Contact Fatigue in Silicon Nitride

A study of contact fatigue in silicon nitride is reported. The contacts are made using WC spheres, principally in cyclic but also in static loading, and mainly in air but also in nitrogen and water. Damage patterns are examined in three silicon nitride microstructures: (i) fine ( F )-almost exclusively fully-developed cone cracks; (ii) medium ( M )-well developed but smaller cone cracks, plus modest subsurface quasi-plastic damage; (iii) coarse ( C )-intense quasi-plastic damage, with little or no cone cracking. The study focuses on the influence of these competing damage types on inert strength as a function of number of contacts. In the F and M microstructures strength degradation is attributable primarily to chemically assisted slow growth of cone cracks in the presence of moisture during contact, although the M material shows signs of enhanced failure from quasi-plastic zones at large number of cycles. The C microstructure, although relatively tolerant of single-cycle damage, shows strongly accelerated strength losses from mechanical degradation within the quasi-plastic damage zones in cyclic loading conditions, especially in water. Implications concerning the design of silicon nitride microstructures for long-lifetime applications, specifically in concentrated loading, are considered.     

Role of Microstructure in Dynamic Fatigue of Glass-Ceramics after Contact with Spheres

Dynamic fatigue data are reported for fine- and coarse-grained micaceous glass-ceramics after contact damage with spheres. The strengths of indented specimens are measured at stressing rates from ∼10⁻² to 10⁴ MPa·s⁻¹ in water. The strength degradation is substantially faster in the coarse-grained structure, and is accelerated further by multicycle contact loading. Failures originate from contact sites in all cases but undergo a progressive transition from classical cone cracks to quasi-plastic microcrack zones with increases in the grain size and the number of contact cycles. The results highlight the particularly deleterious effect of quasi-plastic damage accumulation on lifetime.     

Crack-Bridging Analysis for Alumina Ceramics under Monotonic and Cyclic Loading

Frictional degradation of grain-localized bridges behind a crack tip has been recognized as the major cyclic fatigue mechanism in alumina ceramics. Such a fatigue mechanism implies that the crack growth resistance ( R ) curve behavior during cyclic fatigue is different from that of monotonic loading due to the reduction in crack-tip shielding. A recent crack-bridging theory based on crack compliances is used to study the bridging stresses under monotonic loading and during cyclic fatigue. The bridging-stress distributions of two coarse-grained aluminas under monotonic loading are determined using compliance measurements. Because the interlocking grain bridges at the crack wake are subject to frictional damage from cyclic loading, the bridging-stress distribution evaluated during cyclic fatigue is distinct from that for monotonic loading. These results indicate that it is incorrect to incorporate the R -curve behavior from monotonic loading to the analysis of cyclic fatigue of alumina ceramics.     

Contact Fatigue of a Silicon Carbide with a Heterogeneous Grain Structure

A comparative study of cyclic fatigue damage from Hertzian contacts in silicon carbide ceramics with homogeneous microstructure (fine, equiaxed grains, strong grain boundaries) and heterogeneous microstructure (coarse, contiguous elongate grains, weak interphase boundaries) is presented. Observations of the surface and subsurface damage patterns using optical microscopy reveal fundamentally different cyclic fatigue mechanisns: in the homogeneous material, by slow growth of a well-developed cone crack outside the contact area; in the heterogeneous material, by progressive mechanical degradation within a distributed damage zone below the contact area. Scanning electron micrographs of the latter material show copious fine debris in the damage zone, consistent with a degradation mechanism by frictional attrition by forward-reverse sliding at the weak interphase boundaries. Acoustic emission is recorded during both load and unload half-cycles, confirming hysteresis in the sliding process. Flexure tests indicate initially slight strength losses from the cyclic contact damage in both microstructures, followed by accelerated losses at higher numbers of cycles. The underlying basis for establishing an analytical model of damage accumulation in the heterogeneous microstructure in terms of shear-fault sliding, and for designing micro-structures for optimal properties in fatigue and wear applications, is foreshadowed.     

Cyclic Fatigue from Frictional Degradation at Bridging Grains in Alumina

Tension—tension cyclic loading tests have been conducted on a coarse-grained alumina ceramic that exhibits toughnesscurve behavior by grain-interlock bridging. Fatigue effects are observed in the regions of both short cracks, using indentation flaws, and long cracks, using compact-tension specimens. A true mechanical fatigue effect is demonstrated by running the tests below the static fatigue limit. A custom-made device for in situ observation of crack propagation in the scanning electron microscope enables us to identify bridge degradation as a cause of the fatigue process. "Wear" debris cumulates at the sliding intergranular frictional contact points, indicating a loss of traction at the junction. The basis of a fracture mechanics model describing the effect of this frictional degradation in reducing crack-tip shielding is outlined and fitted to the data. It is suggested that the bridge degradation fatigue mechanism may be widespread in polycrystalline ceramics with pronounced toughness curves.     

