Equivalent Fracture Strain Research Articles

Small punch testing for determining the cryogenic fracture properties of 304 and 316 austenitic stainless steels in a high magnetic field

This paper examines the effect of magnetic field on the fracture properties of austenitic stainless steels at liquid helium temperature (4 K). Small punch tests were performed on cold-rolled 304 and 316 austenitic stainless steels. Previously proposed correlation for small punch and elastic–plastic fracture toughness test methods was applied to predict a small punch test-based fracture toughness from equivalent fracture strain.

Influence of Thermal Aging on Cryogenic Fracture Toughness of JN 1 Steel for Superconducting Magnet Structures of Fusion Reactors

Elastic-plastic fracture toughness tests were conducted at 77K and 4K to examine an influence of thermal aging on fracture properties of an austenitic stainless steel JN 1 at cryogenic temperature, which is a candidate material for superconducting magnet structures of fusion reactors. The tests were applied to solution-treated material and materials aged at 923-1073 K for 5 h. The experimental results showed that the fracture toughness JIC measured on the thermally aged materials were smaller than that on the solution-treated material and decreased with increasing aging temperature. This decrease in fracture toughness due to thermal aging was caused by precipitation and coarsening of grain boundary M23C6 carbides. Additionally, small punch (SP) tests were conducted at cryogenic temperatures to investigate the relationship between the obtained JIC and SP properties. It was found that the equivalent fracture strain eqf in SP specimens was linearly correlated with JIC irrespective of testing temperature. This correlation suggests that the SP testing technique provides a useful tool for evaluating JIC at cryogenic temperature.

Correlationship between JIC and equivalent fracture strain determined by small-punch tests in JN1, JJ1 and JK2 austenitic stainless steels

Small-punch (SP) and fracture toughness tests were conducted at 4 and 77 K on newly developed cryogenic austenitic stainless steels after isothermal aging. Equivalent fracture strain ε qf was determined by measuring the reduction in thickness of SP specimens. A linear dependence of J IC on ε qf was found for these new materials. Regression analysis of experimental data produced the linear relation: J IC =1304.1ε qf +8.09 ( kJ m −2) . An analysis of the correlation was pursued on the basis of material properties and evaluation method of J IC. The results showed that the slope value was influenced by the J IC evaluation method. This linear relation enables the prediction of J IC to be made from the equivalent fracture strain ε qf by conducting small punch tests. Thus, it is expected that the SP testing method provides a useful basis for assessing the degradation of fracture toughness in small areas, such as heat-affected zones in welded components.

332 小型パンチ試験によるオーステナイト系ステンレス鋼溶接熱影響部の極低温破壊特性評価(OS1-6 熱影響部と配管の健全性評価)(OS1 高強度材・表面改質材の疲労と破壊)

This study was performed to demonstrate the feasibility of performing liquid helium temperature (4 K) small punch (SP) tests on austenitic stainless steel weld for Large Helical Device(LHD) superconducting magnets. The SP specimens (10×10×0.5 mm) were prepared from the different locations of electron-beam weld in type 316 stainless steel to examine the variation of the fracture properties in the weld and its heat affected zone. Previously proposed correlations for SP and elastic-plastic fracture toughness test methods were applied to predict a SP test-based fracture toughness from equivalent fracture strain. A finite element analysis was also performed to convert the experimentally measured load-displacement data into useful engineering information. The maximum strain energy density was calculated and correlated with equivalent fracture strain. The heat affected zone with high Vickers hardness at room temperature shows lower fracture toughness, compared to other regions.

Finite Element Analysis and Small Punch Testing for Determining the Cryogenic Fracture Toughness of Austenitic Stainless Steel Welds

Abstract The small punch (SP) test has been used successfully to characterize the elastic-plastic fracture toughness (JIC) of structural alloys and weldments for superconducting magnets in fusion energy systems with thin plate specimens of 10 by 10 by 0.5 mm. This study was performed to demonstrate the feasibility of performing SP tests on austenitic stainless steels and their welds at liq uid helium temperature (4 K). Correlations between SP energy, equivalent fracture strain, and JIC were assessed. All JIC data were obtained using 25 mm thick compact specimens that followed the standard test method for JIC. A finite element analysis was also performed to convert the experimentally measured load-displacement data into useful engineering information. The maximum strain en ergy density was calculated and correlated with JIC.

