Cleavage Crack Propagation Research Articles

Estimating crack arrest under accident conditions

Initiation and arrest stress intensities for cleavage crack propagation in thermal-shock and pressurized-thermal-shock experiments appear to be linearly related. The relation can be used as a predictive tool, provided the fracture mechanism is cleavage.

Cleavage fracture behaviour of carbon steels for different ferrite–pearlite contents

The effect offerrite–pearlite content on the cleavage fracture behaviour of carbon steels was determined via plain tensile tests at temperatures from 12 to −196°C. It was found that, under conditions of uniaxial tension within the test temperature range, the cleavage fracture stress for pearlitic eutectoid steel is independent of temperature, whereas that for mild steel changes with temperature and exhibits a depression at low temperatures. An increase in the proportion of pearlite enhances the effect of pearlite colonies as obstacles to cleavage crack propagation. Only when the volume fraction of pearlite is comparable to that of ferrite does the steel begin to display the fracture features of pearlitic steel. Possible mechanisms manifesting different fracture characteristics for steels of various ferrite–pearlite contents are discussed with the aid of fractographic observations.MST/1075

Cleavage to quasicleavage fracture transition in steels

The cleavage behaviour of plain carbon steels containing from 0· 2 to 0·8%C has been investigated. It is observed that each steel displays two characteristic temperatures at which a transition in the mode of fracture occurs. These are the transition temperatures for cleavage Tc and for general yielding Tg. At temperatures below Tc, the steels fail by pure cleavage. This involves the generation of a cleavage crack nucleus in a carbide particle followed by cleavage crack propagation. The cleavage fracture stress σ f is independent of temperature. Between temperatures Tc and Tg, the steels fail by quasicleavage. This involves the generation of a crack nucleus by a localised fibrous process followed by cleavage crack propagation. The crack nucleation stage is shear stress controlled and therefore the quasicleavage fracture stress σq increases with decreasing test temperature. Above temperature Tg, failure occurs at or after general yielding.MST/1045

An examination of cyclic grain boundary migration and cavitation in an Al3Mg solid solution alloy: Raman, V. and Langdon, T.G. Acta Metall. Feb. 1989 37, (2), 725–737

The purpose of this study was to analyze the microstructure of lath martensite in 0.1C–1.1Si–1.7Mn (wt.%) steel and its effect on mechanical properties and fracture behavior. The microstructure was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back scattering diffraction (EBSD). Charpy V-notch impact samples and compact tension (CT) samples were used to investigate the Charpy impact properties and fatigue crack growth behavior of the steel, respectively. The propagation of cleavage crack and fatigue crack were analyzed to figure out the effective grain size. The results showed that the typical hierarchical lath martensite structure contained prior austenite grains, packets, blocks and laths; packet size and block width were positively correlated to prior austenite grain size, while lath width was maintained at about 0.29 μm. Yield strength was related to prior austenite grain size, packet size and block width, and obeyed Hall–Petch relationship. Grain refinement was effective in improving the resistance to cleavage fracture by introducing barriers to crack propagation; packet boundaries and block boundaries hold similar ability to impede the propagation of crack. Paris model can well describe the FCG behavior of the investigated steel. Block width governs the effective grain size for strength, toughness and fatigue crack propagation.

Fish-eye fracture in high-cycle fatigue at elevated temperatures: Kanazawa, K., Sato, M., Kimura, M. and Nishijima, S. J. Soc. Mater. Sci., Jpn. Dec. 1988 37, (423), 1423–1428 (in Japanese)

The purpose of this study was to analyze the microstructure of lath martensite in 0.1C–1.1Si–1.7Mn (wt.%) steel and its effect on mechanical properties and fracture behavior. The microstructure was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back scattering diffraction (EBSD). Charpy V-notch impact samples and compact tension (CT) samples were used to investigate the Charpy impact properties and fatigue crack growth behavior of the steel, respectively. The propagation of cleavage crack and fatigue crack were analyzed to figure out the effective grain size. The results showed that the typical hierarchical lath martensite structure contained prior austenite grains, packets, blocks and laths; packet size and block width were positively correlated to prior austenite grain size, while lath width was maintained at about 0.29 μm. Yield strength was related to prior austenite grain size, packet size and block width, and obeyed Hall–Petch relationship. Grain refinement was effective in improving the resistance to cleavage fracture by introducing barriers to crack propagation; packet boundaries and block boundaries hold similar ability to impede the propagation of crack. Paris model can well describe the FCG behavior of the investigated steel. Block width governs the effective grain size for strength, toughness and fatigue crack propagation.

