Crack Propagation In Ceramics Research Articles

The effect of a prefabricated crack on the crack growth in ceramics during quenching

The influence of a prefabricated crack on thermal-shock cracking during quenching is studied in real-time. The results show that after the thermal-shock crack extends to the prefabricated crack, the secondary crack may appear at the lower end of the prefabricated crack. The total vertical length of the crack and the probability of the secondary crack occurrence will gradually increase with the prefabricated crack angle. Besides, the influence of the prefabricated crack distance from the edge on thermal-shock crack growth is also considered. The simulation results of meso-damage mechanics are consistent with experimental observation. This article quantitatively investigates the effect of the prefabricated crack on the thermal-shock crack propagation in ceramics, expanding the research on the mechanism of thermal-shock failure.

An adaptive thermo-mechanical peridynamic model for fracture analysis in ceramics

A thermo-mechanical peridynamic model using adaptive grid refinement is developed to investigate crack propagation in ceramics. Compared to a standard peridynamic model, using uniform grid, this approach allows to increase the resolution of analysis only in the critical zones. The performance of this approach in solving 2D thermo-elastic problems is examined and then it is applied to study the fracture of a ceramic disk under central thermal shock. Finally, the proposed approach is adopted to investigate thermal shock in thin rectangular and circular slabs. The accuracy of the method is evaluated by comparing its numerical results with those obtained by applying the finite element method (FEM), a standard peridynamic approach or with experimental data available in the literature. A proper agreement is achieved at a smaller computational cost.

Effect of material’s randomness on scaling of crack propagation in ceramics

Crack propagation in real (quasi)brittle materials demonstrates various signs of stochasticity; a tortuous character of fracture surfaces, multiple cracking and crack branching observed in experiments are a vivid confirmation of it. Traditional approaches of fracture mechanics represent cracks as geometrically smooth objects with straight (or curved) crack fronts, thus usually neglecting morphology of real cracks. An introduction of a direct account for stochastic features of brittle materials into modelling schemes leads to a more adequate description of real fracture processes. The effect of the material’s randomness on crack propagation in ceramics is studied based on the approach, combining these random features with continuum damage mechanics (CDM) and fracture mechanics. CDM describes a macroscopic manifestation of various failure processes developing at lower length scales. The numerical mode-I fracture analysis, based on discretization of a specimen’s cross-section, containing a sharp notch, into rectangular elements, provides detailed information on slow crack propagation. A necessity to describe a crack with its length changing along the front presupposes a transition from a unique stress-intensity factor to a set of its local values. A computational procedure for simulation of crack-damage interaction and crack propagation in alumina specimens at tension is suggested on the basis of a modification of a lattice scheme unified with ideas of CDM and local stress-intensity factors. Inhomogeneity of material properties is modelled in terms of various random spatial distributions of the initial damage in the specimen’s cross-section. Characterization of complicated morphology of cracks is implemented by means of scaling analysis of the crack-front shape.

Global description of crack propagation in ceramics

The subject of crack propagation in brittle materials is sometimes confusing, since many terms or concepts are not always clearly defined, as toughness, sub-critical crack propagation, threshold, R-curve, etc. However, crack propagation can be simply and rigorously analysed by considering the equilibrium point in terms of energy release rate, as defined by the Griffith criteria, based on fundamentals of physics. The departure from this equilibrium leads to crack velocities related to dissipative mechanisms. Reinforcement mechanisms of any type can shift the equilibrium towards higher driving force values, giving an increase of crack resistance. The degradation of this reinforcement, as the case of cyclic fatigue, simply reduces the resistance to propagation. The total description of the behaviour is therefore included in the crack velocity—energy release rate ( v − G ) graph. However, the major point is the equilibrium, because in addition to its theoretical importance, it is of practical interest for brittle materials, since it corresponds to an energy value below which no propagation occurs.

