Mechanism Of Cleavage Fracture Research Articles

Constraint Effects for a Reactor Pressure Vessel Subjected to Pressurized Thermal Shock

Transferability of fracture toughness data obtained on small scale specimens to a full-scale cracked structure involves both in-plane and out-of-plane constraint effects. Both in-plane and out-of-plane constraint effects of a crack in a reference reactor pressure vessel (RPV) subjected to pressurized thermal shock (PTS) are analyzed by two-parameter and three-parameter methods. T11 (the second term of William's extension acting parallel to the crack plane) generally displays a reversed relation to the stress intensity factor (SIF) with the transient time, which indicates that the loading (SIF) plays an important role on the in- plane constraint effect. T33 (the second term of William's extension acting along the thickness) displays a different relation to T11 during the transient. The results demonstrate that both in-plane and out-of-plane constraint effect should be analyzed separately in order to describe precisely the stress distribution ahead of the crack tip. The local approach to fracture, i.e. σ*-A* model is used to predict the in-plane and out-of-plane constraint effect by considering the micro mechanism of cleavage fracture.

Fatigue crack growth of titanium alloy joints by electron beam welding

Electron beam welding (EBW) has been widely used in the manufacture of titanium alloy welded blisk for aircraft engines. Based on fatigue crack growth tests on titanium alloy electron beam welding (EBW) joints, mechanism of fracture was investigated under scanning electron microscope (SEM). The results show that fatigue crack growth rate increases as the experimental load increases under the same stress ratio and stress intensity factor range. At the beginning of crack growth, the extension mechanism of fatigue crack is the typical mechanism of cleavage fracture. In the steady extention stage, crack extends along the weld seam firstly. Then, crack growth direction changes to extend along the base metal. The extension mechanism of fatigue crack in the weld seam is the main mechanism of cleavage fracture and the extension mechanism of fatigue crack in the base metal is the main extension mechanism of fatigue band. In the instantaneous fracture stage, the extension mechanism of fatigue crack is the typical dimple-type static fracture mechanism. Crack growth was simulated by conventional finite element method and extended finite element method.

Low temperature mechanical properties of as-extruded Mg–10Gd–3Y–0.5Zr magnesium alloy

Influence of multi-cycle cryogenic treatment and tensile temperature on microstructure, mechanical properties and fracture mechanism of as-extruded Mg–10Gd–3Y–0.5Zr magnesium alloy was investigated. The results show that there have no significant changes in tensile properties of the tested alloy after 10 d in liquid nitrogen immersion or 10 cycles of high-low temperature treatment at all test temperatures. The room temperature ultimate tensile strength increases from 398 MPa to 417 MPa after 20 cycles of high-low temperature treatments. Compared with the room temperature, the tested alloys exhibit higher tensile properties at low temperatures. At −196 °C, the yield strength and ultimate tensile strength of the as-extruded-T5 Mg–10Gd–3Y–0.5Zr alloy are 349 MPa and 506 MPa, respectively, increasing by about 18% and 27%, respectively. The transgranular cleavage fracture mechanism is observed at room temperature, while at low temperatures both ductile fracture and cleavage fracture behaviors coexist.

Study on the static and dynamic fracture mechanism of different casing-drilling steel grades

Abstract The tensile and impact properties of three different types of casing drilling steels were investigated. Results show that P110 exhibits a higher yield and ultimate tensile strength compared with the N80 and K55 specimens. All tensile fractures follow the dimple fracture mechanism. The impact toughness values of N80 and P110 steels show no remarkable difference. However, both N80 and P110 absorb up to about five times the impact energy absorbed by the K55 specimen. The impact fractures of the K55 and P110 steels exhibit cleavage and ductile fracture mechanisms, respectively, whereas N80 steel primarily follows quasi-cleavage and ductile fracture mechanisms. The microstructures underneath the tensile fracture surface show noticeable deformation compared with those underneath the impact fracture surface. K55 steel microstructure variance in the tensile process is affected by the size of the inclusions as well as by the orientation relative to the inclusion and the normal stress.

