Fractography of biaxial tested Si3N4-specimens

The strength of ceramic materials is limited by flaws which are distributed in the volume or on the surface of the material. Commonly, fractographic investigations are performed after the strength tests to interpret the strength values.The relatively new Ball-on-Three Balls (B3B)-bending test applies a biaxial stress state (which is more searching for cracks than a uniaxial stress state) on the specimen. To identify typical fracture initiating flaws and to get a better understanding of the fracture behaviour of B3B-specimens a systematic fractographic investigation was performed on 260 silicon nitride specimens divided into batches with different surface qualities. It could be shown that in most cases (at least those in which origins could be clearly identified) surface or near surface located defects were responsible for failure. On specimens with poor surface qualities, surface defects were introduced through machining. On specimens with a better surface quality, volume defects, which were exposed on the surface by polishing, could be identified as fracture origins. In only a few cases defects in the bulk were fracture origins.     

Crack Coalescence and Microscopic Crack Growth in the Delayed Fracture of Alumina

Microscopic crack growth in the delayed fracture of alumina is studied. A model for the delayed fracture, based on microscopic cracks and their interaction and coalescence, is proposed as opposed to the conventional single-worst-crack model. The proposed analysis gives a more realistic prediction than the conventional one. It is found that an assembly of microscopic cracks, which may not be observable, may be worse than a single macroscopic crack in delayed fracture and that the coalescence of the microscopic cracks may occur in a very short time without being identified because the critical amount of slow crack growth from the individual inherent flaws can be very small. Applicability of proof-testing concepts is reexamined. It is demonstrated that the existing concepts give nonconservative prediction in time-to-failure; but the nonconservative prediction is partially or sometimes excessively compensated by conservative estimation of the initial crack size.     

双轴压缩下混凝土预制双裂纹扩展与应力应变关系研究

对混凝土预制双裂隙板试件进行了双轴压缩试验,研究了裂隙倾角以及岩桥倾角对双裂隙的扩展演化影响.通过在裂隙尖端贴放应变片,分析了裂隙扩展与应力应变关系,探讨了裂隙尖端应变集中对裂隙扩展演化的作用规律.试验结果表明,裂隙倾角以及岩桥倾角对裂隙的扩展、贯通有较大影响.实验结果共观测到7种裂纹贯通模式(T1和T2;S1和S2;TS1、TS2和TS3)及两种贯通失败模式(剪切失败和拉伸-剪切失败),且随着岩桥角的增加,裂纹贯通模式由剪切裂纹贯通到翼型-剪切复合式贯通,然后再到翼形裂纹贯通逐渐转化.应力应变曲线与裂隙扩展贯通密切相关,拉应变集中是翼形裂纹产生的原因,而压应变集中则是引起剪切裂纹产生的原因.     

Strength Variability of Indented Soda-Lime Glass

Studies of annealed indentation flaws show that specimen strength is inversely proportional to the square root of the surface trace dimension of the strength-controlling radial crack over a range of indentation loads. Using the surface trace as the characteristic dimension in the appropriate fracture mechanics equations, however, underestimates the measured strengths by about 30%. There is also a lack of correlation between the individual strengths and the surface traces at a given indentation load. Despite the complex nature of indentation flaws, strength predictions of individual indentation cracks based on the radial crack depth (not the surface trace) are in agreement with measured strengths. Strength variability of indented glass specimens at a given indentation load is due mainly to the spectrum of crack depths.     

Detection and Analysis of Crack Propagation in PZT Multilayer Ceramics

The flaw propagation in Lead zirconate titanate (PZT) multilayer ceramics under mechanical load was examined using impedance spectroscopy and three‐point bending studies. Initial flaws were generated by applying a positive sinusoidal electric field to the specimens. The cracks were sequentially propagated and after the release of the external mechanical load, impedance spectroscopy was conducted. The shift in the resonance frequencies and the subresonance height of the impedance spectroscopy were used as a measure of flaw extension. A functional dependence of the resonance frequency and the phase shift on the crack length was found. The crack propagation was studied on flaws starting at the positive and negative electrode, respectively. The maximum fracture strength, as well as the crack path, depends on the electrode potential. The variation in the fracture strength was caused by different observed fracture mode: interface cracking, matrix‐cracking, or a combination of both. The morphology of the fracture surfaces was ascribed to a textured microstructure, which is created by the sample processing, for example, by the poling process. A modified poling procedure with a lower poling temperature was analyzed, which yielded a reduction of the anisotropy of the electrode strength. Impedance spectroscopy was found to be a reliable measurement tool for automated flaw detection in PZT multilayer ceramics.     

