Assessment of viscoelastic crack bridging toughening in refractory materials

Viscoelastic bridges can be formed in refractory ceramics while cooling from high temperatures. Such bridges can shield crack tips, thus reducing the effective crack tip stress intensity factors leading to higher resistance to creep and thermal shock. The extent to which the crack tip stress intensity is reduced can be estimated from fracture mechanics models that include experimental measurement of crack bridging and microstructural parameters. In this paper a novel approach is proposed for the assessment of the effective crack bridging toughening from combining destructive and non-destructive test methods. Fracture toughness values were determined applying chevron notched specimen technique and surface damage of the specimen was monitored by image analysis. Different cordierite–mullite compositions characterized by different microstructure morphologies and crack propagation behaviour were investigated. A brief discussion about the correlation between thermo-mechanical properties, microstructure, crack propagation behaviour and thermal shock resistance is presented. Moreover, an empirical model able to determine the presence and effectiveness of the viscoelastic crack bridging ligaments acting in the microstructure under thermal shock conditions and their degradation with increasing thermal shock cycles from parameters measured at room temperature is presented.     

Influence of residual and bridging stresses on the R-curve behavior of Mo- and FeAl-toughened alumina

Alumina matrix was toughened using either metal molybdenum or intermetallic FeAl particles. Mo and FeAl dispersoids were chosen because they have different thermomechanical properties (i.e. Young's modulus, Poisson ratio, as well as thermal expansion coefficient), giving rise to different residual stresses in the matrix. The R-curve behavior of these composites was first studied by stable-crack propagation experiments as a function of the volume fraction of dispersoid. The optimum fraction for toughening was different in the two composites: 25 and 15 vol% addition led to maximum toughness in the Mo- and FeAl added composite, respectively. This difference was ascribed to residual stresses. Microscopic observation of the crack path revealed, in both composites, the systematic presence of dispersoids acting as bridging sites in the crack wake, but only a few of them were plastically stretched. Residual stresses in the Al₂O₃ matrix, after sintering and microscopic bridging tractions during crack propagation, were quantitatively assessed using microprobe fluorescence spectroscopy. Bridging microstresses were assessed in situ by a linear map along the crack profile, at the critical condition for fracture propagation. Experimentally collected residual stresses and bridging stresses were discussed to explain the different fracture behavior of the composites.     

In Situ Measurements of Bridged Crack Interfaces in the Scanning Electron Microscope

A device for in situ SEM examination of crack propagation during loading of compact tension specimens is described, with a specific demonstration on an alumina ceramic. The device facilitates direct qualitative observations of the inception and subsequent frictional pullout of grain-localized bridges at the crack interface. Quantitative data on the bridging mechanism are obtained from measurements of the crack-opening displacements behind the crack tip. The crack profile is found to be closer to linear than parabolic at the bridged interface. Deconvolution of these crack-opening data allow for an evaluation of the closure tractions operative at the crack walls within the bridging zone, and thence the R -curve.     

A model for predicting fracture resistance of fiber reinforced concrete

A theoretical model is presented to predict the crack propagation resistance of fiber reinforced cement based composites. A crack in the matrix is divided into a traction free zone, fiber bridging zone and the matrix process zone. The crack closing pressure due to fibers depends on the (Mode I) crack opening displacement (COD). A method is suggested to estimate this relationship from the pull-out tests. Although calculations of COD are based on linear elastic fracture mechanics concepts, the energy absorbed in the fiber bridging zone is included in the analysis. Theoretical results are compared with the experimental data of notched beam and double cantilever beam specimens.     

金属基聚合物复合材料短纤维桥接界面强化机理

针对金属基聚合物复合材料易诱发界面剥离损伤失效的共性问题,研究了通过多层复合组装注射成型,在聚合物复合层与粘接层界面形成短纤维桥接,实现复合界面强化.基于内聚力剥离损伤模型,构建了短纤维桥接强化界面剥离裂纹扩展断裂失效过程的模拟仿真技术,模拟建立了界面剥离裂纹快速失稳扩展断裂损伤失效临界载荷—桥接纤维特性—界面剥离断裂...     