Biaxial direct tensile tests in a large range of strain rates. Results on a ferritic nuclear steel

Constitutive equations are usually calibrated only trough the experimental results obtained by means of uniaxial tests because of the lack of adequate biaxial experimental data especially at high strain rate conditions. These data are however important for the validation of analytical models and also for the predictions of mechanical behaviour of real structures subjected to multiaxial loading by numerical simulations. In this paper some developments are shown concerning biaxial cruciform specimens and different experimental machines allowing biaxial tests in a large range of strain rates. This experimental campaign has also allowed study of the influence of changing the strain paths. Diagrams of equivalent stress versus straining direction and also equivalent plastic fracture strain versus straining direction are shown.

Assessment of stress corrosion cracking susceptibility by a small punch test

The object of this study is to establish a new test method for evaluating stress corrosion cracking (SCC) susceptibility of high‐strength steel using a small punch (SP) test and acoustic emission (AE). A miniaturized specimen (10 × 10 × 0.5 mm) is adopted for SCC evaluation. The experiments are conducted at various loading rates and at various orientations of the specimen. The cumulative average amplitude of the AE signal per unit equivalent fracture strain (εqf ) increases as the SCC susceptibility increases. Through the load–displacement behaviour, the fracture energy (ESP ), the SEM fractographs, and the correlation between the SCC susceptibility and the AE characteristics, it is proved that the small punch test method combined with AE measurements is a useful method to evaluate the SCC susceptibility of high‐strength steel.

小型パンチ試験による核融合炉超伝導マグネット用構造材料および溶接部の極低温破壊靭性評価

This paper investigates the use of the small-punch (SP) test to estimate the elastic-plastic fracture toughness (JIC) of structural alloys and weldments for superconducting magnets in fusion energy systems. SP testing was performed with thin-plate specimens of 10×10×0.5mm at liquid helium temperature (4K). Correlations between SP energy, equivalent fracture strain, and JIC were assessed. All JIC data were obtained using 25-mm-thick compact specimens that followed the standard test method for JIC. Finite element analysis was also performed to convert the experimentally measured load-displacement data into useful engineering information. The criterion for fracture used is the strain-energy density (strain energy absorbed per unit volume) required to produce crack initiation in a solid, uncracked specimen. The fracture strain-energy density was calculated and correlated with JIC.

Assessment of fracture characteristics from revised small punch test using pre-cracked specimen

Small punch (SP) test has been utilized to analyze the neutron irradiation damage of nuclear vessels. Since this technique is easy, simple, and nondestructive, it can be applied to evaluate the mechanical properties and material degradation of in-service components. Conventional SP test has evaluated the ductile-brittle transition temperature and the equivalent fracture strain by the interpretations of load-deflection curve and the change of specimen thickness, respectively. The assumption that fracture occurs at maximum load is, however, not reasonable because the crack initiates at smaller load. In this study, in order to evaluate quantitatively fracture characteristics based on fracture mechanics, the pre-crack is introduced to SP specimen and acoustic emission is used to determine the crack initiation point. Using the load at crack initiation point, the fracture toughness of thin plate is calculated through bending theory. Therefore, the fracture characteristics of thin plate can be evalualed more reliably by using revised SP test.

A Modified Miniature Disk Test for Determining Material Mechanical Properties

AbstractThis paper briefly describes the apparatus and experimental procedure for the modified miniature specimen test (MMST). The primary focus is on the thought and methodology of converting data. The accuracy and reliability of test results are emphasized. The fundamental relations for converting experimental data are established via analysis of elastic-plastic bulge deformation behavior of a middle-thick circular plate loaded at the center for which the circumference is fixed and points at the boundary can move in a radial direction. The formulas for calculating materials strength and ductility have been derived from the relation. Three types of unirradiated materials whose strengths are different and one kind of 316 SS irradiated by protons were tested with MMST. The test results along with the standard deviations are presented. The comparison of these data with that for uniaxial tension tests is also given. The evaluation of material equivalent fracture strain was also performed with the MMST.

