Evidence is presented for an adiabatic decohesion mechanism of Rapid Crack Propagation (RCP) in polyamide (PA) 12. RCP was induced in pipe specimens using the Small–Scale Steady–State (S4) test. Fractography revealed a surface of “flakes” attached to the bulk through highly fibrillated regions. A temperature rise recorded ahead of the propagating crack tip in combination with the observed fracture appearance indicates a thermally activated disentanglement fracture mechanism for PA12 pipe grades. Using mathematical formulations of the adiabatic decohesion model proposed by Leevers, strong correlation was found between S4 test results and a newly defined analytical ranking parameter R**.