Meeting Abstract

27.3  Saturday, Jan. 4 14:00  Bat wing skin mechanical behavior CHENEY, JA*; BEARNOT, A; SWARTZ, SM; Brown University, Providence, RI Jorn_Cheney@Brown.edu

In addition to the typical protective and regulatory functions of vertebrate skin, in bat, the skin of wings serves as a critical organ for locomotion: during flight, this skin membrane, often less than 50 µm thick, is cyclically loaded with multiple body weights of force. Although bat wings share this function with the wings of other flying animals, wing skin, composed of a double-layer of skin, has little in common with the material of other wings. The skin is very compliant, with an elastic modulus approximately two orders of magnitude less than that of feather keratin and insect wing cuticle. Because of its combination of compliance and thinness, bat wing skin has negligible bending stiffness, unlike bird and insect wings. Thus bats wings cannot support an aerodynamic load without the skin deforming and cambering, until the tension generated by passive deformation supports the load. Understanding the mechanical characteristics of the wing membrane skin can provide insight into both aerodynamic performance and the evolutionary transformation of skin as it acquired locomotor function. To characterize wing skin mechanical properties, we performed uniaxial tensile tests on skin sampled along the wing chord and in the perpendicular direction, along the wingspan. The wing membrane exhibited a J-shaped stress-strain curve, and was anisotropic, with greater compliance and an extended “toe” region in the spanwise direction. We model the wing skin fiber composite with an isotropic matrix of modulus comparable to that of other mammalian skin. The extended “toe” region and greater compliance arises from wrinkling induced by elastin fibers with spanwise orientation. These fibers and the imposed wrinkling create a membrane with excess length, which influences aerodynamic performance by generating larger than expected passive camber.