Estimation of fracture toughness of 20MnMoNi55 steel in the ductile to brittle transition region using master curve method

Fracture toughness is an important material property to assess the critical load for structural integrity of reactor pressure vessel steel. In this paper, master curve method proposed by Kim Wallin is used to estimate the fracture toughness of 20MnMoNi55 steel in the ductile to brittle transition regime. Reference temperature (T₀) is evaluated using both single temperature and multi-temperature method for one inch thick compact tension (1T-CT) specimens. Reference temperature (T₀) is also determined from Charpy V-notch test data and compared. Effect of selection of temperature range and number of test temperatures on the value of T₀ is also studied. It is observed that Charpy test results yield lower values of unirradiated T₀ compared to 1T-CT specimen tests. It is also observed that most of the fracture toughness values fall between 5% and 95% boundary of fracture toughness curves for all the evaluations.     

The growth of fatigue cracks in the creep range from the point of view of ductile fracture

This paper deals with experimental and numerical investigations of crack growth in 1% CrMoV steels in the creep temperature range. For the load and displacement controlled fatigue tests with predominantly plastic deformation amplitudes, centre cracked panels and CT-specimens of different thicknesses were used.For describing the crack growth per load cycle the crack tip displacement δ_t and the J-integral were applied, whereby the question arises whether the various fracture concepts are applicable unrestrictedly or where they meet their limits of validity.Detailed theoretical-numerical calculations using the finite element method were expected to yield information about the path independence of the J-integral. Special importance was therefore attached to investigate the J-integral, both cyclical and with reference to the instantaneous state of deformation, by comparing the different J-integral values (ΔJ, J) with the experimental method of Dowling/Begley.     

Fatigue growth and fracture risk from postulated cracks in pressurized-water reactor piping welds

The necessity of taking quick corrective action when a crack indication is discovered in a nuclear piping weld, has led Framatome to evaluate beforehand the potential risk of such a situation by investigating postulated cracks. Considering pessimistic loading conditions to act on a postulated crack of a given shape and orientation enables the determination of the critical size of such a defect. Introduction of fatigue crack growth then yields the maximum crack size that can be tolerated, given the remaining lifetime of the unit. Additionally, detailed analysis of the scenario that leads to these results contributes to the understanding of the potential risk and helps in alleviating it. In this paper, a review of the basic principles and the application to the case of a branch connection weld are presented.     

Investigation on the interaction effect of two parallel axial through-wall cracks existing in steam generator tube

It is commonly required that steam generator tubes wall-thinned in excess of 40% should be plugged. However, the plugging criterion is known to be too conservative for some locations and types of defects and its application is confined to a single crack. In the previous study, the conservatism of the present plugging criterion of steam generator tubes was reviewed and a crack coalescence model applicable to steam generator tubes with two collinear axial through-wall cracks was proposed. Since parallel axial cracks are more frequently detected during in-service inspections than collinear axial cracks, the studies on parallel axial cracks spaced in circumferential direction are necessary. The objective of this paper is to investigate the interaction effect between two parallel axial through-wall cracks existing in a steam generator tube. Finite element analyses were performed and a new failure model of the steam generator tube with these types of cracks is suggested. Interaction effects between two adjacent cracks were investigated to explain the deformation behavior of cracked tubes.     

Towards a modelling of RPV steel brittle fracture using crystal plasticity computations on polycrystalline aggregates

In the frame of the multi-scale approach of the fracture toughness prediction defined in the PERFECT project, we proposed a new crystal plasticity model and applied it to the computation of stress heterogeneities within a reference polycrystalline aggregate defined in the project RPV material.The proposed crystal plasticity model is able to take into account the effects of temperature and irradiation hardening. The analysis of the results of aggregate computations shows that the distributions of the maximum values of the maximal principal stresses are found to be well described by a Gumbell function. Applying these distributions on a Griffith criterion allows settling the basis of an original fracture criterion. However the increasing resistance to fracture of the steel with temperature can be reproduced only by introducing a temperature dependence of the fracture energy.     

Example for the application of IAES-TECDOC criteria for the brittle fracture safe design of a ductile iron shipping package

A proposed new IAEA regulation describes a procedure for proving a container to be safe against brittle fracture. A very important aspect is the definition of a reference flaw. In non-destructive testing (in this case 100% volumetric US testing) the probability of non-detection of any flaw must be on the order of 1 in 100. In combination with statistics the reference flaw depth will be about 5 times the minimum detectable flaw size. Additional requirements concern the sizes of rejection flaws and critical flaws. Once the reference flaw has been established, it must be shown that even for its most critical location and orientation, the applied stress intensity is smaller than the fracture toughness divided by a safety factor.

The most critical location was found to be a subsurface flaw close to the surface. The relevant loading for the MOSAIK 80T (an 80 ton heavy-section ductile iron shipping vessel) investigated in this paper is the deceleration of 120g resulting from a 9 m side drop into an unyielding target. It was found that this container is indeed safe against brittle fracture.     

