Local Cleavage Fracture Stress Research Articles

Mechanism of Decrease in Impact Toughness in a Low-Carbon MnCrMoNiCu Plate Steel with Increasing Austenitizing Temperature

In order to reveal how microscopic factors affect the toughness and the occurrence of cleavage fracture of a low-carbon MnCrMoNiCu alloyed steel, a series of thermal treatments was performed on the steel employing a thermomechanical simulator. These involved reheating samples at different temperatures (950-1250 °C), producing different prior austenite sizes, followed by a continuous cooling transformation process. The Charpy V-notch toughness was determined, and the effect of austenite grain size on the ductile-to-brittle transition temperatures of the steel was investigated. The microstructural evolution on the austenite sizes was studied, fracture features were characterized, the critical event for cleavage fracture was identified, and the local cleavage fracture stress σf was calculated. The impact toughness decreased as the austenitizing temperature increased. A quantitative relationship between σf and the size of the initial cleavage fracture facet (microcrack nucleus) af in the lathy martensite + bainite microstructure has been developed.

Microstructural Evolution and Ductile-to-Brittle Transition in a Low-Carbon MnCrMoNiCu Heavy Plate Steel

Low carbon MnCrMoNiCu alloyed steels are typically used to produce highly ductile thick plates for offshore structures and bulk shipbuilding. The current study revealed how microscopic factors affect the toughness and the occurrence of cleavage fracture of the steel. In this regard, a series of thermal treatments was performed on the test steel by employing a thermomechanical simulator. These involved reheating samples at different temperatures (1168 K to 1623 K (895 °C to 1350 °C)) producing different prior austenite grain sizes, followed by a continuous cooling transformation process. The Charpy V notch (CVN) toughness was determined, and the effect of the austenite grain size on the ductile–brittle transition-temperatures of the steel was investigated. The microstructural evolution of the austenite grain sizes was studied, fracture features were characterized, the critical event for cleavage fracture was identified, and the local cleavage fracture stress σf was calculated. The CVN toughness and σf were maximized in the steel which was reheated at 1273 K (1000 °C) and containing refined lathlike bainite.

The effects of Silicon and Copper on microstructures, tensile and Charpy properties of weld metals by refined X120 wire

This paper analyzes the micro-mechanism of decrease of impact toughness with increasing Silicon and increasing Copper in weld metal of a low-alloy high-strength X120 steel. By comparing the microstructures, measuring the extending length of the fibrous crack, and calculating the local cleavage fracture stress σf, and then using the basic principles of the micro-mechanism of cleavage fracture, the essential causes of change of toughness in the weld metal of high-strength steel welded joints are revealed in this work.

Micromechanism of Decrease of Impact Toughness in Coarse-Grain Heat-Affected Zone of HSLA Steel with Increasing Welding Heat Input

This paper analyzes the micromechanism of decrease of impact toughness with increasing the welding heat input in coarse-grain heat-affected zone (CGHAZ) of a low-alloy high-strength ship-building steel plate. By comparing the microstructures, measuring the extending length of the fibrous crack, identifying the critical event of cleavage fracture, measuring the critical length, and calculating the local cleavage fracture stress σ f, and then using the basic principles of the micromechanism of cleavage fracture, this work reveals the essential causes of deteriorated toughness in the CGHAZ of high-strength steel welded joints.

Critical cleavage fracture stress characterization of A508 nuclear pressure vessel steels

The critical cleavage fracture stress of SA508 Gr.4N and SA508 Gr.3 low alloy reactor pressure vessel (RPV) steels was studied through the combination of experiments and finite element method (FEM) analysis. The results showed that the value of the local cleavage fracture stress, σF, of SA508 Gr.4N steel was significantly higher than that of SA508 Gr.3 steel. Detailed microstructural analysis was carried out using FEGSEM which revealed much smaller grains, finer and more homogenous carbide particles formed in SA508 Gr.4N steel. Compared with the SA508 Gr.3 steel currently used in the nuclear industry, the SA508 Gr.4N steel possesses higher strength and notch toughness as well as improved cleavage fracture behavior, and is considered a better candidate RPV steel for the next generation nuclear reactors.

