Elastic-plastic Fracture Toughness Tests Research Articles

A study of ductile crack corrections for elastic-plastic fracture toughness tests

This paper critically examines existing ductile crack growth (DCG) corrections for J-integral values calculated with the multi-specimen and elastic compliance approaches, as well as two novel/alternative DCG corrections. The benchmark is represented by the incremental DCG correction prescribed by ASTM E1820-23a for Elastic Compliance tests. Considering calculations performed at both end-of-test and intermediate conditions, most of the existing and proposed approaches tend to under-correct J values, by an amount proportional to the percent of initial cracked ligament. For one of the proposed novel approaches (Pseudo-Resistance Curve Procedure), a tendency to overcorrection was observed. Nevertheless, when values of critical toughness (JQ, JIc) and ductile crack resistance (expressed in terms of tearing modulus, TM) are considered, differences are smaller than the typical uncertainties associated with elastic–plastic fracture toughness test results. Therefore, it does not appear necessary to modify the DCG corrections currently prescribed in the two most widely used fracture toughness test standards (ASTM E1820 and ISO 12135).

Cryogenic fracture behavior of metastable austenitic stainless steel in a high magnetic field

We study the effect of magnetic field on the fracture behavior of the metastable austenitic stainless steels at cryogenic temperatures. Elastic-plastic fracture toughness tests were performed on compact tension specimens in liquid helium at 4 K with and without a magnetic field, and the effect of magnetic field on the cryogenic fracture toughness was discussed. Quantitative phase analysis was also done by magnetic method, and the fracture surfaces were examined by scanning electron microscopy to correlate with the fracture properties.

OS0213 強磁場下におけるオーステナイト系ステンレス鋼の極低温弾塑性破壊特性評価(先端材料システムの力学とメゾスケールモデリング(2))

This paper studies the effect of magnetic field on the fracture behavior of the metastable austenitic stainless steels at cryogenic temperatures. Elastic-plastic fracture toughness tests were performed on compact tension (CT) specimens in liquid helium at 4 K with and without a magnetic field, and the effect of magnetic field on the cryogenic fracture toughness was discussed. Quantitative phase analysis was also done by magnetic method, and the fracture surfaces were examined by scanning electron microscopy (SEM) to correlate with the fracture properties.

Microstructural and mechanical characterization of electron beam welded Al-alloy 7020

The electron beam (EB) welding process is used to weld any metal that can be arc welded with equal or superior weld quality. EB welding is carried out in a high-purity vacuum environment, which results in freedom from impurities such as oxides and nitrides. Thus, pore-free joints can readily be achieved in metallic materials, such as Al-alloys and Ti-alloys. However, autogenous EB welding of some aluminium alloys leads to a significant strength reduction (undermatching) in the fusion zone due to the loss of strengthening phases. For such Al-alloys, the local microstructure-property relationships should be established to satisfy the service requirement of a welded component with strength undermatching. Autogenous EB welding was performed on 5 mm thick aluminium alloy 7020 plate. Microstructural characterization of the weld metals was made by optical and scanning electron microscopy. Extensive microhardness measurements were conducted in the weld regions of the joints which exhibited a hardness loss in the fusion zone due to the loss of strengthening phases. Tensile properties of the joints were determined by testing flat transverse tensile specimens at room temperature without machining the weld profiles. Furthermore, elastic-plastic fracture toughness tests (CTOD) were carried out on the base material and welded joints at room temperature.

CF8M과 SA508 용접재의 열화에 따른 파괴인성에 관한 연구

In a primary reactor cooling system(RCS), a dissimilar weld zone exists between cast stainless steel(CF8M) in a pipe and low-alloy steel(SA508 cl.3) in a nozzle. Thermal aging is observed in CF8M as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330℃, while no effect is observed in SA508 cl.3. The specimens are prepared by an artificially accelerated aging technique maintained for 300, 1800 and 3600 hrs at 430℃, respectively. The specimens for elastic-plastic fracture toughness tests are according to the process in the thermal notch is created in the heat affected zone(HAZ) of CF8M and deposited zone. From the experiments, the JIC value notched in HAZ of CF8M presented a rapid decrease up to 300 hours at 430℃ and slowly decreased according to the process in the thermal aging time. Also, the JIC value presented a lower value than that of the CF8M base metal. And, the JIC of the deposited zone presented the lowest value of all other cases.