Supersmall punch test to estimate fracture toughness JIc and its application to radiation embrittlement of 2.25Cr-1Mo steel

A supersmall punch test has been used to extract fracture strain information on irradiated 2.25Cr-1Mo steel from transmission electron microscopy disc specimens as small as 3 mm in diameter and 0.25 mm in thickness. The test is based on driving a steel ball punch through a clamped specimen. The size effect of biaxial equivalent fracture strain of various kinds of materials and irradiated steel has been demonstrated. The results of fracture strain obtained from specimens 3 mm in diameter has been related almost linearly to the fracture toughness J Ic for elastic and plastic behaviour. The relationship between fracture strain and fracture toughness J Ic has been verified for the irradiated nuclear pressure vessel steel 2.25Cr-1Mo so that large amounts of irradiation space in nuclear reactor could be saved.

Fracture toughness evaluation of fusion reactor structural steels at low temperatures by small punch tests

A small punch (SP) test using miniaturized specimens has been performed for cryogenic austenitic steels at 4.2, 77 and 293 K to evaluate fracture toughness in a fusion material program. An SP testing cryostat for load versus deflection curve measurements has been successfully constructed. A universal relationship between valid fracture toughness, J IC , and equivalent fracture strain, \\ ̄ ge qf , for austenitic steels at different test temperatures has been confirmed and empirical parameters for that relation have been determined using a linear regression model. A linear correlation between valid J IC and \\ ̄ ge qf has been clarified for austenitic steels at low temperatures, where the regression coefficient is found to be 845 kJ/m 2. Using the results of the SP test above room temperature, a new attempt at statistical analysis has been proposed to estimate the relative change in fracture toughness due to neutron irradiation.

Development of a further-miniaturized specimen of 3 mm diameter for tem disk (ø 3 mm) small punch tests

A further-miniaturized specimen punch (SP) test has been developed to extract fracture strain and strength information from TEM disc specimens as small as 3 mm diameter and 0.25 mm in thickness. The test is based on driving a steel ball punch through a clamped specimen. A perfect-rigid plastic analysis has been made to extract fracture strain and the deformation process of the further-miniaturized specimen test is divided into three regions, as in the 10 mm × 10 mm × 0.5 mm specimen. The size effect of biaxial equivalent fracture strain of various kinds of materials has been demonstrated. The results of fracture strain obtained from 3 mm diameter specimens can be related almost linearly to the fracture toughness, JIC, for elastic-plastic behavior. A number of correlations also have been obtained between features of the load-displacement curve of the SP test and the mechanical properties, such as yield stress and ultimate stress.

Characterization of Fracture Behavior in Small Punch Test by Combined Recrystallization-Etch Method and Rigid Plastic Analysis

AbstractThe purpose of this investigation is to understand the deformation process of the small punch test and to use it to characterize the fracture properties of materials. By comparisons between recrystallization-etch results and perfect-rigid plastic analysis, the deformation process of the small punch test has been clearly divided into three regions. Equivalent fracture strain under biaxial stress state has been obtained from 10 by 10 by 0.5 mm thin sheet samples using both a recrystallization-etch technique and a semi-analytical method to characterize the fracture property of the material. The results of biaxial equivalent fracture strain can be related almost linearly to fracture toughness (JIc) for elastic-plastic behavior.

Criteria of Ductile Fracture Strain

Two criteria of ductile fracture strain are suggested. From the theory of plasticity for porous materials, the following criterion of fracture for a triaxial state of stress is obtained: [numerical formula] where εe_qf is the equivalent fracture strain, σm the mean stress, σe_q the equivalent stress, a0 and b0 are constants. Except under certain conditions, this criterion shows reasonable agreement with experiment. To improve the accuracy of the prediction of the fracture strain, the above criterion is modified as follows: [numerical formula] where c0 is a constant. It is found that this criterion provides a greater accuracy for prediction of the fracture strain.