Ductile-brittle Transition Curve Research Articles

Onset of dynamic void coalescence in porous ductile solids

This paper investigates void growth and coalescence in porous ductile solids under dynamic loading condition. A physical definition for the onset of void coalescence in porous ductile solids under dynamic loading is proposed. The onset is deemed to occur when the third invariant of the tensorial form of the Hill–Mandel condition attains a zero value. The definition allows for systematic investigations on the effects of dimensionless stress rate κ and stress state, defined by the stress triaxiality T and Lode parameter L, on the onset of void coalescence via micromechanical analyses. The analyses reveal that the critical macroscopic effective strain for the onset of void coalescence displays an increasing–decreasing transition trend as the dimensionless stress rate increases, for all levels of T and L considered. The macroscopic effective strain at the transition is identified as the “ductile–brittle” transition strain. The dimensionless stress rate at which the transition strain occurs is found to be relatively constant. A mapping in the κ−T space for L=−1, representative of a generalized uniaxial tension typical in spall fracture experiments, is established which depicts regions where coalescence and non-coalescence can take place, as well as the ductile–brittle regions demarcated by a ductile–brittle transition curve. The results also show that the critical void volume fraction and macroscopic effective strain at the onset of void coalescence are insensitive to inertia at high stress triaxialities at L=−1.

Influence of long-term operation of the 17H1S steel on the main gas pipeline on the change of the mechanical properties

The characteristics of strength, ductility and impact toughness of 17H1S steel in the initial state and after 31 years of operation on the main gas pipeline are analysed in terms of their sensitivity to degradation. It is shown that resistance to brittle fracture is the most sensitive of them to changes in the technical state of the metal. Ductile-brittle transition curves were plotted for both steel variants and fractographic features of specimen fracture at temperatures from +20 to –60 ° C. An increase in the temperature of ductile-brittle transition of steel from –40 to –30 ° C after its operation has been established. The ratio of the crystalline relief on the fractures of impact specimens is estimated. It has been established that the obtained data on the mechanical (brittle fracture resistance) and fractographic (fraction of the crystalline relief of the fracture) indicators correlate well with each other. In particular, with a decrease in the index of the mechanical state of the metal due to its operation, the fraction of the crystalline relief at the fracture increases. The data obtained from this study correlate with the previously obtained data for the steels of other gas pipelines.

Influence of long-term high-temperature action on impact toughness of base metal and weld metal of 22K steel welded joint

One of the applications of construction low-carbon 22K steel (AISI 1022 type) is as a material for the vessel of a core catcher (CC) for nuclear power plants with VVER reactors. In the event of severe beyond design basis accident, the CC-vessel will be under conditions of prolonged hightemperature impacts, which can significantly change the structural state and lead to degradation of mechanical properties of the vessel material. Data on the effect of such actions on the mechanical properties and fracture resistance of welds (the properties of which usually differ from those of the base metal) from low-carbon steels are very limited in the literature. This makes it difficult to guarantee the reliability and safety prediction of CC. The purpose of this work was to carry out the comparative Charpy V-notch impact tests of the samples of base metal and weld metal of the 22K steel welded joint before and after long-term high-temperature heat treatment, simulating the thermal effect on the reactor vessel material of nuclear power plants during severe accidents. Welded joints of 22K steel sheets were obtained by the method of automatic argon-arc welding with a consumable electrode (welding wire SV-08G2S was used) in accordance with PNAE G-7-009–89. Based on the test results, the ductile–brittle transition curves were plotted and analysis of fracture surfaces after tests was carried out. The influence of structural factors on the impact toughness has been studied. It is shown that prolonged high-temperature exposure leads to an increase in the temperatures of beginning and end of the ductile-brittle transition by 30 – 50 °C and to the expansion of range of the ductile-brittle transition temperature by 15 – 25 °C of both base metal and weld metal of the welded joint.

