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.