PVP2008-61232 ILLUSTRATION OF THE WPS BENEFIT THROUGH BATMAN TEST SERIES: TESTS ON LARGE SPECIMENS UNDER WPS LOADING CONFIGURATIONS

ABSTRACT A study combining modelling and a series of experiments on large specimens submitted to a thermal shock or isothermal cooling has been performed in CEA-Saclay in order to show the WPS benefit on large scale specimen. The test series, named BATMAN, was made on 18MND5 ferritic steel bars, containing a short or large fatigue pre-crack. For the two performed tests (fast thermal shock creating a gradient across the thickness of the bar or for the gradual uniform cooling), the effect of "Warm Pre-Stressing" was confirmed. In both cases, no propagation was observed during the thermal transient. Fracture occurred under low temperature conditions, at the end of the test when the tensile load was increased. The failure loads then recorded were substantially higher than during prestressing. To illustrate the benefit of the WPS effect, numerical interpretations were performed using either global approach or local approach criteria. The capability of models to predict the WPS effect was clearly shown. INTRODUCTION A large amount of work was performed on ferritic steels to demonstrate the benefit of the Warm Pre-Stressing (WPS) effect. Mainly on small specimens and in the frame of experimental and theoretical studies, the objectives were to show the enhancement of the cleavage fracture toughness at low temperature following the application of a preloading at higher temperature (which exceeds the fracture toughness of the virgin material at low temperature)

PVP2008-61231 BRITTLE FRACTURE CRITERIA TRANSFERABILITY EVALUATION BY TESTING ON DIFFERENT SPECIMEN GEOMETRIES

ABSTRACT This paper describes an experimental and numerical study to assess the transposability of brittle fracture criteria from specimens of one type of geometry to another. The overall "master curve" approach, the Beremin model and a proposed model using the concept of critical stress were accordingly analysed. The experimental work supporting the analysis was made on 16MND5 reactor vessel steel. This was in the form of CT25 specimens, taken as the reference type, SENT specimens, ring specimens and CT por specimens, which are CT specimens with a semi-elliptical surface defect. The analysis itself was made in two stages: the models were first calibrated on the basis of CT25 test results, then they were applied to specimens of other geometries. We then demonstrate that, in all cases, the models correctly replicated the variation of toughness (as measured on a CT25 specimen) with temperature. However, they all failed when applied to SENT and ring specimens, where calculation underestimates the probability of fracture. For CT por specimens, the results are better, the master curve approach and the critical stress criterion give satisfactory results (but it has not yet been possible to apply the Beremin method). This paper concludes with a detailed analysis of the crack tip stress and strain fields, followed by an attempt to explain the differences between the different types of behaviour observed. INTRODUCTION AND GOAL The SEMT/LISN Laboratory of the French atomic energy commission (CEA) at Saclay, with the support of the French institute for nuclear safety and radiological protection (IRSN) are making an in-depth study of the pertinence of brittle fracture criteria. The main question resolved is the transposability of criteria from one geometry to another, from one scale to another (specimen to structure) and from one loading (mechanical or thermal) to another

Fracture properties in dissimilar metal weld junctions experimental methodology of characterization

International audienceWithin the framework of European project MULTIMETAL (Structural performance of multi-metal component), several fracture tests on different types of multi-material specimens have been performed. Present fracture toughness standard methods, e.g. ASTM 1820 are not directly intended for Dissimilar Metal Weld (DMW). Therefore further investigations are needed in order to define the best practice in fracture mechanical tests and their analysis for DMWs.Specimens were taken from welded plates a narrow gap Inconel DMW junction between Ferritic and Austenitic steels, designed and delivered by AREVA France. The work is focusing on the nickel alloy - ferrite steel interface which is the weakest area of such welded pipes regarding ductile tearing. This paper is focusing on the experimental methodology in this particular case of multi-material fracture test specimens.The first difficulty consists in machining specimens with a location of crack plane at the interface or close to the interface. Physically, the interface between the weld in nickel alloy and the ferrite steel is not a straight plane. Some normalized specimens - geometries like CT (Compact Tension specimen), SE(B) (Single Edge Band specimen), or SE(T) (Single Edge Traction specimen) - have been machined in the DMW junction. Some recommendations are discussed about this first step of work.The second task is the procedure of fracture test itself. The present fracture toughness standard method is applied using the method of compliance for the determination of J-et61508;a curves. The plasticity, in Ferritic and Stainless steel parts of specimens, has induced unsymmetrical response. Locally the path of the crack has been influenced by the gradient of properties of the multi-material medium (weld, heat affected zone). The J values, determined at crack initiation, are dependant of the location of crack plane. Results between specimens are compared and discussed. Based on the current standard, this article highlights some shortcomings of this standard when used in heterogeneous samples. The well-known problem of transferability is also pointed out (different values of J for crack initiation depending of the geometry and size of specimen).In conclusion further numerical works are needed to interpret experimental results and provide intrinsic material data, in terms of fracture properties of the Nickel alloy-Ferrite steel interface

A SIMPLIFIED EXPRESSION FOR LOW CLEAVAGE PROBABILITY CALCULATION

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Thermomechanical analysis of thermal shock fracture in the brittle/ ductile transition zone - Part II: Numerical calculations and interpretation of the test results

The integrity of PWR pressure vessels is assured by keeping the crack tip stress intensity factor below the toughness of the material under monotonic isothermal loading. To study the effects of sudden cooling associated with a thermal gradient, a specially modified compact specimen has been developed. This has been used to carry out tests in the transition zone with different loading-temperature sequences liable to call the conventional criteria into question. The test is described in detail in Part I of this article [Chapuliot S, et al. Thermomechanical analysis of thermal shock fracture in the brittle/ ductile transition zone. Part I: Description of the tests. Engng Fract Mech, 72, 2005, 661-73]. The second part describes numerical investigations to estimate the local mechanical fields at the crack tip and the overall parameters of the fracture mechanics. Finite element thermomechanical calculations are used to interpret the results of these new thermal shock tests using the master curve concept [ASTM E 1921-1997. Standard test method for determination of reference temperature To for ferritic steels in the transition range, 1997] and the Beremin statistical model [Beremin FM. A local criterion for cleavage fracture of a nuclear pressure vessel steel. Metall Trans A, 14A, November 1983, 2287-777]. © 2005 Elsevier Ltd. All rights reserved