PSI - Issue 32

V.A. Oborin et al. / Procedia Structural Integrity 32 (2021) 152–157 Author name / StructuralIntegrity Procedia 00 (2019) 000–000

153

2

1. Introduction Determination of the nature of fatigue crack initiation is one of the key problems in various fields of application, especially if it concerns gigacycle fatigue regimes is studied by Bathias (2006), Mughrabi (2006), Sakai (2009) when cracks are formed inside the material. The influence of random static or dynamic loads on the durability of materials under conditions of ultra-high cycle fatigue is currently of great interest to the aircraft engine industry due to the need to assess, for example, the operating life (durability) of gas turbine engine blades under operating conditions accompanied by solid particle impacts is studied by Spanradet al. (2011), Oakleyet al. (2007), Chen (2005), Nowell et al. (2003). A characteristic feature of the development of fracture under gigacycle fatigue conditions is the crucial effect of the fatigue crack initiation stage on the fatigue life of the structure. In this case, the qualitative difference is the formation of a fatigue crack in the volume of the material, which radically changes the formulation of the problem of fatigue strength estimation, methods for studying the stage nature of fracture evolution. In contrast to the traditional approaches used in the field of ultra-high cycle fatigue, where the focus is on the stage of crack propagation, here the central fundamental problem is the initiation of fatigue cracks as a result of multi-scale processes of damage growth associated with defects of different origin (inclusions, localized plastic shear bands, microcracks, pores). Froustey et al. (2010), Bathias (2005) it is noted that the stage nature of failure is characterized by the effects of "irreversibility, associated with the formation of localized shear strains, which play a key role in the fatigue crack nucleation, manifesting itself in the non-linearity of elastic behavior of materials - the "elastic flexibility anomalies" of fatigue specimens. The role of the crack initiation stage is especially important for gigacycle loading modes, which are characterized by the formation of a "fish-eye" fracture zone in the material. The nature of the formation of a special zone involving a fine grain area (FGA) around the internal defect still raises questions is studied by Mughrabi (2006), Bathias (2005). 2. Materials and experimental conditions The preliminary loading of the specimens made of titanium alloy VT6 (Ti-6A1-4V) with a thickness of 15 mm was carried out by impact loading of an aluminum plate with a thickness of 4 mm and a diameter of 120 mm, accelerated by a plane-wave explosive generator to a velocity of ~1400 m/s (Fig. 1) is studied by Antoun et al. (2003) Oborin et al. (2018). The experiment was carried out in the Laboratory of Rheological Properties of Condensed Media under Pulse Loads at the Institute of Problems of Chemical Physics of the Russian Academy of Sciences. The chemical composition and mechanical characteristics of the VT6 alloy are presented in Tables 1 and 2.

Table 1. Chemical composition of VT6 (Ti-6A1-4V) (weight percent %).

Ti

Al

V Cr

Cu

Fe

Mn

Mo

Nb

Sn

Zr

Impurity

90.3

5.42

3.9

0.027 0.012

0.2

0.01

0.03

0.01

0.02

0.01

0.085

Table 2. Quasi-static tensile characteristics of VT6 (Ti-6A1-4V).

Tensile strength (MPa)

Elastic modulus (GPa)

Yield stress (MPa)

Ultimate elongation (%)

115

814

950

16

At the time of impact of the striking plate against the specimen, the diameter of the flat section of collision between the striker and the specimen was not less than 90 mm. The thickness of the striking plate at a given impact velocity was chosen from consideration of maintaining a stationary impact-compressed state through the thickness of VT6 specimens, that is, the maximum amplitude of the compression pulse was the same throughout the specimen. To avoid side and rear unloading of the specimens and to implement the "state maintaining" condition, the former were pressed without gaps into steel rings of diameter of 200 mm.