Sub-size Specimens Research Articles

Using Subsize Impact Bend Specimens for Estimation of Irradiation and Re-Irradiation Embrittlement of VVER RPV Steels

Abstract Results of impact bend tests of subsize Charpy V-notch-type (CVN) specimens are used in Russia for monitoring both re-irradiation embrittlement of the first generation of VVER-440 (including VVER-440/179 and VVER-440/230, i.e., projects B-179 and B-230) reactor pressure vessel (RPV) steels and irradiation embrittlement of VVER-440/213 RPV steels. For the first generation of VVER-440 RPVs, no surveillance program was foreseen and only subsize specimens (5 by 5 by 27.5 mm and 3 by 4 by 27 mm) can be manufactured from samples taken from the inner surface of the RPV for radiation embrittlement monitoring. For VVER-440/213 RPVs, 80 % of the surveillance specimens are irradiated with a lead factor greater than 20. Only four specimens of base metal and four specimen halves of weld seam material are irradiated with a lead factor less than 3. Taking into account that full-size (10 by 10 by 55 mm) CVN specimens were used in surveillance programs, using subsize specimens enables to solve the flux effect problem for VVER-440/213 RPV beltline materials and ensure representativity of surveillance programs. Application of subsize specimens for the estimation of radiation embrittlement requires developing a correlation between transition temperature values evaluated using test results of standard (full-size) and subsize CVN specimens. Such a correlation is discussed.

Application of Subsize Specimens for Irradiation Embrittlement Monitoring of VVER-440/213 RPV Steels

Abstract Results of VVER-440/213 reactor pressure vessel (RPV) surveillance specimens studies showed that all surveillance impact bend specimens were irradiation with a lead factor more than 20 compared to irradiation of the pressure vessel inner surface. This is a special problem of radiation embrittlement assessment for VVER-440/213 RPV materials. Prediction of radiation embrittlement of RPV materials using surveillance specimens is efficient on the basis of accelerated irradiation compared to irradiation of the RPV inner surface. However, the lead factor, which is the ratio of neutron flux affecting surveillance specimens to neutron flux affecting the RPV inner surface in the maximum irradiated zone, should be not too high, when acceleration of steel irradiation would not cause qualitative changes in physical processes of irradiation damage. It is generally assumed that irradiation with a lead factor less than 3–6 provides adequate assessment of radiation embrittlement dependence on the damage dose. In the top part of the chains with VVER-440/213 surveillance specimens, fast neutron flux decreases ∼20 times and the top capsules are irradiated with a neutron flux range close to the flux maximum on the inner surface of the reactor pressure vessel; i.e., the lead factor there is around 1. This concerns both reactors with full core and with dummies on the periphery of the core. The gradient of the neutron flux in the top and bottom parts of chains is great enough; hence, the volume of homogeneously irradiated metal is low. However, there is an opportunity to use material from the top capsules of irradiation sets for correct evaluation of the transition temperature using subsize specimens. As surveillance specimens in the top capsules are irradiated at the same conditions as the pressure vessel, their study allows precise determination of radiation embrittlement of RPV materials. The results of studies of irradiation embrittlement of VVER-440/213 RPV steels using subsize specimens are discussed.

Effect of Temperature and Specimen Thickness on Toughness of Nickel Alloy 22

Nickel Alloy 22, UNS N06022, is being evaluated for use as the material of construction in permanent containers for spent nuclear fuel in Yucca Mountain, NV. To meet nuclear design criteria, Charpy impact data were required for the N06022 plate material, but conventional full-size specimens exceed the energy capacity of typical 400 J impact test machines, which results in stopping the pendulum during the test. Half-size specimens break with about 40% of the machine energy capacity, but their use raises questions concerning energy scaling to full size equivalent data. To address this, a range of subsize specimens were tested at room temperature using a standard 400 J impact test machine, and full-size, 3/4-size, and 2/3-size specimens were tested on a high energy capacity, 950J machine. Additional tests were performed at temperatures ranging from -196 to +200°C. Impact energy and lateral expansion measurements for the various test conditions are presented, their implications are examined, and a new model for absorbed energy correlation between subsize specimens and full size conventional Charpy specimens is proposed.

