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The influence of microstructure of the fiber-matrix interface on the interfacial shear strength, measured using a fiber-pushout technique, has been examined in a sapphire-fiber-reinforced NiAI(Yb) matrix composite under the following conditions: (1) as-fabricated powder metallurgy (PM) composites, (2) PM composites after solid-state heat treatment (HT), and (3) PM composites after directional solidification (DS). The fiber-pushout stress-displacement behavior consisted of an initial "pseudoelastic" region, wherein the stress increased linearly with displacement, followed by an "inelastic" region, where the slope of the stress-displacement plot decreased until a maximum stress was reached, and the subsequent gradual stress decreased to a "frictional" stress. Energy-dispersive spectroscopy (EDS) and X-ray analyses showed that the interfacial region in the PM NiAI(Yb) composites was comprised of Yb203, O-rich NiAI and some spinel oxide (Yb3A15Ol2), whereas the interfacial region in the HT and DS composites was comprised mainly of Yb3A15Ot2. A reaction mechanism has been proposed to explain the presence of interracial species observed in the sapphire-NiAl(Yb) composite. The extent of interfacial chemical reactions and severity of fiber surface degradation increased progressively in this order: PM < HT < DS. Chemical interactions between the fiber and the NiAI(Yb) matrix resulted in chemical bonding and higher interracial shear strength compared to sapphire-NiA1 composites without Yb. Unlike the sapphire-NiA1 system, the frictional shear stress in the sapphire-NiAl(Yb) composites was strongly dependent on the processing conditions.

Measured values of the three low-pressure spall thresholds in copper plate are presented. The two lower thresholds are original with this program. However, the incipient spall threshold or beginning of tensile damage corresponds rather closely in magnitude to the dynamic elastic yield strength of copper in compression. The complete spall threshold corresponds to the value determined by Rinehart for copper.Thresholds are presented in terms of pressure and pulse-width, and the effect of varying the pulsewidth with a given pressure is shown.

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