James M. Staehler

The role of plasticity as a limiting factor in the compressive failure of high strength ceramics

Abstract The behavior of aluminum oxide under compressive loading is investigated over a wide range in strain rate and degrees of confinement. It is shown that plastic flow can be generated in Al 2 O 3 at all strain rates if confinement is sufficient to prevent premature failure via microfracture. Moreover, plastic flow is itself a source of microfracture, and the threshold for multiple slip apparently constitutes the practical ultimate strength for the ceramic. Thus, for sufficiently fine-grained alumina tested under optimum conditions, no confinement is required to generate plastic flow, at which stress the material fails via dislocation-induced general microfracture.

Frequency dependence of high-cycle fatigue behavior of CVI C/SiC at room temperature

Effects of loading frequency on high-cycle fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite were investigated. Tension–tension fatigue tests were conducted at three frequencies, 4, 40 and 375 Hz. Fatigue run out was set to 107 cycles. Applied stress versus cycles to failure (S–N) relationships were developed for these three frequencies. At 4 and 40 Hz, fatigue run out was achieved at a stress level of 375 MPa. At 375 Hz, stress level for run out was 350 MPa. Frequency dependence was observed between the two lower frequencies (4 and 40 Hz) and the higher frequency (375 Hz), but not between two lower frequencies (4 and 40 Hz). This manifested as a reduction in cycles to failure at 375 Hz compared to 4 and 40 Hz at a given stress level. Specimen surface temperature increased due to internal heat generation from sliding friction between constituents of the composite under cyclic loading. This increase was directly related to frequency and/or applied cyclic stress level. There was no clear indication that frequency greatly impacted either the stress-strain response or the overall appearance of fracture surfaces. However, a closer examination of specimens cycled at the highest frequency (375 Hz) showed evidence of the localized oxidation at fiber surfaces that might have attributed to the reduction in fatigue life at this frequency.

Micromechanisms of deformation in high-purity hot-pressed alumina

Abstract A high-strength aluminum oxide was produced by vacuum hot pressing high-purity, submicron-size alumina powders. The uniaxial compressive fracture strength was strongly strain-rate sensitive and varied from 5.5 GPa at 10 −4 s −1 to 8.3 GPa at 10 3 s −1 . A Hugoniot elastic limit of about 11.9 GPa was determined from flyer plate impact tests. The deformation/fracture process was examined using both uniaxial stress and uniaxial strain conditions. Under a uniaxial stress condition, microplasticity was observed in the form of aligned dislocations that appeared similar to shear bands in metals. Under a uniaxial strain condition, extensive dislocation activity, grain boundary microcracking and occasional twins were observed. Based on the experimental results and microscopic observations, possible mechanisms responsible for the observed high strength and high strain-rate sensitivity in this alumina are discussed.

The response of a high purity alumina to plate‐impact testing

Alumina disks which were vacuum hot pressed from a 99.99% pure Al2O3 powder were subjected to flyer‐plate impact testing. VISAR techniques were used to measure rear surface velocities. The Hugoniot elastic limit (HEL) for this alumina was 11.9 GPa. At a precompression of three times the HEL, a remarkably high spall strength of 1.2 GPa was observed. However, a negligible spall strength was found when the alumina was shocked to approximately 1.3 times the HEL. These results indicate that the spall strength of pure polycrystalline alumina goes through a transition, first decreasing in value near the HEL and then increasing again above the HEL. In other flyer‐plate impact tests, manganin stress gauges were used to measure the decay of the HEL with specimen thickness. The HELL for this alumina decreases slightly when the thickness of the specimen was increased but stabilized for specimens thicker than about 9 mm.

Strain-rate effects in high-purity alumina

Ultrahigh-strength alumina specimens made from disks produced by vacuum hot pressing high-purity alumina powders were subjected to uniaxial compressive loads at a range of strain rates. It was observed that the material exhibited a failure strength far superior to commercially available alumina. The failure strength was strongly strain-rate dependent and varied from 5.5 GPa at 10-4 s-1 to 8.3 GPa at 103 s-1. Microscopic studies on the fragments of the specimens deformed under uniaxial strain revealed extensive twinning and dislocation activity. Based on the experimental results and microscopic observations, the factors and mechanism responsible for the observed high compressive strength are discussed.