Grain boundary characteristics and tensile properties of Ti14 alloy after semi-solid deformation

The microstructure and room-temperature tensile properties of Ti14, a new α+Ti2Cu alloy, were investigated after conventional forging at 950°C and semi-solid forging at 1000 and 1050°C, respectively. Results show that coarse grains and grain boundaries are obtained in the semi-solid alloys. The coarse grain boundaries are attributed to Ti2Cu phase precipitations occurred on the grain boundaries during the solidification. It is found that more Ti2Cu phase precipitates on the grain boundaries at a higher semi-solid forging temperature, which forms precipitated zones and coarsens the grain boundaries. Tensile tests exhibit high strength and low ductility for the semi-solid forged alloys, especially after forging at 1000°C. Fracture analysis reveals the evidence of ductile failure mechanisms for the conventional forged alloy and cleavage fracture mechanisms for the alloy after semi-solid forging at 1050°C.

Effects of the strain-hardening exponent on two-parameter characterizations of surface-cracks under large-scale yielding

The influence of strain hardening exponent on two-parameter J–Q near tip opening stress field characterization with modified boundary layer formulation and the corresponding validity limits are explored in detail. Finite element simulations of surface cracked plates under uniaxial tension are implemented for loads exceeding net-section yield. The results from this study provide numerical methodology for limit analysis and demonstrate the strong material dependencies of fracture parameterization under large scale yielding. Sufficient strain hardening is shown to be necessary to maintain J–Q predicted fields when plastic flow progresses through the remaining ligament. Lower strain hardening amplifies constraint loss due to stress redistribution in the plastic zone and increases the ratio of tip deformation to J. The onset of plastic collapse is marked by shape change and/or rapid relaxation of tip fields compared to those predicted by MBL solutions and thus defining the limits of J–Q dominance. A radially independent Q-parameter cannot be evaluated for the low strain hardening material at larger deformations within a range where both cleavage and ductile fracture mechanisms are present. The geometric deformation limit of near tip stress field characterization is shown to be directly proportional to the level of stress the material is capable of carrying within the plastic zone. Accounting for the strain hardening of a material provides a more adjusted and less conservative limit methodology compared to those generalized by the yield strength alone. Results from this study are of relevance to establishing testing standards for surface cracked tensile geometries.

Studies of fracture processes in Cr–Mo–V ferritic steel with various types of microstructures

In this paper, the authors report on analysis of the influence of microstructure on ductile and cleavage fracture mechanisms. The question investigated was whether microstructure observations alone can provide sufficient information to predict the possible fracture mechanism or change in fracture mechanism. Four different microstructures of ferritic steel were tested after four different heat treatments. The microstructures examined were ferritic, ferritic–pearlitic, ferritic–bainitic, and tempered martensitic types. It was concluded that the ratio ( S C/ S 0) of the area covered by carbides to the total area of a ferritic grain (measured by taking into account large carbides) is the only possible quantitative measure that can be used to predict cleavage fracture.

The ductile-to-cleavage transition in ferritic Cr–Mo–V steel: A detailed microscopic and numerical analysis

In this study, the transition from a ductile fracture mechanism to a cleavage fracture mechanism was reanalyzed. Microscopic observations of the domains in front of the cracks were verified by numerical analysis of the stress and strain fields. The stress and strain analyses were performed at different stages of crack propagation: before the onset of a stable crack growth, at the beginning of a stable crack growth, before the onset of a cleavage jump, and after a cleavage jump. Both the influence of the temperature and the in-plane constraint on the mechanisms changes were taken into account. The numerical analyses were carried out using two different crack models: stationary and growing. It was assumed that the strains are finite. The complete yielding of the ligament was observed. It was shown that the actual fracture mechanism depends on both the stress level, which must be greater than the critical value, which depends on the material and temperature, and the critical length, which is a material characteristic.