Contact damage resistant SiC/graphene nanofiller composites

The short-crack domain and contact damage resistances of silicon carbide (SiC) ceramics containing graphene fillers (graphene nanoplatelets -GNPs- or reduced graphene oxide sheets -rGOs) are investigated by performing Hertzian contact tests. A progressive deviation from the linear Hertzian elastic response with increasing graphene content takes place, the composite containing 20 vol.% GNPs being the most deformable material. When adding increasing amounts of GNPs, the damage beneath the contact zone turns from well-defined cone cracks of monolithic SiC to a widespread subsurface damage where microcracks are generated due to the matrix/graphene interface debonding by a shear faulting process. This mechanism enhances the contact damage resistance of the composites, redistributing the stresses at the contact and limiting the long-crack formation. The composite containing 5 vol.% rGOs fully precludes the cone cracks development and enlarges the quasi-plastic damage zone, extraordinarily enhancing the contact damage resistance that approaches to that of a ductile material.     

Quantitative Analysis of Crack Shielding Degradation During Cyclic Fatigue of Alumina

Grain bridging degradation behind a crack tip is the main cyclic fatigue mechanism in nontransforming ceramics. In this work, a compliance function is used to quantify the shielding capacity of grain bridges during cyclic loading of alumina ceramics with different grain sizes. This allows to identify the different stages occurring during cyclic loading. Significant degradation is observed in the coarse grain material and a marked sensitivity to the loading level is outlined. At moderate loads, bridging degradation occurs prior to fatigue crack growth during an incubation period which can reach several million cycles. At low cyclic loads, the shielding capacity can be entirely degraded, leading to a cyclic fatigue threshold equivalent to that of the fine grain material.     

Contact damage tolerance of alumina-based layered ceramics with tailored microstructures

This work demonstrates how to enhance contact damage resistance of alumina-based ceramics combining tailored microstructures in a multilayer architecture. The multilayer system designed with textured alumina layers under compressive residual stresses embedded between alumina–zirconia layers was investigated under Hertzian contact loading and compared to the corresponding monolithic reference materials. Critical forces for crack initiation under spherical contact were detected through an acoustic emission system. Damage was assessed by combining cross-section polishing and ion-slicing techniques. It was found that a textured microstructure can accommodate the damage below the surface by shear-driven, quasi-plastic deformation instead of the classical Hertzian cone cracking observed in equiaxed alumina. In the multilayer system, a combination of both mechanisms, namely Hertzian cone cracking on the top (equiaxed) surface layer and quasi-plastic deformation within the embedded textured layer, was identified. Further propagation of cone cracks at higher loads was hindered and/or deflected owed to the combined action of the textured microstructure and compressive residual stresses. These findings demonstrate the potential of embedding textured layers as a strategy to enhance the contact damage tolerance in alumina ceramics.     

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.     

Role of Microstructure in Hertzian Contact Damage in Silicon Nitride: II, Strength Degradation

In this Part II of a two-part study of the role of microstructure on Hertzian contact damage in silicon nitride we determine strength degradation properties. As previously, three microstructures are investigated: fine ( F ), medium ( M ) and coarse ( C ), representing a progressive transition from brittle to quasi-plastic damage. In both the F and M materials, failures originate from cone cracks (although limited quasi-plasticity is evident in the latter material). These two materials show abrupt losses in strength at the critical contact loads for cone crack initiation, and steady falloff thereafter at higher contact loads. In the C material, failures occur from critical shear faults within the damage "yield" zones. The strengths in this material fall off much more gradually, without abrupt drop, above about twice the critical load for the onset of yield. Fracture mechanics models for each type of failure mode provide explicit relations for the degraded strength as a function of contact load. These models account for all the essential features in the observed strength degradation data. Particular attention is given to indenter size effects in the strength responses. Sphere radius has a profound influence on the critical loads for the onset of degradation, but relatively little influence on the degraded strengths at higher loads. Implications of the results concerning contact fatigue and wear are briefly considered.     

Experimental Observations of Frictional Heating in Fiber-Reinforced Ceramics

The influence of fatigue loading history and microstructural damage on the magnitude of frictional heating and interfacial shear stress in a unidirectional SiC fiber/calcium aluminosilicate matrix composite was investigated. The extent of frictional heating was found to depend upon loading frequency, stress range, and average matrix crack spacing. The temperature rise attained during fatigue can be significant. For example, the temperature rise exceeded 100 K during fatigue at 75 Hz between stress limits of 220 and 10 MPa. Analysis of the frictional heating data indicates that the interfacial shear stress undergoes an initially rapid decrease during the initial stages of fatigue loading: from an initial value over 20 MPa, to approximately 5 MPa after 25 000 cycles. Over the range of 5 to 25 Hz, the interfacial shear stress was not significantly influenced by loading frequency. The implications of frictional heating in fiber-reinforced ceramics are also discussed.     

