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