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    Low Cycle Fatigue Testing Using Digital Random Sequences with Computer Control

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    Over the years there has been an increasing interest in the problem of fatigue damage accumulation in the low cycle fatigue range, as there has been for some time in the high cycle range. In many instances, the amplitude of the cycles of strain or stress are variable. A test technique which demonstrates damage accumulation under both variable and constant amplitude loading would help clarify the differences in fatigue damage buildup in each case. Toward this end, the technique of random loading, which is already attractive for service simulation in many cases, appears useful for fundamental studies of damage accumulation. Since the number of cycles involved in low cycle fatigue (to failure) is low, digital techniques for random loading are especially attractive. They carry with them the added feature of step-by-step monitoring of hysteresis loop energy, as well as close control of the variable involved (usually total strain). In the test program described in this paper, the results of a random walk in strain amplitude (where the polarity is reversed on each step) on the fatigue life of an aluminum alloy are presented, against a background of conventional (zero mean strain) constant amplitude S-N test results. To perform these tests, digital-computer-controlled closed-loop equipment was used in strain control. Interfacing with the test machine required digital-to-analog command conversion, and analog-to-digital transducer information conversion. A subroutine for creating random numbers was developed in the software system for the computer. Constant strain rate (triangular) cycles of strain were used throughout. To characterize the material used in this test program, a separate software program was developed which yielded the true stress-true strain curve. A special dual-range extensometer was developed for this part of the program.


    low cycle fatigue, fatigue tests, random vibration, aluminum alloys, strain tests, cyclic loads, hysteresis energy summation, tests, evaluation

    Author Information:

    Shoe, R. E.
    Systems programmer and manager testing research, MTS Systems Corp., Minneapolis, Minn.

    Swanson, S. R.
    Systems programmer and manager testing research, MTS Systems Corp., Minneapolis, Minn.

    Committee/Subcommittee: E08.03

    DOI: 10.1520/STP48455S