numero25

A. Shanyavskiy, Frattura ed Integrità Strutturale, 25 (2013) 36-43; DOI: 10.3221/IGF-ESIS.25.06 36 Special Issue: Characterization of Crack Tip Stress Field Subsurface metals fatigue cracking without and with crack tip Andrey Shanyavskiy State Centre for Flights Safety, Airport Sheremetievo-1, PO Box 54, Moscow region, Chimkinskiy State, 141426, Russia, shananta@mailfrom.ru A BSTRACT . Very-High-Cycle-Fatigue regime for metals was considered and mechanisms of the subsurface crack origination were introduced. In many metals first step of crack origination takes place with specific area formation because of material pressing and rotation that directed to transition in any volume to material ultra- high-plasticity with nano-structure appearing. Then by the border of the nano-structure takes place volume rotation and fracture surface creates with spherical particles which usually named Fine-Granular-Area. In another case there takes place First-Smooth-Facet occurring in area of origin due to whirls appearing by the one of the slip systems under discussed the same stress-state conditions. Around Fine-Granular-Area or First- Smooth-Facet there plastic zone appeared and, then, subsurface cracking develops by the same manner as for through cracks. In was discussed quantum-mechanical nature of fatigue crack growth in accordance with Yang’s modulus quantization for low level of deformations. New simply equation was considered for describing subsurface cracking in metals out of Fine-Granular-Area or Fist-Smooth-Facet. K EYWORDS . Very-High-Cycle-Fatigue; Subsurface; Mechanisms; Fine-granular-area; First-smooth-facet; Ultra- high-plasticity; Quantum-mechanical; Crack growth. I NTRODUCTION atigue crack growth for metals subsurface cracking in VHCF regime has been investigated in many cases [1-5]. Main idea is about continuous (conventional) fatigue crack propagation process in spite of registered range of increments of crack in one cycle in two orders less than crystallographic lattice. It is physically clear that crack increment cannot be less than one lattice. That is why crack growth process in the considered case is not continuous. Another basically idea is about subsurface cracking process that takes place in vacuum [6]. Special tests in vacuum have been performed for steels and titanium alloys and it was demonstrated similarity in fracture surface patterns for through and subsurface cracks when through cracks did propagation in vacuum. Moreover, it was discovered Fine-Granular-Area (FGA) for the first stage of through crack growth in vacuum. The idea about subsurface fatigue crack propagation in vacuum contradicted with test results of steels having different contents of hydrogen [7]. If hydrogen contents in metals has increasing, subsurface crack initiation has acceleration. Clear that, if hydrogen influenced metals cracking, there is subsurface environment for developed fatigue cracks. In the center of a fish-eye area there has been discovered high level of oxygen [8, 9]. This is the same evidence that subsurface cracking can takes place in oxygen environment. In many cases studied of steels [10] and superalloy [8] subsurface cracking, there were very clear registered fatigue striations on the fracture surface which has been formed before crack evidence on the specimen surface. The fatigue striation formation process has been considered based on the knowledge about environment influence on this process. This consideration based on well-known investigation results which show that fatigue striations have not evidence in vacuum. Consequently, the discussed investigation results demonstrate that subsurface cracking occurs in environment but not in vacuum. F