Dynamic measurement of material strength and life under cyclic loading

DETAILED DESCRIPTION It is therefore an object of the present invention to provide a non-contact method for dynamic fatigue monitoring of polymeric and composite specimens.
It is another object of the present invention to use load and stroke (displacement) signals from an ordinary servo hydraulic test machine to measure quantities such as phase lag and gain to determine damage (% life remaining) and rate of damage in a polymeric or composite specimen.
It is yet another object of the present invention to provide a non-destructive method for determining the residual strength of a polymeric or composite specimen.
According to the invention, a conventional dynamic testing machine is modified to provide for a new method for dynamically monitoring fatigue damage.
In operation, a specimen is gripped at two separate locations and cyclically loaded by applying a tension force that elongates or stretches the specimen.
As the specimen is cycled and damage occurs, the specimen will become less stiff, i.
e.
the modulus of elasticity for the material will begin to degrade.
As the specimen becomes less stiff, it will take longer for a load which is applied at one end of the specimen to be detected at the other end.
In addition, as the specimen fatigues and becomes more elastic, the material will be displaced or stretched a greater distance for a given load.
A cyclic load is applied to a specimen between an upper limit and a lower limit at a predetermined frequency by a conventional dynamic testing machine which has outputs available for measuring load and displacement parameters experienced by the specimen.
In the time domain, the graphs of these parameters appear as sine waves having a phase shift relative to one another.
This relative phase shift has been found to be related to the rate in which the specimen is being damaged.
The gain, or change in the maximum amplitude of the displacement curve over time has been found to be related to the total damage incurred by the specimen.
The phase angle and gain are calculated and plotted for each cycle during the life of the specimen until failure