Strain Gages for Shear Modulus Testing of Composite Materials: Strain Gage Listings

Shear Modulus Sensors are specifically designed to accommodate the unique specimen geometries and strain-field distributions encountered when testing composite materials for shear properties. Two popular specimens for in-plane shear modulus testing of composites are the losipescu and compact designs. The test section for both types is described as the area between two opposing notches. The Iosipescu specimen has a distance between the notch roots of 0.45 in (11.4 mm); for the compact design this distance is 0.75 in (19 mm). Both of these specimens have an inherently nonuniform shear-strain distribution in their test zone. Determining shear modulus requires extracting an average shear-strain value from this nonuniform strain field. Since strain gages have the unique characteristic of integrating the surface strain field under their grids, average specimen strain is automatically obtained by spanning the entire length of either specimen's test section.

Two 500-ohms ±0.4%, ±45° shear-gage configurations are available for both the Iosipescu and compact specimen designs.

The planar configuration, with side-by-side grids, is constructed with a standard N2 backing and Option SP61 (beryllium-copper lead ribbons and polyimide film encapsulation). The stacked configuration is produced with a special backing, A2. This backing is similar to N2 but is fully encapsulated with a polyimide film and includes integral beryllium-copper lead ribbons (like those provided by Option SP61). The stacked configuration is offered to best simulate strain measurement at a point. The stacked gages are supplied in a quarter-bridge arrangement so that independent gage measurements can be made if necessary. When connected in a half-bridge circuit, the stacked construction inherently provides temperature compensation and insensitivity to normal strains. Due to the increased stiffness of a stacked sensor, compared to one having only a single layer, an evaluation of the test conditions and requirements should be made to ensure that the gage will not compromise accuracy by significantly reinforcing low-modulus and/or thin specimens.