughness prediction based on the new local fracture criteria

The stochastic nature of various critical parameters controlling cleavage fracture of nuclear pressure vessel steels has been analysed. On the basis of the performed analysis the local criterion for cleavage fracture proposed by authors [1,2] has been modified into a probabilistic statement. The probabilistic model for fracture toughness prediction has been elaborated on the basis of the modified local criterion for cleavage fracture, the ductile fracture criterion proposed by authors earlier and the approximated solution for problems relating to the stress-strain state near the crack tip. This probablistic model allows one to predict the dependence of K IC on temperature for any given probability of brittle fracture; to describe the effect of the specimen thickness on K IC with material properties has been analysed. Comparison of experimental and calculated results has been performed as applied to a 2.5Cr-Mo-V nuclear pressure vessel steel. The proposed probabilistic model has been shown to describe the lower and upper envelopes for a large number of experimental data.

Brittle fracture of nuclear pressure vessel steels—I. Local criterion for cleavage fracture

Brittle fracture of nuclear pressure vessel steels has been investigated on the basis of physical and mechanical modelling for processes of nucleation, start and propagation of cleavage microcracks in the material substructure, as changed by plastic deformation. It is shown that depending on stress triaxiality, temperature and material properties the brittle fracture in macrovolume may be controlled either by cleavage microcrack nucleation or by the propagation condition. Cleavage microcrack nucleation according to conditions formulated by the authors depends on the effective and maximum principal stress and temperature. Such microcracks, as experimental research has shown, may be nucleated at plastic strains from about 1 to 40% depending on test temperature and stress triaxiality. The brittle fracture critical stress S c is determined as stress for cleavage microcrack start and propagation through the deformation substructure of a material. The dependence describing the variation of S c with plastic strain under various loading histories is theoretically obtained and experimentally confirmed. It is established that S c is a monotonously increasing function of accumulated plastic strain—Odqvist's parameter—and does not depend on loading history. On the basis of experimental and theoretical investigations performed, a new formulation for local criterion of cleavage fracture is proposed.

Local Criterion for Cleavage Fracture : Structural and Mechanical Approach

On the base of physical-and-mechanical modelling for cleavage microcrack behaviour a new local criterion for cleavage fracture is formulated. Cleavage fracture of pressure vessel steel under various loading history is investigated. It has been experimentally shown that for steels after static prestrain by tension and cyclic prestrain brittle fracture critical stress is described by increasing function of Odqvist's parameter, in the same time plastic precompression results in the significant embrittlement of steel and transition from transcrystalline cleavage fracture to intercrystalline one. Proposed criterion application for predicting fracture toughness dependence on temperature K IC (T) and prestrain influence on K IC is shown.

Development and Use of Beremin Local Criterion for Cleavage Fracture in the Steel Industry

This paper describes the use of Local Approach to Fracture in the French steel industry. The criteria used to model cleavage fracture of steels are briefly presented. Various applications covering a wide range of steels are reviewed. These applications illustrate the advantages of the Local Approach, such as scatter prediction, the ability to finely relate microstructure to mechanical properties and to take into account materials with heterogeneous microstructures such as welds. Finally, current developments of the method are presented. These developments will allow a quick and easy implementation of this procedure which will become accessible to industrial laboratories.

Specimen Size Requirements for Fracture Toughness Testing in the Transition Region

AbstractThe article utilizes plane strain elastic-plastic finite element analysis and a local criterion for cleavage fracture to establish specimen size requirements for the ductile-brittle transition region. Critical J and CTOD values, relative to the small-scale yielding value, were predicted as a function of specimen size, strain hardening exponent, and a/W. These analyses predict an increase in the apparent toughness with decreasing specimen size due to a loss in crack tip constraint; this effect is particularly pronounced in shallow notched specimens and low hardening materials. For deeply notched bend and compact specimens, the following size requirement must be met for critical J values for cleavage to be size independent: b,B,a>200JcσY where b is ligament length, B is thickness, a is crack length, and σγ is flow stress. This criterion is eight times more severe than the size requirements in ASTM E 813-87, but it is less stringent than the requirements of ASTM E 399-83. In order for a CTOD value to be nearly size independent, it must be less than 1/300 times the relevant specimen dimensions.The constraint loss in shallow notched specimens is usually far too rapid to obtain J-controlled cleavage fracture, but the analyses presented in this article provide a means for correcting fracture toughness data for constraint loss. Predictions of the effect of a/W on toughness in the transition region agree favorably with experimental data.Future work will consider the effects of specimen thickness and ductile tearing on transition region toughness.

A local approach to cleavage fracture

The derivation of a local criterion for cleavage fracture is presented in detail. In a first part, a brief review of the metallurgical description of cleavage fracture is the first step in the derivation of a local criterion. It is shown that in many cases the critical stress criterion of OROWAN is an adequate modelization. Statistical aspects of test results are presented in detail. The effect of strain, temperature and strain rate are discussed. In a second part, a local criterion derived from WEIBULL's statistics is used. Relationships between the parameters of this criterion and other usual parameters are derived using analytical solutions for the crack-tip situation. It is shown how this local criterion can be implemented in computer programs in order to solve much more complex situations where no analytical solutions are available. Some applications of this last approach are discussed in detail.

A local criterion for cleavage fracture of a nuclear pressure vessel steel

Experiments were performed on three heats of A508 class 3 steel in order to determine the mechanical conditions for cleavage fracture. These tests were carried out on various geometries including 4-point bend specimens and axisymmetric notched tensile bars with different notch radii which have been modelized using the finite element method. In one heat, the temperature range investigated was from 77 K to 233 K. It is shown that the cleavage resistance is increased by tensile straining. Moreover, the probability of fracture obeys the Weibull statistical distribution. All the results can be accounted for in terms of a local criterion based on Weibull theory and which takes into account the effect of plastic strain. In this criterion, the parameters which were experimentally determined are found to be temperature independent over the range 77 K to 170 K. The applicability of the approach proposed for cleavage fracture at the crack tip is also examined. It is shown that the experimental results published in the literature giving the variation of fracture toughness with temperature can be explained by the proposed criterion which predicts reasonably well both the scatter in the experimental results and theKICtemperature dependence.