MTS Series 315 Load Frame - Manual (Page 16)

Series 315 Load Frame 16 Introduction
Note to spring rate
equations
Although the baseplate is modeled as a solid rectangular beam, it actually has a large
hole through it where the actuator is mounted. It is assumed, however, that since the
actuator is attached to the baseplate with fully preloaded fasteners, the stiffness of
the baseplate/actuator assembly is approximately comparable to a solid baseplate
(i.e. with no hole through it). The calculated spring rate values have been found to
be in good agreement with actual measurements on load frame assemblies.
The intent in reporting a spring rate value is to allow comparison between load
frames. For this reason, other, more variable elements in the load train, such as
compression platens, force transducers (load cells) and spacers, are not taken into
consideration. (Typically, different components are used for each application
configuration. Including these elements in the calculation would make direct
comparison between frames difficult.)
Since a servo-hydraulic actuator assembly is a dynamically controlled device that
displays a complex effective “spring rate” (based on oil pressure, oil volume,
cylinder deflection, etc.), it is also not included in the calculation. In general, a high
response servo-hydraulic, closed-loop control system in conjunction with a load
frame properly designed for high stiffness, is sufficient to actively control the
actuator, absorb elastic energy stored in the load frame, and prevent loading system
induced specimen failure. Appendix B discusses in more detail the influence of load
frame stiffness on specimen failure behavior.