Embrittlement behavior of neutron irradiated RAFM steels

The effects of neutron irradiation on the embrittlement behavior of reduced activation ferritic/martensitic (RAFM) steel EUROFER97 for different heat treatment conditions have been investigated. The irradiation to 16.3 dpa at different irradiation temperatures (250–450 °C) was carried out in the Petten High Flux Reactor in the framework of the HFR Phase-IIb (SPICE) irradiation project. Several reference RAFM steels (F82H-mod, OPTIFER-Ia, GA3X) and MANET-I were also irradiated at selected temperatures. The embrittlement behavior and hardening were investigated by instrumented Charpy-V tests with subsize specimens. The neutron irradiation induced embrittlement and hardening of as-delivered EUROFER97 are comparable to those of investigated reference steels, being mostly pronounced for 250 °C and 300 °C irradiation temperatures. Heat treatment of EUROFER97 at higher austenization temperature substantially improves the embrittlement behavior at irradiation temperatures of 250 °C and 350 °C.

Plastic deformation of SUS304 under in situ and post-irradiation fatigue loadings

Side-notched subsize specimens of SUS304 were fatigued in the unirradiated condition, during in situ irradiation with 17 MeV protons at 300 °C and post-irradiation conditions. Nano-indentation tests were conducted on the specimens fatigued for 6, 24, 100 and 200 h in the unirradiated condition, for 24 h in the in situ irradiation condition and for 200 h in the post-irradiation condition. When comparing the distribution maps of nano-hardness for the specimens fatigued for 24 h during in situ irradiation and unirradiated conditions, less significant change of hardness was detected at the notch tip for the in situ irradiation specimen. As for the post-irradiation specimen fatigued for 200 h, the area of higher nano-hardness at the notch tip was smaller than that of the unirradiated specimen fatigued for 200 h. The mechanism of the irradiation effect on fatigue behavior is discussed in terms of the plastic deformation under in situ and post-irradiation fatigue loadings.

Evaluation of the Ductile-Brittle Transition Behaviour of Aging Specimens by the Fracture Toughness Test

The safety and reliability of industrial machineries and structures used in various, and severe conditions has become an increasing concern. This study proposes ductile-brittle transition temperature (DBTT) evaluation technique by the sub-sized specimen test to prevent cleavage fracture. Four classes of the thermally aged 1Cr-1Mo-0.25V specimens were prepared. The fracture toughness tests were performed on sub-sized specimens both at room and low temperatures. The results of the fracture toughness tests were analyzed to obtain the DBT behaviours of the specimen materials

Measurement and Analysis of Impact Test Data for X100 Pipeline Steel

Charpy upper shelf energy is widely used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. The measurement of this fracture criterion particularly for modern steels and its apportion into different components, i.e. fracture and non-related fracture energy, are of great importance for pipeline engineers. This paper presents the results of instrumented Charpy impact experiments on high-grade pipeline steel of grade X100. First, the instrumentation technique including the design and implementation of a strain gauge load-cell and the details of the data-recording scheme are reviewed. Next, the experimental data obtained from the Charpy impact machine so instrumented are presented and discussed. These include the test data from full and sub-sized Charpy V-notched specimens. The instrumented Charpy machine was able to capture the load history in full during the fracture process of the test specimens resulting in a smooth load-time response. This eliminated the need for filtering used in similar test techniques. From the recorded test data the hammer displacement, impact velocity and fracture energy were numerically calculated. The results showed that there was a significant drop in hammer velocity during the impact event. This resulted in a change in the fracture mode from dynamic to quasi-static which was more appreciable for full-size Charpy test samples. As a result, sub-sized specimens might be preferable for impact testing of this steel in order to guarantee the conditions of dynamic crack propagation in the specimen ligament. Accurate analysis of the instrumented impact test data showed that the ratio of crack initiation energy to propagation energy was around 30% for the X100 steel. It can be concluded that in impact testing of high-grade pipeline steel a significant portion of overall fracture energy is consumed in non-related fracture processes. This high fracture initiation energy should be accounted for if the current failure models are going to be used for toughness assessment of highstrength low-alloy gas pipeline steels.

