Study on the effect of the crack length on the JIC value

The ductile fracture tests are carried out using compact type (CT) specimen, three point bend specimen and center cracked tension (CCT) specimens made of A533B steel and aluminum alloy with different crack lengths. The effect of the crack tip constraint on the microscopic fracture behavior is studied by the scanning electron microscope (SEM). It is shown that the apparent J_IC value increases due to the decrease of the crack tip constraint. It is pointed out that the increase of the apparent J_IC value is partly due to the error of the conventional equation to estimate the J value. Based on the FEM analyses, these apparent J_IC values are corrected and are compared with the valid J_IC values. The good co-relation between apparent J_IC value and the Q factor, proposed by O'Dowd and Shih, is shown for all the specimens. The FEM analyses based on the Gurson's constitutive equation is also carried out. The effect of the constraint on the crack tip field is discussed.     

Influences of the dynamic strain aging on the J–R fracture characteristics of the ferritic steels for reactor coolant piping system

The leak-before-break (LBB) design of the piping system for nuclear power plants has been based on the premise that the leakage due to the through-wall crack can be detected by using leak detection systems before a catastrophic break. The piping materials are required to have excellent J–R fracture characteristics. However, where ferritic steels for reactor coolant piping systems operate at the temperatures where dynamic strain aging (DSA) could occur, the fracture resistance could be reduced with the influence of DSA under dynamic loading. Therefore, in order to apply the LBB design concept to the piping system under seismic loading, both static and dynamic J–R characteristics must be evaluated.Materials used in this study are SA516 Gr.70 for the elbow pipe and SA508 Cl.1a for the main pipe and their welding joints. The crack extension during the dynamic and the static J–R tests was measured by the direct current potential drop (DCPD) and the compliance method, respectively. This paper describes the influences of the dynamic strain aging on the J–R fracture characteristics with the loading rate of the pipe materials and their welding joints.     

Use of J–R curves in assessing the fracture behavior of low upper shelf toughness materials

The objective of this investigation was to evaluate the use of small specimen J–R curves in assessing the fracture resistance behavior of reactor vessels containing low upper shelf (LUS) toughness weldments. As required by the U.S. Code of Federal Regulations (10 CFR, Part 50), reactor vessel beltline materials must maintain an upper shelf Charpy V-Notch (CVN) energy of at least 50 ft-lbs (68 J) throughout vessel life. If CVN values from surveillance specimens fall below this value, the utility must demonstrate to the U.S. Nuclear Regulatory Commission (NRC) that the lower values will provide “margins of safety against fracture equivalent to those required by Appendix G of the ASME Boiler and Pressure Vessel Code”. This paper will present recommendations regarding the material fracture resistance aspects of this problem and outline an analysis procedure for demonstrating adequate fracture safety based on CVN values.It is recommended that the deformation formulation of the J-integral be used in the analysis described above. For cases where J-integral fracture toughness testing will be required, the ASTM E1152-87 procedure should be followed, however, data should be taken to 50% to 60% of the specimen remaining ligament. Extension of the crack growth validity limits for J–R curve testing, as described in E1152-87, can be justified on the basis of a “J-controlled crack growth zone” analysis which shows an engineering basis for J-control to 25% to 40% of the specimen remaining ligament. If J-R curve extrapolations are required for the analysis, a simple power law fit to data in the extended validity region should be used. The example analysis performed for low upper shelf weld material, showed required CVN values for a reactor vessel with a 7.8 inch (198 mm) thick wall ranging from 32 ft-lbs (43 J) to 48 ft-lbs (65 J), depending on the magnitude of the thermal stress component.     