A Critical Grain Size Concept to Predict the Impact Transition Temperature of Ti-Microalloyed Steels

Based on the assumption that local principal stress remains the same everywhere within a ferrite grain, a critical value of grain size can be determined for a fixed TiN particle size. When the grain size is smaller than the critical size, grain boundary is expected to resist the propagation of a micro-crack that is initiated from a TiN particle. Using this concept, an attempt has been made to predict the local cleavage fracture stress and 27J impact transition temperature (ITT) of different Ti-microalloyed steels, which were subjected to (instrumented) Charpy impact testing.

Cleavage fracture behavior of a C–Mn vessel steel at various loading rates in notched specimens

In this paper, the cleavage fracture behavior of a C–Mn vessel steel at various loading rates was studied by experiments and FEM calculations. The results show the cleavage fracture mechanism of this steel in two types of notched specimens loaded in two loading modes (four-point bending and three-point bending) at a temperature of −110 °C does not change with loading rates. This leads to the independence of the measured the local cleavage fracture stress σ f and the macroscopic cleavage fracture stress σ F on loading rate, notch geometry and loading mode. The macroscopic notch toughness characterized by the ratio of fracture load to general yield load ( P f/ P gy) of the notched specimens with two notch geometries loaded in the two loading modes at various loading rates can be predicated by the maximum normal stress criteria ( σ yymax⩾ σ F). The σ F can be regarded as an engineering notch toughness parameter of materials, and may be used for assessing integrity of structures with notch defects by the criteria σ yymax⩾ σ F. The σ F values of steels can be simply measured by the Griffiths–Owen's notched specimens loaded in four-point bending or three-point bending at a test temperature and a loading rate.

Fracture behaviour and cleavage initiation in hypoeutectoid pearlitic steel

The paper reports on an investigation of the micromechanism of cleavage fracture in hypoeutectoid pearlitic R7T steel, commonly used for producing railway wheels. The steel possesses extensive Luders deformation, which somewhat complicates finite element (FE) modelling and analyses of fracture behaviour. Standard Charpy V-notch specimens were used in order to analyse the fracture behaviour at quasistatic and impact loading. Finite element 3D calculations were performed and the elastic-plastic behaviour of notched bars up to the fracture was simulated. Detailed fractographic analysis was carried out on a number of Charpy V-notch specimens in order to investigate the origin site of cleavage fracture initiation and its distance from the notch root. The suitability of the three-criterion micromechanical model (Chen et al. Acta Materialia 51:1841–1855, 2003) for cleavage initiation was verified. The R7T steel under investigation exhibited a cleavage fracture stress of 1,837 MPa. Its independence on temperature evidenced the micromechanism of cleavage fracture to be microcrack propagation-controlled. For the investigated blunt-notched bend bars, an active volume exists ahead of the notch root in which pearlite colony-associated initiation sites are located. The cleavage fracture initiation of the steel is thus governed by the sites lying in the active volume. The active volume is determined by the values of three parameters. A plastic strain lying in interval from \({\varepsilon_{\rm pmin}}\) to \({\varepsilon_{\rm pmax}}\) (for the steel investigated from 0.033 to 0.108) is necessary to create a cleavage crack nucleus at any location within the active volume depending on the local pearlite properties. A stress triaxiality parameter ranging from hmin to hmax (from 0.93 to 1.39) is supposed to prevent the blunting process at the site of the cleavage nucleus. Once the main principal stress σ1 exceeds the local cleavage fracture stress σCFmin, an unstable global cleavage fracture occurs in a blunt-notched bar.

Effects of loading rate, notch geometry and loading mode on the local cleavage fracture stress of a C–Mn steel

In this work, notched specimens with two notch geometries were tested in two loading modes (four-point bending (4PB) and three-point bending (3PB)) at various loading rates at a temperature of − 110°C for a C–Mn steel. An elastic–plastic finite-element method (FEM) is used to determine the stress distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ f is measured. The results obtained and combining with previous studies by the authors show that the local cleavage fracture stress σ f is closely related to the cleavage fracture mechanism (critical events) in steels. The σ f values do not change with loading rate, notch geometry and loading mode, as long as the critical event of cleavage fracture does not change at various testing conditions. The σ f is mainly determined by the steel microstructure, and its scatter is mainly caused by the size distribution of the weakest constituent in steels (ferrite grain or pearlite colony with large sizes and large second phase particles) and the change of the critical events in cleavage process. The σ f can characterize the intrinsic toughness of steels and may be used in a “local approach” model for assessing integrity of flawed structures. The σ f values could be measured by both 4PB and 3PB tests.