Mechanical properties and special features of failure of carbides and nitrides of metals of groups IV-V deposited from the gas phase

Brittle fracture of the carbides of the metals of the groups IV–V in the Tbr range is preceded by slight microplastic deformation which reduces the rate of cleavage. The fracture resistance increases and reaches a maximum value. The reduction of the grain size of the carbides has an effect on the propagation of cleavage cracks as a result of the consumption of energy required to overcome the grain boundaries. According to E. Orowan [16], the crack which intersects the boundary splits into a large number of microcracks which propagate in parallel cleavage planes. The fracture resistance increases in these conditions.

Performance of low-upper-shelf material under pressurized-thermal-shock loading

The second pressurized-thermal-shock experiment (PTSE-2) of the Heavy-Section Steel Technology Program was conceived to investigate fracture behavior of steel with low ductile-tearing resistance. The experiment was performed in the pressurized-thermal-shock test facility at the Oak Ridge National Laboratory. PTSE-2 was designed primarily to reveal the interaction of ductile and brittle modes of fracture and secondarily to investigates the effects of warm prestressing, A test vessel was prepared by inserting a cracklike flaw of well-defined geometry on the outside surface of the vessel. The flaw was 1 m long by ≈ 15 mm deep. The instrumented vessel was placed in the test facility in which it was initially heated to a uniform temperature and was then concurrently cooled on the outside and pressurized on the inside. These actions produced an evolution of temperature, toughness, and stress gradients relative to the prepared flaw that was appropriate to the planned objectives. The experiment was conducted in twoseparate transients, each one starting with the vessel nearly isothermal. The first transient induced a warm-prestressed state, during which K I , first exceeded K Ic . This was followed by repressurization until a cleavage fracture propagated and arrested. The final transient was designed to produce and investigate a cleavage crack propagation followed by unstable tearing. During this transient, the fracture events occurred as had been planned.

Statistical model of cleavage crack propagation in spheroidized steels

This contribution presents a model of cleavage crack propagation in spheroidized steels. Microcrack initiation is considered in precipitated carbide particles when the local stress exceed cleavage strength σ f of the carbide given by the Griffith criterion. A major crack is formed by microcrack linking leading to the final fracture instability. Using crack transition probabilities the probability of crack arrest at any given point within the active volume was calculated. The model suggests that the susceptibility of a microstructure to cleavage failure can be assessed by the least value of ( x sσ 2 y K 2 IC ) parameter at which the probability of crack arrest P ii is equal to unity.

Crack opening profile observations for dynamic cleavage crack propagation and arrest

T his paper presents measurements of the crack opening profile during cleavage crack extension and arrest in A533B Class 1 steel specimens subject to a stress and temperature gradient. The crack opening profiles were measured by two independent methods: (1) displacement transducers using Hall effect sensors that measured the crack opening profiles away from the crack tip and (2) a post-test fractographic technique (the fracture surface topography analysis, FRASTA) that reconstructed the details of the fracture process and measured the near crack tip opening profile. The measured evolution of the crack opening profiles during the experiment was consistent with the calculated stress intensity history and provided a basis for interpreting the crack propagation and arrest behaviors. This observation suggests that the crack opening displacement and crack opening angle concepts are promising parameters to characterize dynamic fracture in elasto-plastic or visco-plastic materials. Under the conditions prevailing in the present experiments (increasing toughness, decreasing crack speed) the cleavage crack maintained a sharp tip during propagation. The crack fully arrested before significant blunting of the tip occurred because of dynamic reloading. Comparison of the crack opening profiles measured with either of the two independent methods demonstrated that during dynamic fracture events useful estimates of the near crack tip opening profile can be obtained from remote crack face displacement measurements.