Effect of dynamic strain aging on the leak-before-break analysis in SA106 Gr.C piping steel

The characteristics of dynamic strain aging (DSA) on material properties used in leak-before-break (LBB) analysis were discussed. Using these material data, the effect of DSA on the LBB analysis was estimated through the evaluation of leakage-size crack and flaw stability in SA106 Gr.C piping steel. Also, the results were represented as a form of ‘LBB allowable load window'. In the DSA temperature region, the leakage-size crack length was smaller than that at other temperatures and it increased with increasing tensile strain rate. In the results of flaw stability analysis, the lowest instability load appeared at the temperature corresponding to the minimum J–R curve which was caused by DSA. The instability load depended on the loading rate of J–R data, and decreased with increasing tensile strain rate at the plant operating temperature. These are due to the strain hardening characteristic and strain rate sensitivity of DSA. In the ‘LBB allowable load window', the LBB allowable region at the temperature and loading conditions where DSA occurs was decreased by about 30% compared with that in other conditions.     

Application of the J-integral concept to thermal shock loadings

In the frame of our analytical work the applicability of ductile fracture mechanical J-integral concept on mechanical and thermal shock loaded structures with flaws is investigated. By that the behaviour of possible flaws in components of power plants during accidents can be described (e.g. reactor pressure vessel and piping during emergency cooling).The analyses presented in this paper have been performed with a version of the finite element code ADINA [1] extended by fracture mechanical options. The postanalyses of the first series of pressurized thermal shock experiments (PTSE-1A, B, C) performed at ORNL show stress intensity factors (K_I) calculated from J-integrals which are about 10% lower than values of OCA programs [2] based on the linear elastic K-concept usually used for brittle materials. The discrepancy may be referred to different treatment of the influence of plasticity. The results assessed in the frame of the cleavage fracture concept coincide well with the measured times respectively crack tip temperatures at crack initiation and arrest.In the first thermal shock experiment (NKS-1) performed at the MPA-Stuttgart a circumferentially deep cracked test cylinder with overall upper shelf material conditions has been investigated. The postcalculations based on the J-integral with J_R-controlled crack growth show good coincidence between analytical determined and measured structure and fracture mechanical quantities but they are accompanied with numerical problems due to unloading and large plasticity effects.     

A faster procedure for the calculation of the J(ξ, β) function

One of the biggest difficulties in obtaining an analytical expression for the J(ξ, β) function is its explicit dependence on the Doppler broadening function ψ(x,ξ). The objective of this paper is to present a method for the fast and accurate calculation for the J(ξ, β) function based on the recent advances in the calculation of the Doppler broadening function and on a systematic analysis of its integrand. The methodology proposed uses an analytical formulation for the calculation of ψ(x, ξ) and a representation in series for error functions with complex argument. The results were satisfactory from the accuracy and processing time standpoint and are an option to other calculation methods found in the literature.     

JR curves of the low alloy steel 20 MnMoNi 5 5 with two different sulphur contents in oxygen-containing high temperature water at 240°C

J_R curves of the low alloy steel 20 MnMoNi 5 5 with two different sulphur contents (0.003 and 0.011 wt.%) were determined at 240°C in oxygen-containing high temperature water as well as in air. The tests were performed by the single-specimen unloading compliance technique at load line displacement rates from 1 × 10⁻⁴ down to 1 × 10⁻⁶ mm s⁻¹ on 20% side-grooved 2T CT specimens in an autoclave testing facility at an oxygen content of 8 ppm and a pressure of 7 MPa under quasi-stagnant flow conditions.In the case of testing in high temperature water, remarkably lower J_R curves than in air at the same load line displacement rate (1 × 10⁻⁴ mm s⁻¹) were obtained. A decrease in the load line displacement rate as well as an increase in the sulphur content of the steel caused a reduction of the J_R curves. At the fastest load line displacement rate a stretch zone could be detected fractographically on the specimens tested in air and in high temperature water and consequently J_i could be determined. When testing in high temperature water, the J_i value of the higher sulphur material type decreases from 45 N mm⁻¹ in air to 3 N mm⁻¹, much more than that of the optimized material type from 51 N mm⁻¹ in air to 20 N mm⁻¹ at 1 × 10⁻⁴ mm s⁻¹.     

