Karl Jakus

Appraisal of biaxial strength testing

Abstract Three biaxial strength tests (ring-on-ring, piston-on-3 ball, and ball-on-ring) were evaluated using finite element analysis. Although in all three tests some uncertainties exist regarding the calculation of fracture stresses from the analytical equation, if fracture occurs within the loading ring, the ring-on-ring loading is thought to give the most accurate measure of strength. In addition, it was found that specimen shape (square vs. circular) had no effect on the stress distribution within the supports and that the stress at the edge of the specimen was less than 10% of the maximum stress for an overhang greater than about 40%. Fracture strength measurements on soda-lime glass gave support to these finite element results.

Creep and fracture of a vitreous-bonded aluminium oxide

Creep and creep-rupture behaviour of a commercial grade of glass-bonded, 96% aluminum oxide was characterized as a function of temperature and applied stress. The creep data were fitted to the classical empirical relation usually used to describe this phenomenon. The apparent activation enthalpy, ΔH = 926 kJ mol−1, and the stress exponent,n = 4.8, lie at the high end of the range reported for two-phase materials, primarily as a result of structural modifications that occur during creep. A stress-modified Monkman-Grant relationship was fitted to the creep-rupture data to give a stress exponent of −4.2. None of the available theories of creep rupture provided a satisfactory description of the present set of data. Analytical electron microscopy was used to characterize the composition and structure of this material. In the as-received material the intergranular phase was a glass of nearly uniform composition. During high-temperature exposure, devitrification of the glass resulted in the formation of various crystalline phases within the intergranular region of the material. Devitrification depended on both the proximity to the surface, where it was most pronounced, and on the state of stress. In this regard, flexural creep samples exhibited extensive crystallization within the tensile region of the flexural specimens, but little crystallization within the compressive cross-section. From the composition of the retained glass, estimates of the viscosity of the glass at the grain boundaries were made and used, in combination with microstructural information, to compare the creep behaviour with available theories of creep. The results of this paper are consistent with percolation and solution precipitation mechanisms of creep deformation. By contrast, cavitation did not seem to play a major role in the creep deformation process.

Analysis of fatigue data for lifetime predictions for ceramic materials

Accuracy in data analysis is of utmost importance because lifetime predictions are extremely sensitive to experimental uncertainty in the crack growth parameters. The limitations of the conventional data reduction techniques used for analysing static and dynamic fatigue data are reviewed and new, statistical methods of data reduction that offer advantages over the conventional techniques are discussed.

Application of fracture‐mechanics theory to fatigue failure of optical glass fibers

The fatigue behavior of optical glass fibers was determined in air at 23°C and 55% relative humidity by the dynamic‐fatigue test technique in which strength is measured as a function of stressing rate. The good correlation found between the fatigue test data and fracture‐mechanics theory indicates that failure is controlled by slow crack growth of preexisting flaws and that fracture‐mechanics theory can be used in making failure predictions for optical glass fibers.

Effect of microstructure on the erosion and impact damage of sintered silicon nitride

The erosion rates and impact damage of two sintered silicon nitride materials with identical compositions but different microstructures were determined as a function of impacting particle (SiC) kinetic energy and temperature (25–1000° C) using a slinger-type erosion apparatus. The coarse-grained silicon nitride had significantly better resistance to impact damage than the fine-grained material. Crack-microstructure interactions were characterized using scanning electron microscopy and showed that crack-bridging was an important toughening mechanism in the coarse-grained material. Post-impact strength data were significantly less than those predicted from the indentation-strength data, due to impact flaws linking up prior to fracture. Consistent with its greater fracture resistance, the erosion rate of the coarse-grained material was less than that of the fine-grained material for erosion at 25 deg, and was independent of erosion temperature.

The mechanics of matrix cracking in fiber reinforced ceramic composites containing a viscous interface

Abstract A detailed fracture mechanics analysis of matrix cracking in a fiber reinforced ceramic composite is presented for the case where the fiber—matrix interface exhibits viscous flow as can be the case when ceramic composites containing amorphous interfacial layers are subjected to loads at elevated temperatures. The analysis considers the case where matrix cracks are fully bridged by fibers, and the role of the viscous interface is to introduce a time dependence into the stress-intensity formulations. Such time-dependence arises because the bridging fibers are able to pull out of the matrix by viscous interfacial flow, with the result that the crack opening, as well as the actual (or shielded) matrix crack-tip stress-intensity factor, increase with time under the action of a constant externally applied load to the composite. The differential equation governing the mechanics of the fiber pull-out is derived. This is then applied to obtain expressions for the time-dependence of the crack opening and the effective crack-tip stress-intensity factor in terms of material and microstructural factors. These expressions predict that the matrix crack will exhibit stable crack growth, with the crack growth rate being essentially crack length (and time) independent and a function only of the applied stress and of material and microstructural factors. It is also shown that the composite lifetime is independent of the sizes of pre-existing cracks and is dependent only on a critical microstructure dependent flaw size, applied stress and microstructural factors.

Failuer of fused silica fibers with subthreshold flaws

Abstract It has been proposed that substreshold Vickers indentations simulate the response of naturally occurring flaws in high quality glass fibers. Accordingly, the inert strength of bare fused silica fibers with Vickers subthresholds flaws was studied as a function of identation load. These strengths are significantly higher than those predicted by simple extrapolation of data from post-threshold identation flaws. A simplistic fracture mechanics model, incorporating residual stresses associated with the contact impression, is developed to explain the results. The residual contact stresses are shown to control the flaw instability. The model gives relations for the subthreshold inert strength as a function of load and for the threshold indentation size. These relations are shown to be consistent with both present and earlier silica glass data.

Temperature fluctuations in a turbulent flame

Abstract The instantaneous temperature fluctuations in a turbulent stoichiometric methaneair flame were measured with an iridium hot-wire. Near the flame front, high amplitude narrow temperature “spikes” were observed and their probability distribution function and average time duration were studied in detail. The nature of the low amplitude random temperature fluctuations was also explored using space-time correlation techniques.

Failure of amorphous polystyrene

The failure behaviour of amorphous polystyrene was studied in methanol and ambient air under constant load and strain rate conditions. The good correlation found between fracture mechanics theory and the test results of both crazing and fracture indicates that fracture mechanics theory can be used in predicting failure of amorphous polystyrene. From the fracture mechanics analysis of the results it is inferred that the kinetics associated with craze initiation and crack propagation are similar and that the inherent flaw responsible for failure first initiates the craze in which a crack is then formed. Both the distribution of inherent flaws and the kinetics of crazing and fracture are dependent on the test environment.

Maximum likelihood estimation techniques for concurrent flaw subpopulations

Failure of structural materials is often caused by the presence of two or more types of defect subpopulations. The maximum likelihood estimation technique for evaluating the Weibull parameters of these underlying subpopulations in the case of known fracture origin is presented. The maximum likelihood estimation equations are derived, and solved by means of nonlinear programming. The estimators obtained therefrom are tested for both accuracy and consistency against a series of simulation runs. For data sets containing a relatively small sample size, the advantage of the method of maximum likelihood over two established nonparametric techniques is demonstrated.