High-dose neutron irradiation embrittlement of RAFM steels

Neutron irradiation-induced embrittlement of the reduced-activation ferritic/martensitic (RAFM) steel EUROFER97 was studied under different heat treatment conditions. Irradiation was performed in the Petten High Flux Reactor within the HFR Phase-IIb (SPICE) irradiation project up to 16.3dpa and at different irradiation temperatures (250–450°C). Several reference RAFM steels (F82H-mod, OPTIFER-Ia, GA3X and MANET-I) were also irradiated at selected temperatures. The impact properties were investigated by instrumented Charpy-V tests with subsize specimens. Embrittlement and hardening of as-delivered EUROFER97 steel are comparable to those of reference steels. Heat treatment of EUROFER97 at a higher austenitizing temperature substantially improves the embrittlement behaviour at low irradiation temperatures. Analysis of embrittlement in terms of the parameter C=ΔDBTT/Δσ indicates hardening-dominated embrittlement at irradiation temperatures below 350°C with 0.17⩽C⩽0.53°C/MPa. Scattering of C at irradiation temperatures above 400°C indicates no hardening embrittlement.

Fatigue crack propagation properties from small sized rod specimens

Mechanical properties characterization is needed in many industrial applications yet sufficient amount of material for fabricating standard-sized testing specimens is often not available. Techniques for testing miniaturized specimen must be adopted. Much effort has been made to develop techniques for impact, fracture toughness and tensile properties of sub-sized specimens. Work on the testing of fatigue properties is more limited. In this study, fatigue crack propagation behavior is evaluated from the growth of surface crack in a cylindrical rod under tension. Rods of various lengths and diameters were tested. As the size of the rod specimen is reduced, the fatigue crack growth rate tends to increase when correlated using the stress intensity factor range. This increase is explained largely by the decrease in the degree of premature crack closure in the small specimens. Valid fatigue crack growth data can be obtained among the specimens examined except on the crack growth on the surface of the smallest specimen, which has a length of 26 mm and diameter of 8 mm. Even so, valid data can still be elucidated on the latter specimen if the interior growth is considered. The dimensions of the latter specimen allow fatigue properties to be evaluated using broken remnants from impact or other test specimens.

Dynamic Finite Element Modeling of Fracture in Charpy V-Notch Specimens of Weld Material 72W

Abstract Dynamic finite element modeling of the fracture behavior of fatigue-precracked and V-notch Charpy specimens was performed using ABAQUS Explicit. The purpose of this modeling effort was to predict the upper shelf energies of full-size precracked and Charpy V-notch full-size and sub-size specimens based on the upper shelf energy (USE) of precracked specimens. To achieve this end, finite element model prediction of the USE of a precracked Charpy specimen was calibrated to the corresponding experimental data. Based on this calibration, a material model incorporating a stress-strain curve and fracture strain properties was developed for the nuclear reactor pressure vessel 72 material. The calibrated material model was then applied to the modeling of crack extension and propagation for predicting the USE of both full-size and sub-size Charpy precracked and V-notch specimens. Finite element modeling of Charpy V-notch models has not been performed previously using this methodology. It was found that the predicted USE of standard and sub-size specimens in both notched-only and precracked conditions based on the USE of third-size specimens were in reasonable agreement with the corresponding experimental data. However, when full-size precracked USE was used as the calibration point, full-size notched USE matched well, half and third-size precracked USE were within normal scatter and half and third-size notched USE did not acceptably match with the corresponding experimental values.

Impact properties of sintered and wrought 17–4 PH stainless steel

Charpy V notch (CVN) impact testing was conducted on full size and subsize specimens of sintered and wrought 17–4 PH stainless steel (17–4 PH SS) in the as sintered and H900 heat treated conditions. Test geometries correspond to the American Society for Testing and Materials (ASTM) and Metal Powder Industries Federation (MPIF) impact testing standards. Merits of a notched specimen compared with an unnotched specimen were analysed for both the wrought and sintered materials. The notched ASTM standard bars had a lower coefficient of variance for impact energy than the unnotched MPIF standard bars and displayed greater toughness. Porosity and grain size have a detrimental synergistic effect on impact toughness for the sintered material. Following a discussion about the differences in the wrought and sintered microstructures, it is recommended that impact testing of the injection moulded and sintered specimens should be evaluated according to the ASTM test specifications.