Crack Propagation Behavior of Polycrystalline Alumina under Static and Cyclic Load

Various causes for cyclic-loading fatigue in ceramics have been proposed. Degradation in the grain-bridging effect is the most important cause for cyclic-loading fatigue in nontransforming ceramics. Cyclic- and static-loading crack propagation behavior in terms of crack propagation rate, load—strain curve, and R -curve in compact tension specimens of polycrysalline aluminas with two types of average grain size is reported. Significant bridging is observed in coarse-grained alumina. The results are consistent with the proposal that grain bridging increases with grain size and that degradation in grain bridging is the most important cause for cyclic-loading fatigue in alumina ceramics.     

Bridging and Damage Zones in Crack Growth

The equivalence between bridging and damage zones that may occur at crack tips during crack propagation is demonstrated. The physical insights provided by both approaches are introduced. Finally, some results for the crack velocity during the creep rupture of a material containing an amorphous phase are discussed.     

Retardation of Fatigue Crack Growth in Ceramics by Glassy Ligaments: A Rationalization

In high-temperature fatigue crack growth (FCG) experiments on ceramic materials containing amorphous grain boundary phases, the crack growth rates under cyclic loads were observed to be lower than those predicted solely on the basis of crack growth velocities measured under static loads. In this paper, a rationalization was offered for such a behavior by means of a phenomenological glass-bridging model which takes the relaxation behavior of glass into account. In ceramics which exhibit subcritical crack growth through cavitation ahead of the crack tip, the maximum stress intensity factor of the fatigue cycle required to initiate FCG was observed to be always greater than or equal to the threshold stress intensity factor for crack growth under sustained far-field loads. This trend was also explained with the aid of the glass-bridging model and invoking the equivalence between bridging and damage zones. The elevated temperature FCG behavior of nitride-based ceramics which exhibit grain bridging in the wake during crack propagation was discussed and contrasted with oxide-based ceramics which show glass bridging.     

Model for Fatigue Crack Growth in Grain-Bridging Ceramics

A model for fatigue crack propagation based on sliding wear of bridging grains is analyzed for polycrystalline ceramics. Taking into account damage development and crack tip energy balance, we have obtained rigorous solutions for equilibrium and compatibility equations in the crack wake under monotonic and cyclic loading/unloading conditions. Fatigue mechanics in ceramics is found to be formally similar to elastic-plastic mechanics of a path-dependent hardening material, due to the frictional resistance to reverse sliding. It features a load-displacement hysteresis causing energy dissipation and wear, and a longer cohesive zone required for supporting the same peak load with the wear-reduced bridging stresses. The unloading crack opening displacement is more strongly dependent on K _max than on Delta K ; such displacement causes wear on the bridging grains. Meanwhile, incremental crack growth brings in new bridging grains that has a shielding effect on the crack tip stress field; such an effect is strongly dependent on K _max but independent of Delta K . At steady state, when shielding accumulation and shielding degradation are balanced, the fatigue crack growth rate has a form d a /d N = A ( K _max) ^b (Delta K ) ^c , where A, b , and c are material-dependent parameters. Fatigue is predicted to have a very high b , a modest c , a higher fatigue resistance for tougher ceramics, and a stronger K _max dependence for less tough ceramics. These predictions are in agreement with experimental observations.     

Crack Propagation Law Affected by Natural Fracture and Water Jet Slot under Blast Loading

The natural fracture of rock has a strong effect on the law of explosion stress wave transmission and crack propagation during blasting. Based on the stress wave theory, the influential mechanism for both the law of transmission of the stress wave and of crack propagation due to natural fracture and water jet slot are analysed. Next, an experiment is conducted to understand the crack propagation law because of the effect of an explosion shock wave, and the evolution law of the blast stress wave and blast-induced crack propagation is simulated by ANSYS/LS-DYNA. The results indicate that the existence of the water jet slot not only promotes the generation of the main crack along its direction, but also promotes the generation of the secondary crack near the water jet slot because of the explosion shock wave. The direction of propagation of the secondary crack and the main crack are seriously affected by the natural fracture. In addition, if the distance between the blast hole and the natural fracture is too small, a smash area is formed; and with an increase in the distance between the blast hole and the natural fracture, the smash area becomes smaller, and the effect on the blast-induced crack becomes weaker.     