Characterization of the local brittle layer formed in electro-gas weld metals

The microstructural characteristics of the local brittle layer (LBL) reported to be formed at the weld center of tandem electro-gas (EG) weld made with EG70T-G wire have been investigated to identify the critical factors causing its brittleness and thus facilitate the development of welds free from the local brittle layer. The results showed that in addition to the directional characteristics of columnar grain structures, the proportion of grain boundary ferrite played an important role in LBL formation and in controlling the impact toughness of electro-gas welds at the weld center. These results led to investigate the additional electro-gas welds containing less amounts of grain boundary ferrite with an aim to improve the toughness at weld center. In addition, the relative brittleness of the local brittle layer was evaluated in terms of Charpy V-notch ductile-brittle transition curves and a stepwise transition unusually obtained from local brittle layer was discussed from the perspective of the microstructural characteristics of LBL.

Construction of correlation dependences between the results of standard and sub- size Charpy specimens impact tests

The object of this paper is a construction of correlation dependences between the results of standard and sub-sized Charpy specimens impact tests. The world experience shows, that the standard basis available in Ukraine needs improvements for more correct interpreting of strength and durability of the nuclear power-station 1-st contour elements. Nowadays different numerical and experimental methods are being developed and improved for solving the problems of broadening the power-stations equipment operation life looking for the additional margin of safety. Charpy specimens impact tests are one of more simple and inexpensive method to determine the mechanical properties of materials. The use of sub-size specimens for radiation embrittlement estimation of NPP’s reactor pressure vessel metal is very relevant due to saving of scarce metal. Material properties obtained from tests depend on the specimen size. It is necessary to build correlation dependencies between standard and sub-size Charpy specimens to obtain conservative results. A two series of Charpy specimens impact test from structural carbon steel 45 were carried out in the temperature range of –50 ÷ +150ºС. Tests were conducted on instrumented vertical impact machine, which can record a full diagram of specimen’s deformation and fracture. The results of these tests can be used to estimate the strength and durability of large-scale structures. An analysis of the impact tests results of standard and sub-size Charpy specimens from steel 45 were carried out. Approximation ductile-brittle transition curves from the function of the Oldfield hyperbolic tangency were constructed with definition of function parameters. The analysis of known correlation dependencies between the results of standard and sub-size Charpy specimens impact tests was carried out. The ORNL method was used to calibrate the test results of sub-size specimens from steel 45 and comparison with the results of standard Charpy specimens was fulfilled.

A constraint-equivalent approach for assessing fracture toughness of RPV steels under neutron irradiation

Neutron irradiation damages reactor pressure vessel (RPV) steels. As a result of material degradation, the mechanical properties of the material are changed and the crack tip stress and deformation fields are altered. The reduction of fracture toughness due to neutron irradiation is shown to be equivalent to a “constraint shift” exhibited by the ductile–brittle transition curve. Using the J–A2 two-parameter fracture theory to quantify the crack tip stress field and a critical crack opening stress as the fracture criterion, the fracture toughness of an irradiated RPV steel can be predicted from its nonirradiated value. The procedure and examples are given in the paper.

Evaluation of fracture behavior of recrystallized and aged high-Cr ODS ferritic steels

The effect of aging on the ductile to brittle transition behavior of recrystallized high-Cr ODS ferritic steels were investigated using small-punch test technique. In order to reduce the anisotropy of microstructure in the as-received ODS steels, they were recrystallized after cold rolling. The ODS steel without Al hardly recrystallized at 1200°C for 90min, while the ODS steel with Al significantly recrystallized. Ductile–brittle transition curves were drawn for as received, recrystallized, and thermally aged (at 450°C for 1440h). Before aging, both materials had good impact properties with low ductile–brittle transition temperatures and high USEs. The ductile–brittle transition temperatures are not dependent of the grain size. The aging degraded their impact properties for ODS steel with Al. However, almost no degradation occurred for the ODS steel without Al. SEM observation showed an influence of the cold-rolling direction on the fracture behavior especially at low temperatures.