Fracture Toughness with Thermal Aging in CF8M/SA508 Welds

In a primary reactor cooling system, a dissimilar weld zone exists between cast stainless steel (CF8M) in a pipe and low-alloy steel (SA508 cl.3) in a nozzle. Thermal aging is observed in CF8M as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330, while no effect is observed in SA508 cl.3. The aged specimens are prepared by an artificially accelerated aging technique maintained for 300, 1800 and 3600 hrs at 430, respectively. The specimens for elastic-plastic fracture toughness tests are prepared two types, which notch is created in the center of deposited zone and the heat affected zone of CF8M. From the experiments, the plastic-elastic fracture toughness values (JIC) with the increase of aging time decrease as the notch is created in the HAZ of CF8M, while that is different slightly as the notch is created in the deposited zone. Also, JIC values in the deposited zones are smaller than the HAZ of CF8M at all aged specimens.

Evaluation of the Fracture Characteristics with Thermal Aging on Dissimilar Welds in Reactor Coolant System

A dissimilar weld zone exists between the pipe and nozzle in a primary reactor cooling system (RCS). Thermal aging is observed in cast stainless steel, CF8M used in a pipe as the RCS is exposed for a long period of time to a reactor operating temperature between 290 and 330°C. No effect is observed in low-alloy steel. SA508 cl.3 is used in a nozzle. The artificially accelerated aging specimens are prepared to maintain for a temperature of 430°C for 300, 1800, and 3600hrs, respectively. Then, various mechanical tests such as hardness, tension, impact test, are performed in virgin and aged specimens in order to determine the existence of dissimilar weld zones. The specimens for elastic-plastic fracture toughness tests are prepared for one type, where a notch is created in the heat affected zone of CF8M. From the experiments, it was found that J-integral values decrease as age increases.

Fracture and Adiabatic Heating of Thick-Section Weldment in Fe-24Cr-15Ni-4Mn-0.32N Stainless Steel with Ni-Base Filler Metals at 4 K

Abstract Type JN1, a nitrogen-strengthened austenitic stainless steel [Fe-24Cr-15Ni-4Mn-0.32N (wt%)], is emerging as the preferred structural material for superconducting fusion magnet casings to operate in liquid helium at 4 K. In order to evaluate the fracture toughness and the adiabatic heating of thick-section (200 mm) weldment in JN1 forged stainless-steel plate with Ni-base filler metals, elastic-plastic fracture toughness tests were performed with 25-mm-thick compact tension specimens at 4 K. Thermocouples were used to measure the temperature rise near the crack tip. The effect of specimen location and nitrogen content on the fracture toughness parameters, and the temperature rise during dynamic crack growth, are discussed.

Fracture behavior of thick-section weldment in Fe-12Cr-12Ni-10Mn-0.24N stainless steel at liquid helium temperature

Type JJ1, a nitrogen-strengthened austenitic stainless steel (Fe-12Cr-12Ni-10Mn-0.24N), is emerging as the preferred structural material for superconducting fusion magnet casings to operate at liquid helium temperature (4 K). This article describes an examination of the cryogenic fracture behavior of thick-section (200 mm) weldment in JJ1 forged stainless-steel plate. The compact tension geometry was used for the elastic-plastic fracture toughness tests at 4 K. The through-thickness fracture toughness was investigated with particular attention to the influence of the nitrogen and inclusion contents. The magnitude of the 4-K fracture toughness decreased with increasing inclusion content. In addition to fracture toughness data, temperature rise near the crack tip during fracture was measured. Significant adiabatic heating occurred, and internal temperature rises up to 52 K were reported.

表面粗さの相違を利用したストレッチゾーン幅の定量解析

A stretched zone is formed as a result of blunting and stretching at the tip of the crack during the fracture process. Consequently, the fracture surface becomes fine and smooth. Paying attention to this characteristic, the authors proposed a new method to evaluate the stretched zone width quantitatively, based on the difference in the fracture surface roughness.To verify the validity of the proposed method, elastic-plastic fracture toughness tests were performed on compact tension specimens cut out of carbon steel pipes. After the fracture toughness tests, the stretched zone was observed with a high-resolution scanning electron microscope to measure the critical stretched zone width (SZWc) for stable crack initiation quantitatively. The critical stretched zone widths determined by the proposed method were compared with those reported in the references. The comparison led the conclusion that this method enabled us to measure SZWc quantitatively, which had been difficult to measure in the absence of skilled microscope operators. The method also showed the possibility to estimate the failure load from the fracture surface by using fracture mechanics parameters.

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.

