The influence of loading rate on fracture resistance of notched and precracked ISO-V-specimens

Higher loading rates are expected to have a disadvantageous influence on fracture parameters. This opinion derives probably from experience with ductile-cleavage transition whose temperature increases with increasing loading rate. As far as results on the influence of loading rate on crack resistance are published in the literature they show mostly increasing initiation values as well as increasing crack resistance with increasing loading rate.The paper deals with the influence of loading rate on fracture resistance of notched and precracked ISO-V-specimens measured on different materials. The crack resistance of notched specimens has been determined in analogy to that one of the precracked specimens. The influence of loading rate was separately investigated with regard to initiation as well as to tearing modulus.The results demonstrate that the ratio of dynamic to quasi-static values decreases with increasing toughness, the initiation values for dynamic loading may drop at high toughness below those for quasi-static loading, the slope of the crack resistance curve for dynamic loading is mostly larger than for quasi-static loading and the ISO-V-specimens fall well into line with the results of valid tests.     

Determination of crack initiation and crack resistance using compact specimens, and comparison with results of numerical simulation of dynamically loaded wide plates

Static and dynamic crack resistance curves for the ferritic steel 20 MnMoNi 5 5 and the austenitic steel X6 CrNi 18 11 were determined for compact tension specimens, using a modified key curve method which contains a numerical calculation of the key curves. The method is described and verified under quasi-static loading. The J integral evaluation according to the ASTM standard (which at present is only applicable for ferritic steels under quasi-static loading) did not show in any case a considerable difference compared with the energy release rate approach. The numerical simulation of tests was carried out for wide plates made of austenitic material on the 12 MN high speed tensile testing machine at MPA Stuttgart.     

Determination of the dynamic fracture toughness using a new stress pulse loading method

This paper presents a method for the determination of the dynamic fracture toughness K_Id of metallic materials at loading rates K_I of about . The method is derived from the known split Hopkinson pressure bar technique and uses a well-defined stress pulse for the loading of a fatigue precracked specimen. The interpretation of the experimental data is strictly based on a numerical analysis of the specimen under the given dynamic loading conditions. It is shown, that a conventional quasi-static approach would yield incorrect fracture toughness values. The results for some steels confirm, that the fracture toughness decreases with increasing loading rate. Therefore, in some sense the fracture toughness versus temperature curve determined with the presented stress pulse method can be regarded as lower bound curve.     

The effect of irradiation on the toughness of pressurized water reactor vessel steels under different service conditions

The different toughness tests performed on two pressure vessel steels with very different upper shelves served to make a number of observations concerning the shifts in the transition temperature due to the effect of irradiation, as well as changes in toughness with temperature in the ductile region.With respect to shifts in the transition temperature, the following was observed: the shift obtained with precracked charpy test specimens was narrower than with the others; the shift obtained with charpy V impact tests was substantially equal to that obtained with CT test specimens — wider in the case of steel A, but slightly narrower in that of steel H.With respect to toughness values in the ductile region: the toughness values obtained using precracked charpy test specimens are significantly higher than those obtained with CT test specimens for static tests; 25and 12.5 mm thick CT test specimens display comparable variations in J_1C and dJ/da, but with wide scattering; the effect of irradiation, if any, is of the same order of magnitude as the scattering of the results — however, a test temperature effect is observed; the variation in toughness with temperature is determined more easily by considering a J value corresponding to a stable crack propagation of 1 mm, so that ; this variation of J_Δal with temperature is substantially the same for both steels, or about −30% at 70 or 80°C, and −50% at 290°C.     

