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

A limitation in virtually all planar biaxial studies of soft tissues has been the inability to include the effects of in-plane shear This is due to the inability of current mechanical testing devices to induce a state of in-plane shear, due to the added cost and complexity. In the current study, a straightforward 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 axes and on rotating carriages to allow the specimen to undergo in-plane shear freely. To demonstrate the method, Jive glutaraldehyde treated bovine pericardium specimens were prepared with their preferred fiber directions (defining 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 strain stales, and the resulting biaxial darn re-expressed in material awes coordinate 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 system and c and A(i) are constants. While W was able to fit the data very well, the constants A, and A, were Sound nor to contribute significantly to the fit and were considered unnecessary to model the shear strain response. In conclusion, while not able to control the amount of shear strain independently or induce a state of pure shear, the method presented readily produces a state of simultaneous in-plane shear and normal strains. Further the method is very general and can be applied to any anisotropic planar tissue that has identifiable material axes.