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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This paper provides an overview of recent progress in probabilistic modeling of cleavage fracture phrased in terms of a local approach to fracture (LAF) and the Weibull stress concept. Emphasis is placed on the incorporation of plastic strain effects into the probabilistic framework by approaching the strong influence of constraint variations on (macroscopic) cleavage fracture toughness in terms of the number of eligible Griffith-like microcracks which effectively control unstable crack propagation by cleavage. Some recent results based on a modified Weibull stress model to predict specimen geometry effects on Jc-values for pressure vessel grade steels are summarized in connection with an engineering procedure to calibrate the Weibull stress parameters. These results are compared against corresponding fracture toughness predictions derived from application of the standard Beremin model. Finally, the robustness of LAF methodologies, including specifically the Weibull stress approach, is critically examined along with a discussion of key issues and challenges related to engineering applications in fracture assessments of structural components.
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The presentation will briefly review the history of the development of fracture mechanics from 1921 to the present, including the evolution of its basic concept. Arguments will be made that Griffith’s basic concept, properly implemented in the context of modern non-equilibrium thermodynamics, remains valid.
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The scale- or the size-dependence of mechanical strength in many brittle materials appears to follow a ‘universal law,’ of the form: strength proportional as:L^-n or V^-n, where n is a number, L is the length and V is the volume of the specimen or structure. Broadly known as the “size-effect” in geology, civil engineering, mining and materials science, this behavior determines the strength of large structures such as ice sheets, rock formations, coal pillars in mines and concrete beams and columns in civil infrastructure. As of now, there is no reliable scientific basis or theory to explain the size effect or for determining a reliable value of ‘n’. This has been the missing link in strength of materials for nearly a century since the Griffith’s crack theory Here, we show that the change in net-section strain energy, due an initial crack in a structure, and its dissipation within a crack layer of finite thickness, leads to the necessary and sufficient physical basis to explain the size-dependence of strength as L^-0.5. Further, size-independence of strength is explained simultaneously when the crack layer volume approaches the specimen volume.
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