PSI - Issue 18

Prokhorov A. et al. / Procedia Structural Integrity 18 (2019) 802–808 Author name / Structural Integrity Procedia 00 (2019) 000–000

803

2

not only from the sample surface, as is customary in the high-cycle fatigue mode, but also in the bulk of the material, the so-called gigacycle (very high cycle) fatigue region Bathias C. (2005), Wang C. et al. (2012). Over the past decades, this area of fatigue has been the subject of study by many groups of researchers Zhu X. et al.(2006), Botvina L.(2005), Sakai T.(2009). The main feature in this area of fatigue is the onset of fracture under the surface of the material. The initiator of fatigue crack development in the gigacycle fatigue mode is heterogeneity in the structure of the material, inclusion or accumulation of pores in the bulk of the material Plekhov O. et al.(2007) The development of fatigue cracks in such a system leads to the formation of a fish-eye structure. On the fracture surface, there are several areas with characteristic scales of the grain structure. The process of evolution of the structure of the material to the stage of critical growth of a macroscopic crack is still a controversial issue. Currently, there are several models of this phenomenon Naimark O. et al (2000). The development and verification of the proposed models require a detailed experimental study of the material failure process under gigacycle loading. The investigation of lifetime equals 10 7 cycles or more require considerable time when testing on classic fatigue servo-hydraulic testing machines with a loading frequency of 100 Hz. Modern test resonant ultrasonic machines Bathias C. (2005), allow testing at a frequency of 20 kHz, which significantly reduces the test time, but has drawbacks associated with overheating of samples during the test. Investigations of the structure of the fracture surface after the test allowed us to answer a number of questions related to the mechanisms of formation of defects Naimark O. et al (2000), but they cannot answer questions related to the kinetics of the development of a defective structure at various stages of destruction. The in-situ monitoring the specimen integrity in this work carried out based on analysis of alternating magnetic field interaction with specimen. The detector allows us to observe the peculiarity of material behavior under very high cycle fatigue testing. 2. Physical and mechanical properties of material The experimental investigation on very-high cycle fatigue were performed under high strength steel AISI 420. Chemical composition of material is reported in Table 1.

Table 1. Chemical composition of the AISI 420 steel Element C Mn

Ni

Cr

S

P

Mass (%)

0.35-0.44

< 0.6

< 0.6

12-14

< 0.025

<0.03

(a) (c) Fig. 1. Specimen geometries for (a) tension test; (b) Luong test, (c) VHCF test. (b)