Capacitance Methods for Measuring Properties of Adhesives in Bonded Joints

Abstract

Two capacitance‐type methods are described, one of which can be used for measuring the stress‐strain behavior and the other for the strain‐sensitive electrical properties of the adhesive material used to bond two metallic adherends. In cases where the electrical properties of the adhesive are insensitive to strain, both techniques can be used to measure the stress‐strain behavior. The first method uses an externally mounted, air gap, capacitance‐type extensometer mounted on the adherends very close to the adhesive joint. In the second method, the adherends are the capacitor plates and the adhesive serves as the dielectric medium. Load‐unload tests using butt joints have been made in tension and compression. The data are obtained as X‐Y recordings where the X axis is the output of a capacitance bridge and the Y axis is the load applied to the joint. Capacitance sensitivities are on the order of 5×10⁻⁴ pF/div. (8 div./cm on the X axis). When calibrated for extension, the X axis sensitivity is about 1.3×10⁻⁶ mm/div. Thus, capacitance‐load and extension‐load behavior may be recorded. When the adherends are used as the capacitor plates, the observed extension is from the adhesive material alone; deformation in the adherends is not recorded. When a load is applied, the capacitance changes because the plate separation is altered and the electrical properties of the adhesive material may be affected by strain. For those materials whose electrical properties are strain‐sensitive, the data are backed up by the air gap extensometer which establishes the plate separation due to loading, independent of any electrical property changes in the adhesive material. Thus, the strain sensitivity of the electrical properties may be established. Typical results on three different polymeric adhesives are described; these results illustrate the use of both techniques.