Fig. 5. Measured and predicted breaking stresses as a function of crack length, a, for (A) M. flaccida, (B) P. occidentalis and (C) U. expansa. In each plot, data points indicate breaking stress measured as a function of crack length in single-edge-notch, pull-to-break tests. Data points on ordinate (open circles) are for un-notched test pieces. Solid line indicates predicted breaking stress as determined from Eqn 5 using critical strain energy release rate calculated from estimated stored strain energy density. Broken line indicates predicted breaking stress based on critical strain energy release rate calculated from total strain energy density. Curve equations are as follows, with breaking stress in units of MPa and crack length, a, in m: (A) solid line: breaking stress=3510a^–0.81, r²=0.933; broken line: breaking stress=9530a^–0.66, r²=0.935; (B) solid line: breaking stress=2123a^–0.90, r²=0.0033; broken line: breaking stress=8668a^–0.69, r²=0.0030; (C) solid line: breaking stress=17650a^–0.58, r²=0.712; broken line: breaking stress=28790a^–0.51, r²=0.724.