Rate dependent mechanical response of avian skeletal muscle under quasi-static and dynamic uniaxial tension

Aircraft bird strikes pose a substantial threat to aircraft and passenger safety. In the event of bird-aircraft collisions, the forces exerted on aircraft components can cause extreme damage, severely impacting system performance or even causing catastrophic failure. The magnitude and severity of the forces exerted by birds on aircraft components during these violent events can vary drastically dependent on the initial conditions of impact. To better simulate these impact forces, robust quantitative data characterizing the rate and orientation dependent material behaviors of anatomical components such as skeletal muscle is desired. This data can only be gathered through thorough experimentation of biological components under a wide range of external stimuli. Despite the stress-stretch response of skeletal muscles being well documented to behave anisotropically, fewer studies have documented skeletal muscles' high levels of strain rate dependency. In avian skeletal muscle tissue, studies characterizing the muscles' strain rate dependent and anisotropic material behavior under dynamic tension have yet to be made publicly available. In this descriptive study, tension experiments were conducted across a broad range of strain rates via a hydraulic load frame (10^-2-10^-1 s^-1) and a modified Split-Hopkinson bar (1300-2000 s^-1). This range of loading rates allowed quantification and comparison of rate dependency of skeletal muscles extended in-line with fiber direction and muscle extended transverse to fiber direction. Results from this study highlighted that the observed mechanical response of avian skeletal muscle tissue under tension is highly dependent on both strain rate and direction of extension in relation to muscle fibers. Furthermore, it was observed that strain rate dependency was impacted by both the direction of load and whether or not the muscle tissue had been extended beyond the initial nonlinear (toe) region. The results discussed in this work have broad implications in the interpretation of the anisotropic stress-stretch behavior of skeletal muscle as a whole, particularly when reviewing quasistatic-only studies.