Statistical scatter in the fracture toughness and Charpy impact energy of pearlitic steel

The effect of microstructure on the fracture characteristics of high carbon hypo eutectoid steel was studied under conditions of quasistatic and dynamic loading. Experimentally determined sets of fracture toughness and Charpy impact energy values were statistically treated. A relationship was found between fracture toughness and Charpy impact energy. In the very brittle domain, the fracture toughness increases slightly with increasing Charpy impact energy. In the domain where the fracture toughness is higher, the rise in fracture toughness with increasing Charpy impact energy is more pronounced. Detailed SEM examination of fractured compact tension (CT) and Charpy V-notch (CVN) specimens showed that the fracture at ambient temperature occurred almost exclusively by cleavage. There were no visible differences in the morphology of cleavage facets on the fracture surfaces of Charpy and CT specimens. Mechanisms of cleavage initiation were revealed by the fractographical investigation of fracture surfaces. Whereas the fracture surfaces of broken CT specimens exhibit a number of cleavage origins, the fracture surfaces of CVN specimens usually show only one.     

Analysis of impact properties of A533 steel for nuclear reactor pressure vessel by instrumented Charpy test

Instrumented Charpy impact test can become more useful by connecting the digital memory and microcomputer, where yield load (P_y), maximum load (P_m), premaximum load energy (E_i) and post-maximum load one (E_p) are automatically and rapidly analyzed. In such analysis, cyclic oscillations in a load signal are corrected and smoothened by using a method of moving averages (the nonrecursive low pass digital filter). Dynamic J_1c value (J_1d) of A533 steel can be measured in a fatigue precracked type Charpy specimen, provided that a true deflection of specimen and a true crack initiation point in the load-deflection curve are known. For this purpose, elastic compliance values of the testing machine and the specimen are measured dynamically by the elastic low blow test to correct the apparent deflection. Crack initiation point, on the other hand, is detected by the plastic low blow test. It is shown that the crack initiation in the fatigue pre-cracked specimen of this material occurs prior to the maximum load, and that the relation between crack initiation energy (E_Δα) and pre-maximum load energy (E_i) is $\text{E}{}_{\mathrm{\Delta \alpha }}\text{E}{}_{\mathrm{i}}\text{? 0.8}$.     

Grain size effects in the charpy impact energy of a thermally embrittled RPV steel

The Charpy impact energies of a reactor pressure vessel (RPV) steel in the as-received and several thermally embrittled conditions have been assessed on the basis of microstructural parameters. It has been concluded that the parameters controlling the impact-absorbed energy of pre-cracked and side-grooved Charpy test specimens are the equivalent grain size of dual-phase annealed microstructures, and the bainite packet size of single-phase quenched and tempered materials. The Charpy energy has been correlated very well with the reduction in area and true fracture strain of tensile specimens, which could be inferred as grain size governed mechanical properties.     

Dislocation structure and impact toughness of reactor pressure vessel steel 15Kh2NMFA upon ultrasonic treatment

The paper addresses the influence of high-temperature ultrasonic treatment on the structure and impact toughness of steel 15Kh2NMFA. The observed increase of impact toughness and decrease of ductile-brittle transition temperature for the steel upon ultrasonic treatment of various durations are due to the formation of a uniform defect structure.     

舰船结构钢的夏比冲击韧性与断口形貌

论述了从夏比冲击韧性分解出来的断裂扩展功与断口形貌的关系，指出冶金因素对夏比冲击韧性α＿ｋ值和扩展功的影响不完全是一致的，提出采用α＿ｋ，值和断口纤维率作为韧性指标的互补性，建议在我国的舰船结构钢韧性指标中增加断口纤维率的要求。     

Low-temperature fracture mechanisms in a spheroidised reactor pressure vessel steel

The micromechanisms of fracture of a spheroidised A533B reactor pressure vessel steel over the temperature range of −190°C to + 60°C were investigated by performing uniaxial tensile tests on double-notched cylindrical specimens. Failure was by quasi-cleavage at temperatures between −190°C and −145°C. Quasi-cleavage fracture surfaces are characterised by clusters of planar facets that are separated from other facets either by large voids or by clusters of microvoids. At temperatures between −145°C and −25°C failure was by mixed microvoid coalescence and cleavage while complete microvoid coalescence was observed at temperatures higher than −25°C. Over the whole temperature range studied, fracture nucleation was either from large single voids or localised regions of microvoids.     

