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.

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! iThe influence of specimen thickness and width on the elongation in 2 in,~•.f-w8"s studied on copper, AI~~ .. }.g20 steel r ':7 and ~~~.:~~r.~ated•-5i.i_te~k:> J The results conform approximately to Templin's equation, El • CAn • The constant n, a measure of the variation of elongation in 2 in. with specimen area, is shown to be related to the log of the ratio of the zero gage length (~~cture strain) to the infinite gage length (uniform strai9) elongations. irA method is shown for predicting the elongation in 2 inMor a bar of any thickness (or width) from measurements on another bar of the same material. The reason for specimen area being of greater importance than absolute values of width or thickness in controlling elongation is demonstrated by studying the strain distribution near the fractur~' The methods outlined in this report will enable specification and inspection personnel to determine how elongation values will vary with sheet thickness.