Environmentally assisted crack growth in a low alloy boiler steel in high temperature water containing oxygen

The crack growth behaviour of the higher strength steel 17 MnMoV 6 4 in deionized high temperature water containing oxygen was investigated with respect to static loading. The tests were performed in an autoclave testing facility at an oxygen content of 8 ppm, a pressure of 70 bar and a temperature of 240°C under quasi-stagnant flow conditions. The stress intensities at the beginning of the tests were 17, 20, 27, 34, 40 and 58 MPa√m. In contrast to the higher loaded specimens no evidence of environmentally assisted cracking was found at stress intensities of 17 and 20 MPa√m.The maximum crack growth rate in the range where only environmetally assisted cracking occured amounted to about 4 x 10^-5 mm/s. The critical stress intensity K_IJ for the onset of stable ductile crack growth in air is 100 MPa√m. On the basis of fractographic studies the crack development found can be ascribed to the “Strain Induced Corrosion Cracking (SICC)” mechanism. This mechanism, used as a working hypothesis, gives a satisfactory explanation for the occurence of stress corrosion cracking of unalloyed and low alloyed steels in high temperature water. SICC is particularly characterized by aggravated corrosive attack occuring as soon as the magnetite/haematite protective layer has been locally disturbed. The stress concentration then just becomes so great that in the region of the resulting crack tips, yield/creep deformation within the critical range of strain rates occurs.     

Effect of loading rate upon fracture behavior of ferritic steel under large scaled yielding

Dynamic loading to ferromagnetic materials and large scaled yielding result in peak or valley and non-linear curve, respectively, on the Direct Current Potential Drop (DCPD) versus Crack Opening Displacement (COD) plots, which make it difficult to determine the crack initiation point. In this work high intensity of current up to 100 A was applied to the specimens of SA106Gr.C ferritic steel and the crack growth behavior was directly monitored by a high speed camera to obtain the crack initiation point. The effects of loading rate up to 1200 mm min⁻¹ upon the fracture resistance were explored. As the results, it has been shown that, although no substantial difference was seen in the load–COD plots, the crack initiation and then J_i and J–R curve were quite sensitive to the loading rate. That is, under the loading rate of 300 mm min⁻¹ the material showed the worst fracture resistance than under static loading and even under the higher loading rates of 600 and 1200 mm min⁻¹. Also applying the high speed camera and high current source have been proved to be an effective way to find out the accurate crack initiation point and to compensate the pulse of DCPD due to the ferromagnetic effect.     

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The focus of the investigations on ferritic nodular cast iron with varying graphite morphology was the determination of fracture mechanical characteristics and crack resistance curves of the J-integral and Crack Tip Opening Displacement (CTOD) concept under static and dynamic load. The experiments were performed at various temperatures down to 40 degrees below zero. The results show a close correlation between static crack initiation toughness values as well as crack resistance curves and quantitative graphite morphology parameters. The practised low-blow technique allowed the reproducible detennination of dynamic R curves of the J-integral and CTOD concept. First results of the R curve investigations under dynamic load draw attention to a correlation between crack initiation toughness and graphite morphology that is comparable to the results under static load. A comparison of static and dynamic crack initiation toughness data shows a loss of toughness with the dynamic values due to the higher loading rate. An influence of temperature on the static fracture mechanical crack resistance data in the investigated range of temperatures was found to depend on the size of the graphite nodules.     

Irradiation dose and temperature dependence of fracture toughness in high dose HT9 steel from the fuel duct of FFTF

To expand the knowledge base for fast reactor core materials, fracture toughness has been evaluated for high dose HT9 steel using miniature disk compact tension (DCT) specimens. The HT9 steel DCT specimens were machined from the ACO-3 fuel duct of the Fast Flux Test Facility (FFTF), which achieved high doses in the range of 3–148 dpa at 378–504 °C. The static fracture resistance (J-R) tests have been performed in a servohydraulic testing machine in vacuum at selected temperatures including room temperature, 200 °C, and each irradiation temperature. Brittle fracture with a low toughness less than 50 MPa √m occurred in room temperature tests when irradiation temperature was below 400 °C, while ductile fracture with stable crack growth was observed when irradiation temperature was higher. No fracture toughness less than 100 MPa √m was measured when the irradiation temperature was above 430 °C. It was shown that the influence of irradiation temperature was dominant in fracture toughness while the irradiation dose has only limited influence over the wide dose range 3–148 dpa. A slow decrease of fracture toughness with test temperature above room temperature was observed for the nonirradiated and high temperature (>430 °C) irradiation cases, which indicates that the ductile–brittle transition temperatures (DBTTs) in those conditions are lower than room temperature. A comparison with the collection of existing data confirmed the dominance of irradiation temperature in the fracture toughness of HT9 steels.     

