The Challenges of Adhesive Bonding Shape Memory Alloy Structures

Adhesive bonding offers a high strength, lightweight means of integrating shape memory alloy (SMA) actuator elements into adaptive aircraft structures.  In most cases, the SMA phase change occurs in the cure temperature regime creating complications in how to maintain configuration control throughout the bonding process. This paper addresses the challenges of manufacturing bonded SMA structures and lessons learned from bonding trials.  Materials development was conducted to find durable ways to bond SMA materials. Coupon-level testing was performed to validate sol-gel surface preparation techniques to treat the nickel-titanium (Nitinol) bonding surface.  A variety of structural epoxy adhesives (ranging from room-temperature to 350°F cure temperatures) were screened to find the appropriate balance of adhesive strength and toughness that can survive at the SMA transition temperature while minimizing the impact of adhesive cure temperature on the SMA integration process. Manufacturing trials were conducted to understand the effects SMA fixturing during the bonding process on SMA performance.  Bonded SMA tube, beam, and fiber-metal laminate actuators were tested for joint strength, shape change, and thermal-mechanical durability.