Assessment of cracks in lateral supports of the magnet system of Wendelstein 7-X

The superconducting coils of the magnet system of Wendelstein 7-X (W7-X) are bolted onto a central support ring and interconnected with five so-called lateral support elements (LSEs) per half module. After welding of the LSE hollow boxes to the coil cases cracks were found in the vicinity of the welds that could potentially limit the allowed number N of electromagnetic (EM) load cycles of the machine.In response to the appearance of first cracks during assembly, the stress intensity factor (SIF) of theoretical cracks of various sizes in potentially critical position and orientation were predicted in a fast approach. For each crack size, N was based on the SIF, derived from beam theory, and on Paris’ law parameters determined in fatigue crack growth rate (FCGR) tests, thus leading to tolerable maximal crack sizes and distances between cracks. It was proved that the actual crack dimensions remained below these values or turned out to be only superficial. Afterwards, (extended) finite element method (XFEM and FEM) and boundary element method (BEM) models were developed to project the SIF of most critical tolerated cracks, considering new FCGR tests and the local stress state in more detail. N appeared highly sensitive to the assumptions which were therefore critically reviewed.Finally, the limit for load combinations of different amplitudes was determined using Miner's rule. As a result it was shown that the predefined number of W7-X operation cycles is not jeopardized by any of the detected cracks.     

Investigation of the behaviour of cracks in a dissimilar weld

Using the finite-element method the behaviour of an axial and a circumferential crack in a dissimilar weld of the ferritic steel X20 CrMoV 12 1 and the austenitic X10 NiCrAITi 32 20 has been investigated. Stresses due to internal pressure and stresses resulting from temperature cycling due to different thermal expansion coefficients have been calculated. The circumferential crack located within the austenite in the region of maximum longitudinal stress is loaded at its deepest point by a stress intensity factor of 473 The axial crack is loaded up to a stress intensity of 325 N mm^−3/2 not at its deepest point, but within the X20 CrMoV 12 1. The resulting crack growth for both cracks has been estimated and compared with a component test with artificial defects in a dissimilar weld undergoing similar loadings.     

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.     

Influence of temperature, strain rate and specimen geometry on the microscopic cleavage fracture stress

The aim of this work is to determine the influence of temperature, strain rate and specimen geometry on the microscopic cleavage fracture stress σ_f*. Besides, the dependence of the initiation temperature for shear fracture T_i and the temperature for general yield T_gy on strain rate is investigated. Finally, the local values of stress triaxiality and equivalent plastic strain at the occurrence of cleavage fracture for several steels and specimen types are compared with the failure curve for ductile fracture to check the validity of the theory of stress controlled cleavage fracture and the strain/triaxiality controlled shear fracture in the transition region. Based on experimental tests, the results are obtained by finite element analyses. The investigation shows, that σ_f* is dependent on temperature and strain rate and increases with decreasing test temperature and increasing strain rate. The transition temperatures T_i and T_gy increase with increasing strain rate. The theories of stress controlled cleavage fracture and of shear fracture controlled by equivalent plastic strain and stress triaxiality seem to be valid. That fracture mechanism occurs for which the critical condition is reached first.     

Generalized procedures for the use of plane strain and elastic-plastic fracture mechanics options in the modernization of nuclear standards

In the design of nuclear power plants and the selection of required structural materials, the assurance of reliability in operation is an essential consideration. The need for analytical criteria for defining the adequacy of fracture toughness is particularly acute for pressure vessel materials. The 1972 revision to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Section III) has adopted linear elastic fracture mechanics (LEFM) methods as a means of assuring fracture-safe operation for nuclear vessels and components. It is noted that LEFM can be used to analyze only the behavior of metals subject to plane strain constraint (i.e. brittle behavior), while many steels when used in structural applications will behave in a ductile fashion. Thus, in the modernization of nuclear codes, there is the additional need to include the full range of fracture mechanics options for system design, that is, elastic-plastic and fully plastic fracture mechanics as well as the linear elastic procedures. The choice of a particular toughness regime for application of the metal (e.g., plane strain or elastic-plastic) can then be made by the designer or regulatory body. It follows that this decision will have a major implication on the selection of nuclear structural materials.This paper describes recent developments in the means for defining the full range of plane strain, elastic-plastic, and plastic fracture mechanics options available to the designer. Comparisons are made between these options and the fracture toughness requirements of the ASME Nuclear Code (Section III). Exisiting dynamic plane strain, K_Id, data for structural metals are analyzed in concert with dynamic tear (DT) test trends. The limited temperature region of K_Id applicability for these materials is shown to presage the elastic-plastic regime through which sharply increasing stress is required for fracture propagation whereby a leak-before-fail condition is ultimately attained. This phenomenon highlights the need to extend the analytical capabilities for fracture assurance into the non-brittle regime. The DT test is an effective engineering tool which, like the crack opening displacement (COD) concept, can be used to define the elastic-plastic and plastic-constraint transitions. The DT test procedure is fully rationalizable in terms of section size parameters and can be used independently or together with the K_Id-temperature trend to predict the onset of the elastic-plastic and plastic regimes as a function of temperature and section thickness.     

Evaluation of crack opening times and leakage areas for longitudinal cracks in a pressurized pipe Part II. Application of proposed model and fracture dynamics

The present study (Part I and Part II) deals with a method of evaluating the average time to crack opening as well as the maximum leakage areas in the case of longitudinal through-wall cracks in a cylinder with internal pressure. Part I deals with the leakage areas. Starting from the linear elastic theory as applied to the case of a central crack in a plate, leak areas are evaluated in a cylinder under elastoplastic conditions by using an amplification factor and a plasticity correction factor. A reasonable upper bound is proposed which takes into account the interaction between plasticity and curvature effects as a first approximation and considers the crack opening uniform all over the crack surface. The method is validated using the available experimental and/or computational results.