Quantification of the Effectiveness of Various Conductive and Non-Conductive Epoxies as an Attachment Method for Small SMA Wires

Abstract

The discovery of thermoelastic behavior in shape memory alloys (SMA) such as Nitinol enabled the conceptualization of many applications where actuators and sensors are embedded into a structural component. These so called “adaptive structures” can be extremely compact and energy efficient. SMA wires show characteristics similar to that of muscle fibers, and thus enable replication of an animal’s musculoskeletal system. Also, SMA wires are extremely attractive in the biomedical field where compact robotic elements can reduce the invasiveness of some surgical procedures. Unfortunately, widespread implementation of adaptive structures actuated by small SMA-wires in industry has been hindered by two significant challenges. First, SMA wires exhibit hysteretic behavior during heating and cooling that can be difficult to model without substantial computation. Second, attaching a small (∼50–100 μm diameter) SMA wire is extremely difficult because standard methods such as crimping are infeasible. The goal of this study is to quantify the effectiveness of using adhesives to hold a small SMA wire. A wide range of commercially available adhesives are tested under the conditions relevant to an application where SMA-wires are embedded directly within a structure. Epoxies are tested so that the adhesive will bond to both plastic and metallic elements. The experimental setup is designed to test the failure shear stress between an SMA wire and adhesive. A wire is encapsulated in a small drop of adhesive. Then the area of the wire that is exposed to the adhesive is measured under a microscope and the wire is pulled as a load cell measures the tensile load. The force that causes failure of the bond is recorded and used to calculate the failure shear stress between the adhesive and wire. The effects of using wires with different surface temperatures, and handling procedures (i.e. washing with acetone and handling with latex gloves) are also tested. Measurements under each set of parameters are repeated and quantitative results are obtained.