Cryogenic fracture and adiabatic heating of austenitic stainless steels for superconducting fusion magnets

This paper discusses the fracture and the adiabatic heating of austenitic stainless steels that may be used in cryogenic structures for the superconducting magnets of magnetic fusion reactors. Elastic–plastic fracture toughness tests were performed on compact specimens in liquid helium at 4 K. Adiabatic heating was detected by measuring the internal temperature. The effect of test specimen size and side grooving on the cryogenic fracture toughness, and the temperature rise during dynamic crack growth are examined. A size independent toughness parameter results in the case of side-grooved specimens. The effect of inclusion content on the fracture mechanics parameters is also discussed.

An evaluation of cast stainless steel (CF8M) fracture toughness caused by thermal aging at 430°C

Cast stainless steel may experience embrittlement when it is exposed approximately to 300°C for a long period. In the present investigation, the three classes of the thermally-aged CF8M specimen were prepared using an artificially-accelerated aging method. After the specimens were held for 300, 1800 and 3600hrs. at 430°C, respectively, the specimens were quenched in water which is at room temperature. Load versus load line displacement curves andJ- R curves were obtained using the unloading compliance method,JIC values were obtained using the ASTM E 813–87 and ASTM E 813–81 methods. In addition to these methods,JIC values were obtained using the SZW (stretch zone width) method described in JSME S 001–1981. The results of the unloading compliance method areJQ = 543.9kj/m2 for virgin materials. The values ofJIC for the degraded materials at 300, 1800 and 3600hrs. are obtained 369.25kJ/m2, 311.02kJ/2, 276.7kJ/2, respectively. The results obtained by the SZW method are compared with those obtained by the unloading compliance method. Both results are quite similar. Through the elastic-plastic fracture toughness test, it is found that the value ofJIC is decreased with an increase of the aging time.

EXPERIMENTALLY DETERMINED KEY CURVES FOR FRACTURE SPECIMENS

Crack extension during fracture toughness tests of ferritic structural steels cannot be determined from measurements of unloading compliance or electric potential change when the specimen is dynamically tested. Measurements of crack extension in fracture toughness tests are also very difficult when the test temperature is high or the test environment is aggressive. To circumvent this limitation, researchers for years have been developing key curve and normalization function methods to estimate crack extension in standard elastic-plastic fracture toughness test geometries. In the key curve method (Ernst et al., 1979; Joyce et al., 1980) a load-displacement curve is measured for a so-called `source' specimen that is sub size or has a blunt notch so that the crack will not initiate during elastic-plastic loading. The load and displacement are then converted to normalized stress-strain units to obtain a key curve that can be used to predict crack extension in geometrically similar `target' specimens of same material loaded at similar loading rates and tested under similar environmental conditions. More recently Landes and coworkers (Herrera and Landes, 1990; Landes et al., 1991) proposed the normalization data reduction technique – Annex A15 of ASTM 1820 specification – that presents an alternative to the standard E1820 unloading compliance procedure. Although the normalization method works well in many cases, it has serious drawbacks: the load, displacement and crack length at the end of the test must be measured; the prescribed functional form that is fitted to the initial and final data may not be accurate for all materials; and the iterative method of inferring crack length from the combination of the data and the normalization function is complex. The compliance ratio (CR) method developed in this paper determines key curves for predicting crack extension as follows. First, a statically loaded source specimen with the unloading compliance procedure specified in ASTM 1820. Second, the so-called CR load-displacement curve is calculated for the source specimen, which is the load-displacement record that would have been obtained if the crack had not extended. Third, non-dimensionalizing the CR load by the maximum load and the displacement by the elastic displacement at the maximum load, P*i/Pmax and vi/vel max from the source specimen yields the adjusted key curve. Analysis of extensive data shows that the key curve is independent of notch type, initial crack length and temperature. But it is dependent on specimen size and steel type. Assuming that the key curves of the source and target specimens are one and the same, the compliance of the target specimens are calculated with a reverse application of the compliance ratio method, and the crack length is obtained using the equations in ASTM E1820. The CR Method is found to be much simpler than the normalization method described in the Annex A15 of ASTM 1820. With the compliance ratio method, Joyce et al. (2001) successfully predicted crack extension in dynamically loaded specimens using a key curve of a statically loaded specimen.

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.

