Elastic properties of a boron-filament-reinforced aluminum-matrix composite were studied experimentally. Assuming transverse-isotropic elastic symmetry, five independent elastic constants were measured using a piezoelectric composite-oscillator method. Two constants, Young’s modulus along the filament axis and the torsional modulus perpendicular to the filament axis, were determined between 300 and 4 K. The composite’s elastic-constant values are between the values of boron and aluminum, but closer to that of aluminum. Along the filament axis, Young’s modulus is 2.30×1011 N/m2. Cooling from 300 to 4 K increases all the elastic stiffnesses by as much as 11%.
An automated fatigue crack growth rate (FCGR) test system has been developed that can be used for tests of constant-load-amplitude FCGR above 10−8 m/cycle [ASTM Test Method for Constant-Load-Amplitude Fatigue Crack Growth Rates Above 10−8 m/Cycle (E 647-83)] at normal (∼ 10 Hz) or low (∼ 0.1 Hz) cyclic frequencies and for tests of near-threshold and variable-load-amplitude FCGR. The test system consists of a minicomputer, a programmable arbitrary waveform generator, a servo-hydraulic test frame, and a programmable digital oscilloscope. The crack length is measured using the compliance technique; the FCGR and the stress-intensity factor range are calculated and plotted automatically during the test.
A comparison has been made between the wearout rates of [0/± 45/0] boron/epoxy and boron/aluminum composites in low-cycle fatigue at 295 and 76 K. Using degradation of modulus as the wearout criterion, these preliminary data indicate that cryogenic temperatures have a negligible effect on either tensile or compressive fatigue performance of boron/epoxy. A similar indication was obtained for boron/ aluminum in tensile fatigue. These conclusions are substantiated by high-cycle fatigue results on boron/epoxy.
The fracture of seven austenitic stainless steels with varying nickel and nitrogen contents were studied at 4 K. Smooth, 6-mm-diameter tension specimens and 22-mm-thick compact specimens were used. Nitrogen content controlled the yield strength and influenced fracture toughness by its effect on yield strength. Increasing the nickel content increased the fracture toughness at a constant nitrogen content. Observations of the fracture surfaces and polished cross sections through the fracture surfaces of test specimens showed that nucleation controlled the dimpled rupture fracture process. A critical stress criterion for nucleation that depends on both the applied stress and strain was developed and applied to the fracture toughness test. This fracture criterion explained the increase in fracture toughness with increasing nickel and the decrease in fracture toughness with increasing yield strength for strengths over 600 MPa.
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