The Analysis of Fracture Mechanisms of Ferritic Steel 13HMF at Low Temperatures

Abstract In this paper, the transition from a ductile fracture mechanism to a cleavage fracture mechanism is analyzed. Microscopic observations of the domains in front of a crack were followed by numerical analysis. The microscopic observations were accompanied by analysis of the fracture surfaces. Both the influences of temperature and an in-plane constraint on the mechanism transition were taken into account. The numerical analyses were carried out using two different models of a crack: Stationary and propagating. The direct, simultaneous influence of the level of the opening stress maximum, the characteristic distance in front of the crack tip, and the level of plastic strain on the ductile-cleavage transition have been confirmed.

Micromechanism of cleavage fracture at the lower shelf transition temperatures of a C–Mn steel

This paper investigates the mechanism of cleavage fracture in precracked specimen made of Q-T C–Mn steel at the lower shelf of ductile-to-brittle transition curves. The results reveal that the lower shelf region can be divided into three domains: (1) at the lower temperature end (−150 to −130°C), cleavage is controlled by crack nucleation in the microscopic scale and is driven by increasing applied load which blunts the precrack tip in the macroscopic scale; (2) at moderate low temperatures (−130 to −90°C) cleavage fracture is controlled by propagation of a second phase particle-crack into contiguous grains in the microscopic scale and is also driven by increasing applied load which further blunts the precrack tip in the macroscopic scale; (3) at the upper temperature end (−90 to −60°C) the cleavage fracture is controlled by the propagation of the carbide crack into contiguous grains in the microscopic scale, yet in addition to the crack tip blunting for compensating the drop of the yield stress a short fibrous crack is needed to extend from the precrack tip to increase the peak normal stress and to move it closer to the precrack tip.The results also reveal that work hardening plays a key role in triggering cleavage fracture in this temperature region. The normal stress induced by dislocation pile-up impingement, which greatly enhances the applied normal stress, is the essential part of the cracking mechanism and is enhanced by increasing the applied load through strain-hardening effects.

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.

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.

Investigation on the Weibull parameters identification for local approach application in the ductile to brittle transition regime

Fracture of BCC metals is a temperature dependent process characterized by a temperature transition regime where material failure is the result of the competing action of two different mechanisms of rupture, namely, cleavage and ductile fracture. In this paper, two micromechanics based methodologies, the local approach and a non-linear CDM model, are used to model fracture processes in the temperature transition regime for a low alloy steel. The study revealed that approaching the nihil ductility temperature (NDT), the identification procedure for the material parameters needed in the local approach becomes no longer stable resulting in a non unique set of σ W and m values. This occurrence can be used to discriminate in the experimental data set those data failed in a non-complete brittle manner. Using m exponent equal to 4, a linear dependency, between σ W and K J , also in upper shelf regime, is demonstrated.

Spark plasma sintering reaction synthesized TiB reinforced titanium matrix composites

Abstract The in situ TiB particle reinforced titanium metal matrix composites were prepared using MA followed by park plasma sintering (SPS) with a heating rate of 50 °C/min. The in situ TiB were synthesized by chemical reaction between TiB2 and Ti during the SPS process. The microstructure of the composites sintered at 800, 1000 and 1200 °C were investigated. TiB whiskers are needle-shape and uniformly dispersed in the titanium matrix. The interface between the in situ TiB needles and the titanium matrix is singular and faceted. The composite sintered at 1000 °C has a maximum value of relative density of 99.6%, and good mechanical properties with a flexural strength of 1007 MPa, Young's modulus of 146 GPa and fracture toughness of 8.64 MPa·m1/2, respectively. The fractographs of all composites exhibit a predominantly brittle cleavage fracture mechanism. The elastic modulus of synthesized TiB was estimated.

Observations of cleavage initiation at (Ti,V)(C,N) particles of heterogeneous composition in microalloyed steels

The mechanism of cleavage fracture initiated by microcracking of (Ti,V)(C,N) particles of heterogeneous composition has been investigated in Ti–V–N and V–N microalloyed forging steels. Preferential microcracking of the (Ti,V)(C,N) particles was observed to occur across the interface which separates regions richer in V from lower V content regions.