Measurement of Microscopic Bridging Stresses in an Alumina/Molybdenum Composite by In Situ Fluorescence Spectroscopy

The R -curve behavior of an Al₂O₃ ceramic with 25 vol% of molybdenum-metal particles added was studied by using fracture-mechanics experiments and in situ piezospectroscopic measurements of microscopic bridging tractions. Cracks were propagated by using a crack stabilizer, which allowed stable crack growth in a bending geometry. Microscopic bridging stresses were measured in situ during fracture propagation by detecting the shift of the Cr³⁺ fluorescence lines of Al₂O₃. Laser spots ∼1 µm in diameter and ∼10 µm deep were focused at the ceramic/metal interface of the bridging sites, and the closure stresses that acted on the crack faces were recorded as a function of external load. The maximum stress that was experienced by the stretched metal particles prior to final failure was ∼0.4 GPa. The maximum stress magnitude was not markedly different in relatively small (i.e., <5 µm) metal particles, failing with large ductility, as compared with larger particles which, instead, fractured in semibrittle fashion. A map of bridging tractions along the crack wake was constructed under a constant stress intensity factor, almost equal to that which is critical for crack propagation. Using this map to theoretically predict the rising R -curve behavior of the composite led to results that were consistent with the fracture-mechanics experiments, thus enabling us to explain the observed toughening, primarily in terms of a crack-bridging mechanism.     

Direction-dependent adhesion of micro-walls based biomimetic adhesives

The adhesive properties of a biomimetic anisotropic micro-structured surface are investigated. The system is constituted by parallel elastic wall-like structures topped with a thin film. The micro-walls are assumed in perfect contact with a rigid substrate and the adhesive interaction is modeled by considering full contact conditions. Because of its crack trapping behavior, this geometry, when loaded with an external moment acting perpendicularly to the walls direction, shows enhanced adhesive properties compared to the simple flat surface.In the present paper, we study how the adhesive properties depend on the direction of crack propagation. In particular, we determine how the applied moment needed to detach the adhesive depends on the angle that the crack propagation direction makes with the micro-walls one. We find that crack trapping occurs only when crack propagates perpendicularly to the walls. In all the other cases, the system compliance linearly increases with the crack length, causing the energy release rate at the crack to be constant during the crack propagation: crack trapping cannot occur in such conditions.We also propose a simplified analytical model, based on the Euler–Bernoulli beam theory, to calculate the adhesion strength of the system. Analytical solution and numerical calculations show perfect agreement for all directions of crack propagation and values of geometrical parameters.     

Fracture and Fatigue Behavior at Ambient and Elevated Temperatures of Alumina Bonded with Copper/Niobium/Copper Interlayers

Interfacial fracture toughness and cyclic fatigue-crack growth properties of joints made from 99.5% pure alumina partially transient liquid-phase bonded using copper/niobium/copper interlayers have been investigated at both room and elevated temperatures, and assessed in terms of interfacial chemistry and microstructure. The mean interfacial fracture toughness, G _c, was found to decrease from 39 to 21 J/m² as temperature was raised from 25° to 1000°C, with failure primarily at the alumina/niobium interfaces. At room temperature, cyclic fatigue-crack propagation occurred both at the niobium/alumina interface and in the alumina adjacent to the interface, with the fatigue threshold, Δ G _TH, ranging from 20 to 30 J/m²; the higher threshold values in that range resulted from a predominantly near-interfacial (alumina) crack path. During both fracture and fatigue failure, residual copper at the interface deformed and remained adhered to both sides of the fracture surface, acting as a ductile second phase, while separation of the niobium/alumina interface appeared relatively brittle in both cases. The observed fracture and fatigue behavior is considered in terms of the respective roles of the presence of ductile copper regions at the interface which provide toughening, extrinsic toughening due to grain bridging during crack propagation in the alumina, and the relative crack propagation resistance of each crack path, including the effects of segregation at the interfaces found by Auger spectroscopy.     

