Precracked Charpy V-notch Specimens Research Articles

Study of dynamic crack formation in nodular cast iron using the acoustic emission technique

In this study, acoustic emissions (AE) were analyzed to detect the onset of crack growth in nodular cast iron during dynamic loading at room temperature, 50°C and 65°C. To determine the AE signal from the crack growth, two types of specimens were used: Charpy V-notch specimens without a fatigue crack (CVN), and precracked Charpy V-notch specimens (PCVN). While the uncracked specimens showed only elastic and plastic deformation, the precracked specimens exhibited crack formation and extension, additionally. Damage at the notch/crack tip was analyzed by means of scanning electron microscopy. Investigations revealed that local cleavage fracture on the notch of the fatigue precrack occurred before the start of ductile stable crack. Acoustic emission measurements on Charpy impact test were complemented with the AE measurements on quasi-static bend test. The acoustic emissions, emitted from plastic deformation and the ductile crack growth during the impact test were not measurable. In contrast, local cleavage fracture was determined by means of AE even if it had a small area. The amplitude of AE signal correlates with the size of the cleavage areas.

이축하중을 받는 십자형 시편의 파괴인성 및 구속효과 평가

Current guidance considers that uniaxially loaded specimen with a deep crack is used for the determination of the ductile-to-brittle transition temperature. However, reactor pressure vessel is under biaxial loading in real and the existence of deep crack is not probable through periodic in-service-inspection. The elastic stress intensity factor and the elastic-plastic J-integral which were used for crack-tip stress field and fracture mechanics assessment parameters. The difference of the loading condition and crack geometry can significantly influence on these parameters. Thus, a constraint effect caused by differences between standard specimens and a real structure can over/underestimate the fracture toughness, and it affects the results of the structural integrity assessment, consequentially. The present paper investigates the constraint effects by evaluating the master curve <TEX>$T_0$</TEX> reference temperature of PCVN (Pre-cracked Charpy V-Notch) and small scale cruciform specimens which was designed to simulate biaxial loading condition with shallow crack through the fracture toughness tests and 3-dimensional elastic-plastic finite element analyses. Based on the finite element analysis results, the fracture toughness values of a small scale cruciform specimen were estimated, and the geometry-dependent factors of the cruciform specimen considered in the present study were determined. Finally, the transferability of the test results of these specimens was discussed.

Validation and application of the Master Curve and reconstitution techniques to a Spanish nuclear vessel

In this work, a methodology based on the Master Curve approach and the reconstitution of specimens is validated and applied to the vessel base metal of the currently in service Spanish boiling water reactor of the Santa María de Garoña nuclear power plant. The extensive experimental program performed consisted in the characterisation of the ductile to brittle transition region with standard and reconstituted specimens using subsized compact tension and Pre-Cracked Charpy V-notch specimens, under non-irradiated and irradiated conditions. Experimental results validated the reconstitution technique down to inserts of 10 mm which allows specimen reorientation and therefore, the comparison of LT and TL material orientations. The 110 specimens tested in this program allowed the Master Curve approach to be validated for the base steel of Santa María de Garoña nuclear power plant. By comparing the results for the compact and Pre-Cracked Charpy V-notch specimens, the existence of a systematic bias between these two geometries has been analysed. Although the neutron irradiation effect on the condition studied is predicted to be very limited (due to the small fluence), the results did allow the irradiation-induced shift of the ductile to brittle transition temperature to be detected. Comparison between the directly measured fracture toughness and the conventional semi-empirical approach proposed by the ASME Code reveals the overconservatism of the latter approach.

PCVN 시편 파괴인성의 균열 깊이 영향에 대한 Scaling 모델 해석

Standard procedures for a fracture toughness testing require very severe restrictions for the specimen geometry to eliminate a size effect on the measured properties. Therefore, the used standard fracture toughness data results in the integrity assessment being irrationally conservative. However, a realistic fracture in general structures, such as in nuclear power plants, may develop under the low constraint condition of a large scale yielding with a shallow surface crack. In this paper, cleavage fracture toughness tests have been made on side-grooved PCVN (precracked charpy V-notch) type specimens (10 by 10 by 55 mm) with various crack depths. The constraint effects on the crack depth ratios were evaluated quantitatively by the developed scaling method using the 3-D finite element method. After the fracture toughness correction from scaling model, the statistical size effects were also corrected according to the standard ASTM E 1921 procedure. The results were evaluated through a comparison with the T0 of the standard CT specimen. The corrected T0 for all of the PCVN specimens showed a good agreement to within 5.4℃ regardless of the crack depth, while the averaged PCVN T0 was 13.4℃ higher than the real CT test results.