Moment modified compact tension specimen for measuring crack arrest toughness

Crack arrest toughness in reactor vessel steels in the transition and Charpy upper shelf energy temperature range are of particular interest to the nuclear industry to aid with the analysis of the phenomenon known as pressurized thermal shock (PTS). A test specimen and analysis technique have been developed to measure crack arrest toughness at temperatures from the transition region up to and beyond the Charpy upper shelf energy level. The moment modified compact tension (MMCT) specimen combines a thermal gradient with mechanical loadings to initiate a crack in brittle material below NDT and then have arrest take place in hot, ductile material. A finite element model was used to help design the specimen and fixturing geometry as well as calculate the arrest toughness. Tests have been conducted on ASME SA533 Grade B Class 1 steel plate with a variety of loadings confirming the veracity of the technique and developing valuable data. Crack arrest toughness has been measured from 0°F to 110°F (−18°C to 43°C). This work has been part of a research program performed by C-E, Windsor and funded by the Electric Power Research Institute.     

Influence of loading rate on the fracture toughness versus temperature curve

This paper presents recent experimental investigations on the influence of loading rate on the fracture toughness K_Ic for different structural steels. The loading rate in terms of increasing in stress intensity factor was changed from quasistatic up to dynamic conditions (

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). The results show significant differences in the amount of temperature shift between K_Ic -T curve obtained after static and dynamic loading for the materials investigated. Based on microscopic fracture criteria for cleavage fracture correlations were made between fracture toughness and yielding behaviour depending on temperature and strain rate. The experimental results were also compared with the predictions given by different models. The most promising results were achieved by a correlation between transition temperature shift in K_Ic caused by dynamic loading and strain rate sensitivity of steels.     

Improvement of impact toughness of the SA 508 class 3 steel for nuclear pressure vessel through steel-making and heat-treatment practices

Comparative investigations of the effects of steel-making practices on impact and fracture toughness were studied. From these examinations, impact and fracture toughness of the steels by vacuum carbon deoxidation (VCD) offered the required values; however, those of the steels by modified VCD and silicon-killing practices were secure. The fracture toughness (K_IC) was significantly improved by the silicon-killing and the modified VCD. These resulted from the fineness of austenitic grain size and reduction of sulphidic inclusion. It was observed that the grain size of steels by modified VCD and silicon-killing practices was 20 μm, while that of steel by VCD was 50 μm. The sulphidic inclusion contents were reduced in the steels by modified VCD and silicon-killing practices. Furthermore, the effects of cooling rates from austenitizing temperature on the impact toughness in the steel by VCD were also investigated. The impact toughness of the steel by VCD was closely related to the cooling rate. To obtain the secure impact toughness in the steel by VCD, it seems that the recommended minimum cooling rate from the austenitizing temperature should be 15°C min⁻¹.     

Development of a database of pipe fracture experiments

Over the last 35 years, researchers worldwide have conducted hundreds—if not thousands—of pipe fracture experiments. In the early years, researchers focused their attention on studying the failure pressure and crack propagation behavior of axially cracked pipe loaded by internal pressure. The earliest work was sponsored by the oil and gas industry and, as such, involved relatively thin-walled, low toughness carbon steel pipes. This work was eventually followed up by efforts in the USA and Germany on nuclear piping with axial cracks. In recent years, attention has turned to understanding the behavior of circumferentially cracked nuclear piping subjected to both pressure and bending loads. The loading histories for these experiments range from the relatively simple case of quasi-static, monotonic displacement control to the more complex cases of dynamic cyclic loading, and pipe system experiments. In this paper, two of the leaders in this research, i.e. Battelle in the USA and MPA Stuttgart in Germany, have collaborated to develop a database of pipe fracture experiments. The database includes data from other organizations as well as the data from Battelle and MPA. In addition, as part of this paper, an example of how the database was used to assess the failure pressure of axially cracked pipe is given.     

