Gordon A. Sargent

Statistical analysis of the Hertzian fracture of pyrex glass using the Weibull distribution function

Hertzian fracture tests were carried out on specimens of ground-and-polished Pyrex glass using polished Pyrex glass balls of 6 and 8 mm diameter. The results were analysed according to the theory of flaw statistics originally proposed by Weibull. The Weibull parameters m and σ0 were found to be independent of ball size; σu however decreased with increase in ball size. The parameters σu,σ0 and m obtained from the Hertzian tests differed from those obtained from a four-point bend test. The predicted mean fracture stress and the mean fracture location for Hertzian fracture using the derived Weibull parameters agreed reasonably well with the experimental values.

Hertzian fracture of Pyrex glass under quasi-static loading conditions

Hertzian fracture tests were conducted using an Instron on Pyrex glass specimens with various surface conditions, including lubricants, employing steel, Al2O3, WC and Pyrex glass indentors of 0.79 to 12.7 mm radius under ambient air and high vacuum environments at cross-head speeds of 8.5×10−6 to 2.1×10−4m sec−1. The results were not in strict accord with Auerbachs law, nor any of the existing energy-balance Hertzian fracture theories. Rather, they indicated that surface roughness and friction modified the Hertz stress field so that the maximum tensile stress at the surface occurred outside the contact circle. Further, they indicated that Hertzian fracture occurred by the direct, unstable growth into a cone crack of a pre-existing flaw at the displaced site of the maximum tensile stress, the flaw size responsible for the fracture decreasing with decrease in ball size (contact radius). Once a cone crack occurred, its length and growth were described reasonably well by Roeslers theory; however, his constant appears to be too high by a factor of about 5. A surface energy of @ 4 J m−2 was derived from bend tests on specimens similar to those used in the Hertzian fracture tests. Using this value, the crack sizes which lead to fracture were estimated to range between 0.6 and 3.5 μm for the conditions investigated here. The increase in the critical load for Hertzian fracture with indentation velocity was concluded to be due to kinetic effects of water vapour acting at the tip of the crack.

The effect of heat treatment on the mechanical properties and Young's modulus of Beta III titanium alloy

Abstract A wide variety of mechanical properties can be produced in Beta III titanium alloy by thermomechanical processing. Measurements of the mechanical properties, including Youngs modulus, have been made after various aging times and temperature. The results are discussed with regard to previous microstructural observations.

Hexagonal dislocation networks in titanium

Hexagonal dislocation networks which occurred in as-annealed commercial Ti-A 50 rod and following partial extrusion under hydrostatic pressure at room temperature were studied using transmission electron microscopy. For the as-annealed condition networks were observed on the prism, basal and {2•1•1x} planes, while for the extrusion networks were only observed on the basal plane. The various stages in the development of the networks are presented and mechanisms by which they form are proposed. The self energies of the dislocation hexagons constituting the various networks are calculated using the equations proposed by de Wit and Ruff. They increase in the order: a) networks on the basal plane in the as-annealed condition, b) networks on the prism plane in the asannealed condition and c) networks on the basal plane in the partially extruded condition.

Adsorption of CO on, and S poisoning of, a perfect Ni(111) single crystal and A Ni(111) crystal with small angle boundaries

Abstract A Ni(111) crystal with small angle boundaries was used to examine the adsorption of CO. The adsorption of CO on a perfect Ni(111) single crystal was used for reference. Auger spectra show that the boundary lines on the sample surface provide favorable sites for the adsorbed CO to dissociate at temperatures as low as 25°C. The post-dissociation carbon appears mostly in the form of a nickel carbide on the surface. After heating the crystal to 850°C, sulfur diffused to the surface and blocked the surface adsorption sites uniformly. The boundary-enhanced dissociation of absorbed CO is no longer observed after the diffusion of sulfur to the crystal surface. AES depth profiling of sulfur concentration at different positions on the crystal with respect to the boundary lines show no evidence that the boundary lines provide an enhanced path for sulfur diffusion.

A computer simulation of the time dependence of the erosion of Pyrex glass by glass beads

With the aid of scanning electron microscopy (SEM) observations of the surface of Pyrex glass, eroded by spherical glass beads, a recursive algorithm has been formulated which predicts the weight-loss from the specimen as a function of time. This method takes into account three mechanisms of material removal: material chipped off when cone cracks form on the surface; materials removed due to interaction of cone cracks on the flat surface; and materials removed due to chipping of exposed cone frustrums and the underlying rough surface. Multi-particle erosion tests were performed on the Pyrex glass target material employing spherical glass beads as impacting particles. Four different particle velocities and average particle sizes were employed as the varaibles in the experiments. An analysis of the experimental results in conjunction with the mathematical model showed a very good agreement between experimental results and theoretical prediction.

