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.     

Estimation of ASTM E-1921 reference temperature from Charpy tests: Charpy energy-fracture toughness correlation method

In this paper, some of the older and newer Charpy-fracture toughness correlations have been examined and new correlations have been developed for predicting the ASTM E-1921 standard reference temperature, T₀. The results have been applied to some selected new steels and compared with measured T₀, where available. It is found that the predicted reference temperature from the new procedure gives reliable and acceptably conservative engineering estimate of T₀. Predicted values of reference temperatures under dynamic conditions from these T₀ using Wallin’s strain rate shift equation agree well with measured dynamic values for a few steels giving added support to the new procedure.     

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.     

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 .     

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.     

Characterization of fracture behavior of a Ti alloyed supermartensitic 12%Cr stainless steel using Charpy instrumented impact tests

In this work, the toughness of a Ti-alloyed supermartensitic stainless steel with 12%Cr was evaluated by instrumented Charpy impact tests at − 46 °C. The material was heat treated by quenching and tempering at 500 °C or 650 °C. The temper embrittlement phenomena was detected in the specimen tempered at 500 °C, while the specimens as quenched and quenched and tempered at 650 °C presented a ductile fracture with high impact energy values. The predominance of cleavage fracture instead of intergranular cracks suggests that the temper embrittlement was caused by fine and disperse precipitation observed in the specimen tempered at 500 °C. The dynamic initiation fracture toughness (J_Id) was calculated from the force versus deflection curves using three different methods suggested in the literature to obtain the initiation energy.     

用仪器化的冲击试验系统评定材料的动态性能

由于仪器化的Ｃｈａｒｐｙ冲击试验方法简单经济，已被广泛地用于评价材料的冲击韧性。本文以引进的ＷＯＬＰＥＲＴ冲击试验系统为背景，详细地介绍了计算机辅助的仪器化冲击试验系统（ＣＡＩ）的原理和面貌，对动态断裂韧性Ｋ_（Ｉｄ）的测试原理和方法亦作了较为细致的描述。最后，指出了ＣＡＩ系统几个方面的用途。     

A method of extracting dynamic fracture toughness from CT tests

Recent research on methods of determining the dynamic fracture resistance for rapidly propagating cracks has progressed to the point where standardized testing procedures are being proposed. Because direct measurements of dynamic fracture resistances cannot be made, the resistances must be inferred from other measurement. In the present paper, an analysis which is fundamental to a method whereby the dynamic fracture toughness as a function of crack speed can be inferred from measurements of the specimen's end displacement at initiation of crack growth and of the total crack extension is described. All of the reference curves required for making the inference are developed for a standardized CT specimen. The procedure for using these curves to establish the dynamic fracture toughness-crack speed relationship and the minimum fracture toughness is given.

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Résumé Les recherches récentes sur les méthodes de détermination de la résistance à la rupture dynamique en présence de fissuration en cours de propagation rapide ont progressé à ce point que des procédures d'essai standards sont à présent proposées. Comme des mesures directes des résistances à la rupture dynamique ne peuvent être effectuées, les résistances doivent être déduites d'autres mesures. Dans le mémoire, on décrit une analyse qui constitue la base d'une méthode par laquelle la ténacité à la rupture dynamique en fonction de la vitesse de fissuration peut être déduite de mesures de déplacement de l'extrémité de l'échantillon lors de l'amorçage de la propagation de la fissure, et de mesures de l'extension totale de la fissure. Les courbes de référence requises pour établir cette déduction sont développées dans le cas d'éprouvettes standards CT. La procédure d'utilisation de ces courbes pour établir les relations qui lient la ténacité à la rupture dynamique et la vitesse de propagation de la fissure ainsi que la ténacité minimum à la rupture sont fournies.

     

Determination of dynamic fracture toughness of heat affected zone of 9Cr–1Mo steel from instrumented drop weight tests

Abstract

The dynamic fracture toughness (K_1d) of the heat affected zone (HAZ) of 9Cr–1Mo steel at and below the nil ductility transition temperature has been estimated from instrumented drop weight test results. The presence of a significant microstructural gradient in the HAZ, comprising coarse, fine, and intercritical regions with sharp toughness differences, is reflected in the presence of distinctive load peaks in the load–time traces; the K_1d estimates for the coarse, fine, and intercritical regions are 53, 85, and 128 MPa m^1/2 respectively. The results from the 9Cr–1Mo steel were compared with those for AISI grade 403 martensitic stainless steel. The lack of distinctive multiple load peaks in the load–time traces for the latter is attributed to the absence of a steep toughness gradient, owing to a more or less uniform martensitic microstructure in the HAZ.     

Correlation between Charpy impact energy and J fracture toughness for thermally embrittled reactor pressure vessel steel

Abstract

The Charpy impact energies of a reactor pressure vessel steel in the as received and several thermally embrittled conditions have been tentatively correlated to three J fracture toughness parameters derived under quasi-static loading regimen. Very good correlation has been achieved over the whole fracture resistance range of the structural steel, as obtained by the application of special heat treatments. It has been established that the parameters controlling the impact energy absorption capacity of the materials are the equivalent grain size of dual phase (ferrite/bainite) annealed microstructures and the bainite packet size of single phase quenched and tempered materials. The dependence of the Charpy impact energy of precracked, side grooved bend bars on the representative grain size of the microstructures tested has been disclosed as a Hall-Petch relationship.     

