Fatigue crack growth in polyethylene: Material dependence. I: Tensile/compressive loading

Dynamic fatigue crack growth has been investigated in polyethylenes of vari ous molecular weights and branch concentrations. Square load waveforms were applied to sharply notched samples and the load was adjusted so that for a ll tests the maximum stress intensity factor was constant and tensile, K-ma x, but different experiments featured different values of minimum stress in tensity factor K-min, which was either zero or compressive. The crack growt h rate increased as K-min became more compressive. Subsidiary experiments w ere made using static loads and these confirmed that, as previously reporte d, the slow crack growth rate decreased with increasing branch concentratio n and molecular weight. In all the materials studied, fatigue crack growth rates were significantly greater than those under steady loads. However, ma terials possessing higher branch concentrations showed a relatively greater increase in crack growth rates. For negative values of K-min, increasing b ranch concentration has a deleterious effect on the fatigue crack resistanc e. In the fatigue loading experiments studied here it was found that for K- min/K-max approximate to -1, the ranking of materials in terms of crack gro wth resistance was reversed with respect to that seen under steady loads.