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.
EXTENDED ABSTRACT

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.
EXTENDED ABSTRACT

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.
EXTENDED ABSTRACT

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.
EXTENDED ABSTRACT

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.
EXTENDED ABSTRACT

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.
EXTENDED ABSTRACT

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.
EXTENDED ABSTRACT

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.
EXTENDED ABSTRACT