Brittle failure by transgranular and intergranular mechanisms is commonly addressed by probabilistic methods based on the weakest-link concept. For homogeneous materials this approach is straightforward and well established. Different methods have been proposed in the past to incorporate the presence of heterogeneities, e.g. due to welding or segregated zones. A key issue in this context is the length that characterizes variations in the heterogeneous microstructure in relation to a representative size of the zone where brittle fracture typically has been observed to occure, i.e., fracture process zone (FPZ). Here, a new approach for weakest-link modelling of heterogeneous materials is proposed that accounts for the interplay between the different scales.
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Charpy impact tests are used in the nuclear industry to certify forging processes. However, the results of these tests may exhibit a strong variability in the context of large metal parts manufactured by Framatome. Preliminary studies have shown that the steel is highly heterogeneous at the millimeter scale in certain areas of forged parts. These heterogeneities are surmised to be the main cause of the variability observed in the results of impact tests. The aim of this study is to qualify and numerically quantify the effect of these heterogeneities on the distribution of fracture energies thanks to an innovative computational approach featuring deep learning to generate 3D realizations of the mechanical properties from sparse experimental results, and high-fidelity modeling of brittle fracture in heterogeneous Charpy specimens.
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A computational strategy to evaluate the Weibull stress for the Beremin model
is proposed to simultaneously solve problems caused by volumic locking and extreme element
distortion at the crack tip. It is based on the use of mixed elements and remeshing. It is shown that
a single simulation can be used to evaluate the Weibull stress for any range for the CTOD at failure.
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Ductile-To-Brittle transition modeling for ferritic steels used in the nuclear industry has been studied for years. This paper proposes a two-step coupled modeling representing ductile crack growth thanks to a gradient-enhanced GTN model and applying a modified Beremin model to evaluate the probability of failure of CT specimens at -50°C.
Both models have been calibrated separately at temperatures where there is no coupling. Beremin model is first studied at low temperature. It is shown that the model could be applied to different geometries. GTN model is then calibtrated at -20°C. The identified GTN model could be transferred to lower temperatures. Finally the coupling is studied.This work emphasizes the necessity of a special treatment of the stress field resulting from the GTN model to compute the Weibull stress and the use of a modified Beremin model accounting for the void volume fraction.
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The dynamic fracture of a brittle ceramic, B4C, is investigated using our in-house split-Hopkinson pressure bar (SHPB), and Sandia’s peridynamics simulation code, PERIDIGM. In order to study the dynamics of this particular SHPB, the initial boundary value problem (IBVP) is solved for a 1-D impact in which a finite striker bar collides with the front face of a stationary incident bar bonded to a specimen of finite thickness, with the back face of the specimen bonded to another finite transmission bar; this is the classic SHPB experiment. Laplace transform domain solutions are numerically inverted to the time domain using a modified Dubner-Abate-Crump algorithm. The new IBVP solutions for particle velocity in the SHPB composed of maraging steel bars, and B4C specimen, are used to verify the commercial FE codes COMSOL and ABAQUS, and PERIDIGM. Subscale SHPB simulations are conducted using PERIDIGM on jacketed/unjacketed B4C specimens with a critical stretch failure condition proportional to the ceramic’s critical energy release rate, Gc; also investigated is the effect of initial defect populations governed by, Weibull, uniform random, and Bobaru critical stretch distributions, on the ceramic failure behavior. Computationally expensive full-scale PERIDIGM simulations are also currently underway to compare with the subscale simulation results.
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The new generation of advanced high strength steels for oil & gas transportation exhibit better mechanical response and fracture toughness not only in corrosive media, but also in arctic environments. In particular, under these latter conditions, X65 Q&T pipeline steels do not reveal a clear ductile-to-brittle transition (DBT) temperature and, in some cases, inverse fracture. It is still unclear the actual causes of this phenomenon typically observed in impact tests such as Charpy and drop-weight tear experiments. This study aims at the understanding of the underlying mechanisms controlling this abnormal behavior, which leads mostly to disqualifying a particular material for a certain engineering application. In general, thorough mechanical and material characterizations are intended to be conducted in order to unveil the relationship between microstructure characteristics and structural configurations. By means of a phenomenological fracture model, the statistical nature of the brittle fracture and the size effects will be deemed into a more general computational damage framework incorporating also ductile fracture from the upper shelf energy region.
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The oil and gas industry favors to use less conservative fracture toughness measured from a single edge notched tension (SENT) specimen in the end-clamped conditions in terms of J-integral or crack-tip opening displacement (CTOD) or their resistance curves, where the elastic unloading compliance technique is usually utilized to monitor the incremental crack growth during the single specimen test. Several numerical solutions of crack mouth opening displacement (CMOD) compliance obtained from the finite element analysis (FEA) are available for the end-clamped SENT specimens. However, they have different accuracies and different applicable ranges of crack length ratio a/W, and they may be inconsistent with the existing solutions of their stress intensity factor (K) solutions for the same end-clamped SENT specimen because both the compliance and the K factor were determined separately by FEA. Based on a full-range analytical K solution, this work develops a more accurate, analytical solution of CMOD compliance equation for the end-clamped SENT specimens. Comparisons with various existing FEA results confirm the higher accuracy of the proposed analytical compliance solution. As a result, the proposed CMOD compliance solution can be used to determine more accurate crack length for the SENT testing.
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Stress intensity factors are viewed as specializations of a family of drivers of crack propagation, defined on three-dimensional stress fields, to two-dimensional stress fields. The question of which driver is best suited for the prediction of crack propagation in three dimensions will have to be decided on the basis of evidence developed through the application of a model development process. The procedure for rational choice of a predictor of crack propagation in metals, caused by cyclic loading, is addressed.
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A novel modified cantilever beam method and modified clamped beam method with DIC (Digital Image Correlation) is developed to measure the interface fracture energy of ceramic/metal interface. The experimental execution for these geometries is demonstrated on an Air Plasma Sprayed (APS) YSZ coating on a steel substrate. For modified cantilever method, a pre-crack is first made along the interface, followed by the interface test. These methods use the same geometry for both pre-cracking and testing. The value of interface fracture energy is obtained as the critical energy release rate, Gc, using numerically computed values of J-integral. The results of both the geometries are compared.
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