Crack initiation life in notched Ti-6Al-4V titanium bars under uniaxial and multiaxial fatigue: synthesis based on the averaged strain energy density approach

Alberto Campagnolo,Filippo Berto,Keisuke Tanaka,Giovanni Meneghetti

doi:10.3221/igf-esis.41.02

Abstract

The fatigue behaviour of circumferentially notched specimens made of titanium alloy, Ti-6Al-4V, has been analysed. To investigate the notch effect on the fatigue strength, pure bending, pure torsion and multiaxial bending-torsion fatigue tests have been carried out on specimens characterized by two different root radii, namely 0.1 and 4 mm. Crack nucleation and subsequent propagation have been accurately monitored by using the direct current potential drop (DCPD) technique. Based on the results obtained from the potential drop technique, the crack initiation life has been defined in correspondence of a relative potential drop increase ?V/?V0 equal to 1%, and it has been used as failure criterion. Doing so, the effect of extrinsic mechanisms operating during crack propagation phase, such as sliding contact, friction and meshing between fracture surfaces, is expected to be reduced. The experimental fatigue test results have been re-analysed by using the local strain energy density (SED) averaged over a structural volume having radius R0 and surrounding the notch tip. Finally, the use of the local strain energy density parameter allowed us to properly correlate the crack initiation life of Ti-6Al-4V notched specimens, despite the different notch geometries and loading conditions involved in the tests.

Highlights

I n the recent literature [1,2,3,4,5,6], an unexpected notch-strengthening phenomenon was found in circumferentially notched specimens under pure torsion or combined tension-torsion multiaxial fatigue loadings
The fatigue life of notched bars resulted longer than that of smooth ones, the higher the stress concentration factor under the same load amplitude. This notch-strengthening effect has been observed by fatigue testing cylindrical specimens made of austenitic stainless steels [1,2,3], NiCrMo steel [4], pure titanium [5], but it was not found in carbon steels [3,7,8]
To investigate the effect of the notch geometry and the loading condition on the fatigue strength of the titanium alloy, pure bending, pure torsion and multiaxial bending-torsion fatigue tests have been carried out on specimens characterised by two different notch root radii, namely 0.1 and 4 mm

Summary

INTRODUCTION

I n the recent literature [1,2,3,4,5,6], an unexpected notch-strengthening phenomenon was found in circumferentially notched specimens under pure torsion or combined tension-torsion multiaxial fatigue loadings. As recently discussed by Tanaka [6], this effect can be explained on the basis of different morphologies of the fracture surfaces: in circumferentially notched bars under torsion fatigue loadings, factory-roof type fracture surfaces have been found; the sliding contact and the meshing between crack surfaces cause the retardation of crack propagation [9,10,11,12,13]. In this context, the fatigue behaviour of circumferentially notched specimens made of titanium grade 5 alloy, Ti-6Al-4V, has been analysed in the present contribution. The technical fatigue crack initiation life can be defined as the number of loading cycles at which the normalized electrical potential V/ V0 shows a given increase, assumed here about equal to 1%

40 ΔV0 growth monitor fatigue tested specimen

CONCLUSIONS