AbstractFatigue crack propagation (FCP) rates are studied in 6 mm thick specimens of high density polyethylene (HDPE) containing razor notches, Centrally‐notched plates and single‐edg notched bars are subjected to sinusoidal tension‐compressio or tension‐zero cycling at 0.5 or 2.0 Hz under load control a room temperature; crack growth is monitored using a travelling microscope. After many thousands of cycles with no observable damage at the tip of the razor notch, a craze like zone begins to form. This zone grows slowly until it reaches the length characteristic of a mature crack at the same ΔK. Crack growth proper then begins. The number of cycles to initiate crack growth falls linearly with increasing ΔK at the razor notch Subsequent crack growth is determined both by the current value of ΔK and by loading history. When ΔK is increasing, FCP rates follow a standard Paris law curve. However, reduced, FCP rates are observed following an overload.