Cyclic contact fatigue behavior of baria-silicate glass-ceramics as a function of crystal aspect ratio

Ceramic materials are potentially useful for dental applications because of their esthetic potential and biocompatibility. However, evidence of contact fatigue damage in ceramics raises considerable concern regarding its effect on the survival probability predicted for dental prostheses. To simulate intraoral conditions, Hertzian indentation loading with steel indenters was applied in this study to characterize the fatigue failure mechanisms of ceramic materials. Baria silicate glasses and glass-ceramics with different aspect ratios of crystals were selected because the glass and crystal phases have similar density, elastic modulus, and thermal expansion coefficients. Therefore, this system is a model ceramic for studying the effect of crystal geometry on contact cyclic fatigue failure. The subsequent flexural strength results show that the failure of materials with a low fracture toughness such as baria-silicate glass (0.7 MPa m^1/2) and glass-ceramic with an aspect ratio of 3.6/1 (1.3 MPa m^1/2) initiated from cone cracks developed during cyclic loading for 10³ to 10⁵ cycles. The mean strengths of baria-silicate glass and glass-ceramics with an aspect ratio of 3.6/1 decreased significantly as a result of the presence of a cone crack. Failures of baria-silicate glass-ceramics with an aspect ratio of 8.1/1 (K_c = 2.1 MPa m^1/2) were initiated from surface flaws caused by either grinding or cyclic loading. The gradual decrease of fracture stress was observed in specimens with an aspect ratio of 8.1/1 after loading in air for 10³ to 10⁵ cycles. A reduction of approximately 50 % in fracture stress levels was found for specimens with an aspect ratio of 8.1/1 after loading for 10⁵ cycles in deionized water. Thus, even though this glass-ceramic with an 8.1/1 crystal aspect ratio material is tougher than that with a 3.6/1 crystal aspect ratio, the fatigue damage induced by a large number of cycles is comparable. The mechanisms for cyclic fatigue crack propagation in baria-silicate glass-ceramics are similar to those observed under quasi-static loading conditions. An intergranular fracture path was observed in glass-ceramics with an aspect ratio of 3.6/1. For an aspect ratio of 8.1/1, a transgranular fracture mode was dominant.     

Chipping of ceramic-based dental materials by micrometric particles

Chipping caused by micrometric particles poses a threat to the structural integrity of modern dental prosthetic materials. It can degrade their fracture strength and cause wear of both artificial crowns and antagonist teeth. Here, surface chipping of the main types of commercial ceramic-based dental materials at the microcontact/particle level is investigated by means of indentation tests. Conical tips of different sizes (radii 20 and 200 μm) under axial and sliding loading are employed to simulate individual microcontacts. Both decreasing particle size and adding a lateral contact force decrease the chipping load below typical bite forces. Specific damage mechanisms are identified as predominantly brittle fracture in ceramics with small, equiaxed crystals, with significant quasi-plastic damage in ceramics containing large, elongated crystals and composites. Critical loads for the occurrence of chipping are quantified (lowest values in equiaxed glass–ceramics; greatest in zirconia) and analyzed within the framework of fracture mechanics. The brittleness index (BI) is proposed as a simple indicator of the resistance to chipping of dental materials—the lower the BI, the greater the resistance. Special attention is paid to the effect of the materials’ microstructure, which can result in transformation toughening (as in zirconia) or quasi-plastic behavior (as in lithium disilicate), both highly beneficial to increasing the chipping resistance. Finally, practical implications for the selection of current dental materials as well as for the development of novel materials with improved durability are discussed.     

Push-Pull Fatigue of Sintered Silicon Nitride Ceramics at Elevated Temperatures

The fatigue tests under push-pull completely reversed loading and pulsating loading were performed for silicon nitride ceramics at elevated temperatures. Then the effects of stress wave form, stress rate, and cyclic understressing on fatigue strength, and cyclic straining behavior, were examined. The cycle-number-based fatigue life is found to be shorter under trapezoidal stress wave loading than under triangular stress wave loading, and to become shorter with increasing hold time under the trapezoidal stress wave loading. Meanwhile, the equivalent time-based life curve, which is estimated from the concept of slow crack growth, almost agrees with the static fatigue life curve in the short and intermediate life regions, showing the small cyclic stress effect and the dominant stress-imposing period effect on cyclic fatigue life. The fatigue strength increased in stepwise stress amplitude increasing test, where stress amplitude is increased stepwise every given number of stress cycles, at 1100° and 1200°C. Occurrence of cyclic strengthening was proved through a gradual decrease in strain amplitude during a pulsating loading test at 1200°C in this material, corresponding to the above cyclic understressing effect on fatigue strength.     

