Dinesh K. Shetty

Indentation fracture of WC-Co cermets

Indentation fracture of a series of well-characterized WC-Co cermets was studied with a Vickers diamond pyramid indenter. The resulting crack length-indentation load data were analysed in terms of relations characteristic of radial (Palmqvist) and fully developed radial/median (half-penny) crack geometries. The radial crack model gave a better fit to the data on all the alloys studied. Crack shapes determined by repeated surface polishing confirmed the radial nature of the cracks. An indentation fracture mechanics analysis based on the assumption of a wedge-loaded crack is shown to be consistent with the observed linear relation between the radial crack length and the indentation load. The analysis also predicts a simple relation among the fracture toughness (Klc), the Palmqvist toughness (W) and the hardness (H) of the WC-Co alloys.

Mixed-mode fracture in biaxial stress state: Application of the diametral-compression (Brazilian disk) test

Abstract Mixed-mode fracture of soda-lime glass was studied using a diametral-compression test that features disk specimens with symmetric through-cracks. The test enables one to study fracture under pure mode I loading, pure mode II loading, or any combination of mode I and mode II loading by a simple alignment of the crack relative to the diameter of compression loading. The disk specimens were precracked with the aid of both chevron notches and water-assisted subcritical crack growth. The directions of noncoplanar crack extensions and the relative magnitudes of mode I and mode II stress-intensity factors for mixed-mode fracture under inert conditions were compared to the predictions of three different mixed-mode fracture theories. None of the theories was completely adequate to explain the experimental observations, but a maximum hoop stress criterion modified to include second order, nonsingular term in the series solution for the crack-tip region stress gave reasonable agreement with the experimental results.

Plastic deformation of aluminum under repeated loading

The plastic deformation behavior of high purity (99.999 pct) polycrystalline and single crystal aluminum under repeated stressing was investigated by studying the creep behavior. The creep behavior under repeated stressing (cyclic creep) was compared with the static creep behavior at identical peak stresses. The influence of such experimental variables as the applied stress, the amplitude of cyclic stress, the test temperature and the static creep rate prior to stress cycling were systematically examined. The most important experimental observation in this study was that the cycling of the creep stress could either enhance or retard the creep deformation, depending upon the combination of the experimental variables. The experimental variable that had the most significant influence on the cyclic creep behavior was the applied stress; the enhancement of the creep rate was observed above a threshold stress, while the cyclic stress retarded the creep deformation at lower stresses. The threshold stress was found to depend sensitively on temperature. The implications of the threshold stress were examined by an analysis of the work-hardening behavior.

Toughening of layered ceramic composites with residual surface compression: effects of layer thickness

Abstract Shielding of edge cracks in multilayer single-edge-notch-bend (SENB) specimens, with alternating layers designed to produce residual compression and balancing tension, was calculated by the method of weight function. Investigation of the effects of the relative thickness of the layers and of the number of layers revealed that the maximum crack shielding and the maximum apparent fracture toughness were achieved in three-layer composites with the thickness of the compression layer constituting one-half of the specimen height with an edge crack extending to the first interface. These theoretical predictions were confirmed with measurements of apparent fracture toughness in the range, 28–32 MPa√m, on three-layer SENB specimens of alumina–zirconia composites.

Transformation yielding, plasticity and crack-growth-resistance (R-curve) behaviour of CeO2-TZP

Transformation yield and plasticity, transformation zone sizes at crack tips and rising crack-growth-resistance (R-curve) behaviours were studied in a commercial-grade ceria partially stabilized zirconia polycrystalline material (CeO2-TZP). The yield stresses measured in three-point bending decreased from 390 to 176 MPa when the sintering temperature was varied from 1425 to 1525° C. The corresponding total plastic strain to fracture increased with decreasing yield stress. Crack-tip transformation zones in precracked, annealed and loaded single-edge-notch-bend specimens decreased significantly in size with increasing transformation yield stress; however, the R-curves were relatively insensitive to the yield stresses or the transformation zone sizes. The measured zone sizes and R-curves were examined in terms of both crack shielding and plastic yield strip zone models.

