Abstract
Abstract Background In this paper the mechanical properties of armco-iron are studied experimentally. The ultrasonic testing machine USF-2000 was used to carry out fatigue tests involving 106–1010 loading cycles. The frequency of loading is 19.5 ± 0.5 kHz. Based on the experimental data, the Wöhler curve was obtained. Methods For the analysis of physical condition, two new sensors were designed, one of which measured the electric resistance of the sample and the other measured the magnetic permeability. Second measuring method can be applied to ferromagnetic materials only, though it may be useful for studying the mechanical properties of metals in general. Results It was shown that the significant changes in physical processes accompanying the evolution of structural defects in the material were observed in the final stages of the experiment. Conclusions The applied measurement techniques allowed us to exactly determine the time of fatigue crack initiation below the surface the material, which cannot be monitored by the standard optic methods.
Highlights
In this paper the mechanical properties of armco-iron are studied experimentally
The characteristic features of the response of the material to giga cyclic fatigue test mode are the initiation of subsurface fatigue cracks and splitting of the Weller curve (Naimark et al 2014; Sakai 2009; Wang et al 1999)
It has been shown that in the region of gigacyclic fatigue, the cyclic deformation of pure iron can lead both to the formation of a stable shear band on the sample surface, and characteristic subsurface structures of the “fish eye” type
Summary
In the magnetic method the ferromagnetic properties of the material is used to monitor processes that accompany the evolution of the material structure. To switch the magnetizing coil, a mini amplifier of signals was assembled based on the TDA2030a chip (18 W HiFi power amplifier class AB). This amplifier provides high output current, has small harmonic and intermodulation distortions, wide bandwidth of the amplified. An alternative method of monitoring the nucleation of the fatigue crack is the method of controlling the voltage drop (four-point measurement of the electrical resistance). In this method four contacts are placed on the sample, which is set fixed inside the ultrasonic testing machine. The instrument accuracy of the electrical resistance is 30 uΩ
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