Biaxial testing of membrane biomaterials: testing equipment and procedures.

A testing facility for measuring the biaxial mechanical properties of highly deformable membranes is described. Forces are applied, via strain-gauge force transducers, to four points on each side of an initially square 12 to 25 mm membrane sample to produce biaxial extensions of up to 80 percent of undeformed length. Strain is estimated from the displacement of markers bounding a 1 to 2 mm central square. The accuracy of stress and strain field measurements has been assessed by finite element analysis of a biaxially-loaded isotropic elastic membrane. Major advantages of the present system over those previously described in the literature are that 1) sample mounting procedures are simplified, 2) there is provision for independent adjustment of stress field uniformity and measurement of the applied point forces and 3) faster strain rates can be imposed on the relatively small samples tested.