Specimen loading determined by displacement measurement in instrumented Charpy impact test

An experimental method based on the specimen displacement measurement with a commercially available laser sensor is proposed in order to determine the actual specimen loading in instrumented impact test. The prediction resulting from this approach is compared with results deduced from dynamic analysis of impact tests with mass-spring model.     

Assessment of an instrumented Charpy impact machine

The dynamic responses of the standard Charpy impact machine were studied experimentally using strain gauges and accelerometer attached to the striker and the rotary position sensor fixed at the rotating axis and numerically with the finite element analysis. The fracture propagation was simulated with the cellular automata finite element approach developed earlier. A series of low velocity as well as full capacity Charpy tests were analysed. It was found that the strain gauge signal recorded close to the tup edge and the acceleration recorded at the back of the striker do not match. The energy calculated with the strain gauge data agrees well with the dial reading, while the energy calculated with the accelerometer signal is never near it. Frequencies close to the first natural \hbox{frequency} of the Charpy sample have high modal magnitudes in the acceleration signal but are effectively damped in the strain gauge response. Vibrations of the striker arm have highest modal magnitudes in the rotary position sensor. A low-pass filter is used to obtain the striker movements. The finite element analysis partly supports the experimental observations but also suggests that acceleration at the tup edge suffers higher oscillations than strain.     

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.     

The development of an instrumented impact test machine

The variables to be considered and constraints to be fulfilled in the design of an impact tester are discussed. Solutions are proposed and the final design of the machine is described, including associated micro-computer based analysis-equipment. Some of the assumptions made by earlier workers are avoided.     

Fracture mechanics considerations in the Charpy impact test

Instrumentation of the Charpy impact test permits an analysis of the data in terms of linear elastic fraeture mechanics. Measurements of time to fracture are converted to estimated crack opening displacements at fracture and thus to values of the fracture toughness parameter KC. The variation of these values with temperature shows a transition which is interpreted as a change from plane strain to plane stress fracture.An expression is derived which relates the critical cleavage stress and the plane strain fracture toughness (K_IC) and allows this latter quantity to be calculated from dynamic stress measurements in those Charpy tests which give fractures at conditions where general yield just occurs. It is noted that the values of K_IC so obtained are significantly lower than the values obtained from time-to-fracture measurements, and the differences are discussed. It is concluded that the K_IC values obtained from the stress measurements are of more general interest, and the uncertainties involved in these estimates are outlined.

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Zusammenfassung Infolge der Instrumentierung der Charpy-Kerbschlagprufüng können die gelieferten Daten durch linear-elastische Bruchmechanik analysiert werden. Messwerte der vorm Bruchvorkommen ablaufenden Zell werden zu geschätzten Rissöffnungswerten (crack opening displacements) beim Bruch, also zu Werten des Bruchwiderstandsparameters KC, umgerechnet. Die Temperaturabhägig dieser Werte zeigt einen Übergang, der nach der Deutung der Autoren einer Änderung der Bruchweise entspricht (zweiachsige Spannung wird zu zweiachsiger Verformung).Ein Ausdruck wird abgeleitet, der die kritische Trennbruchspannung und den der zweiachsige Verzerrung entsprechenden Bruchwiderstand (K_IC) in Wechselbeziehung bring und die Berechnung dieses letzten aus dynanamischen Spannungswerten bei jinen Charpy-Prüfungen ermöglicht, bei denen der Bruch gerade bei der Einstellung des weitverbreiteten Fliessens vorkommt. Es wird bemerkt, dass die so erhaltenen K_IC-Werte bedeutend niedriger sind als die, die aus Zeit zum Bruch Messwerten erhalten werden. Die Unterschiede werden besprochen. Es wird beschlossen, dass die aus den Spannungsmessungen bestimmten K_IC-Werte von weiterem Interesse sind, and die mit diesen geschätzten Werten verbundenen Unsicherheiten werden kurz beschrieben.

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Résumé À force d'instrumenter Fessay Charpy du dureté au choc on se met en position de faire l'analyse des résultats qu'il fournit en utilisant le méchanique linéar—elastique de la rupture. De la valeur du temps avant la rupture se laisse calculer approximativement l'ouverture du dout de la fissure (crack opening displacement) au moment de la rupture et de la valeur du paramètre de la résistance à la rupture, K_C. La dépendence de cette valeur de la température montre une transition considérée comme changement du mode de la rupture de deformation planaire en contrainte planaire.On développe une expression qui met en corrélation la contrainte critique du clivage et la résistance à la rupture dans les conditions de la déformation planaire et permet le calcul de cette quantité au moyen du mesurage de la contrainte dynamique dans les essais Charpy où la rupture se produit lors de l'écoulement général. On remarque que les valeurs de K_IC ainsi obtenues sont d'une manière significative inférieures à celles que l'on obtient du temps avant la rupture, et les différences sont discutées. Il est conclu que les valeurs de K_IC que l'on obtien en mesurant la contrainte sont d'un interêt plus général et les incertitudes qu'entraînent ces calculs approximatifs sont brièvement décrites.

     

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.     

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。     

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 $\text{E}{}_{\mathrm{\Delta \alpha }}\text{E}{}_{\mathrm{i}}\text{? 0.8}$.     

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 .     

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.     

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.     