The effect of crack location and maximum stress position on the failure of a cracked piping system

The failure of a circumferentially cracked stainless steel piping system can be predicted by assuming that failure conforms to a net-section stress criterion, using as input an appropriate value for the critical net-section stress together with a knowledge of the anticipated loadings. The usual procedure is to calculate the stress acting on the cracked section via a purely elastic analysis based on the piping system being uncracked. However (a) the piping is built-in at the system ends into a larger component, and (b) the onset of crack extension requires some plastic deformation. Consequently, use of the net-section stress approach to predict the onset of crack extension, can give overly conservative failure predictions. Earlier work by the author has quantified the extent of this conservatism, and has shown how it depends on the material ductility and the geometry of the cracked section. It has also been demonstrated that the extent of conservatism depends on the elastic flexibility of the system, and in particular on the location of the cracked section within the system. The present paper extends these earlier studies, and shows that the location for which crack extension is favoured is not necessarily that location where the stress (calculated on the basis of an uncracked system analysis) is a maximum, as is implicitly assumed when the net-section stress approach is used.     

Prediction of the obstacle effect on film-boiling heat transfer

A correlation has been developed to account for the effect of obstacles (simulating the spacing devices in bundles) on heat transfer in dispersed-flow film boiling. The correlation is expressed as a modification factor to the reference geometry without any obstacles. The basic form of the correlation is an exponential decay function that resembles the diminishing effect on turbulence enhancement. The coefficients and constants in the correlation have been optimized with heat-transfer data of low-pressure single-phase (air) flow and high-pressure steam-water flow at film-boiling conditions.An experiment has been set up to obtain validation data with a heated tube of 4.1 mm inside diameter. HFC-134a was used as the working fluid. Three types of obstacles with the same blockage-area ratio of 37.8% were tested. The results showed that the obstacles exhibited a strong enhancement effect on the film-boiling heat-transfer coefficient at locations downstream of the obstacles. A comparison between predictions of the correlation for the spacing-device effect and experimental data showed an underprediction of the heat-transfer rate at locations downstream of the obstacle. The underprediction is due mainly to the rewetting of the heated surface at the location of an obstacle, beyond which the developing film-boiling effect becomes dominant. The agreement between prediction and data is significantly improved after accounting for the critical heat flux (CHF) enhancement and developing film-boiling effects in the calculations.     

The effect of heat treatment on the microstructure and the corrosion resistance of Zircaloy-4 in 450°c steam

The effect of beta treatment and the subsequent cold work and alpha annealing on the microstructure and corrosion resistance of Zircaloy-4 in 450°C steam was investigated. The distribution of second-phase particles in test specimens were found to be not uniform and the effect of cold work and alpha annealing was the growth and redistribution of second-phase particles. By using SEM-EDX, the particles were identified as Zr-Fe-Cr and Zr-Fe. One cycle of the cold work and alpha annealing process improved the uniform corrosion resistance but enhanced the nodular corrosion. The difference was explained by a nodular growth mechanism which was related to the distribution of second-phase particles, the build-up of hydrogen pressure, the weakening of oxide after rate transition, and crack propagation along grain boundaries.     

A study of the effect of rod-bowing on critical heat flux

An experimental study was carried out to determine the effect of rod-bowing on critical heat flux, using an electrically-heated rod cluster. In this experiment, rod-bow was set to occur in the severest subchannel and axially at the middle between the last two spacers, with uniform axial heat flux. The minimum gap between the outer and inner rods was reduced variously to 1.6 mm, 1.0 mm and zero from the nominal value of 2.1 mm. Other experimental conditions were as follows: pressure 7 MPa; mass velocity 640–2600 kg/m² sec; inlet subcooling 40–560 kJ/kg.Experimental results show only a slight rod-bowing effect, if any, compared with normal spacing, as confirmed by analysis of three-dimensional heat conduction around the rod-bowing area and by the local steam quality deviations calculated by subchannel analyses.     

