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The control of propagating cracks in structures first requires an understanding of the interrelationships of microstructure, fractographic features, dynamic stress intensity factor, and crack velocity in materials. Conventional techniques for measuring crack velocity suffer in varying degrees from imprecision, excessively stringent synchronization requirements or poor resolution due to data compression on a single oscilloscope sweep. A new instrumentation concept has evolved from consideration of the advantages and limitations of earlier techniques. High-speed digital data processing techniques are used to measure time of sequential events 300 ns apart to an accuracy of ±50 ns. Temporary storage, high-speed sampling, and a 3:1 multiplexing scheme have been combined with a random access memory structure to enable recording times of as many as three simultaneous events per microsecond. Special templates have been designed to vapor deposit on sample metallic propagation grid configurations that take full advantage of the electronic capabilities of the system. These have 250-μm slits precisely spaced 5 deg apart in radial arrays and 1 mm apart in parallel arrays. Applications to fast fracture, crack branching, fatigue cracking, and stress-corrosion cracking are discussed.
crack propagation, dynamic fracture, crack velocity, time measurement, digital instrumentation, fatigue (materials), stress corrosion, fracture properties
Physicist, Pitman-Dunn Laboratory, Frankford Arsenal, Philadelphia, Pa.
Professor, Drexel University, Philadelphia, Pa.