A method for planar biaxial mechanical testing that includes in-plane shear

A limitation in virtually all planar biaxial studies of soft tissues has be en the inability to include the effects of in-plane shear This is due to th e inability of current mechanical testing devices to induce a state of in-p lane shear, due to the added cost and complexity. In the current study, a s traightforward method is presented for planar biaxial testing that induces a combined state of in-plane shear and normal strains. The method relies on rotation of the test specimen's material axes with respect to the device a xes and on rotating carriages to allow the specimen to undergo in-plane she ar freely. To demonstrate the method, Jive glutaraldehyde treated bovine pe ricardium specimens were prepared with their preferred fiber directions (de fining the material axes) oriented at 45 deg to the device axes to induce a maximum shear state. The test protocol included a wide range of biaxial st rain stales, and the resulting biaxial darn re-expressed in material awes c oordinate system. The resulting biaxial data was then fit to the following strain energy function W: W = c/2 [exp(A(1)E'(2)(11) + A(2)E'(22)(2) + 2A(3)E'E-11'(22) + A(4)E'(2)(12) + 2A(5)E'E-11'(12) + 2A(6)E'E-22'(12)) - 1] where E-ij' is the Green's strain tensor in the material axes coordinate sy stem and c and A(i) are constants. While W was able to fit the data very we ll, the constants A, and A, were Sound nor to contribute significantly to t he fit and were considered unnecessary to model the shear strain response. In conclusion, while not able to control the amount of shear strain indepen dently or induce a state of pure shear, the method presented readily produc es a state of simultaneous in-plane shear and normal strains. Further the m ethod is very general and can be applied to any anisotropic planar tissue t hat has identifiable material axes.