Effect of specimen configurations on the fracture resistance curve

An experimental study on the constraint effect of fracture resistance curve (J–R curve) caused by specimen size variation was carried out with base metal of SA515 Gr. 60 carbon steel and SA516 Gr. 70 carbon steel and SA240 TP304 stainless steel and weld metal of SA516 Gr. 70 carbon steel and SA312 TP347 stainless steels. The resulting J–R curve increased with increasing plane size and decreased with increasing crack length. The test results show that the effects of plane size and crack length were significant, however, the effects of specimen thickness and size were trivial.     

Investigations on strength and ductility of base metals and welded joints of steels for power plants at elevated temperatures

Dynamic strain aging can appear at low alloyed ferritic steels and their welded joints. It influences the strength as well as toughness of the steels. It causes a relative maximum of impact energy at temperatures around 300°C. It also causes a distinct dependence of the strain rate on the shape of crack resistance curves. A lower slope of J_R-curves is to be found at higher deformation rates. The technical initiation J_Ic has a relative minimum in the range between 200 and 300°C.     

J-R fracture properties of SA508-1a ferritic steels and SA312-TP347 austenitic steels for pressurized water reactor’s (PWR) primary coolant piping

J-R fracture resistance of SA508-1a and SA312-TP347 steels, which are both rather peculiar as PWR primary coolant piping materials, were evaluated for application of leak-before-break methodology to the design basis of nuclear power plant piping. Archive materials from various heats of both steel pipes showed apparent heat-to-heat variations in ductile fracture resistance at the operating temperature 316°C. The SA508-1a ferritic steels showed relatively good J-R curve properties although they varied with the microstructures depending on the manufacturing process and chemical compositions. On the other hand, ductile crack growth resistance of SA312-TP347 austenitic stainless steel was unexpectedly poor when carbon content is moderately high. It was found that coarse niobium carbides deteriorated the ductile fracture resistance, so that more rigorous specification is needed in carbon and niobium contents to improve fracture properties of Type 347 stainless steels.     

Study on fracture toughness evaluation method of SA738Gr.B steel heat affected zone in ductile-brittle transition region

The Master Curve (MC) method is widely used to evaluate the fracture toughness of ferritic steels for reactor pressure vessels in the ductile-brittle transition zone, but this method is only applicable to macroscopically uniform ferritic steels. Based on the measured fracture toughness data of China-produced SA738Gr.B steel Heat-Affected Zone (HAZ) in the as-welded and heat-treated state, the problems of the standard MC method for characterizing the fracture toughness of the HAZ are discussed. Then the Bi-modal master curve method (BMC), SINTAP method, and Single point estimation method (SPE) were used to characterize the fracture toughness of the HAZ of SA738Gr.B steel in the ductile-to-brittle transition zone. The comparative analysis found that the BMC method is better in terms of the accuracy of the estimation and the over-dispersion envelope was caused by the heterogeneity. The estimation results of the SINTAP and SPE models are conservative but can be used for the safety evaluation of the actual structure. The study provided technical support for SA738Gr.B steel to exempt post-weld heat treatment issue.     