Insitu TEM studies of the mechanisms of crack nucleation and propagation in fully lamellar microstructures

The mechanisms of crack nucleation and propagation of the lamellar microstructures found in pearlite were studied by in situ transmission electron microscopy. It was found that, in addition to the predominant mechanism of cleavage fracture in the cementite plates, ductile fracture of the cementite plates is another cracknucleation mechanism. The process of failure in the fully lamellar microstructure studied in the present investigation consists of four stages: dislocation pile-ups; single crack nucleation; distribution of crack nucleation; and crack growth, followed by final fracture. Through this study, a description of the lamellar microstructure fracture mechanism has been developed for numerical simulation in future studies.

Development of the Euro fracture toughness dataset

Ten European laboratories have generated the Euro fracture toughness dataset in order to provide an experimental data base sufficiently large to study specimen size and temperature effects on cleavage fracture toughness in the ductile-to-brittle fracture transition regime. The Euro fracture toughness dataset quantifies the fracture behaviour of the quenched and tempered pressure vessel steel DIN 22NiMoCr37. This material is frequently used in nuclear power plants. About 800 fracture toughness tests were performed using compact tension specimens with a size range from 1/2T to 4T. In the lower shelf temperature regime, no significant specimen size effects on cleavage fracture toughness scatter was observed. At higher temperatures, the lower tails of the toughness scatter bands are not significantly effected by the specimen size but with decreasing specimen size the toughness scatter increases due to the fact that the upper part of the scatter band is extended. The presence of a specimen size effect on fracture toughness scatter coincides with the appearance of single cleavage initiation sites at the fracture surface. At the lower shelf temperature both, cleavage initiation sites and size effects are not observed whereas at higher test temperatures both phenomena are present. The specimen size effect trends and the corresponding fracture surface morphology support a weakest-link type cleavage fracture mechanism in the ductile-to-brittle transition regime. A unique correlation between the amount of ductile tearing and cleavage fracture toughness was observed for the steel investigated. This result offers the possibility to determine cleavage fracture toughness from post-test fracture surface examinations. Due to the large number of tests and the wide range of testing conditions, the Euro fracture toughness dataset gives a comprehensive insight into specimen size effects and temperature effects on ductile-to-brittle transition fracture. The Euro fracture toughness dataset includes a large set of raw test data such as load versus load line displacement curves and raw tensile test data for deriving stress–strain curves. The Dataset can be downloaded from the internet via the address ftp://ftp.gkss.de/pub/eurodataset.

Application of acoustic emission to the study of cleavage fracture mechanism in a HSLA steel

Application of acoustic emission to the study of cleavage fracture mechanism in a HSLA steel

Cleavage fracture in bainitic and martensitic microstructures

This paper addresses the mechanisms of cleavage fracture in the pressure-vessel steel A533B. Microstructures of single bainite, single auto-tempered martensite and a mixture of bainite-plus-martensite have been investigated. Upper-bainite microstructures exhibit a higher propensity for brittle cleavage fracture than do those of auto-tempered martensites. The K 1 c values of mixed microstructures are determined by the statistical distribution of the two phases and the range of the values is bounded by limits set by those for the single-phase microstructures. The results are explained in terms of the RKR model, which involves a local cleavage stress σ F * and a distance ahead of the macrocrack tip, X, as two critical parameters. It is found that the carbides or carbide “colonies” act as critical microcrack nuclei, and hence play a key role in determining the fracture toughness, although packet boundaries in bainite may give rise to “pop-in” arrests in displacement-controlled tests.

Advances in the mechanism of cleavage fracture of low alloy steel at low temperature. Part I: Critical event

The critical event of cleavage is variable for different types of specimens made of the same steel. In notched specimens (Charpy V or 4 PB) over a wide temperature range as low as -196°C, the critical event is the propagation of a ferrite grain-sized crack (30-40µm). In precracked specimens at a moderately low temperature (around -110°C) it is the propagation of a second phase particle-sized crack (< 10µm). At ever lower temperatures (-150°C - -196°C) the cleavage fracture is nucleation-controlled.