Evaluation of dynamic fracture toughness parameters of locomotive axle steel by instrumented Charpy impact test

The analysis of the energy of fracture of specimens from steel OSL, which is widely used for the manufacture of railway axles under shock loading, is performed. The nature and quantitative parameters of the typical stages of the processes of plastic and brittle fracture, depending on the test temperature and stiffness of the stress state at the tip of the crack‐like defect, are established. It is shown that impact loading at 20 °C leads to the formation of the local zone of plasticity and ductile–brittle fracture of the material. An increased stiffness of the stress state at the tip of the defect at ?40 °C causes brittle fracture. An approach is developed, which is based on using the size of shear lips as a quantitative parameter of fracture under normal and low temperatures, similar in its physical essence to deformation approaches of nonlinear fracture mechanics. Based on this approach and the quantitative analysis of specimen fracture zones, the physical and mechanical scheme of specimen fracture is proposed in the presence of localized plasticity and in its absence near the tip of the concentrator.     

Analysis of warm prestressing effect on fracture toughness of reactor pressure vessel steels

We have performed finite element calculation of precracked Charpy-like three-point bending specimen, in order to analyze the effect of warm prestressing on fracture toughness value of reactor pressure vessel steels. Two different hardening laws were applied in the calculation. J-integral was determined in both cases and comparative analysis was made.     

On the information about fracture characteristics obtained from instrumented impact test of A533 steel for reactor pressure vessel

Analysis of the information obtained from instrumented impact test and of its correlations with static and dynamic J integral values and with various tensile properties is made in this study.     

Measurement of fracture initiation toughness and crack resistance in instrumented Charpy impact testing

A new method has been developed involving direct measurement of the load-line displacement during instrumented Charpy testing. The method uses a laser interferometer to measure displacement in addition to the load-line displacement derived from the load signal. Tests were conducted using fatigue precracked and V-notched test pieces in the temperature range +23°C to −80°C on a conventional ship grade steel, a pressure vessel steel and two welded joints. Good correlation was found between the J_0.2 initiation fracture toughness determined by the multi-specimen method and the J_i fracture toughness determined from single specimens using the new method to detect ductile fracture initiation.     

Micromechanical modelling of reactor pressure vessel steel

Safe operation of nuclear power plants is one of the major concerns and relies primarily on the integrity of the reactor pressure vessel. Neutron exposure induces temperature dependent embrittlement and can lead to loss of fracture toughness of reactor pressure vessel steel. Conventional fracture mechanics suffers from a serious limitation of non-transferability of fracture data from specimen level to component level as fracture resistance data obtained is largely geometry dependant. This difficulty is largely overcome by Gurson–Tvegaard–Needleman (GTN) model which models the drop in load carrying capacity of a material with the increase in plastic strain, considering nucleation, growth and coalescence of micro-voids in the material. However, determination of the model-parameters with experiments is extremely difficult and doubtful. Hence, a parametric study was undertaken to find out the effects of different parameters on material behaviour of reactor pressure vessel steel and to asses the parameter for best practical result with least metallographic study using a hybrid approach. The model parameters were finally verified by predicting ductile fracture in compact tension and three point bend specimen made from reactor pressure vessel steel.     

Evaluation of dynamic fracture toughness KId by Hopkinson pressure bar loaded instrumented Charpy impact test

A novel method for measuring the dynamic fracture toughness, K_Id, using a Hopkinson pressure bar loaded instrumented Charpy impact test is presented in this paper. The stress intensity factor dynamic response curve (K_I(t)−t) for a fatigue-precracked Charpy specimen is evaluated by means of an approximate formula. The onset time of crack initiation is experimentally detected using the strain gauge method. The value of K_Id is determined from the critical dynamic stress intensity factor at crack initiation. A K_Id value for a high-strength steel is obtained using this method at a stress-intensity-factor rate () greater than 10⁶ MPa .     

