Fig. 5 Comparison of crack growth trajectories of three polymer materials tested by Osorio, 1981. PVC and DARVIC show two mechanisms while Epoxy shows one.
two show two mechanisms governing FCG, one at low Rs and the other at high Rs. PVC-Pipe grade material shows a pure fatigue mechanism at low Rs as the trajectory falls on the pure-fatigue line. For all other cases, the trajectories deviate toward the Kmax-axis depending on the extent of the superposition of the Kmax-dependent mechanism.
We next analyze another polymer called PMMA, a polymethyl methacrylate, known as acrylic or acrylic glass, with different trade names. It is a transparent thermoplastic used as an alternative to glass. The FCG studies were made on a commercial PMMA material at three different R-ratios, one Kmean = constant test and one Kmax = constant test by Clark et al. [22]. The crack growth rates are plotted in terms of ΔK, Fig. 6a, and Kmax, Fig. 6b. The growth rates in terms of ΔK run all over the plot while the same data in terms of Kmax get compacted into a narrow band. The ΔK-R plot at crack growth rates close to the threshold is shown in Fig. 6c. All the crack growth data is falling on a straight line indicating of fully Kmax-controlled mechanism governing the crack growth. As before, in these figures, we chose to draw a horizontal line at R = 0.8 to define the minimum ΔK needed for crack growth. Based on this, a corresponding ΔK-Kmax plot is shown in Fig. 6d. All the experimental data is falling on the vertical line with a constant Kmax value. Interpolated points and assumed constant ΔK value provide the horizontal line defining the selected ΔK constant minimum required for crack growth. Even here, the data from the constant Kmean test and constant Kmax test also fall on the same L-shaped curve indicating the intrinsic