ELECTRIC-POTENTIAL TECHNIQUE FOR DETERMINING SLOW CRACK GROWTH
ARMY MATERIALS RESEARCH AGENCY WATERTOWN MA
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Slow crack growth was followed as a function of applied load during fracture toughness testing of high-strength sheet materials 4340 and 300M steels by means of continuous measurement of electric potential field changes across the crack. The instrument employed is a commercially available millionmeter and uses the ammetervoltmeter measurement method with a minimum of current passing through the test specimen. Voltage drop across the crack, rather than resistance, is measured. Electronic and mechanical testing techniques are described. Typical potential field distribution diagrams illustrate the potential change with crack growth. A single calibration curve for crack size versus potential drop may be obtained independent of material and specimen configuration by maintaining geometrical similarity of current and potential contacts. Calibration data are readily obtainable for any specimen design by duplicating the specimen in aluminum foil and simulating crack growth with a razor.