Fracture toughness of irradiated modified 9Cr–1Mo steel

The effects of irradiation on fracture toughness of modified 9Cr–1Mo steel in the transition region were investigated. Half size precracked Charpy specimens were irradiated up to 1.2 × 10 21n/cm 2 ( E > 0.1 MeV) at 340 °C and 400 °C in the Korean research reactor. The irradiation induced transition temperature shift for a modified 9Cr–1Mo was evaluated by using the Master Curve methodology. The T 0 temperature for the unirradiated specimens were measured as −67.7 °C and −72.4 °C from the tests with standard PCVN (precracked charpy V-notch) and half sized PCVN specimens, respectively. The T 0 shifts of specimens after irradiation at 340 °C and 400 °C were 70.7 °C and 66.1 °C, respectively. The Weibull slopes for the fracture toughness data obtained from the unirradiated and irradiated modified 9Cr–1Mo steels were determined to confirm the applicability of master curve methodology to modified 9Cr–1Mo steel.

Analysis of Fracture Toughness in the Transition Temperature Region of API X70 Pipeline Steels Rolled in Two-Phase Region

In this study, fracture toughness in the transition temperature region of three API X70 line pipe steels rolled in the two-phase (α + γ) region was analyzed in accordance with the ASTM E1921-05 standard test method. Elastic-plastic cleavage fracture toughness (K Jc ) was determined by three-point bend tests, using precracked Charpy V-notch specimens, and then the measured K Jc values were interpreted by the three-parameter Weibull distribution. The fracture toughness test results indicated that the master curve and the 98 pct confidence curves explained the variation in the measured fracture toughness values well. Reference temperatures obtained from the fracture toughness tests as well as index temperatures obtained from the Charpy impact tests were lowest in the steel cooled at 500 °C, which did not contain brittle martensite and had a small effective grain size. In this steel, the absence of martensite led to the higher resistance to cleavage crack initiation, and the smaller effective grain size led to the higher possibility of crack arrest, thereby resulting in the best overall fracture properties.

Fracture toughness and the master curve for modified 9Cr−1Mo steel

Modified 9Cr−1Mo steel is a primary candidate material for the reactor pressure vessel of a Very High Temperature Gas-Cooled Reactor (VHTR) in the Korean Nuclear Hydrogen Development and Demonstration (NHDD) program. In this study, the T0 reference temperature, J-R fracture resistance and Charpy impact properties were evaluated for commercial Grade 91 steel as part of the preliminary testing for a selection of the RPV material for the VHTR. The fracture toughness of the modified 9Cr−1Mo steel was compared with that of SA508−Gr.3. The objective of this study was to obtain the pre-irradiation fracture toughness properties of the modified 9Cr-1Mo steel as reference data for an investigation of radiation effects. Charpy impact properties of the modified 9Cr-1Mo steel were similar to those of SA508−Gr.3. T0 reference temperatures were measured as −67.7 and −72.4°C from the tests with standard PCVN (pre-cracked Charpy V-notch) and half-sized PCVN specimens respectively, which were similar to the results for SA508−Gr.3. The KJc values of the modified 9Cr-1Mo steel with the test temperatures are successfully expressed by the Master Curve. The J-R fracture resistance of the modified 9Cr−1Mo steel at room temperature was nearly identical to that of SA508−Gr.3; in contrast, it was slightly higher at an elevated temperature.

Fracture toughness analysis in transition temperature region of API X70 pipeline steels

This study is concerned with the analysis of fracture toughness in the transition temperature region of API X70 pipeline steels according to ASTM E1921 standard test method. Two kinds of steels were fabricated by varying finish rolling temperatures, and their microstructures and mechanical properties were examined. Elastic–plastic cleavage fracture toughness, K J c was determined by three-point bend tests of precracked Charpy V-notch specimens, and then the measured K J c values were interpreted by the three-parameter Weibull distribution. Fractographic observation indicated that the critical distance from a precrack tip to a cleavage initiation site linearly increased with increasing critical J value. Effects of martensite–austenite constituent (MA) and effective grain size on fracture toughness in the transition region were analyzed from the standpoint of cleavage crack initiation and propagation. The higher volume fraction of MA led to the lower resistance to cleavage crack initiation, and the larger effective grain size led to the lower resistance to cleavage crack propagation. Thus, the steel rolled in the two-phase region had the higher fracture toughness in the transition region and the lower reference temperature and energy transition temperature than the steel rolled in the single-phase region because of the lower volume fraction of MA and the smaller effective grain size.

