This study proposes a new fracture model to correlate cleavage fracture toughness with microstructure of steel having bainitic structure with/without M-A constituent based on the Local Approach. In this model, a new fracture parameter to predict fracture toughness is derived through the proposal of microstructural characteristic of the material to control fracture toughness and on the basis of weakest link theory assumed Griffith crack. The material properties required for applying the fracture model are microstructural properties, those are 1) representative volume and 2) maximum size distribution of micro-crack nuclei, 3) mechanical properties and 4) effective energy release rate of matrix material. The applicability of the theoretical fracture model is demonstrated by experiments for upper bainitic steel with different microstructural morphology. This model can correlate materials properties which are microstructural and mechanical properties with fracture toughness.
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Fracture of brittle solids is ultimately executed by atomistic-scale, discrete, and ultrafast bond-breaking mechanisms along the crack path. Here, we show new fracture behavior and properties of brittle materials, based on macroscopic fracture cleavage experiments of silicon crystal specimens and atomistic-scale semi-empirical model for bond-breaking mechanisms along the curved crack front, to relate micro to macro in fracture.
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In this talk, we will describe damage accumulation and failure of free-standing micro-cantilevers made of 7YSZ coatings during cyclic bending in a nanoindentation system in both, the as-sprayed condition as well as after low-temperature thermal cycling up to 700 oC while attached to the substrate. The technique has been established as a means of tracking elastic modulus, hysteresis/creep, and fracture behavior as a function of coating densification during isothermal treatment at high temperatures. In contrast, low-temperature thermal cycling is designed to simulate operating conditions during which crack healing and sintering, which are known to lead to stiffening, are minimal. The load-displacement curves typically display hysteretic behavior with an increasing permanent residual displacement (ratcheting) after each cycle which increases with an increase in load, accompanied by a reduction in stiffness that is characteristic of damage accumulation. Failure appears to result from the formation of macrocracks after a critical amount of ratcheting. The number of mechanical cycles to failure reduces with the number of prior thermal cycles and with increasing maximum load/stress. Thus, mechanical cycling can act as a proxy for thermal cycling in evaluating progressive damage accumulation in TBCs.
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This study proposes the new fracture model to assess the fracture toughness under complex loading mode subjected to cracked component on the brittle fracture toughness assuming combined stress state in plastic zone near crack-tip. This model newly considers non-linear energy release rate named Local-J as the elastic-plastic local fracture driving force for micro-crack nucleus in plastic zone. The effect of 3-dimentioinal combined stress state on local-J, which is different from the effect on the linear elastic energy release rate for Griffith crack, is formulated as the Local-J equivalent stress by conducting numerical analysis of unit-cell including a penny-shaped crack. Based on weakest link theory assuming this new model under combined stress field, Extended Weibull stress is derived as a new fracture parameter for cracked component. The characteristics of the proposal model is examined by predicting the critical load for pure mode II or III from fracture toughness assumed under pure mode I load. Fracture toughness assessed by this new model under mode II or III load is smaller than that assessed by conventional model. This result of numerical analysis implies the possibility of rational assessment of the effect of loading mode by applying the new model.
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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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