Issue 35

O. Plekhov et alii, Frattura ed Integrità Strutturale, 35 (2016) 414-423; DOI: 10.3221/IGF-ESIS.35.47 414 Focussed on Crack Paths The study of a defect evolution in iron under fatigue loading in gigacyclic fatigue regime O. Plekhov ( O. Naimark Institute of Continuous Media Mechanics UB RAS, 614014 Perm, Russia M. Narykova, A. Kadomtsev, V. Betechtin The Ioffe Institute, 26 Politekhnicheskaya, 194021 St Petersburg, Russia A BSTRACT . The work is devoted to the study of the damage accumulation in iron under gigacyclic fatigue (VHCF) regime. The study of the mechanical properties of the samples with different state of life time existing was carried out on the base of the acoustic resonance method. The damage accumulation (porosity of the samples) was studied by the hydrostatic weighing method. The obtained results show the accumulation of porosity in the volume of the sample during fatigue loading and corresponding decrease of the elastic properties. A statistical model of damage accumulation was proposed in order to describe the damage accumulation process. The model describes the influence of the sample surface on the location of fatigue crack initiation. K EYWORDS . Gigacycle fatigue; Damage accumulation; Fatigue crack initiation. I NTRODUCTION he evolution of structural defects in metals under deformation is observed at all spatial scales and leads to the irreversible deformation and fracture. Nowadays we can observe a rise of interest to study of the defect evolution in metals under VHCF regime [1-12]. The structural investigation of iron tested under HHCF regime was carried out in [13]. One of the key features of a fracture in the VHCF regime is the undersurface fatigue crack initiation. Under this loading condition the crack initiation cannot be detected by a traditional fatigue crack monitoring technique. Due to this fact this regime can be considered as a most dangerous loading regime for real engineering structures. To develop a model of the defect evolution under small stress amplitude we need to choose the basic physical level of the description of the material microstructure and describe the geometry of the elementary defects. Analysis of the experimental results of the study of structural levels of plastic deformation and fracture and recent experimental investigation can hypothesize that scale level with the size of submicrocracks 0.1-0.3 mm plays a key role in this process [14]. It was experimentally shown that the defect kinetics is different near the specimen surface and within its volume. The rate of the microcrack initiation in the surface layers is from one to three orders of magnitude higher than in the volume of the material. T