Application of instrumented impact testing for studying dynamic fracture behaviour of rotor steels

The viability of the instrumented Charpy impact testing for studying dynamic fracture behaviour of rotor steels is investigated. This encompasses determination of dynamic fracture toughness (K_Id) and dynamic J-integral (J_Id), establishing correlation between oscilloscope profiles and fracture morphology of the ruptured samples and identifying fracture mechanisms involved. The predicted oscilloscope profiles for common fracture modes, their experimental counterparts, and the inferences drawn from these concerning operating fracture mechanisms are in good accord with the fractographic observations made on broken samples. Thus, the respective oscillographs vividly manifest the observed variations in the fracture processes. Fracture mechanics analysis of load-time and energy-time records of pre-cracked Charpy samples gave dynamic fracture toughness (K_Id) values of 43, 74 and 124MN/m3/2, and dynamic J-integral (J_Id) values of 0.008, 0.03 and 0.06 MJ/m² at −180°, 26° and 96°C respectively. It is possible that the deduced J_Id values correspond to a small but finite amount of crack extension instead of Zero Crack extension, in line with the emerging trends of J_Id estimation. Apart from increasing with temperature, both parameters recorded a true transition around 35°C which is attributed to the combined influence of a change in the fracture mode and relaxation of crack tip constraint. Another significant outcome of this investigation concerns about the existence of a minimum crack depth ratio for valid J_Id determination which, based on a detailed fractographic study, is interpreted in terms of the collective influence of crack tip plasticity and notch constraint.     

A method for dynamic fracture toughness determination using short beams

This paper deals with dynamic fracture toughness testing of small beam specimens. The need for testing such specimens is often dictated by the characteristic dimensions of the end product. We present a new methodology which combines experimentally determined loads and fracture time, together with a numerical model of the specimen. Calculations are kept to a minimum by virtue of the linearity of the problem. The evolution of the stress intensity factor (SIF) is obtained by convolving the applied load with the calculated specimen response to unit impulse force. The fracture toughness is defined as the value of the SIF at fracture time. The numerical model is first tested by comparing numerical and analytical solutions (Kishimoto et al., 1990) of the impact loaded beam. One point impact experiments were carried out on of commercial tungsten base heavy alloy specimens. The robustness of the method is demonstrated by comparing directly measured stress intensity factors with the results of the hybrid experimental-numerical calculation. The method is simple to implement, computationally inexpensive, and allows testing of large sample sizes, without restriction on the specimen geometry and type of loading.     

Dynamic deformation and fracture properties of simulated weld heat affected zone of 9Cr-1Mo steel from instrumented impact tests

Abstract

Dynamic deformation characteristics such as dynamic yield stress and dynamic strain hardening exponent along with dynamic elastic-plastic fracture toughness for the different microstructural regions of heat affected zone (HAZ) of a nuclear grade 9Cr-1Mo steel have been evaluated by instrumented Charpy tests. Isothermal heat treatment at different temperatures was used to simulate the different microstructural regions in the HAZ of this steel, namely the over tempered base metal, intercritical, fine prior austenitic grained martensitic, and coarse prior austenitic grained martensitic regions. Effects of interparticle spacing on the dynamic deformation and fracture properties have been studied. It has been observed that the dynamic yield stress and the dynamic fracture toughness follow power law relationships with the interparticle spacing whereas the strain hardening exponent follows a linear fit.     

Automated dynamic fracture procedure for modelling mixed-mode crack propagation using explicit time integration brick finite elements

Several fracture codes have been developed in recent years to perform analyses of dynamic crack propagation in arbitrary directions. However, general-purpose, commercial finite-element software which have capabilities to do fracture analyses are still limited in their use to stationary cracks and crack propagation along trajectories known a priori . In this paper, we present an automated fracture procedure implemented in the large-scale, nonlinear, explicit, finite-element code DYNA3D which can be used to simulate dynamic crack propagation in arbitrary directions. The model can be used to perform both generation- and application-phase simulations of self-similar as well as non-self-similar dynamic crack propagation in linear elastic structures without user intervention. It is developed based on dynamic fracture mechanics concepts and implemented for three-dimensional solid elements. Energy approach is used in the model to check for crack initiation/propagation. Dynamic energy release rate and stress intensity factors are determined from far-field finite-element field solutions using finite-domain integrals. Fracture toughness is input as a function of crack-tip velocity, and when the criterion for crack growth is satisfied, an element deletion-and-replacement re-meshing procedure is used along with a gradual nodal release technique to update the crack geometry and model the crack propagation. Direction of crack propagation is determined using the maximum circumferential stress criterion. Numerical simulations of experiments involving non-self-similar crack propagation are performed, and results are presented as verification examples.     

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.     

Prediction of failure probabilities for cleavage fracture from the scatter of crack geometry and of fracture toughness using the weakest link model

The weakest link model proposed by Landes and Shaffer[1] is generalized in order to predict the failure probabilities of real structures with surface cracks and to include the scatter in crack size and crack geometry. The failure probabilities and the distributions of crack depth, of depth to length ratio and of fracture toughness of the failed structures are calculated in this model and compared with the results of the usual probabilistic calculations.