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    Rational Basis and New Methods for Proportional Limit, Machine Stiffness, Critical Stress Intensity, and Crack Velocity Measurements

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    This paper is a summary of some new measurement techniques that have been developed recently at the National Bureau of Standards for measurement of proportional limit of elasticity, machine stiffness, critical stress intensity, and crack velocity. These seemingly unrelated measurements all result from the rational basis of a single type of equation relating specimen and machine kinetics. In a regime of linear elasticity, the concept of a limit beyond which the specimen elastic gage length displacement is no longer proportional to load is referred to as proportional limit if the gage length begins to exhibit plastic deformation, and is referred to in a precracked specimen as critical stress intensity if the crack begins to extend in length. Multiaxial plastic yielding is also discussed. Machine stiffness and crack velocity are other measurements which can be obtained through the specimen-machine equation. The relationship of the new stiffness measurement to the International Standards Organization (ISO) proposed method and advantages of the new method are discussed. Methods of measuring stiffness in constant load rate and constant strain rate tests are outlined. A method of measuring the proportional limit or critical stress intensity which does not use a displacement gage of any type is also given; this should prove useful in hostile environments. Other related topics discussed include loading transients and rate sensitivity.


    mechanical tests, measurement, proportional limit, determination of stress, crack propagation, plastic deformation

    Author Information:

    Clough, RB

    Committee/Subcommittee: E28.08

    DOI: 10.1520/STP27853S