Ductile crack growth in pre-cracked CVN specimens: numerical studies

This study describes plane strain, finite element analyses to model ductile crack extension in pre-cracked Charpy specimens subjected to static and impact loading. The Gurson–Tvergaard (GT) dilatant plasticity model for voided materials describes the degradation of material stress capacity. Fixed-size, computational cell elements defined over a thin layer along the crack plane provide an explicit length scale for the continuum damage process. Outside of this layer, the material remains undamaged by void growth, consistent with metallurgical observations. The finite strain constitutive models include the effects of high strain rates on the material flow properties. Parametric studies focusing on numerically generated R-curves quantify the relative influence of impact velocity, material strain rate sensitivity, and properties of the computational cells (thickness and the initial cell porosity). In all cases, impact loading elevates significantly the R-curve by increasing the amount of background plasticity. The strong effects of impact loading on the driving force for cleavage fracture are illustrated through evolution of the Weibull stress. The analyses suggest a negligible, additional effect of tearing on the Weibull stress under impact loading. Validation of the computational cell approach to predict loading rate effects on R-curves is accomplished by comparison to static and impact experimental sets of R-curves for three different steels.

Loading rate effects on cleavage fracture of pre-cracked CVN specimens: 3-D studies

Specimen size and loading rate effects on cleavage fracture of ferritic steels tested in the ductile-to-brittle transition region remain key issues for the application of pre-cracked Charpy specimens. This investigation employs 3-D, nonlinear finite element analyses to assess crack-front stress triaxiality in quasi-static and impact-loaded pre-cracked CVN specimens, with and without side grooves. Crack-front conditions are characterized in terms of the Weibull stress which reflects the statistical effects on cleavage fracture. These 3-D computations indicate that a less strict size/deformation limit, relative to the limits indicated by previous plane-strain analyses, is needed to maintain small-scale yielding conditions at fracture under quasi-static and impact loading conditions. For impact toughness values which violate these size/deformation limits, a toughness scaling methodology is described to remove the effects of constraint loss. The new scaling model also enables prediction of the distribution of quasi-static fracture toughness values from a measured distribution of impact toughness values (and vice versa). This procedure is applied to experimental data obtained from a CrNiMoV pressure vessel steel and accurately predicts quasi-static fracture toughness values in 1T-SE(B) specimens from impact-loaded, pre-cracked CVN specimens. These 3-D analyses also yield η-total values for use in impact testing to infer thickness average and mid-thickness J-values from measured work quantities.

Fracture toughness of maraging steel from Charpy ?V? notch specimens

Compact tension (CT) and Charpy “V” notch (CVN) (impact and three-point bend) specimens of 18 Ni 1800 MPa maraging steel (parent metal and weldment) were used to determine plane strain fracture toughness (K IC) and CVN impact energy (CVNIE), respectively. Using an empirical equation,K IC-CVNIE correlation is attempted which could be advantageously utilized for routine quality control of inward material to effect savings in cost and time. Investigations reveal betterK IC-CVNIE correlation for tests using the precracked CVN specimens. Scanning electron microscopic (SEM) observations reveal good correlation between fractographic features and fracture toughness.

Brittle-ductile transition curve determined by charpy impact testing on CVN, precracked CVN and side-grooved precracked CVN specimens

Brittle-ductile transition curve determined by charpy impact testing on CVN, precracked CVN and side-grooved precracked CVN specimens

Determination of the Fracture-Energy Criterion JIc of Two Steels by Static and Dynamic Testing

Abstract The fracture energy criterion JIc was determined on two carbon-manganese steels, SA-516-70 and SA-105-B, from a single specimen. A graphic method is proposed for verifying the validity of measured values of JIc. Static JIc and dynamic JIcD experiments on precracked Charpy V-notch specimens at ambient temperature enabled the observation of the effect of loading rate on the value of JIc. For these two steels, the effect of side grooves was detected only in dynamic testing. For SA-516-70 steel, JIcD &amp;gt; JIc, while JIcD &amp;gt; JIc for SA-105-B steel, because of transition temperature fracture behavior. For SA-516-70, static testing was performed on three-point bend specimens of different dimensions to determine the effect of geometrical parameters on the value of JIc. A ligament 1-cm long is the lower limit for the determination of JIc.

The Determination of the J-Integral on Precracked Charpy Specimens by Instrumented Impact Testing and Slow-Bend Testing

The fracture criterion JIC is determined on SA-516-70 steel using precracked CVN specimens. The addition of an appropriate side-groove results in a better plane-strain condition at the crack tip and removes a major part of the absorbed energy due to lateral deformation. The value of JIC static is calculated from a single three-point-bend experiment. The displacement of the cracked front is followed by the measurement of the electrical resistance. We have shown that a single specimen is sufficient for determining JIC. Experiments on an instrumented Charpy machine were used for the calculation of the value of JICD. We assumed, for elasto-plastic behavior, that the maximum point of the load-displacement curve corresponds to the instability threshold of the crack. The values of JIC and JICD obtained by these two methods are compared and discussed.