Dynamic performance characteristics of the liquid metal strain gage

Abstract

Performance characteristics of the liquid metal strain gage (LMSG) were evaluated by both static and dynamic bench testing. Statically, the devices were found to have outputs closely proportional to engineering strains, up to strain levels of 40%. While individual gage factors varied appreciably (up to 50%), each of the gages studied showed excellent reproducibility of behavior. Dynamically, the response to sinusoidal strain inputs was frequency-independent up to 50 Hz, and there was no detectable phase shift. Similarly, the LMSG response to constant-speed displacement inputs was velocity-independent over the range of nominal strain rates from 20 s −1 to 0.02 s −1. The devices proved capable of maintaining stable outputs when held stretched to fixed lengths, even if such tests were performed immediately following stepwise displacement inputs. Thermal artifacts were found to be modest (0.185% apparent strain per °C), and there was no appreciable sensitivity to non-axial strains. When mounted on an in vitro ligament preparation, the LMSG measured apparent ligament strain similar to that detected by a video dimension analyzer. A protocol by which an implanted LMSG could be used to infer in vivo muscle forces was demonstrated, based on recordings of tendo-Achilles strains developed by a rabbit during slow hopping.