Threshold intensity factors as lower boundaries for crack propagation in ceramics

BackgroundSlow crack growth can be described in a v (crack velocity) versus KI (stress intensity factor) diagram. Slow crack growth in ceramics is attributed to corrosion assisted stress at the crack tip or at any pre-existing defect in the ceramic. The combined effect of high stresses at the crack tip and the presence of water or body fluid molecules (reducing surface energy at the crack tip) induces crack propagation, which eventually may result in fatigue. The presence of a threshold in the stress intensity factor, below which no crack propagation occurs, has been the subject of important research in the last years. The higher this threshold, the higher the reliability of the ceramic, and consequently the longer its lifetime.MethodsWe utilize the Irwin K-field displacement relation to deduce crack tip stress intensity factors from the near crack tip profile. Cracks are initiated by indentation impressions. The threshold stress intensity factor is determined as the time limit of the tip stress intensity when the residual stresses have (nearly) disappeared.ResultsWe determined the threshold stress intensity factors for most of the all ceramic materials presently important for dental restorations in Europe. Of special significance is the finding that alumina ceramic has a threshold limit nearly identical with that of zirconia.ConclusionThe intention of the present paper is to stress the point that the threshold stress intensity factor represents a more intrinsic property for a given ceramic material than the widely used toughness (bend strength or fracture toughness), which refers only to fast crack growth. Considering two ceramics with identical threshold limits, although with different critical stress intensity limits, means that both ceramics have identical starting points for slow crack growth. Fast catastrophic crack growth leading to spontaneous fatigue, however, is different. This growth starts later in those ceramic materials that have larger critical stress intensity factors.

A study on Crack Propagation in Ceramics under Cyclic loading Conditions

A study on Crack Propagation in Ceramics under Cyclic loading Conditions

Environmentally-controlled non-equilibrium crack propagation in ceramics

A general framework is developed for environmentally-controlled non-equilibrium crack propagation and applied to ceramic materials that exhibit microstructurally-controlled fracture resistance variations. Increasing fracture resistance with crack length, arising from frictional interlocking of predominantly intergranular fracture surfaces, is modelled by the influence of a localized line force behind the crack tip. An indentation fracture mechanics analysis incorporates the fracture resistance variation to describe the inert strength of ceramic materials as a function of dominant flaw size. Non-equilibrium fracture is modelled as the competition between thermally-activated bond-rupture and bond-healing processes, in which the activation barriers are modified by the net mechanical energy release rate acting on a crack. The resulting dependence of crack velocity on mechanical energy release rate is used to describe the strength of ceramic materials as a function of applied stressing rate in a reactive environment. The deconvoluted crack velocity behavior allows both the macroscopic reactive environment fracture resistance and the atomistic lattice traps for fracture to be determined. An implication is that fracture resistance variations are more important in determining observed fracture behavior in reactive environments than in inert environments.

Crack propagation in ceramics under cyclic loads

Stable crack growth is observed in notched plates of polycrystalline alumina subject to fully compressive far-field cyclic loads at room temperature in a moist air environment andin vacuo. The fatigue cracks propagate at a progressively decreasing velocity along the plane of the notch and in a direction macroscopically normal to the compression axis. The principal failure events leading to this effect are analysed in terms of notch-tip damage under the far-field compressive stress, microcracking, frictional sliding and opening of microcracks, and crack closure. An important contribution to such Mode I crack growth arises from the residualtensile stresses induced locally at the notch-tip when the deformation within the notch-tip process zone leaves permanent strains upon unloading from the maximum nominal compressive stress. It is shown that the phenomenon of crack growth under cyclic compressive stresses exhibits a macroscopically similar behaviour in a wide range of materials spanning the very ductile metals to extremely brittle solids, although the micromechanics of this effect are very different among the various classes of materials. The mechanisms of fatigue in ceramics are compared and contrasted with the more familiar examples of crack propagation under far-field cyclic compression in metallic systems and the implications for fracture in ceramic-metal composites and transformation toughened ceramic composites are highlighted. Strategies for some important applications of this phenomenon are recommended for the study of fracture mechanisms and for the measurement of fracture toughness in brittle solids.

Microstructural aspects of crack propagation in ceramics

X-ray microradiographic examination supported by optical and SEM observations was used to study crack propagation in various ceramics, including glasses and cubic and noncubic polycrystalline bodies of different grain sizes. The nature of crack propagation in ceramics was often extremely complex. While cracks in glassy materials were generally simple, as would be expected, in cubic and non-cubic polycrystalline specimens both wandering and branching of cracks was observed. In cubic materials, wandering and branching occurred on the scale of the grain size, while in fine grain, non-cubic materials these were on a multi-grain scale. Results are consistent with the grain size dependence of fracture energy. Elastic anisotropy and thermal expansion anisotropy were suggested as major factors in crack wandering and branching.

Microstructural aspects of crack propagation in ceramics

Microstructural aspects of crack propagation in ceramics