Influence of Subsurface Defects on Fatigue Properties of Titanium Alloys.
Abstract
A program was conducted to evaluate the influence of two specific defects on the low cycle fatigue properties of selected alpha:beta titanium alloys. The formation of subsurface fatigue cracks and subsequent propagation to produce failure can lead to a characteristic fracture topography which has been designed a 'cleavage rosette'. This type of failure was first recognized in the alloy Ti-6Al-5Zr-0.5Mo-0.25 Si(IMI 685) beta processed to give a coarse acicular structure, in this case a very large fatigue debit is observed. The fatigue properties of IMI 685 determined under a range of stress amplitudes and frequencies are presented together with the results of detailed analysis of the chemistry of crack origins and fracture topography. Experiments to simulate rosette type failures by the introduction of artificial subsurface nucleation sites in IMI 685 and other alloys are described. Although some features of a rosette fracture could be reproduced, the magnitude of the fatigue debit was not. Models to account for the kinetic features of cracking in IMI 685 are proposed and compared with the experimental results. The second type of defect studied was a local structural heterogeneity, known as 'beta fleck', that can occur in incorrectly processed alpha:beta titanium alloys. The periodic chemical fluctuation responsible for regions of beta structure in an alpha:beta processed Ti-6Al-4V alloy are documented. Fatigue testing, under both stress and strain control, have established that property-debits associated with beta flecks are only observed in specimens with surface residual stresses. The magnitude of this debit is dependent upon the specific test conditions.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jul 01, 1977
- Accession Number
- ADA052883
Entities
People
- C. C. Law
- M. J. Blackburn
Organizations
- Pratt & Whitney