Wide-plate crack-arrest tests utilizing a prototypical pressure vessel steel

Wide-plate crack-arrest tests are being performed at the National Bureau of Standards (Gaithersburg, MD) under the Heavy-Section Steel Technology (HSST) Program and are designed to provide fracture-toughness measurements at temperatures approaching or above the onset of the upper-shelf regime, in a rising toughness region and with increasing driving force. The test specimens are 1 × 1 × 0·1 m and possess a single-edge notch (crack) that initiates in cleavage propagation at low temperature and arrests in a region of increased fracture toughness. The toughness is achieved through a linear transverse temperature profile across the plate. Results obtained using a prototypical reactor pressure vessel steel (A533 grade B class 1 material) exhibit a significant increase in toughness at temperatures near and above the onset of Charpy upper shelf. Additionally, cleavage crack propagation and arrest at temperatures above the onset of Charpy upper shelf have been demonstrated.

Statistical analysis of cleavage fracture ahead of sharp cracks and rounded notches

Mechanisms of initiation and unstable propagation of transgranular cleavage cracks are compared for brittle fracture ahead of sharp cracks and rounded notches, e.g. for fatigue pre-cracks and Charpy V-notches, respectively, in standard toughness specimens. The comparison is made over a range of temperatures, from the lower shelf into the ductile/brittle transition region, for a single phase material containing a known distribution of particles where weakest link statistics can be used to model the onset of catastrophic failure. Using linear and nonlinear elastic solutions for the stress distribution ahead of a sharp crack, and slip-line field solutions, modified for a power hardening material, for the rounded notch, statistical modelling is employed to define the critical dimensions ahead of the crack or notch tip where initial cracking events are most probable. The analysis provides an interpretation of the role of stress gradient in governing microscopic fracture behavior. Predictions are evaluated by comparison with experimental results on the low temperature flow, Charpy V-notch and plane strain fracture toughness behavior of a low carbon mild steel with simple ferrite/grain boundary carbide microstructures.

Effective grain size of dual-phase steel

The effect of second-phase martensite on cleavage crack propagation was studied to define the effective grain size of dual-phase steels which controls the ductile-to-brittle transition temperature. It has been shown that the introduction of second-phase martensite into the ferrite matrix has no effect on changing the direction of cleavage crack propagation; therefore the effective grain size of dual-phase steels is the same as that of the initial structure prior to two-phase annealing, regardless of changes in the ferrite mean free path.

Fractographic aspects of cyclic cleavage

A number of aspects of cleavage crack propagation occurring during fatigue were identified from a fractographic study of low cycle fatigue samples of a quenched renitrogenized mild steel, tested at 20 and 100°C. The most striking aspect was the frequent presence of cyclic cleavage striations, whose visibility increased with an increase in the plasticity accompanying cleavage. Cyclic cleavage results in a locally accelerated crack growth rate and often in a local crack front. Increasing hindrance to the continuation of cyclic cleavage often results and is responsible for transitions from brittle to semibrittle to ductile striations. This observed continuity between the three types of striation strongly indicates that the first two types of striation correspond to temporary crack arrest sites.

Nucleation and propagation of cleavage fracture in iron

The effect of oxygen segregation and of oxide precipitation on the initiation and propagation of cleavage cracks at 77 K has been studied in binary iron-oxygen alloys. Results indicate that fracture is initiated by intergranular cracks, which are nucleated by the intersection of slip initiating from the surface with the segregated grain boundary. It has been shown that for low oxygen contents, the fracture is coincident with general yield and is classified as initiation controlled cleavage. The nucleation of cracks in alloys containing oxide inclusions takes place by the growth of voids along associated grain boundaries. The dispersed oxide particles (~ 1 μm) promote local plasticity which effectively raises the plastic work associated with crack propagation. It has been shown that fracture at high oxygen levels is growth controlled cleavage and can only be achieved at stresses exceeding the general yield stress. A direct correlation has been established between the macroscopic behaviour characterized by the ratio σf σy and the micromechanisms of cleavage fracture. Based on these observations a fracture identification diagram for iron at 77 K has been proposed.