Cleavage fracture of steel in the upper ductile-brittle transition region

Abstract Stable-crack growth precedes cleavage fracture in the upper ductile-brittle transition region. It is suggested that the conversion from one fracture mode to the other occurs at random locations in the microstructure, denoted trigger points. It also is suggested that K Ic for cleavage should be calculated from the elastic energy in the specimen at the point of conversion. Experiments are described that showed that the amount of stable-crack growth increases with increasing temperature and with decreasing load-train compliance.

Analysis of creep deformation under cyclic loading conditions

Abstract A mathematical model for creep deformation under cyclic loading is reformulated with the objective of achieving more quantitative comparison with experimental results. Introduction of a more general creep relation and inclusion of a cyclic hardening effect in the improved model lead to prediction of creep behavior in good agreement with experimental results. Experimental methods are indicated whereby materials constants appearing in the formulations can be independently determined and used in the prediction of cyclic creep behavior. The application of this improved model is demonstrated for the case of interstitial free iron at room temperature.

Transient wear of silicon nitride in lubricated rolling contact

Abstract Wear of silicon nitride balls and rods was studied in lubricated rolling contact using a ball-on-rod rolling contact fatigue (RCF) tester. The wear response of both the elements, balls and rods, was transient. The nature of the transient was dependent on the initial surface roughness of the rod, while the total volume loss increased with the initial contact stress. Bearing-grade balls ( R a =0.005 μm) rolling on as-ground ( R a =0.18 μm) rods produced a high initial wear rate in both elements that decreased exponentially with time or number of loading cycles. Improving the surface finish on the rods by lapping ( R a =0.05 μm) produced delayed wear in both elements; i.e., the wear rate was initially zero during an incubation period, then, increased and, finally, decreased. The role of decreasing contact stress in the transient wear, and the micromechanisms of wear involving a tribochemical reaction are discussed.

Stress-relaxation technique for deformation studies in four-point bend tests: application to polycrystalline ceramics at elevated temperatures

The advantages of the stress-relaxation technique can be effectively realized by applying the procedure to conventional four-point bend tests usually employed in the deformation studies of ceramic materials. A test system and procedure for determining plastic strain rate-stress relationships at elevated temperatures (up to 1600° C) by the stress-relaxation method is described. An analysis to calculate true plastic strains and stresses from measured deflections and loads is presented and it is shown that such an analysis requires minimum assumptions regarding the materials behaviour. Preliminary results obtained on an iron-doped MgO specimen are discussed and compared with the constant load test results obtained on identical specimens. Sources of error in the four-point bend stressrelaxation tests and the methods to minimize them are also discussed.

Prediction of indentation-load dependence of fracture strengths from R-curves of toughened ceramics

Rising-crack-growth resistance or R-curves evaluated by direct measurements of crack lengths associated with indentation flaws in a sintered silicon nitride and a sintered and isostatically hot pressed (HIPed) SiC (whisker)-reinforced alumina were used to predict the corresponding indentation-load dependence of fracture strengths. Two empirical R-curve functions, a power law and an exponential function, were fitted to the R-curve data for the analysis. Fracture strengths were calculated by a combined numerical and graphical procedure that determined the point of tangency between crack-driving forces and the R-curves. The results revealed that the exponential function gave a better prediction of the measured log fracture-strength versus log indentation-load relation than the power law. The exponential function also predicted a nearly linear relation between log fracture-strength and log indentation-load, thus indicating that the apparent linearity of this plot is not adequate evidence to assume a power-law R-curve function. This study, therefore, reinforces the case for R-curve evaluations to be based on direct crack-length measurements rather than strength measurements, because the indentation-load dependence of the fracture strength is not sufficiently sensitive to discriminate between potential R-curve functions.