Fatigue life testing of swaged pneumatic artificial muscles as actuators for aerospace applications

Pneumatic artificial muscles are a class of pneumatically driven actuators that are remarkable for their simplicity, lightweight, and excellent performance. These actuators are essentially a tubular bladder surrounded by a braided sleeve and sealed at both ends. Pressurization of the actuators generates contraction and tensile forces. Pneumatic artificial muscles have traditionally been used for robotics applications, but there has been recent interest in adapting them to a variety of aerospace actuation applications where their large stroke and force, which are realized at minimal weight penalty, create potential performance improvements over traditional technologies. However, an impediment to wide-spread acceptance of pneumatic artificial muscles is the relatively short fatigue lives of the actuators reported in the literature (typically, less than 18,000 actuation cycles before damage occurs). The purpose of this study is to develop a new construction method designed to greatly increase the number of fatigue cycles before damage occurs. The fabrication methodology employs a swaging process to provide smooth and distributed clamping of the bladder and braided sleeve components onto the end fittings. This approach minimizes stress concentrations and provides high mechanical strength, which can be experimentally validated via testing for the ultimate tensile failure load. Finite element analysis was used to refine the design of the swaged end fittings before extensive fatigue testing began. Long-term fatigue testing of the actuators under realistic operating conditions showed a substantial increase in actuator life, from a maximum of less than 18,000 cycles in previous research studies to more than 120,000,000 cycles in this study.