Dynamic fatigue crack growth has been investigated in polyethylenes of various molecular weights and branch concentrations. Square load waveforms were applied to sharply notched samples and the load was adjusted so that for all tests the maximum stress intensity factor was constant and tensile, Kmax, but different experiments featured different values of minimum stress intensity factor Kmin, which was either zero or compressive. The crack growth rate increased as Kmin became more compressive. Subsidiary experiments were made using static loads and these confirmed that, as previously reported, the slow crack growth rate decreased with increasing branch concentration and molecular weight. In all the materials studied, fatigue crack growth rates were significantly greater than those under steady loads. However, materials possessing higher branch concentrations showed a relatively greater increase in crack growth rates. For negative values of Kmin, increasing branch concentration has a deleterious effect on the fatigue crack resistance. In the fatigue loading experiments studied here it was found that for Kmin/Kmax ≈ −1, the ranking of materials in terms of crack growth resistance was reversed with respect to that seen under steady loads.

Fatigue crack growth rates have been determined for a series of polyethylenes through the application of various periodic loading and unloading programs in which the load was held constant for tON seconds and unloaded to a lower constant value for tOFF seconds in each cycle. The crack growth rate, da/dN, was given by where the parameters γ, k, β and n are material dependent and β and n are also geometry dependent. It was found that β and n described the closure of the crack during the unloaded period as well as the kinetics of the increase in crack growth rate. It is proposed that this equation is consistent with a mechanism in which the crack growth rate increases because of damage sustained by the craze during the unloaded period as it closes and is compressed under the recovery of the surrounding material.