A Two-Parameter Method for Determining the Fracture Toughness of Materials from Subsized Specimens

The fracture toughness of brittle materials can be determined easily using the Linear Elastic Fracture Mechanics (LEFM) approach, and the only demand for change is the desire to decrease specimen size requirements. Specimen size effect on the normally measured KQ and PMAX/PQ values was revisited in order to establish the suitability of a relationship between these parameters. A linear dependence was confirmed from experimental data of materials as different as isotactic polypropylene, AISI 4140 steel, a titanium alloy, and an aluminum alloy. The load ratio was interpreted as a second parameter, and thorough this dependence, KQ values measured on subsized specimens were extrapolated to linearity conditions PMAX/PQ = 1, determining a value identified as K1.0. When some simple conditions were met, this K1.0 value matched the toughness of evaluated materials. For the analyzed materials, this toughness parameter could be obtained from subsized specimens with important size reduction compared with that needed for KIC valid value determination. Moreover it permitted a simple characterization of material as difficult to characterize as isotactic polypropylene, which initially motivated this study.

High-Cycle Fatigue Behavior of Type 316L Stainless Steel

High-cycle fatigue tests were conducted to investigate the effects of temperature, stress ratio (R), specimen orientation, welding and specimen size on the fatigue behavior of type 316L stainless steel. The high-cycle fatigue test results indicated that the fatigue limits significantly decreased when the stress ratio (R) decreased. The corresponding fatigue limits were reduced to lower values when tests were conducted at 300 � C, compared to those obtained at room temperature. The fatigue behavior and fatigue limits of standard and subsize specimens were observed to be consistent at both room temperature and 300 � C. The constant life diagram was established from the S–N curves acquired. The fatigue limit strongly depended on the materials strength, which was a function of specimen orientation, test temperature, and welding processes. The dimension of the fatigue damaged area on a fracture surface increased as the stress ratio decreased. In the case of R ¼� 1:0, the fatigue damaged region extended over the whole fracture surface. The subgrain boundaries after high-cycle fatigue tests were clearly demonstrated by their diffraction patterns, which were related to the dynamic recovery of multiple dislocations.

Evaluation of Ductile-Brittle Transition of Fracture Toughness by Material Degradation

When huge energy transfer systems like nuclear power plants and steam power plants are operated for long times at high temperatures, mechanical properties change and ductile-brittle transition temperature increases by degradation. So we must estimate the degradation in order to assess safety, life expectancy, and other operation parameters. The sub-sized specimen test method using the surveillance specimen, and BI (Ball Indentation) method were developed for evaluating the integrity of metallic components. In this study, we will present the evaluation of the ductile-brittle transition temperature using the BI test and the sub-sized specimen test. The four classes of the thermally aged 1Cr-1Mo-0.25V specimens were prepared using an artificially accelerated aging method. The tensile test, the fracture toughness test, and the BI test were performed. The results of the fracture toughness tests using the sub-sized specimens were compared with those of the BI test. The evaluation technique of the ductile-brittle transition temperature using the BI test was also discussed. Our results show that the ductile-brittle transition temperatures rose as the aging time increased. We suggested that the fracture toughness results of the sub-sized specimen test and the IEF results of the BI test could be used in the estimation of the ductile-brittle transition temperature as material degrades.

Impact Characterization of Sub-Size Charpy V-Notch Specimens Prepared from Full-Size Certified Reference Charpy V-Notch Test Pieces

Abstract To investigate the feasibility of the production of sub-size Charpy reference test pieces, tests were performed on sub-size samples (3 × 4 × 27 mm3) carefully machined from full-size (10 × 10 × 55 mm3) Charpy V-notch certified reference test pieces. The measured absorbed energy (KV) values indicate a highly satisfactory homogeneity of the sub-size specimens. Sub-size specimens were prepared from full-size reference test pieces of 30, 60, 80, 120, and 150 J nominal absorbed energy; and values between 2 J and 7.2 J were obtained, covering almost fully the range of interest for sub-size Charpy tests. A linear relationship is observed between the KV values of sub-size and full-size samples of the same material.

C-Specimen Fracture Toughness Testing: Effect of Side Grooves and η Factor

Elastic-plastic crack front fields in arc-shaped tension specimens (C-specimens) were analyzed by a three-dimensional finite element method. The effect of side grooves on the ductile fracture behavior was investigated by studying the J-integral distribution, plane-strain constraint parameter, and development of plastic zones and comparing to experimental data. The applicability of the η factor (derived for use with compact tension specimens) for the calculation of J-integral values for the C-specimen was also investigated. The results show that side grooves promote and establish near plane strain conditions at the crack front in sub-size specimens. It was also found that a two-dimensional plane-strain analysis in conjunction with the standard American Society for Testing and Materials (ASTM) tests was sufficient to determine the fracture toughness values from side-grooved C-specimen. The results indicate the η factor for compact tension specimen as specified in the ASTM standards appears to produce reliable results for the calculation of J of C-specimens.