Effects of Notch Tip Geometry on the Cleavage Fracture of RPV Steel

A series of fracture tests and elastic-plastic FEM analysis were carried out for three-point bend specimens with different notch root radius in the cleavage temperature region of a RPV low alloy steel. The cleavage initiation distance (CID) from the notch tip was measured by SEM photographs on the fracture surface of each specimen. The local cleavage fracture stress σf * was defined as the stress σyy at the cleavage initiation site. The σf * increased with decreasing notch root radius. The CID’s were larger in specimens with bigger root radius. This implies that a larger volume of material and possibly bigger microcracks should be involved in the cleavage fracture process of those specimens. The σf * is considered to have a certain relationship with the level of stress-strain concentration in different geometry specimens.

Effects of loading rate on the local cleavage fracture stress σf in notched specimens

Four point bending (4PB) notched specimens with different notch sizes are tested at various loading rates at a temperature of −110 °C for a C–Mn steel. An elastic–plastic finite element method (FEM) is used to determine the stress and strain distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ f is measured. The results show that the local cleavage fracture stress σ f does not essentially change with loading rate V and notch size. The reason for this is that the cleavage micromechanism does not change in the different specimens at various loading rates. The cleavage micromechanism involves competition of two critical events of crack propagation and crack nucleation in the high stress and strain volume ahead of notch root. The large scatter of σ f and notch toughness are mainly caused by the different critical events in different specimens.

Effects of geometry of notched specimens on the local cleavage fracture stress σf of C–Mn steel

An elastic–plastic three-dimensional finite element method analysis is used to determine the stress and strain distributions ahead of notches of four-point bending (4PB) specimens with various sizes ( W, B and a) and widths ( B). By measuring the location of the cleavage initiation sites for a C–Mn steel, the local cleavage fracture stress σ f is accurately determined. With increasing specimen sizes and widths the fracture load P f increases considerably, but σ f remains nearly constant. The reason that the σ f of the specimen with minimum size is slightly larger than that of the other specimens is analyzed by an active zone model of cleavage fracture for notched specimens. The critical event for cleavage fracture is the propagation of a ferrite grain-sized crack into the neighboring matrix, and is independent of specimen sizes and widths. σ f is mainly determined by the length of the critical microcrack, and the specimen sizes and widths have little effect on it.

ON THE MEASUREMENT AND PHYSICAL MEANING OF THE CLEAVAGE FRACTURE STRESS IN STEEL

Elastic-plastic two-dimensional (2D) and three-dimensional (3D) finite element models (FEM) are used to analyze the stress distributions ahead of notches of four-point bending (4PB) and three-point bending (3PB) specimens with various sizes of a C-Mn steel. By accurately measuring the location of the cleavage initiation sites, the local cleavage fracture stress σf and the macroscopic cleavage fracture stress σF is accurately measured. The σf and σF measured by 2D FEM are higher than that by 3D FEM. σf values are lower than the σF, and the σf values could be predicted by σf=(0.8−−1.0)σF. With increasing specimen sizes (W,B and a) and specimen widths (B) and changing loading methods (4PB and 3PB), the fracture load Pf changes considerably, but the σF and σf remain nearly constant. The stable lower boundary σF and σf values could be obtained by using notched specimens with sizes larger than the Griffiths–Owen specimen. The local cleavage fracture stress σf could be accurately used in the analysis of fracture micromechanism, and to characterize intrinsic toughness of steel. The macroscopic cleavage fracture stress σF is suggested to be a potential engineering parameter which can be used to assess fracture toughness of steel and to design engineering structure.

