| Abstract: | Do crosslinks between the molecular chains of a glassy polymer impede the growth of fatigue cracks? Three polymers H, M, and L of equal chemical composition but differing in crosslinking densities were prepared by the polyaddition reaction of stoichiometric mixtures of 1,4-butanediol diglycidyl ether, N,N'-dimethylethylenediamine, and 1,4-diaminobutane. The growth of the fatigue cracks under cyclic loading (2 Hz) was studied in the glassy polymers at -100°C and -60°C with the help of strain gauges bonded to the back side of the compact tension specimens. Crack growth rate da/dN ranged from a few nm to 10 μm depending on the stress intensity amplitude K̂1/2 at the crack tip. Log (da/dN) plotted against K̂1 revealed a linear relationship in accordance with the model calculation. The slopes, m, of the straight lines characterizing the fatigue crack growth were found to scale with M̄-1/2 in the case of the studied polymers. (M̄c: average molar mass of chain segments between crosslinks). Glassy polymers with a low degree of crosslinking are less prone to fatigue crack growth than are the more tightly crosslinked polymers, as shown by the presented examples. |
