Small fatigue crack propagation in a near alpha titanium alloy, IMI 829

• Main Author: Spence, S. H.
• Published: Swansea University 1991
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.639092

Small crack growth rates are assessed for the near alpha titanium alloy, IMI 829, with beta processed microstructures. Constant load amplitude tests were conducted using three test piece geometries which encompassed plain surfaces and stress concentrations. Two computerised crack length measurement techniques were used: A switched d.c. potential drop method and a photomicroscopic system. The p.d. system together with a clip gauge were used to assess crack closure. Crack growth is structure sensitive, characterised by crack path tortuosity, crack bifurcation and a faceted fracture surface. Colony and prior beta grain boundaries provide barriers to propagation which result in temporary arrest in many instances. Small crack growth rates were compared and contrasted with available long crack data for the same alloy and microstructure. A transition, ΔK<SUB>T</SUB>, was observed for the small crack growth rate data. This transition is most marked for through thickness cracks at notches and least marked for part through cracks in plain surfaces. Small crack growth behaviour was observed for values of Δ<i>K</i> less than ΔK_T with growth rates up to a factor of 10 greater than long crack rates. For values of Δ<i>K</i> greater than ΔK<SUB>T</SUB> the data merge, although in some instances rates for through thickness small cracks were slower. The transition from small to long crack behaviour occurs as the monotonic crack tip plastic zone approaches the widmanstatten alpha colony size. At low Δ<i>K</i> there was no evidence of a threshold even at 3.6 MNm^-3/2. Load ratio was not observed to affect growth rates. The results suggest that the significant difference between crack voltage signals during a load cycle, for Δ<i>K</i> above and below ΔK<SUB>T</SUB>, could be due to crack tip plasticity rather than closure in this study. Crack closure was not evident from crack opening displacements obtained using the clip gauge.