Effects of precracked specimen geometry on local cleavage fracture stress σf of low alloy steel

An elastic-plastic finite element method (FEM) is used to analyze the stress distributions ahead of crack tips for three types of COD specimens with different precracked depth a/W and height W of a low alloy steel, and the tensile and COD tests are carried out at various temperatures. By accurately measuring the distances of the cleavage initiation sites from the blunted crack tips, the local cleavage fracture stresses σf are measured. With increasing precrack depth a/W, specimen height W and test temperatures in a certain range, it was found that the σf essentially does not change. The σf is a steady inherent parameter of the material whose value is independent of the precracked specimen geometry.

Effects of notch geometry on the local cleavage fracture stress σf

An elastic–plastic finite element method (FEM) is used to analyse the stress and strain distributions ahead of notches with various depths and flank angles in four‐point bending (4PB) specimens of a C–Mn steel. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σf is measured. By increasing the notch depth and notch flank angle from 2.25 to 8.25 mm and 10 to 90°, respectively, the distributions of high stress and strain at the moment of fracture show considerable variations. However, the value of σf stays relatively constant. The critical fracture event is thus shown to be identical, i.e. the propagation of a ferrite grain‐sized crack into the neighbouring matrix. It is concluded that σf is mainly determined by the length of the critical microcrack, while the notch geometry and its associated stress volume have little effect on the value of σf . The cleavage site ahead of a notch is determined by the stress distributions and the positions of the weakest grains.

The effect of stress triaxiality on the local cleavage fracture stress in a granular bainitic weld metal

The local cleavage fracture stress σF measured at the cleavage initiation sites of a granular bainitic weld metal is only an apparent fracture parameter dependent on stress triaxialities at the local sites in the propagation controlled cleavage regime. This dependence can be explained by the stress triaxiality modified Smith equation in which the intrinsic cleavage fracture stress σF0 is introduced, considered to be an invariant characterizing a material's fracture property. In the nucleation controlled cleavage regime σF is temperature dependent and coincident with the local yield stresses defined by Von Mises criterion at the local sites. It is suggested that the modified Smith model would provide a unified physical basis for the intrinsic correlation among various macroscopic fracture properties as well as their temperature transition behaviors from the temperature dependence of material's yield strength.

Brittle fracture: Weakest link or process zone control?

Does brittle fracture instability start from a single event such as a cracked carbide or sulfide or does it start from a series of discontinuous cleavage events, which coalesce together? In fact, either of these may occur depending on the test temperature (low or high), the amount of stored elastic energy at a crack tip (blunt or sharp), and the microstructure of brittle second phases (coarse or fine). The present study was to sort out which of these processes occurs in a coarse-grained high strength, low alloy (HSLA) steel tested at—80 °C (193 K). Statistical distributions of grains and fracture origins were identified in this 108 µm grain size steel. This was accomplished with compact tension specimens equipped with plezoceramic transducers to evaluate microcleavage onset. Termination of the test at various stress intensity levels, KI, followed by fatigue cracking at ΔK << K,I, allowed isolation of multiple cleavage origins. Both the amount of cleavage fracture and a preliminary estimate of fracture toughness could be interpreted in terms of process zone concepts. These rely on a local cleavage fracture stress controlled by microgeometry and a quasi-static equilibrium governed by the process zone size and strength. The conclusion for the steel in the present study is that a discontinuous process zone approach is most descriptive of the stochastic events leading up to brittle fracture.

Some observations on the assessment of short, sharp cracks at the roots of blunt notches, and their effect on brittle fracture

The paper is concerned with effects of short, sharp quench cracks at the roots of blunt notches on cleavage fracture in a coarse-grained martensitic microstructure. At test temperatures between −196 and −140°C, such cracks control cleavage fracture. The values of local stress intensities calculated by loading such short cracks with the stress field ahead of the blunt notch are closely similar to the values of K IC measured in long-crack fracture toughness tests. At the test temperature of −100°C, such sharp cracks are blunted out (by ductile crack growth) and appear to play no part in the subsequent failure mechanism, which is now better characterised by values of the local cleavage fracture stress ω F * , below the notch. At the test temperature of −120°C, a ‘transition’ region is seen with a large observed scatter in faulure load. In a fine-grained martensitic microstructure tested in the ST-L orientation brittle plates of manganese sulphide inclusions have been shown to promote cleavage fracture at −196°C.