Cleavage Fracture Of Steel Research Articles

Computer simulation of cleavage fracture surface morphologies in steel plates

A computer simulation model has been developed to reproduce brittle fracture surface morphologies. Local stress intensity factor along a propagating crack front is calculated considering the fracture surface irregularity and crack front non-straightness. Shear stress exerted at fracture surface steps and crack closure stress by uncracked ligament are also taken into consideration in calculating the local stress intensity factor. The local stress intensity factor is compared with assumed critical value to judge if a crack propagate or arrests at each crack front location. The model was found to reproduce chevron markings and shear-lips, both of which are typical of cleavage fracture in steels. The present model was applied to wide-plate crack arrest test with temperature-gradient. Changes of fracture surface morphologies and arrest crack length were well reproduced by the present model.

Fracture toughness of steels with nickel content in respect of carbide morphology

This paper is focused on the influence of Ni addition on the microstructure and fracture toughness of structural steels after tempering. Nickel is known to increase the resistance to cleavage fracture of steel and decrease a ductile–brittle transition temperature. The medium carbon, low alloy martensitic steels attain the best combination of properties in low tempered condition, with tempered martensite, retained austenite and transition carbides in the microstructure. In the present research, four model alloys of different Ni contents (from 0·35 to 4·00%) were used. All samples were in as quenched and tempered condition. Quenching was performed in oil at room temperature. After quenching, samples were tempered at 200°C for 2 h. An increase in nickel content in the investigated model structural steels causes a decrease in ε carbide volume fraction in their microstructure. Cementite nucleates independently in the boundaries of martensite laths and in the twin boundaries in the areas where the ε carbide has been dissolved. It was stated that stress intensity factor KIc significantly decreases in the case of the presence of dispersive elongated cementite precipitations at the boundaries of the prior austenite grains.

Estimation of Single-grain Properties of Steel through Inverse- engineering of Microindentation Experiments: Uniqueness of the Solution

Cleavage fracture of steel is governed by micromechanical mechanisms, but it is modelled with simple continuum- level models. These models are starting to reach the limits of their applicability, so there is interest to look deeper into micromechanical modelling of fracture processes. In order to model fracture on the microstructural level, a plasticity model of the grains themselves is needed. In prior studies, a method of finding the single-crystal properties by an inverse engineering approach has been described. By this method, a single crystal is indented, and the material input to a FE model of the experiment is iteratively changed until agreement is achieved. These prior studies have shown that methods like this provide ambiguous results. In this paper, the uniqueness of the solution is assessed by evaluating the difference between experiments and simulations for a variety of material input parameters. The results show that with a power law hardening relationship, martensite produces two unique minima (at n=0 and 0.1). This may represent the Lüder's plateau and the subsequent hardening, respectively. The ferrite material featured a minimum that was flat, but the flatness was over a very short range of hardening exponents.

Effects of Cleavage Triggers and Constraint on Brittle Fracture in Steel Weld Heat Affected Zone

Cleavage fracture in steel is usually evaluated by using the stress criterion. However, some researchers indicated the necessity of strain in the stress criterion according to the cleavage fracture test results of steel base metal. If so, the cleavage fracture criterion should be also applicable to steel welded joints. In this study, cleavage fracture tests, SEM fracture surface observations and FEA were conducted using the smooth and notched round bar tensile specimens of the simulated HAZ microstructure of JIS SM490A. Cleavage trigger types did not influence the cleavage fracture properties of the specimens obviously. FEA results suggested that the critical condition of cleavage fracture initiation was associated not only with stress but also with strain.

Experiments and predictions of the effects of load history on cleavage fracture in steel

A series of experiments conducted on two steels, A533B and A508, are summarised. Tests were conducted to explore the influence of different room temperature pre-loading cycles on subsequent low temperature (−150 °C and −170 °C) cleavage fracture. In all cases the low temperature fracture toughness was modified, with tensile pre-loading increasing the toughness and precompression reducing the toughness. Results from finite element simulation of the pre-loading cycles are illustrated. Tensile pre-loading created compressive residual stresses and precompression generated tensile residual stresses. The residual stresses were adopted in a stress based local approach to fracture model using Weibull statistics and applied to the experimental results. The parameters in the Weibull model were calibrated for the virgin steels prior to its application to prior loading cases. The model is found to be successful in predicting the change in toughness relative to the virgin material for pre-loading in tension of A533B steel. The model underestimated the change in toughness for tensile pre-loading of A508 steel and overestimated the toughness change for precompression of both steels.

A statistical, physical-based, micro-mechanical model of hydrogen-induced intergranular fracture in steel

Intergranular cracking associated with hydrogen embrittlement represents a particularly severe degradation mechanism in metallic structures which can lead to sudden and unexpected catastrophic fractures. As a basis for a strategy for the prognosis of such failures, here we present a comprehensive physical-based statistical micro-mechanical model of such embrittlement which we use to quantitatively predict the degradation in fracture strength of a high-strength steel with increasing hydrogen concentration, with the predictions verified by experiment. The mechanistic role of dissolved hydrogen is identified by the transition to a locally stress-controlled fracture, which is modeled as being initiated by a dislocation pile-up against a grain-boundary carbide which in turn leads to interface decohesion and intergranular fracture. Akin to cleavage fracture in steel, the “strength” of these carbides is modeled using weakest-link statistics. We associate the dominant role of hydrogen with trapping at dislocations; this trapped hydrogen reduces the stress that impedes dislocation motion and also lowers the reversible work of decohesion at the tip of dislocation pile-up at the carbide/matrix interface. Mechanistically, the model advocates the synergistic action of both the hydrogen-enhanced local plasticity and decohesion mechanisms in dictating failure.