Some advances in fracture studies under the heavy-section steel technology program

Recent results are summarized from HSST studies in three major areas that relate to assessing nuclear reactor pressure vessel integrity under pressurized-thermal-shock (PTS) conditions. These areas are irradiation effects on the fracture properties of stainless steel cladding, crack run-arrest behavior under non-isothermal conditions, and fracture behavior of a thick-wall vessel under combined thermal and pressure loadings.Since a layer of tough stainless steel weld overlay cladding on the interior of a pressure vessel could assist in limiting surface crack extension under PTS conditions, its resistance to radiation embrittlement was examined. A stainless steel overlay cladding, applied by a submerged arc, single-wire, oscillating-electrode method, was irradiated to 2 × 10²³ neutrons/m² (> 1 MeV) at 288°C. Yield strength increases up to 27% and a slight increase in ductility were observed. Charpy V-Notch data showed a ductile-to-brittle transition behavior caused by temperature-dependent failure of the 8-ferrite phase. The type 308 cladding, microstructurally typical of that in reactor pressure vessels, showed very little degradation in either upper-shelf energy or transition temperature due to irradiation.Crack-arrest behavior of A533 grade B class 1 steel was examined for temperatures extending above the onset of Charpy upper-shelf. Crack-arrest experiments that use wide-plate specimens have shown crack arrest occurring prior to transition to tearing or tensile instability. High values of crack-arrest toughness have been recorded (static values above 400 MPa that are well above the maximum value that safety assessment criteria assume such materials can exhibit.A validation experiment was performed by exposing an intentionally flawed HSST intermediate test vessel to combined pressure and thermal transients. The experiment addressed warm-prestressing phenomena, crack propagation from brittle to ductile regions, and crack stabilization in ductile regions. Test and analysis results are summarized.     

中国低活化马氏体钢室温和150℃下1/2CT断裂韧性实验研究

利用卸载柔度法参考ASTM E1820-11标准对聚变堆候选结构材料中国低活化马氏体(CLAM)钢在室温和1 50℃条件下的断裂韧性进行了测试分析,同时对断面进行了扫描电子显微镜(SEM)微观分析。结果显示,CLAM钢1/2CT样品在室温和1 50℃条件下测试的断裂韧性J_Q分别为287 kJ/m~2和256 kJ/m~2,在这两个测试温度下CLAM钢均表现出较高的断裂韧性,且随着测试温度的升高断裂韧性有所降低。断面SEM观察显示韧窝布满整个断面,裂纹稳定扩展区域为韧性断裂。     

Yielding- and fracture behaviour of ferritic steels in the transition region of quasistatic to dynamic loading

This paper presents results of investigations on the influence of loading rate on yielding and fracture behaviour of ferritic steels. The range of loading rates was below a certain level at which a special stress wave analysis is required. Concerning the yielding behaviour it was found, that the yield strength can be predicted by the model of thermally activated flow. The strain hardening dσ/dε appeared to be independent of strain rate, if adiabetic heating can be neglected. Concerning the fracture behaviour it is demonstrated that the K_lc-T-curves are shifted to higher temperatures with increasing loading rate. The temperature shift could be correlated with the strain rate sensitivity m = d lnσ/d lnε. The ductile/brittle transition temperature increases with incrasing loading rate. For the upper shelf region crack resistance curves as a function of loading rate are presented. It is shown that the crack length can be determined using the key-curve-method. A slightly increasing tendency of the evaluated J_R-curves was found.     

Elastic-plastic fracture-mechanics assessment of a CrMoNi pressure vessel steel

This program involved the testing both of SENB- and CT-specimens and model vessels from pressure vessel steel 18CrMoNi5.4 V with the aim of studying specimen size and flaw size effects on the toughness behaviour. A comparison was made between the R-curves J-Δa of usual specimens in fracture-mechanics investigation (CT, SENB, partly 20% side-grooved) and Charpy specimens with static and dynamic loading. The crack blunting line was determined experimentally from SZW-measurements and used as a reference line for determining the crack initiation point. The R6-procedure was used to estimate the influence of flaw size in cylindrical model vessels with axial surface flaws loaded by internal pressure. A failure curve was constructed for option 2 with regard to the material deformation properties by the Ramberg-Osgood law.