Indentation Fatigue in Random and Textured Alumina Composites

The effect of grain orientation on contact fatigue behavior has been investigated by using alumina with elongated grains as a model system. Two kinds of composite microstructures, textured and random, were prepared by controlling the processing conditions. The textured material has the platelets aligned parallel to the surface, and the random material has the platelets randomly oriented. The Hertzian indentation results show that, although both materials exhibit damage accumulation with increasing number of cycles due to frictional degradation at the microstructural level, the damage evolution rate is much lower for the textured material. This suppression of fatigue damage in the textured material appears to result from the lower shear stress concentration along the textured weak interfaces between elongated alumina grains. The implication of the present results for structural design in improvement of contact-fatigue resistance is also addressed.     

复合材料胶接修补结构疲劳性能的数值和试验研究

为研究室温下复合材料胶接修补结构的疲劳性能,以三维渐进损伤理论为基础,创建了复合材料胶接修补模型,利用材料损伤判断子程序实现对修补结构的静拉伸失效载荷及剩余强度的预测分析,并进行了相关试验的对比分析。采用5种不同尺寸的圆形补片来评价修补效果,并利用超景深仪对修补试件的疲劳损伤扩展模式进行微观测量。结果表明:静载拉伸中,尺寸为3.5r的修补结构承载能力最好;疲劳循环中,尺寸为2.5r的修补结构剩余强度提升效果最好;疲劳载荷下,当循环次数较低时,修补结构的主要损伤为基体开裂,而随着循环次数的增大,主要损伤为纤维断裂。     

Fatigue Analysis of Adhesive Joints Under Vibration Loading

Adhesively bonded joints have been used extensively for many structural applications. However, one disadvantage usually limiting the service life of adhesive joints is the relatively low strength for peel loading, especially under dynamic cyclic loading such as impulsive or vibrational forces. Moreover, accurately predicting the fatigue life of bonded joints is still quite challenging. In this study, a combined experimental–numerical approach was developed to characterize the effect of the cyclic-vibration-peel (CVP) loading on adhesively bonded joints. A damage factor is introduced into the traction-separation response of the cohesive zone model (CZM) and a finite element damage model is developed to evaluate the degradation process in the adhesive layer. With this model, the adhesive layer stress states before and after being exposed to various CVP loading cycles are investigated, which reveals that the fatigue effect of the CVP loading starts first in the regions close to the edges of the adhesive layer. A good correlation is achieved when comparing the simulation results to the experimental data, which verifies the feasibility of using the proposed model to predict the fatigue life of adhesively bonded joints under the CVP type of loading.     

Cyclic Fatigue in Ceramics: A Balance between Crack Shielding Accumulation and Degradation

Cyclic fatigue growth rates in R-curve ceramics have been observed to depend very strongly on the maximum applied stress intensity, K _max, and only weakly on the stress intensity range, Δ This behavior is rationalized through measurement of crack wake shielding characteristics as a function of these fatigue parameters in a gas-pressure-sintered silicon nitride. In particular, evidence for a mechanical equilibrium between shielding accumulation by crack growth and shielding degradation by frictional wear of sliding interfaces is found for steady-state cyclic fatigue. This equilibrium gives rise to a rate law for cyclic fatigue. The data suggest that the accumulation process is the origin of the strong K _max dependence, and that the degradation process is the origin of the weak Δ dependence. These features are shown to be related to the "cyclic" R -curve and to the cyclic crack opening displacement, respectively.     

Fatigue behaviour and residual strength evolution of 2D C/SiC Z-pinned joints prepared by chemical vapour infiltration

Z-pinned joints prepared by chemical vapour infiltration are widely used in ceramic matrix composite components. Excellent fatigue behaviour is important for structural safety. In this study, 2D C/SiC Z-pinned joints were loaded in axial direction of the pins under static and cyclic loading. Internal damage was monitored in situ by an acoustic emission system. The binding force between pin and hole is relatively strong. Meanwhile, the joints exhibite promising resistance to fatigue. The residual strength increased first with the fatigue cycles then decreased after 10⁵ cycles. Microstructural analysis indicated that full-developed cracks and local stress redistribution resultes in the increase in the strength of the joints. The acoustic emission analysis also provides a supplementary understanding of the damage mechanism. The results show that damage fully develops at the early stage of fatigue. When the specimen is reloaded, less AE events are collected before the fatigue maximum stress.