Where l is the length of the specimen between two supports. If the relationship of V and S can be determined, the rotation factor r can be calculated by (1). Figure 2 is the measurement of S and V for several polymers and a steel with dimensions of 10x10x55mm and l=40mm. A very good linear relationship is exhibited for the materials except for a little nonlinearity at the beginning. The marks e and m on the lines indicate the elastic and maximum loads in correspondence with P-S curves of the specimens, see for example, polymer PVC in Fig. 3. It can be seen that the linearities are kept in the whole fracture process which can not be displayed fully in the figure. The fact is that the nonlinearities are below the elastic points, which can be thought of as, no crack growth taking place when the load is lower than the elastic point. Two circumstances result in the nonlinearity, one is the bending deformation of the beam contributing to the displacement S and the other is the rotation factor r changing from zero to the current value in this region. Table 1 displays the measurements of the slopes V/S and r calculated from (2). The values in Table 1 indicate that r is not a constant. It depends on materials and crack length. Because V/S is a constant, the r will vary with