Apparatus and method for force-controlled fatigue testing

DETAILED DESCRIPTION It is one object of this invention to provide an apparatus and method for force-controlled fatigue testing of a specimen wherein a force feedback loop is used to continuously correct a difference between a desired and a measured force profile.
It is another object of this invention to provide an apparatus and method wherein physical property changes of the test specimen are measured over time and the load force applied to the test specimen is maintained at a constant magnitude, even during physical property changes of the test specimen.
It is another object of this invention to provide an apparatus and method which accommodates tension, compression and bending tests.
It is still another object of this invention to provide an apparatus and method which can be used for fatigue testing and fatigue crack propagation testing of plastics, metals, ceramics, composites, adhesives, and other natural materials in various loading modes for various geometries of the test specimens.
The above and other objects of this invention are accomplished with an apparatus that includes an actuator for applying the load force to the test specimen.
Physical property changes of the specimen are measured over time and are used to generate a feedback signal as a function of the measured physical property change.
A motion command signal is computed as a function of the feedback signal and a computer emits the motion command signal to the actuator.
A driver of the actuator responds to the motion command signal and thus maintains the load force at an approximately constant magnitude, even during physical property changes of the test specimen.
The stability and accuracy of the apparatus and process according to this invention stem from an adaptive force control mechanism which automatically adjusts the stiffness of the control loop in order to match the slowly time-varying stiffness of the test specimen.
It is an important aspect of this invention to adjust the load force applied to the test specimen in order to compensate for the dynamic changes in the stiffness of the test specimen and to accommodate for the wide ranges of materials being tested