Correlation between stretched zone size and fracture toughness

The topography of the stretched zone was measured from stereo electron micrographs. The stretched zone reflects crack tip blunting preceding fracture. The depth of the stretched zone of fourteen fracture toughness specimens of some Al-Zn-Mg alloys appeared to correlate reasonably with the critical crack tip opening displacement. The width of the stretched zone appeared to be in the order of magnitude of the ligament spacing in Krafft's model.     

Weibull master curves and fracture toughness testing Part III Master curves for the evaluation of dynamic Charpy impact tests

The existence of specimen-size-independent quasi-static Weibull master curves for macroscopically homogeneous solids characterizing strength and failure of both purely brittle materials and rather tough materials, which undergo an amount of stable crack growth prior to failure, has already been proved in earlier publications. In this paper, the concept of Weibull master curves is extended to the case of dynamic testing conditions, being typical for Charpy impact tests performed in the ductile-to-brittle transition temperature-range of ferritic-martensitic steels. Dynamic Weibull master curves can be constructed, if the stress-distributions, which are built up in the process zone of the specimens during the Charpy impact tests, can be described with a dynamic quasi-equilibrium approach. In this case, the dynamic Weibull master curves can be related to the quasi-static Weibull master curves with the help of the toughening exponent , characterizing the rate of toughness increase with increasing crack length. Characteristic magnitudes, being most convenient to estimate the capacity of the tested materials to undergo stable crack growth, microcracking and crack-tip shielding prior to rupture, can be derived as well from dynamic Weibull master curves as from quasi-static Weibull master curves.     

Predicting reference temperature from instrumented Charpy V-notch impact tests using modified Schindler procedure for computing dynamic fracture toughness

Reactor pressure vessel (RPV) steels are increasingly being characterised in terms of the reference temperature T ₀ and the associated Master Curve (MC) Procedure, following the ASTM E-1921 standard. Though correlations have been proposed to predict the T ₀ from Charpy transition temperature T _28J or instrumented impact test parameters like T _4kN, none can be taken as a universal correlation. Here we are proposing a new correlation of T ₀ with T _0Sch ^dy, where T _0Sch ^dy is the reference temperature corresponding to a median K _Id=100 MPa√m evaluated by the ASTM E1921 procedure applied to K _Id vs T data, and K _Id has been calculated from instrumented CVN impact test data using modified Schindler relations. This will provide a reliable method for determining T ₀ from instrumented CVN tests alone. T _0Sch ^dy provides a conservative alternative to T ₀ ^dy for application of the ASTM E 1921 MC procedure in dynamic situations. Since the above procedure depends only on instrumented CVN data, it will be less costly to apply (no precracking is necessary) and will also obviate the difficulties associated with determining T ₀ ^dy from precracked CVN testing (because of severe size limitations, associated scatter and signal oscillations from the mechanics of the test, there needs to be precise control over test temperatures and test velocity for obtaining valid data from limited number of specimens). The RT _NDT(est) from the suggested procedure (or its modifications based on future work) will provide an acceptable alternative to RT _NDT for application of the ASME K _IR curve based on instrumented CVN tests alone. For low-uppershelf steels, the new reference temperature estimate T _0.075 and its correlation to T _0Sch ^dy will provide a methodology for application of MCs to such steels. Further comprehensive work is needed to validate the procedures and correlations suggested in this paper.     

Relationship between fracture topography and fracture toughness of a high strength steel

The relationship between fracture topography and the plane strain fracture toughness of Comsteel En25 tempered at nine different temperatures and with crack planes in the R-L, R-C and L-R orientations has been studied. The results show that fracture toughness is qualitatively relatable to the fracture morphology. Due to the shape and alignment of the elongated MnS inclusions, fracture toughness in the L-R orientation was found to be 1.8 times those in the other two orientations. Terrace-type fracture prevailed in the R-L orientation, but this frequently was observed to change to zigzag type fracture in the R-C orientation. However, both these fracture mechanisms were absent in the L-R orientation.