Dye Impregnation Method for Revealing Machining Crack Geometry

The palladium nitrate dye penetrant method for revealing surface microcracks was investigated and applied to display the geometry of machining cracks in silicon nitride flexure test specimens. This method used elemental mapping with an electron probe microanalyzer to detect the presence of the dye and, thereby, display the crack geometry. A previously used bending method and a method developed in this study in which the specimen surface is exposed to the dye under pressure were used to facilitate dye penetration. Prior to applying the method to study machining cracks, carefully controlled Knoop indentation cracks introduced into flexure specimens were used to verify penetration of the dye to the crack tip. During these experiments it was found that the palladium nitrate dye resulted in a reduction in flexure strength, which, on further study, was attributed to the dilute nitric acid solution used to formulate the dye. Exposure to carbon tetrafluoride plasma etching prior to applying the pressurized dye method also resulted in a detectable decrease in flexure strength. Although there was clear evidence that exposure to dye and plasma etching resulted in a small but measurable decrease in flexure strength for the silicon nitride material studied, there was no detectable change in observed crack geometry. The reduction in flexure strength was apparently caused by a decrease in resistance to initiate crack propagation. It was concluded that the palladium nitrate dye method is an accurate and useful means for determining the geometry of small, otherwise difficult to observe surface microcracks. Nevertheless, caution should be exercised with the use of this method during strength measurements. When applied to machining cracks, the complex nature of these shallow, elongated, sometimes joining cracks was unambiguously revealed.     

Indenter Flaw Geometry and Fracture Toughness Estimates for a Glass-Ceramic

Shapes of cracks associated with Vickers indenter flaws in a glass-ceramic were assessed by stepwise polishing and measuring surface traces as a function of depth. The cracks were of the Palmqvist type even at200-N indentation load. The toad dependence of crack lengths and fracture toughness estimates were examined in terms of relations proposed for Palmqvist and half-penny cracks. Estimates based on the half-penny crack analogy were in closer agreement with bulk fracture toughness measurements despite the Palmqvist nature of the cracks.     

Mixed-Mode Fracture of Hot-Pressed Si3N4

The mixed-mode fracture of hot-pressed Si₃N₄ was investigated using inclined indentation surface flaws in bending and large crack geometries in combined tension/torsion. Non-coplanar fracture was observed in all cases. Values of K_Ic, K_IIc, and K_IIIc stress intensity factors were obtained, with ratios K_IIc/K_Ic= 0.79 and K_IIIc/K_Ic= 1.55 observed. For large cracks, mode II conditions had more of an effect on mode I fracture than mode III conditions. The mixed-mode I-II fracture of surface flaws was significantly different from that for large cracks, suggesting surface flaw shear resistance effects. A model describing these effects was derived, based on the ratio of the crack-opening displacement to the crack surface asperity height.     

Effect of Indenter Geometry on Controlled-Surface-Flaw Fracture Toughness

Fracture toughness values obtained using both Knoop and Vickers-indentation-produced controlled surface flaws were compared as a function of indentation load for a well-characterized glass-ceramic material. At the same indentation load, Knoop cracks were larger than Vickers. As-indented K_c values calculated from fracture mechanics expressions for surface flaws were higher for Knoop flaws than Vickers, but both types gave low K_c values due to indentation residual stress effects. Analysis suggested that theoretical formalisms for indentation residual stress effects based on fracture mechanics solutions for a center-loaded penny crack in an infinite medium should apply to both indentation types. K_c values calculated using the residual stress approach were identical for Knoop and Vickers controlled surface flaws when a "calibration" value for a constant term in the expression for K_c was used for both indentation types.     

