Urethane Tensile Specimens with Embedded Strain Gages

Prewired strain gages were embedded in urethane tensile specimens. Intent of embedding was to verify that gages operate as expected once embedded. The goal is to verify that the gages can be located accurately using the SDM process and attain good adhesion between the gage and the urethane. The tensile specimens are to be tested on an Instron machine under know loads and cycled to ensure measured values match predicted values with no appreciable hysteresis.

The first specimens were designed in AutoCAD based on the ASTM specifications for testing plastics. The specimens are molded urethane in a wax substrate. The insulated wires supplied on the gages proved cumbersome and locating the specimens repeatably and accurately in the channel was not guaranteed. The channel utilized a 0.5mm high shelf to support the gage and the channel was machined all the way down to the wax substrate. The rounded corners machined in the channel did not allow the gages to sit squarely. specimen in process (1), specimen in process (2), specimen in process (3)
Only one of the three specimens stayed in the mold to completion but the wire placement created a stress concentration where the specimen subsequently broke.broken specimen--bottom, broken specimen--top
A second design for molding 6 specimens included a small channel with a "T" shape at the end to allow the gage to sit squarely, a shallow layer of urethane remains in the bottom of the channels to ensure the wires remain encapsulated, and the wire is replaced with 37gage copper wire. The channel is then filled with urethane, and machined to thickness. Some of the specimens chattered while machining resulting in a tapered grip. On two of these, the wires also snapped off. Four of the six specimens are ready for testing and those gages and wiring have continuity.