The effective fracture energy for cleavage crack propagation in B.C.C. iron

The effective fracture energy for cleavage crack propagation in b.c.c. iron is generally believed to be appreciably greater than the macroscopic surface energy. It has been suggested that this is because dislocations are nucleated and yet do not move beyond a few lattice spacings from the crack tip where they would be able to propagate to much greater distances and thereby to blunt the crack tip. As the crack tip moves forwards, these dislocations are assumed to move back towards the crack faces as a result of image forces. In this paper we analyse a simple model that simulates the effects of this incipient shear deformation and demonstrate that the effective fracture energy can indeed be increased appreciably by such a deformation.

Electron microscope study of surface markings left by unsteady cleavage crack propagation in NaCl single crystals

Abstract Surface markings left by an unsteadily propagating cleavage crack in NaCl single crystals have been studied by electron microscopy as a function of accelerations and decelerations of the crack. As a result, a comprehensive description of the sequence of markings present between two successive stopped crack fronts has been obtained. Models arc proposed illustrating the manner in which the activation of the longitudinal and transverse slip systems and their interaction can account for the cleavage markings observed.

Tensile Properties and Fracture of Two Phase Iron Alloy of Austenite and Ferrite

The tensile properties and fracture behaviors of 32 Cr-8 Ni steel with ferrite matrix and stable austenite phase were studied at room temperature. The specimens were prepared in two different conditions: The one consisting of ductile ferrite and austenite was obtained by quenching and the other having strengthened ferrite was obtained by ageing at 475°C after quenching. The grain size of the specimens varied from 0.8 to 26.9μ. The main results obtained are as follows:(1) For the aged specimens, 0.2 proof stress of the coarse grained specimens is below the value expected from the results of the fine grained specimens by extrapolation using the Petch's relation. This may be caused by the formation of deformation twins in the ferrite of the coarse grained specimens.(2) The fracture surfaces of all the quenched speimens show dimples, while those of the aged ones show both quasi-cleavage and dimples. Quasi-cleavage fracture occurs in the initial stage of fracture irrespective of grain size and is characterized by initiation and propagation of cleavage cracks in ferrite, followed by tear or dimple fracture in austenite. In the specimens with coarse grain, a crack propagates in the same manner as mentioned above until the final fracture. However in the specimens of fine grain the fracture by progress of quasi-cleavage cracks formed in the initial stage changes to dimple fracture in both ferrite and austenite. The size of this dimple region increases with a decrease in grain size and the reduction in area also increases with an increase in dimple region.

Fracture Toughness Phenomena in an Unaged Beta Titanium Alloy

The fracture toughness behavior of a commercial beta titanium alloy, Ti-3Al-8V-6Cr-4Zr-4Mo, has been examined between −196°C and 380°C, The observed tough-brittle transition results from increasing amounts of intergranular and cleavage fracture with decreasing temperature. Crack initiation is associated with particle/matrix interface parting, while cleavage crack propagation at low temperature requires the attainment of a critical stress over a characteristic distance. Micromechanical modeling of the principal fracture event associated with critical stress cleavage crack propagation does appear to accurately predict the magnitude of this distance, although it is limited to initiation controlled cleavage fracture events.

Initiation of crack propagation in KCl

Abstract Initiation of {100} cleavage crack propagation in single-crystal KCl was studied using a constant-moment, double-cantilever beam technique at 22°C and 40–50% relative humidity. KCl crystals doped with Sr+2 or γ-irradiated to increase their yield strengths exhibited variations in fracture energy (γc) versus critical resolved shear stress (τy) lying within the boundaries γc α τy 2 and γc independent of τy. Slip on two {110} ⟨110⟩ systems was observed to intersect the cleavage cracks with increased applied stress. The fracture energy measured for crystals having constant τy was dependent on the loading rate and the presence of mobile dislocations. Dislocation mechanisms for crack tip blunting and for initiation of crack propagation described the observed fracture energy-critical resolved shear stress behaviour.

Propagation of cleavage cracks in single crystals of lithium fluoride

Propagation of cleavage cracks in single crystals of lithium fluoride