A Study on the 43$0^{\circ}C$ Degradation Behavior of Cast Stainless Steel(CF8M) (III) - Evaluation of Elastic-Plastic Fracture Toughness -

A cast stainless steel may experience an embrittlement when it is exposed to approximately 30 for long period. In the present investigation, The three classes of the thermally aged CF8M specimie n are prepared using an artificially accelerated aging method. Namely, after the specimen are held for 300, 1800 and 3600hrs. at 43 respectively, the specimens are quenched in water to room temperature. Load versus load line displacement curves and J-R curves are obtained using the unloading compliance method. values are obtained following ASTM E 813-87 and ASTM E 813-81 methods. In addition to these methods, JIC values are obtained using SZW(stretch zone width) method described in JSME S 001-1981. The results of the unloading compliance method are =485.7 kJ/m for virgin material, of the degraded materials associated with 300, 1800 and 3600hrs are obtained 369.25 kJ/m, 311.02 kJ/m, 276.7 kJ/m, respectively. The results of SZW method are similar to those of the unloading compliance method. Through the elastic-plastic fracture toughness test, it is found that the value of is decreased with increasing of the aging time. The results obtained through the investigation can provide reference data for a leak before break(LBB) of reactor coolant system of nuclear power plants.

Elastic-Plastic Fracture Toughness of Steel Tube.

The strength evaluation of ductile and brittle fracture characteristics of high pressure steel tubes have to be comfirmed for the hydraulic pressure tests. The elastic-plastic fracture tough-ness;Jin tests were carried out with the small size arc-shape specimens from 2 kinds of steel tubes of different materials. The new tubes were made in several years before, and used tubes were made in 30 years before. (STBA 12, the chemical compositions are different from each other and used tubes might have the creep damage under 480°C, 2×105 hours in boiler super heater). And Jin characteristics were discussed on these rusults of elastic-plastic fracture toughness test.

A study on the fracture toughness evaluating method for cryogenic structural material

This paper was undertaken to develop the fracture toughness testing method using small and single specimen compared to the conventional method in evaluating elastic-plastic fracture toughness of the superconducting magnet structural material at cryogenic temperature. The elastic-plastic fracture toughness test was conducted by using the unloading compliance method recommended by ASTM E813-89 to accomplish the above purpose. And, the 20% side-grooved 0.5TCT and 1TCT specimens were used to evaluate the fracture toughness by using as possible as miniaturized CT specimen. The unloading compliance method was a very useful method in evaluating elastic-plastic fracture toughness at cryogenic temperature. It could be taken valid fracture toughness values by using 20% side-grooved 0.5TCT specimen recommended by ASTM E813-89.

An Experimental Study on the Evaluaiton of Elastic-Plastic Fracture Toughness under Mixed Mode I-II-III Loading Using the Optical PSD

In this paper, as elastic-plastic fracture toughness test under mixed mode loading was proposed using a single edge-cracked specimen subjected to bending moment(M), shearing force(F), and twisting moment(T). The J-integral of a crack in the specimen is expressed in the form J=+ $J_II$, where , and are the components of mode I, mode II and mode III deformation, respectively. , and can be estimated from M- (;crack opening angle), F-U(U; crack shear displacement) and T- (;crack twisting angle). In order to obtain the the M$\theta$, F-U and T- diagram inreal time, a new deformaiton gage for mixed mode loading was proposed using the optical position sensing device(PSD). The elastic-plastic fracture toughness test was carried out with an aluminum alloy. The loading apparatus was designed and manufactured for this experiment. For the loading condition of the crack initatio in the mixed mode, the MMT -3(mode I+ mode II+ mode III) has the lowest values out of the all specimens. This implies that MMT-3 is possible of the crackinitation at lower load, if the specimen acts on together with the torque under the same loading condition. An elastic-plastic fracture toughness test using the PSD brings a successful experimentation in measuring the crack deformation(mode I+ mode II+ mode III).

Factors Influencing Fracture Mechanism of IN718 at Cryogenic Temperature.

An elastic-plastic fracture toughness test was conducted at 4 K and 290 K to investigate the factors influencing fracture behavior of IN 718. First, the effect of grain size on the fracture toughness and tearing modulus was clarified. It was found that at 4 K the pop-in behavior was dominant in both coarse- and fine-grained materials. However, the fracture surface examination showed that ductile dimple formation prevailed in the coarse-grained meterial while alternating narrow dimple bands and cleavage facets were observed in the fine-grained material. Results showed that fracture toughness increases with increasing grain size. Second, IN 718 with few inclusions was used to clarify the effect of cleanliness. It was revealed that at 4 K the fracture toughness was comparatively increased due to the presence of few inclusions while the tearing modulus was decreased since the presence of many fine brittle carbides contributed to sudden crack growth.