Influence of gap size on the dynamic behaviour of piping systems

This paper addresses the effects of dynamic events induced by support motion on piping systems with snubbers having variable gap sizes. The investigation consists of 3 parts: (i) Mathematical examination of a linear I DOF mass-spring-snubber system with gap size zero or infinity. (ii) Numerical analysis of a piecewise linear I DOF mass-spring-snubber system with varying gap sizes, by means of a simple computer code. (iii) Numerical analysis of a realistic three-dimensional piping system with 3 snubbers, each having 4 different gap sizes, with the aid of a non-linear F.E. code.     

Effect of heat treatment on the corrosion resistance of zirconium alloys

Some data on the effect of heat-treatment conditions on the corrosion resistance of Zr–1.0% Nb and Zr–2.5% Nb alloys are presented. It is shown that double annealing imparts a higher corrosion resist-ance to the alloys than other forms of heat treatment.The greatest improvement in corrosion resistance occurs for the Zr–2.5% Nb alloy.Translated from Atomnaya Énergiya, Vol. 20, No. 4, pp. 330–333, April, 1966.     

CLAM钢TIG焊后的热处理工艺研究

为了解决利用钨极氩弧(TIG)焊方法焊接的CLAM钢在焊缝区存在的强度、硬度增加而韧性减弱的问题,从回火温度、回火时间以及重复回火等方面研究了回火对焊缝区硬度的影响,得到了回火温度选择为760℃、回火时间选择为2 h和在760℃保温1 h回火,重复进行两次的两种最佳热处理工艺.     

Evaluation of different fracture-mechanical J-integral initiation values with regard to their usability in the safety assessment of components

Determining fracture-mechanical material characteristic values on the basis of the J-integral is described and stipulated in a variety of standards and guidelines. The individual specifications differ in terms of procedure when determining the characteristic values and, therefore, also in terms of the meaningfulness of the results. This paper presents the different procedures, suggested in the course of the development of test methods in the field of elastic—plastic fracture mechanics, used to characterize crack initiation behaviour with regard to their features as material characteristic values and their usability in the safety assessment of components.     

Experimental study of the effect of non-circular flow geometry on the critical heat flux

An experimental study of the effect of flow geometry (circular, rectangular, triangular, and dumb-bell shaped) on the critical heat flux (CHF) was performed using R-134a as a coolant. The CHF is affected by the following geometric parameters: hydraulic-equivalent diameter, heated length, gap size, channel shape, and curvature. It may also be affected by the thermal conductivity of the wall material and wall thickness. The effect of flow geometry on CHF is influenced by flow parameters. The effect of these parameters on CHF was examined, and recommendations for predicting the CHF in non-circular geometries have been made.     

Comments on the probability of leakage in piping systems as used in PRA's

In risk analysis of power reactors the leakage or failure of piping structures has to be taken into account as a possible cause of loss of coolant.As part of the SMiRT post conference seminar on “PRA of NPP for External Events” the present practice of selecting pipe failure rates as initiating or related events for PRA's has been discussed.For pipe failures as initiating events an approach has been developed in the framework of the risk study for German PWR's. As compared to NUREG-1150 some significant differences are identified.For external events the effect of seismic induced loads on pipe failure has been a subject of considerable efforts in research. Several studies have demonstrated that for moderate siting conditions the effect of seismic induced loads on pipe failure rates of large diameter high pressure piping does not lead to significant contributions to the overall risk.The main subject for future research on pipe failure mechanisms is the detailed assessment of the influence of the water chemistry conditions.     

Experimental Observations of the Chemistry of the SiO2/Si Interface

Changes in silicon surface preparation prior to thermal oxidation are shown to leave a signature by altering the final SiO2/Si interface structure. Surface analytical techniques, including XPS, static SIMS, ion milling, and newly developed wet-chemical profiling procedures are used to obtain detailed information on the chemical structure of the interface. The oxides are shown to be essentially SiO2 down to a narrow transitional interface layer (3-7 ?). A number of discrete chemical species are observed in this interface layer, including different silicon bonds (e.g., C-, OH-, H-) and a range of oxidation states of silicon (0 ? +4). The effect of surface preparation and the observed chemical species are correlated with oxide growth rate, surface-state density, and flatband shifts after irradiation.