Crack-tip Degradation Processes Observed during in situ Cyclic Fatigue of Partially Stabilized Zirconia

It is proposed that reduced transformation zone widths in Mg-PSZ in cyclically versus critically propagated cracks are due to reductions in the crack-tip toughness, consistent with an intrinsic cyclic fatigue mechanism. Cyclic fatigue crack growth in Mg-PSZ was observed in situ in a SEM. Following cyclic fatigue, the samples were critically broken and the fracture surfaces observed. Extensive crack bridging by the precipitate phase was observed near the crack tip, and it is proposed that this crack bridging significantly affects the material's intrinsic toughness. Frictional degradation of the precipitate bridges occurs during cyclic loading and hence reduces the critical crack-tip stress intensity factor for crack propagation. Reductions in the critical crack-tip stress intensity factor also lead to reductions in the transformation zone widths during cyclic loading and hence the level of crack-tip shielding caused by phase transformation. This appears to be the mechanism of cyclic fatigue. A degree of uncracked ligament bridging was also observed and is linked with the frequency of random large precipitates. However, analysis shows that its effect upon crack growth rates under cyclic load is limited.     

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.     

Delamination Cracking in a Laminated Ceramic-Matrix Composite

Delamination crack propagation has been investigated in a laminated fiber-reinforced ceramic-matrix composite. The crack growth initiation resistance has been shown to be dominated by the critical strain energy release rate for the matrix. However, the resistance increases with crack extension because of bridging effects associated with intact fibers and, in some cases, intact segments of matrix. The delamination cracks also assume a steady-state trajectory within a 0° layer close to the 0°/90° interface.     

Crack-Interface Interactions in a Tungsten-Yttria-Stabilized-Zirconia Directionally Solidified Eutectic

A directionally solidified eutectic (DSE) of W-ZrO₂(Y₂O₃) consists of faceted fibers of tungsten embedded in yttriastabilized cubic zirconia. The W-ZrO₂ interfaces in this system are devoid of any impurity phases or a reaction product. Microindentation-induced cracks and their interactions with interphase interfaces are investigated using scanning and transmission electron microscopy. Crack-interface interactions are found to fall into one of four categories: propagation along the metal-ceramic interface, deflection away from the interface, crack-interface bridging, and shearing of tungsten fibers. Further crack propagation is investigated in situ in a TEM with a propagation induced by local electron beam heating. These crack path selections are analyzed within the framework of phenoin-enological understanding of crack-interface interactions in composites.     

Influence of crystal structure on crack propagation under cyclic electric loading in lead–zirconate–titanate

Crack propagation under cyclic electric loading was studied in two non-commercial compositions of lead–zirconate–titanate and compared to earlier results from a commercial composition. These materials were chosen to provide a well-defined variation in crystal structure, ranging from rhombohedral to tetragonal, including a composition from the morphotropic phase boundary. The results are presented in terms of crack propagation as a function of various electric load amplitudes. While the crack propagation rates were of the same order of magnitude in all three compositions, fracture occurred in an either trans- or intergranular manner with crack extension either in the form of a singular crack, a microcrack zone or with extensive secondary cracking. These differences in crack propagation are discussed in the context of different piezoelectric material properties.     

Crack-Interface Grain Bridging as a Fracture Resistance I, Mechanism in Ceramics: I, Experimental Study on Alumina

Direct microscopic evidence is presented in support of an explanation of R -curve behavior in monophase ceramics by grain-localized bridging across the newly formed crack interface. In situ observations are made of crack growth in tapered cantilever beam and indented flexure specimens of a coarsegrained alumina. The fractures are observed to be highly stable, typical of a material with a strongly increasing resistance characteristic, but are discontinuous at the microstructural level. Associated with this discontinuity is the appearance of overlapping segments in the surface fracture trace around bridging grains; the mean spacing of such "activity sites" along the trace is about 2 to 5 grain diameters. These segments link up with the primary crack beneath the specimen surface, and continue to evolve toward rupture of the bridge as fracture proceeds. The bridges remain active at large distances, of order 100 grain diameters or more, behind the crack tip. Scanning electron microscopy of some of the bridging sites demonstrates that secondary (interface-adjacent) microfracture and frictional tractions are important elements in the bridge separation process. Evidence is sought, but none found, for some of the more popular alternative models of toughening, notably frontal-zone microcracking and cracktip/internal-stress interaction. It is suggested that the crackinterface bridging mechanism may be a general phenomenon in nontransforming ceramics.