Multiparticle Erosion of Pyrex Glass

Multiparticle erosion tests were performed on Corning 7740 Pyrex glass using alumina powder of two grit sizes. Erosion was measured as a dimensionless parameter in terms of the loss in weight of the target per unit weightof particles impinged. The erosion characteristics of the Pyrex glass were investigated over a range of particle velocities, angles of impingement, and times of erosion. It was found that maximum erosion occurred at a normal angle of impact for the 240-grit powder (mean particle size = 30 μm); however, with the 400-grit powder (mean particle size = 10 μm), the maximum was found to occur at a slightly lower angle of about 80 deg. At all angles of impingement it was found that amount of erosion decreased as the erosion time increased. Erosion (E) was found to be dependent on the average particle velocity (V) by a power relationship of the form EαV n where n is a material constant. The values of the velocity exponent, n, in the present experiments were found to be between 2.2 and 2.7 and were independent of time of erosion. The time (t) dependence of erosion was also found to obey a power law relationship of the form Eαt c where c is a material constant. The time exponent c was found to be -0.1 under all conditions of velocity, particle size, and angle of impingement. The decrease in erosion with increase in time has been interpreted in terms of a change in the surface roughness. Scanning electron microscopy (SEM) has been used to establish that the effective average angle of impingement decreased as time of erosion increased until a constant surface roughness was achieved, at which time it was found that the average angle of impingement was about 55 deg instead of 90 deg. The experimental data obtained in the present work are discussed in terms of existing theories for the erosion of brittle materials.

Omega phase formation in RMI (38-6-44) beta titanium alloy

The stabilized beta titanium alloy RMI (38-6-44) was designed to limit the formation of omega-phase during quenching and subsequent ageing. It has been shown in the present paper, however, that both athermal and thermal omega do form and that the growth kinetics of the thermal omega follows aD=Ct0.45 relationship.As expected from the alloy additions, the maximum volume fraction of omega phase observed is considerably less than in other beta titanium alloys and the severe embrittlement due to omega phase formation is not a serious problem in this alloy. This is an important consideration for engineering applications in the service temperature range below 900‡F (480‡C).

Sulfur poisoning of a Ni(111) crystal with small angle boundaries (SAB) and its effect on CO adsorption

Abstract The sulfur poisoning of a Ni(111) crystal with small angle boundaries (SAB) and its effect on CO adsorption were studied using the techniques of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The sulfur poisoned Ni(111)-SAB surface was prepared by heating the crystal to 850°C for 20 min. The bulk sulfur impurity was diffused to the surface. The sulfur Auger peak intensities taken at and between boundary lines, show that sulfur distribution is uniform on the Ni(111)-SAB surface. No LEED pattern due to sulfur presence was observed. Sulfur atoms do not form an ordered structure in the Ni(111)-SAB surface. The Auger spectra showed that the boundary lines at the sulfur-free crystal surface provide favorable sites for the adsorbed CO to dissociate at temperatures as low as 25°C. After sulfur is diffused to the surface, it blocks the active sites as well as the boundary sites. The boundary line enhanced dissociation of CO is no longer seen. Sulfur and nickel Auger intensities were taken during the CO adsorption. From the change of the intensities, sulfur only interacts with nickel and physically inhibits the CO adsorption and, thereby, further CO dissociation.

Relationship between the structure and mechanical properties in ß III titanium alloy

The ageing reactions that take place during the heat-treatment of solution-treatedβ III titanium (11.5 wt% Mo, 6 wt% Zr, 4.5 wt% Sn, balance Ti) were followed by detailed structure characterization using electron microscopy. The variations in mechanical properties with heat treatment were also followed systematically. The electron microscopy investigations indicated that the omega phase formed on quenching. The size and volume fraction of the omega phase increased on subsequent ageing,β,ω andα phases were found to co-exist at ageing temperatures between 800 and 900° F (427 and 482° C) for short ageing times. From the observations of interfacial dislocations at theβ/ω interface and the precipitation of fine alpha near the omega particles, with a morphology that is characteristic of the priorω morphology, it is suggested that theα-phase forms directly from the omega phase. The observed increase in yield strength over the solution-treated condition, due to the precipitation ofω phase, was found to agree well with that predicted by the Orowan hardening mechanism. Since the precipitation of fine ellipsoidalα-phase was found to increase the yield strength of the alloy with reasonable ductility, it is suggested that the optimum heat treatment to produce high strength with good ductility inβ III titanium is to age at 900° F (482° C) for 10 to 25 h.