Strain rate effects on viscoelastic crack bifurcation

The effects of applied strain rate on the viscoelastic crack bifurcation phenomenon in Polymethyl Methacrylate (PMMA) were investigated. It was still verified that the product $\text{σ}{}_{\mathrm{f}}\text{C}{}_{\mathrm{b}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ was constant, as was already observed by Congleton and Petch, and Anthony, Chubb and Congleton, for brittle elastic materials, for any strain rate, where σ_f = the gross fracture stress and C_b= the main crack length until the bifurcation starts. However, it was found that the higher strain rate increases the main crack length C_b resulting in the decrease in the gross fracture stress σ_f and vice versa. This might be interpreted that the higher stress concentration at the initiation crack tip, which is realized by becoming more brittle due to the higher strain rate owing to the predominance of the elastic element in the viscoelastic material, decreases the gross fracture stress leading to the longer main crack length.