Mechanisms of Failure from Surface Flaws in Mixed-Mode Loading

Mechanisms of failure from surface cracks in combined tension and shear are identified by directly observing the cracks during failure testing. Under the combined influences of residual contact stresses and applied loading, indentation cracks propagate stably and realign normal to the principal applied tension prior to failure. Annealing of indentation flaws causes relaxation of the residual stresses and thereby leads to a change in the mechanics of fracture; unstable propagation occurs from the initial crack at a critical applied loading, with an abrupt change in fracture plane. Strengths of indentation flaws and machining damage in both the as-formed and annealed states are measured as a function of flaw orientation relative to an applied uniaxial tension. Strength variations of indentations and machining flaws are similar. The results are assessed in terms of various proposed mixed-mode fracture criteria, and the implications of the results for nondestructive testing using scattering of surface acoustic waves are discussed.     

Rising Crack-Growth-Resistance (R-Curve) Behavior of Toughened Alumina and Silicon Nitride

R -curves for a sinter/HIPed SiC(whisker)-reinforced alumina and a sintered silicon nitride were assessed by direct measurements of lengths of cracks associated with Vickers indentation flaws. The fracture toughness measurements based on (a) initial (as-indented) crack lengths, (b) equilibrium growth of cracks during increasing far-field loading, and (c) crack lengths corresponding to unstable fracture showed definitive trends of R -curves for both materials. The fracture mechanics analyses employed an indenter-material constant that was independently estimated using a physical model for the residual driving force and a free surface correction factor that accounted for the effects of size and shape of the cracks on stress intensity. It is shown that R -curve estimations based on crack length measurements have the intrinsic advantage that crack length dependence of fracture toughness is not assumed a priori as is done in conventional analysis based on strength. The measured fracture toughness of SiC(whisker)-reinforced alumina was in agreement with the prediction of a toughening model based on crack bridging by partially debonded whiskers.     

Quasi-Static Adhesive Fracture

The concept of quasi-static crack propagation is used in the present paper to study quantitatively the effects of environmental fluids on fracture in adhesive joints. The mechanisms and mechanics of environmental adhesive fracture under rising loads are discussed. Two types of cracking behaviour were observed. (1) When the dissolution or the “surface energy reduction” mechanism prevailed, the fracture toughness of the adhesive joint in the environment was reduced. (2) However, when environment-enhanced crazes were formed in the adherend at the crack tip region, the local fracture toughness of the adhesive joint would be increased. But cracking was usually unstable so that crack velocities were not readily measurable.

Except in so far as the adhesive surfaces may have considerable effects, the fracture toughness of an adhesive joint is independent of the specimen geometries used in the present work. Also, the variation of fracture toughness with crack velocity for an Aluminum/ Araldite joint in a carbon tetrachloride solution is reported.     

High strength concrete — Freeze/thaw testing and cracking

Cracks due to rapid freezing and thawing in water (ASTM C666 procedure A) of non-air entrained high strength concretes have been investigated using a procedure designed to avoid creating cracks during specimen preparation. Polished sections impregnated by immersing virgin water saturated slices in ethanol containing fluorescent dye (Fluorescent Liquid Replacement — FLR) were inspected in ultra violet light in an optical microscope. The fluorescent impregnated polished sections showed no cracking in any of the concretes before frost exposure, and extensive cracking after deterioration. The amount of cracks on the polished sections showed a good relationship to Durability Factors (DF) measured in the rapid freeze/thaw test. The crack volume calculated using a simple square grid model of cracks correlated reasonably to measured volume increase at frost testing.     

Onset of Cone Cracking in Ion-Exchanged Glass

A simple fracture mechanics approach is used to predict the critical crack size required to initiate Hertzian cracks in a brittle material in the absence and presence of a residual surface stress. To test the approach, the load and location of ring/cone cracks are measured in a silicate glass before and after ion exchange. The crack sizes estimated by this technique are, however, different before and after the ion-exchange process. It is concluded that the ion-exchange process changes the nature of the surface flaws, which is a key assumption in previous theoretical analyses in using Hertzian contacts as a means to estimate surface stress.     

