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Glenn Research Center at Lewis Field Random Vibration Testing of Hardware Tutorial 3 Notes taken from “Dynamic Environmental Criteria” NASA-HDBK-7005, March 13, 2001 N = c S b T F = c σ b x Inverse Power Law Model (time-to- failure related to rms dynamic loading) Equivalency Equation for two rms dynamic loads and times Idealized S-N Curve for Structural Materials Special case of the inverse power law describing the S-N curve (ignoring the fatigue limit) N = number of loading cycles to failure S = peak stress b and c are material constants

Fatigue Life Chart_1

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  • Glenn Research Center at Lewis Field

    Random Vibration Testing of Hardware Tutorial

    3

    Notes taken from Dynamic Environmental Criteria

    NASA-HDBK-7005, March 13, 2001

    N = c S b

    T F = c

    b

    x

    Inverse Power Law Model (time-to-

    failure related to rms dynamic loading) Equivalency Equation for two

    rms dynamic loads and times

    Idealized S-N Curve for Structural Materials

    Special case of the inverse power

    law describing the S-N curve

    (ignoring the fatigue limit)

    N = number of loading cycles to failure

    S = peak stress

    b and c are material constants

  • Glenn Research Center at Lewis Field

    Random Vibration Testing of Hardware Tutorial

    4

    Fatigue Life Assessment Requires

    Prototype Hardware Program

    Prototype programs allows an assessment to be

    made of remaining fatigue life:

    Qualification testing of the qualification hardware establishes

    the test demonstrated fatigue life for the hardware design.

    The identically-designed flight hardware may confidently be

    acceptance tested and launched, if these events do not

    extend beyond the previously demonstrated fatigue life.

    Any remaining fatigue life can be allotted to additional ground

    tests, reflights, etc.

    Protoflight programs have no test demonstrated

    fatigue life.

    Unknown remaining fatigue life.