Charpy impact test on A508-3 steel after neutron irradiation

Reactor pressure vessel (RPV) is the critical un-changeable component of the reactor during its service lifetime, which prevents the radioactive leak of the nuclear power plant core. The irradiation test (about 10 × 10¹⁹ cm^− 2, E > 1 MeV) in research reactor of the pressure vessel material was carried out, and the charpy impact test has been carried out before and after the neutron irradiation. Analysis of the impact energy and the fracture morphology has been done to estimate the embrittlement due to neutron irradiation. It shows that the main effects of neutron irradiation to fracture are the crack initiation and stable expansion process. And there also are a small amount of intergranular fracture in the unstable crack expansion after neutron irradiated which aware us pay more attention to the increasing intergranular fracture behavior of higher neutron fluence level after 60a nuclear power plant operation.     

Characterization of the dynamic failure behaviour of a glass-fiber/vinyl-ester at different temperatures by means of instrumented Charpy impact testing

Instrumented Charpy impact testing is used to investigate the strength and failure properties of a glass-fiber/vinyl-ester composite. The test technique, originally developed for testing of steel specimens, is presented in its basic aspects; reported are the conventional procedures for determining load, displacement and energy absorption that a specimen experiences, over the entire phase of loading and subsequent failure of the specimen. Techniques are described for generating data of sufficient accuracy when applying the test to composites. In particular, the necessity of utilizing measurement chains of sufficiently high frequency response and striker tups of sufficiently high sensitivity is emphasized. Tests are performed with glass-fiber/vinyl-ester specimens, provided with notches oriented in two different directions with respect to the plies of woven glass fiber rovings. Two different types of failure result: fiber breakage ahead of the notch due to tensile stresses, and delaminations of the interface planes between the plies of woven glass fiber rovings due to shear stresses. Specifically, energies absorbed by the specimen over the entire failure process and values of maximum load occurring during the impact process are measured over a large range of temperatures. The data are correlated with the observed failure phenomena. The high level of information obtained in characterizing the failure behaviour by means of a test which requires limited technical effort proves the instrumented Charpy impact test to be a simple but effective tool for quantifying the quality of a composite in practical applications, as e.g. in surveillance programs for controlling processes such as manufacturing or aging of the material.     

Numerical simulation of the Charpy impact testing

This paper deals with the dynamic effects understanding of the impact three point testing. In the first step, the transverse vibrations of the discretized simply supported beam are investigated and compared to the open literature. In the second step, a modal analysis is performed on the specimen-tup mechanical system. A linear contact stiffness at the impact point is introduced.     

The Charpy impact properties of beryllium

Notched and unnotched Charpy impact measurements have been made on two grades of hot pressed beryllium over a range of densities between 80% and 100% theoretical density. The beneficial effect of ≈ 5–10% porosity on notched impact properties is demonstrated and the effects of annealing out machine-induced surface stresses is shown. In unnotched specimens this annealing leads to an increase of a factor of six in absorbed energy. Possible reasons for this phenomenon are discussed.     

The Hopkinson pressure bar: an alternative to the instrumented pendulum for Charpy tests

In the instrumented Charpy pendulum, strain gauges mounted on the hammer react to strain waves resulting from the impact against the specimen by providing a jagged strain-time trace. This is electronically filtered to provide a smooth curve taken to represent the variation with time of the load applied to the specimen. The Hopkinson pressure bar (HPB) described here provides an alternative method of testing that does not require electronic filtering and simplifies data collection and interpretation.

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Résumé Dans un mouton pendule Charpy instrumenté, les jauges de contrainte disposées sur le marteau réagissent à l'onde de déformation provenant de l'impact sur l'éprouvette en produisant une trace irrégulière sur un graphe déformation/temps. Un filtrage électronique permet d'obtenir une courbe régulière, que l'on adopte pour représenter la variation dans le temps ou la charge appliquée à l'éprouvette. Le dispositif HPB présenté dans l'étude constitue une autre méthode d'essai, qui ne nécessite pas de filtrage électronique, et qui simplifie la collecte et l'interprétation des données expérimentales.

     

Computer simulation of the Charpy V-Notch toughness test

We calculated the impact of a striker with the ASTM Type A Charpy V-Notch specimen. We used an elastic-plastic constitutive law with J₂-flow theory and isotropic hardening to model A-533 Grade B class 1 steel at 100°C. Large strain and rotation are accounted for. We found that the notch stress-strain state was independent of the three-point loading type (round- or flat-tip striker or pressure push) and was most strongly correlated with notch-opening displacement. The dynamic stress-strain state at the time of fracture initiation was obtained by comparing the calculated deformed shape with that obtained in interrupted Charpy-V-notch tests where cracking had started. This time agreed with a computed prediction of fracture initiation obtained using a computer model of material damage. Cracking starts at the notch at about 40% of the maximum load found in an instrumented Charpy test. We have also calculated the distribution and partition of specimen energy. Adiabatic heating and strain rate are discussed.     

Analysis of the Charpy V-notch test for welds

The ductile–brittle transition for a weld is investigated by numerical analyses of Charpy impact specimens. The material response is characterized by an elastic–viscoplastic constitutive relation for a porous plastic solid, with adiabatic heating due to plastic dissipation and the resulting thermal softening accounted for. The onset of cleavage is taken to occur when a critical value of the maximum principal stress is attained. The effect of weld strength undermatch or overmatch is investigated for a comparison material, and analyses are also carried out based on experimentally determined flow strength variations in a weldment in a HY100 steel. The predicted work to fracture shows a strong sensitivity to the location of the notch relative to the weld, with the most brittle behavior for a notch close to the narrow heat affected zone. The analyses illustrate the strong dependence of the transition temperature on stress triaxiality.