Issue 35

A. Nikitin et alii, Frattura ed Integrità Strutturale, 35 (2016) 213-222; DOI: 10.3221/IGF-ESIS.35.25 218 titanium alloy were compared with previous results [17] obtained on the material but under fully reversed tension, Fig. 5b. In terms of Von Mises equivalent stress amplitude, like for the forged alloy, the slope of the SN-curve is more important under torsion than under tension-compression. However, unlike the forged alloy, fatigue strength of extruded titanium is higher (about 50 MPa) in torsion than in tension-compression. This shows that the Von Mises equivalent stress is not suitable to describe the VHCF strength of this alloy. In order to try to explain such mechanical behavior the analysis of the fracture surface was carried out both by optical microscopy and SEM. Unlike tension-compression, where only subsurface crack were observed, in case of torsion load a transition from surface to subsurface crack initiation was found. Comparison of results on forged and extruded titanium alloy (Fig.4 and Fig.5) shows that fatigue strength of extruded titanium alloy is higher compared to the forged one that is in good agreement with mechanical properties of materials (Tab.2). Crack initiation Cracks in torsion specimens were first observed on a lateral surface of specimens by optical microscopy. It was pointed out, that for most of the investigated specimens the first stage of crack propagation was observed on a plane experiencing the maximum shear stress amplitude i.e. perpendicular or parallel to the specimen’s longitudinal axis. Further, when the crack became longer it bifurcated (Stage II) and propagated in Mode I on plane(s) of maximum normal stress (i.e. on plane having an angle about 45° with regard to the specimen’s longitudinal axis). The propagation of long crack was never observed in the plane of maximum shear stress amplitude up to the final length (corresponding to the end of the test). It should be noted, that crack growth in the plane of maximum normal stress may be found as in a single plane, as well in two planes at the same time (X-type cracks), Fig.6b. This is similar to HCF regime on many metals. (a) (b) Figure 6 : Optical microscopy at the surface of torsion specimens: (a) single crack in the plane of maximum normal stress and (b) two cracks in two 45° orientated planes of maximum normal stress or X-type crack. Crack growth under torsion loading in the plane of maximum normal stress looks to be quite sensitive to material microstructure. Comparing the surface crack path of extruded and forged titanium alloy it is notable, that branching of the crack is higher for forged titanium alloy represented by less homogenous microstructure. An example of crack path in extruded titanium alloy is showing on the Fig.6a. In this case the crack is quite well orientated on the 45° plane, while in case of forged titanium alloy, Fig.6b, the crack path is more ‘zigzag’ (alternative branching mode I and mode II). Sometimes crack make a clear ‘steps’ or even sometimes it is propagating in a two parallel 45° planes. An observation on a fracture surface of torsion specimens (after opening) shows two types of crack for both alloys: (1) surface crack and (2) subsurface crack. In the case of extruded VT3-1 these two mechanisms are more clear, Fig.7 In the case of surface crack initiation the roughness of the fracture surface is lower. That can be concluded based on a more homogeneous color. In the case of subsurface crack initiation, a roughness of fracture surface in the area of subsurface and surface crack propagation is not the same, that clearly seen by pronounceable color change. Subsurface crack initiation under torsion loading leads to forming a well known (in push-pull) ‘fish eye’ pattern [3]. But unlike push- pull loading, torsion cyclic loading produced an oval ‘fish-eye’