Visualization of Subsurface Damage in Silicon Nitride from Grinding by a Plasma Etching and Dye Impregnation Method

In order to develop a spatial morphology model of subsurface damage in grinding of hot-pressed silicon nitride, the damage has been investigated using visualization techniques, plasma etching, and a dye impregnation method developed in our previous work. The plasma etching technique resolved subsurface damage formed by a single abrasive grit on a grinding wheel surface. The damage consists of a grain-released area, which has an overall shape approximately that of a blade like a willow leaf, and a series of bow-like crack series that appear repeatedly along the grit-scratching direction. By coupling these results with previous conclusions about the geometry of the strength-controlling cracks in a flexural test specimen, we were able to develop a spatial model of subsurface damage morphology. The most distinctive feature of the model is the absence of a transverse crack, whose presence is commonly accepted in existing work on subsurface damage by grinding. The applicability of the model to the fracture behavior of a longitudinally ground flexural test specimen was verified by fractography and dye impregnation.     

In situ double torsion fracture studies of cement mortar and cement paste inside a scanning electron microscope

The characteristics of microcracking in cement and high strength mortar have been investigated by fracture experiments of small (33×11×1.1mm) double torsion (DT) type specimens, inside the specimen chamber of a conventional scanning electron microscope. The control of incremental crack growth, at less than a micron, is finer than has been reported before. The cracking path is tortuous and quite branched both around and through fine aggregate particles, thus absorbing significantly more energy than a corresponding straight crack. In the vicinity of the main crack tip a zone of diffuse microcracking was observed which may be regarded as a “process zone”. These two observations suggest that fracture mechanics models utilising discrete straight single cracks should only be used with circumspection. In cement paste there also appeared to be a transition from predominantly intergranular to transgranular fracture with increased age of hydration.     

Experimental Study of Fracture of Glass:I, The Fracture Process

A concept of fracture is developed from experimental data. Fractures are found to originate at flaws or cracks of finite size, most of which are at the surface. The mechanism is one of crack propagation which begins when the local stress at the crack exceeds a minimum value. The rate of propagation increases with crack growth until a critical stress is reached at the crack tip which coincides with a limiting crack velocity. This limiting condition is identified with the boundary of the mirror surface of the fracture. From calculations to be presented in Part 11, the critical stress is estimated to be several million pounds per square inch.     

Stable Crack Growth in Polycrystalline Magnesia under Monotonic and Cyclic Loads

The quasi-static, stable growth of cracks in polycrystalline magnesia under various loading conditions (cyclic compression, monotonic tension, and tension–compression fatigue) was studied. Crack length was monitored continuously through a conductive film attached to the specimen surface. The fracture surfaces and the crack path were examined by scanning electron microscopy. The different fracture mechanisms responsible for the observed stable crack growth in each loading condition are discussed.     

Failure of Glass with Subthreshold Flaws

Conventional postthreshold crack analysis cannot be used to predict the strength and fatigue behavior of glass with subthreshold flaws. Therefore, a fracture mechanics model for failure of glass with subthreshold indentation flaws was developed. This model accounts for both the near- and farfield residual stresses associated with the indentation impression. It is shown that these stresses play a major role in the initiation and subsequent propagation of cracks that eventually cause failure. The model predicts "pop-in" of a well-developed crack and failure under continuous and discontinuous crack growth in both inert and fatigue conditions. The results of experiments with bare fused silica fibers with indentation subthreshold flaws in inert and fatigue (water) environments were in good agreement with the predictions by the model.     

The Mechanism of Crack Formation in Rubbers under the Effect of Ozone

Abstract

The formation and growth of cracks under an effect of ozone were investigated in different types of rubbers, such as NR, PBR, BMSR, SBR and in composite materials based on these rubbers. The study was carried out by the reflection-light optical microscopy method with using the MMP-4 microscope. In the presence of microinhomogeneities in a surface structure the crack formation is shown to start from those defects. In the absence of inhomogeneities the cracks are formed uniformly over the whole surface of a specimen. It is found that even insignificant quantities of low-modulus rubber in the PBR-based composite material may result in changing the shape of a crack during its growth. An attempt is undertaken to describe the shape of formed cracks quantitatively. The calculational and experimental data are presented.

The influence of deformation on the number of cracks per surface unit, on their shape, geometrical size, cracking degree is studied for example of natural rubber specimen.