Effect of crack depth on the shift of the ductile–brittle transition curve of steels

Nuclear reactor pressure vessel (RPV) steels degrade due to neutron irradiation during normal operation. As a result, the ductile–brittle transition curve of the steel shifts to higher temperature which decreases operation margins in both the temperature and pressure. The loss of these margins however can be offset somewhat by appealing to arguments based on constraint of potential/postulated shallow cracks. In this paper, it is demonstrated that the fracture toughness values for shallow flaws are higher than those determined from standard deep cracked test specimens based on constraint consideration. The J– A 2 three-term solution is used to characterize the crack-tip stress field where J represents the level of loading and A 2 quantifies the level of constraint. Based on the RKR cleavage model, procedures to quantify the temperature shift between specimens with different constraint levels are developed. The experimental data by Sherry et al. [Sherry AH, Lidbury DPG, Beardsmore DW. Validation of constraint based structural integrity assessment methods. Final report, Report No. AEAT/RJCB/RD01329400/R003, AEA Technology, UK, 2001] for the A533B RPV steel are used to demonstrate the procedure and it is shown that the ductile–brittle transition curve shifts to lower temperature from high constraint to low constraint specimens.

Static Strength of Cracked Square Hollow Section T Joints under Axial Loads. I: Experimental

A series of static strength tests were performed on full-scale precracked square-to-square hollow section welded T joints. These joints were subjected under brace end axial load until failure using a special purpose designed test rig. An alternating current potential drop technique was employed to monitor the crack propagation during the tests. In addition, three sets of linear variable displacement transducers were used to record the crack mouth opening displacements. The tests were performed at room temperature; therefore, the material is on the upper shelf of the ductile-brittle transition curve where failure occurs in a ductile manner. In all these tests, a stable ductile tearing was observed with a small tearing region. Based on the yield line theory, the plastic collapse load is estimated, and then applied to the failure assessment diagram (FAD) of BS7910. The results show that BS7910 Level 2A FAD provides an adequate procedure for the assessment of cracked square-to-square hollow section T joints.

Increased Temperature Margins Due to Constraint Loss

Enhanced levels of toughness due to loss of crack tip constraint have been related to temperature shifts in the ductile–brittle transition curve. An argument to quantify the temperature shift is developed using the self-similarity of near-tip stress fields under contained yielding combined with scaling techniques developed by Dodds and co-workers (12) for cleavage. This allows the temperature changes which give the same stress field at failure in constrained and unconstrained fields to be determined. The procedure is illustrated using the data of Sherry et al. (3) for an A533B pressure vessel steel. The results are consistent with empirical expressions proposed by Wallin (4), and enable a discussion of the micromechanics of cleavage.

Ductile to brittle transition of an A508 steel characterized by Charpy impact test: Part I: experimental results

This study is devoted to the ductile–brittle transition behavior of a French A508 Cl3 (16MND5) steel. Due to its importance for the safety assessment of PWR vessels, a full characterization of this steel with Charpy V-notch test in this range of temperature was undertaken. The aim of this study is to provide a wide experimental database and microstructural observations to supply, calibrate and validate models used in a local approach methodology. Mechanical and fracture properties of the steel have been investigated over a wide range of temperatures and strain-rates. Effects of impact velocity on ductile–brittle transition curve, on ductile tearing and on notch temperature rise are presented and discussed. A detailed study of ductile crack initiation and growth in Charpy specimens is also carried out. From fractographic investigations of the microvoids nucleation around carbide second phase particles, a plastic strain threshold for nucleation is determined for this material. A508 Cl3 steels undergo a transition in fracture toughness properties with temperature, due to a change in fracture mode from microvoids coalescence to cleavage fracture. A systematic investigation on the nature and the position of cleavage triggering sites and on any change in the ductile to brittle transition (DBT) range has been carried out. This leads to the conclusion that manganese sulfide inclusions do not play an increasing role with increasing test temperature as recently mentioned in other studies on A508 Cl3 steel with a higher sulfur content. In a companion paper [Tanguy et al., Engng. Fract. Mech., in press], the numerical simulation of the Charpy test in the ductile–brittle transition range using fully coupled local approach to fracture is presented.