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 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.

Mechanism of crack formation at hard brittle particles in steels

Hard brittle particles can have an important influence on cleavage fracture in steels, and can act as cleavage initiators. Considering the strain elastic energy and the dislocation pile-up elastic energy to be released, and surface energy to be produced, during the hard brittle particle cracking, the mechanism of crack formation at hard brittle particles in steels was studied by a thermal dynamic model. It has been demonstrated theoretically that both normal stress and dislocation pile-up at the hard brittle particles are necessary for its cracking. Furthermore, a criterion for hard particle cracking has been derived, by which the critical cracking stress for the hard brittle particle was predicted not only to depend on its size and surface energy, but also to have correlations with the grain size. Their correlations are the same as the relationship between the fracture stress determined experimentally and the grain size in the case of finer grain constituents. It can be indicated that in such a situation the hard brittle particle cracking is responsible for the ultimate cleavage fracture of the material. In the case of coarser grain constituents, the critical cracking stress for the hard brittle particle is smaller than the fracture stress determined experimentally. This indicated that propagating a particle-size crack into the surrounding ferrite matrix is the critical event for the ultimate failure of the material.

The scatter in KIC-results

Scatter in K IC -results can often be quite extensive, and to make reliable interpretations of the results it is of great importance to understand the nature of it. Cleavage fracture in steels is of a statistical nature and therefore the scatter in K IC -results will behave similarly. Two different approaches, one based on a microstructural statistical model and an other based on the Weibull distribution are applied to evaluate the theoretical scatter in K IC -results. With both methods it is shown that the theoretical value of the relative scatter described through the Weibull slope factor is constant and equal to four. The reason for the discrepancy between the theoretical value and the experimentally determined values of the slope factor is shown to be caused by inadequate number of experimental K IC -measurements. The existence of a lower limiting K min value is verified and a simple procedure for conservative estimation of the K Ic - mean and lower bound values is presented.

Analysis of cleavage fracture potential of martensitic stainless steel fusion structures: Part I. Micromechanical models and material properties

The major disadvantage of martensitic stainless steels for structural applications in fusion reactors is currently considered to be their potential for low temperature brittle cleavage fracture. This study attempts to review the current understanding of cleavage fracture in steels and the role of microstructure in dictating material resistance to this type of fracture. A parametric analysis of cleavage fracture in a surrogate steel, A533B, is made and the results are used in conjunction with general cleavage fracture theory to establish some potential guidelines for future research in developing the martensitic stainless steels.

Statistical aspects of cleavage fracture in steel

The fracture of mild steel in the cleavage range has been evaluated using a weakest link statistical model, assuming the preexistence of a distribution of cracked carbides. The model provides a rationale for the critical fracture distance, viz., the distance from the crack tip at which the probability of cleavage cracking exhibits a maximum. The critical distance depends on the size distribution and volume fraction of carbides. The model also predicts trends in K,ic with material properties: flow strength, cracked carbide size and volume fraction, and grain size. The resultant temperature dependence of K,ic is shown to derive exclusively from the temperature dependence of the flow stress, as in prior models. The effects of microstructure on K,ic depend primarily on the size distribution of cracked carbides, with additional influences of the grain size and of the volume fraction of carbides.

Cleavage fracture of steel in the upper ductile-brittle transition region

Stable-crack growth precedes cleavage fracture in the upper ductile-brittle transition region. It is suggested that the conversion from one fracture mode to the other occurs at random locations in the microstructure, denoted trigger points. It also is suggested that K Ic for cleavage should be calculated from the elastic energy in the specimen at the point of conversion. Experiments are described that showed that the amount of stable-crack growth increases with increasing temperature and with decreasing load-train compliance.

Influence of prestrain and strain aging on cleavage fracture in C–Mn steel

Abstract The influence of prestrain and static strain aging on the cleavage fracture stress σf in a C–Mn steel has been investigated. It is shown that σf increases with increasing amount of prestrain and that the effect of strain aging is to reduce slightly the value of σf. Smith's original theory for cleavage fracture in steel containing grain-boundary carbides is used to determine the effect of prestrain and strain aging on the effective surface energy γeff of crack propagation. It is shown that γeff is approximately prestrain independent and that strain aging makes γeff decrease slightly.

Cleavage fracture of steels at low temperatures

Model of cleavage fracture: deformation twin creates microcrack in cementite lamella; critical event in fracture process is extension of crack into ferrite under the action of twin and applied stresses. Stress analysis of the model yields fracture stress. Mechnical tests with tension, compression and notched bend specimens at temperatures 4 K < T < 100 K. Evaluation of local fracture stress by finite element analysis. Comparison of theoretical and experimental results.