A study of the effects of mechanical and environmental variables on fatigue crack closure

Using the potential drop technique, fatigue crack closure has been monitored in pin loaded SEN specimens of -titanium, a titanium alloy and EN24 steel. The specimens were tested in tension-tension under conditions closely approximating to plane strain, and closure was only detected in vacua of better than 133 mN m^?2 (10^?3 torr). No significant closure was detected in air. The extent of the crack area closed at minimum load varied with air pressure, applied stress, R ratio (R = L_min/L_max), crack length, material, and loading mode. Additional experiments made with a dip gauge showed that the COD/applied load response of the crack was non-linear in vacuum above minimum load in the fatigue cycle confirming that crack closure was occcurring. It is shown that for a given material, loading mode and air pressure, the effect of loading and crack length variables on crack area closed at minimum load can be characterised in terms of the parameter (K²_min-ΔK², this being proportional to the calculated COD at minimum load. The extent of closure in vacuum is influenced extensively by this parameter.