Effects of degradation on the mechanical properties and fracture toughness of a steel pressure-vessel weld metal

A degradation procedure has been devised to simulate the effect of neutron irradiation on the mechanical properties of a steel pressure-vessel weld metal. The procedure combines the application of cold prestrain together with an embrittling heat treatment to produce an increase in yield stress, a decrease in strain hardening rate, and an increased propensity for brittle intergranular fracture. Fracture tests were carried out using blunt-notch four-point-bend specimens in slow bend over a range of temperatures and the brittle/ductile transition was shown to increase by approximately 110 °C as a result of the degradation. Fractographic analysis of specimens broken at low temperatures showed about 30% intergranular failure in combination with transgranular cleavage. Predictions have been made of the ductile–brittle transition curves for the weld metal (sharp crack) fracture toughness in degraded and non-degraded states, based on the notched-bar test results and on finite element analyses of the stress distributions ahead of the notches and sharp cracks. The ductile–brittle transition temperature shift ( ΔT=110 ° C) between non-degraded and degraded weld metal at a notch opening displacement of 0.31 mm was combined with the Ritchie, Knott and Rice (RKR) model to predict an equivalent shift of 115 °C for sharp-crack specimens at a toughness level of 70 MN/m 3/2.

A model for predicting fatigue crack growth behaviour of a low alloy steel at low temperatures

In the present study, a model to predict the fatigue crack growth (FCG) behaviour at low temperatures is proposed for a low alloy steel (16 Mn). The experimental results indicate that fatigue ductile-brittle transition (FDBT) occurs in 16 Mn steel and the FDBT temperature ( T FDBT ) is about 130 K. When T > T FDBT , the FCG mechanism in the intermediate region is the formation of ductile striation and the FCG rates decrease with decreasing temperature. When T ≈ T FDBT , the FCG mechanism changes into microcleavage and the fatigue fracture toughness K fc of the steel decreases sharply. The FCG rates tend to increase as the temperature is further reduced. The test data of the FCG rates are well fitted by the formula developed by Zheng and Hirt. An approximate method to predict ΔK th of the steel at low temperatures is proposed and then a general expression of the FCG rates is given at temperatures ranging from room temperature to T FDBT . By means of the expressions proposed in this paper, the FCG rates at low temperatures can be predicted from the tensile properties if the endurance limit σ −1 and δk th , at room temperature are known. Finally, a model for FDBT is tentatively proposed. Using this model, one can predict T FDBT from the ductile-brittle transition curve determined from impact or slow bending tests of cracked Charpy specimens.

微小シャルピー衝撃試験片によるＣｒ‐Ｍｏ‐Ｖ鋳鋼の経年劣化評価

There are two methods of evaluation of material degradation, that is, destructive and non-destructive. In this study, the degradation of Cr-Mo-V casting steel which has been used for about ten years as the internal casing of turbine was evaluated using a miniature Charpy impact specimen. The miniature specimen (thickness: 1.5mm, width: 1.5mm, length: 16mm) was taken from the Cr-Mo-V casting steel and the ductile-brittle transition curve was measured by the instrumented Charpy impact test. Moreover, the effects of specimen dimension on the ductile-brittle transition temperature (DBTT) and upper shelf energy and a relationship between the change of DBTT and micro-structural properties were investigated. The results of this study are summarized as follows:(1) When the specimen dimension becomes smaller, the difference of DBTT due to the difference in degradation becomes smaller.(2) The degradation of material can be successfully evaluated with a 1/6 size specimen.(3) The change of DBTT can be predicted from the changes of hardness, innergranular hardness, diameter of precipitated carbide and depth of etched grain boundary groove.

含りん鉄系焼結材料の機械的性質

The authors have previously investigated the sintering process and spheroidization of the pores of iron and ferrophosphorus mixed powder compacts. In this paper, the tensile and impact properties of Fe-P and Fe-P-C are investigated, and the correlation between the spherical pore and the properties is discussed.Addition of finer ferrophosphorus powder improves the tensile properties; particularly ductility increases with sintering above eutectic temperature (1050°C) as a result of appearance of small and spherical pores. On the other hand, the ductile-brittle transition curve appears more evidently with the ferrophosphorus additions, and the transition temperature increases with increase of phosphorus content. Though favourable combinations of strength and ductility can be attained with Fe-0.4 wt%P, the limiting concentration of phosphorus is 0.3 wt% in view of practical toughness. Addition of graphite to iron compacts containing low phosphorus content increases the strength, while it causes a drop of both ductility and toughness. However, the addition of graphite to that of high phosphorus content above 0.4 wt% improves somewhat the toughness as a result of grain refinement.