Near-threshold fatigue propagation of physically short and long cracks in Titanium alloy

Abstract

Damage tolerance is today imposed by the regulations to dimension aeronautical turbine critical parts. Thus resistance of these parts to potential anomalies is assessed. It requires to study, besides standard long cracks, the propagation of cracks of little dimension (some tenth of mm for initial length) which can have an atypical behavior consisting in crack propagation at lower stress intensity factor range than the threshold for long crack. In this context, this study is focused on a bimodal Titanium alloy (TA6V) tested at different temperatures: room temperature and 400°C at a low load ratio R of 0.1, which corresponds to loading of manufactured compressor impellers in these materials.Tests are run on compact tension specimens initially pre-cracked at constant applied stress intensity factor range (ΔK) and then cracked to threshold in view of obtaining a long crack of a/W˜0,5. Physically 2D through-thickness short fatigue cracks are created by gradually removing the plastic wake of this long crack in order to obtain a crack length as short as possible (0.1 mm). Next the short crack obtained is propagated in view of getting different thresholds for different crack lengths according to a procedure of load decrease. Crack closure contribution is systematically measured using the compliance variation technique with numerical data acquisition and filtering for accurate detection of the stress intensity factor (SIF) at the crack opening. 2D short crack propagation behavior is compared to long crack behavior with a special attention given to crack closure. The threshold evolution in function of the crack length is investigated using a Kitagawa diagram approach for determining a non-propagation criterion of short cracks.