Slow crack growth was followed as a function of applied load during fracture toughness testing of high-strength sheet materialrby means of continuous measurement of electric pntential field changes 4 cross the crack. The instrument employed is a commercially available milliohmmeter and uses the ammeter-voltmeter 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. Slow crack growth has been followed i 4340 and 300M steels eat treated to several strength levels as well s cold rolled and a~ed after-heat treatment. Typical curves of load versus potential drop relate the various stages in the fracturing process.

The influence of tempering, prestraining, and retempering on the strength and toug .ness of martensitic and bainitic 4340 steel rods was investigated. The initial neat treatment consisted of a quench and temper to obtain martensite in one group and formation of bainite at 575 F followed by tempering in the second group. Reduct .ons of area of 4 tc percent were effected by cold drawing through a die with and without backpull, followed by a retempering treatment. Ultimate ter.sile strengths approaching 385,000 psi for the martensitic stricture and 283,000 psi for the bainitic structure resulted with yield-tensile strength ratios approaching umity. Notch tensile strength of the bainite was superior to that of martensite at comparable strength levels. The prestraining also caused a yield point drop and/or serrations in the flow curves. This was influenced by specimen test temperature and strain rate, tempering temperature, and retempering temperature.

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Security Classification Pfl. U2L