On the Possibility of Avoiding the Use of a Clip Gage When Testing Precracked Charpy-V Specimens in the Ductile-to-Brittle Transition Region

Abstract Fracture toughness testing using bend-type samples, such as precracked Charpy specimens, requires using a clip gage for measuring crack mouth opening displacement (CMOD). This paper demonstrates that it is possible to avoid using a clip gage and use instead machine crosshead displacement (MCD) as an approximation of load-line displacement; this simplifies considerably the test procedure, both in terms of mechanical operations and specimen instrumentation. This is particularly desirable for irradiated materials and when testing subsize specimens. Results obtained in the ductile-to-brittle transition regime from the analysis of 151 toughness tests show that reference temperatures calculated using the Master Curve methodology with CMOD and MCD are fully equivalent within a few degrees. The investigation has been conducted on different materials, test machines, full-size, and subsize specimens.

On the Identification of Critical Damage Mechanisms Parameters to Predict the Behavior of Charpy Specimens on the Upper Shelf

Abstract The use of sub-sized Charpy-V specimens to monitor the mechanical properties of reactor pressure vessel (RPV) steels is receiving increasing attention. It is motivated by the fact that a smaller amount of material can be used to obtain the fracture properties, which can be of great economical interest when dealing with activated materials. Prior to use reduced-size specimens, investigations are still necessary to verify that the properties obtained from sub-sized specimens are comparable to those obtained from standard size specimens. Amongst the approaches that are available, the local approach to failure, which combines a description of the material microstructure (void density, distance between inclusions…) and empirical observations, is a promising way to investigate the transferability of mechanical properties. It has been successfully used to describe the macroscopic behavior of a wide range of cracked or notched specimens and components of various dimensions but restricted so far to quasi-static loading conditions. The results discussed in the paper are focused on the upper shelf behavior of the French RPV material, equivalent to the American ASTM A508 Cl.3 material. Modeling of the material is achieved using the Rousselier coupled damage constitutive equations. This model is implemented in CAST3M, a general-purpose finite element code developed by the French Atomic Energy Agency. Comparisons between the numerical results and the experimental observations are given and the computed local temperature elevations and plastic strain rates discussed to explain the behavior of the specimens.

Embrittlement behaviour of low-activation alloys with reduced boron content after neutron irradiation

Ferritic/martensitic steels for fusion applications have been irradiated up to 2.4 dpa in the Petten high flux reactor (HFR); their embrittlement behaviour was investigated by instrumented Charpy-V tests with subsize specimens. The aim of this mid-dose range programme was a comparison of low-activation alloys subjected to different heat treatments and with reduced B contents (down to 2 wt ppm). In the present report, the results of different OPTIFER alloys (Ia, II, IV, V, VI), as obtained in Phases IA and IB of the HFR-irradiation programme (2.4 dpa, at 250–450 °C), are analysed and assessed in comparison to the results of the former MANITU programme. The evaluation clearly shows the eliminated embrittlement problem for the advanced European reduced-activation alloys in comparison to international reference steels. This improvement can be clearly correlated to the reduction of the boron content. Furthermore, the influence of different heat treatments on the impact properties is presented.

Wear properties of Co–Cr–Mo–N plasma-melted surgical implant alloys

This paper examines the feasibility of using metal-on-metal bearings to reduce the wear debris which commonly occurs in UHMWPE (ultra-high molecular weight polyethylene) joint materials in knee replacements. HPPM (high pressure plasma melting) was used to make high nitrogen Co–Cr–Mo implant alloys. The melted ingots were subsequently hot-forged and heat-treated. Sub-size specimens were used to evaluate tensile strength and wear properties. Experimental results indicated that the highest nitrogen content of ingots observed in this study was 0.288 wt.%, presenting better tensile and wear properties than those of specimens with less nitrogen. Homogenization treatment can further improve the wear resistance and ductility of the alloys.