Issue 35

A. Nikitin et alii, Frattura ed Integrità Strutturale, 35 (2016) 213-222; DOI: 10.3221/IGF-ESIS.35.25 214 achieved during in-service due to high loading rate [3]. Analysis of in-service loads for aeronautical applications [4] has shown that fatigue life of compressor blades may reach more than 10 9 cycles that is in the very-high cycle fatigue regime. Since the beginning of the 1990th, this fatigue regime is under investigations [5]. Mainly the VHCF properties of structural materials were investigated under axial push-pull loading, but some different testing systems were developed to reproduce different in-service loading and conditions [6]. In the recent years, investigations on fatigue properties of metals under torsion loading become more and more actual topic [7, 8]. Results of ultrasonic tests on high-strength aluminum shows that torsion fatigue crack in VHCF regime have a qualitative similarity to crack in HCF regime [9]. It has been shown that crack initiation under torsion is located at the surface of specimen. The first stage of growth is found in the plane of maximum shear stress with further formation of circumferential crack. However, a few years after, it has been shown that under torsion loading in VHCF range as surface, as well a subsurface cracks may also appears if material contains a non-metallic inclusions. In reference [10] high-strength steel was studied up to a fatigue life of 10 9 cycles. In the case of surface initiation, the fracture surface of 100C6 steel shows a similar to HCF 'factory roof' pattern, but in the case of subsurface crack initiation, an initiation mechanism is more similar to push-pull fatigue. In this case a 'fish-eye' pattern is formed around an elongated non-metallic inclusion, fig.1b. The torsion fatigue crack does not show a significant branching and 'factory roof' fracture is absent, fig.1a. Unlike high-strength aluminum, a 38MnSV5S steels shows a surface crack initiation on the plane of maximum normal stress (45° by the specimen's axis) [11] and further propagation in inclined (by the specimen's length) plane with tendency to turn back to the maximum shear stress plane. (a) (b) Figure 1 : Subsurface fatigue crack initiation in 100C6 steel under ultrasonic torsion, shear stress is 360 MPa, Nf is 10 9 cycles [10]. Ultrasonic torsion VHCF data on titanium alloy are not available in the literature. The study of Ti-alloys under torsion loading in VHCF regime is a very interesting subject, because titanium is defect free metal which has a quite complex micro-structure that may produce internal crack initiation, like shown under push-pull fatigue [12]. Present paper is focused on the study of crack initiation mechanisms in aeronautic titanium alloy VT3-1 under ultrasonic torsion in VHCF (10 6 – 10 9 cycles). Two main questions are discussed: (1) does a VHCF torsion loading may produce and internal crack initiation in VT3-1 titanium alloy; (2) does fatigue crack initiation an early crack growth stage in VT3-1 under torsion loading will be similar to ones that were observed under push-pull loading. E XPERIMENTAL PROCEDURE Materials aterial for present investigation is two phase titanium alloy VT3-1 (similar to Ti-6Al-4V) which is commonly used in aircraft engine industry. Its chemical composition is presented in Tab. 1. Two sets of specimens were used for present investigation. The first set was machined from a real compressor disk of Tu-154 aircraft. This disk was produced by forging technology for D30 engine. This compressor disk was in service for 6000 flight cycles (takeoff – landing). An estimate in service time is about 18000 hours. After in-service the disk was checked for damage tracks by non-destructive control methods. This analysis did not show any fatigue damage due to in-service loading and the disk was transmitted to fatigue tests. M