ABSTRACT
A typical cyclic mechanical test of biological so tissues, for example indentation, can be seen in Figure 8.1, where several phenomena can be observed clearly. First, the loading and unloading curves for each cycle are dierent, called hysteresis. Second, the loading curve typically shows the shape of a bowl bottom of clear nonlinearity. ird, the loading-unloading curve of each cycle at the beginning of the test moves towards the right side of the x-axis and this curve becomes more and more consistent as the number of cycles increases. is phenomenon is called pre-conditioning. Because of the existence of pre-conditioning, a pre-test (several cycles of loading and unloading) is commonly adopted before collecting real data for analysis. Because of nonlinearity, a toe region with a very small stiness
is always observable at a small value of deformation. erefore, a pre-load is always necessary to reduce the eect of the toe region in practical measurement of so tissue elasticity. A common phenomenon observed for the nonlinearity of so tissue is the increase of tissue stiness with the increase of strain level, which is called ‘strain hardening’. A good example is the elasticity of cornea, which increases with the increase of intraocular pressure (IOP), thereby inducing a strain in the cornea. During mechanical testing, if the tissue behaviour signicantly deviates from linearity, then nonlinearity should be considered.
