Peculiarities of Fatigue Crack Growth in Pseudoelastic NiTi Alloy

Authors

  • V. P. Yasnii Ternopil Ivan Puluj National Technical

DOI:

https://doi.org/10.31649/1997-9266-2020-151-4-120-124

Keywords:

pseudoelastic alloy, fatigue crack grow rate, density, stress ratio, fracture mechanisms

Abstract

There have been analyzed the peculiarities of fatigue crack growth in pseudoelastic NiTi alloy at stress ratios R = 0,2 and 0,5. The fatigue crack growth resistance was determined on cylindrical specimens with a diameter of 8 mm with a unilateral segmental notch to a depth of 0,6 mm in their diametrical cross section at a temperature of 20°C in air. Using the servohydraulic testing machine STM-100, the specimens were preloaded by three-point bending in order to grow a fatigue crack from the notch, and the fatigue crack growth kinetics was determined on a cylindrical specimen with one lateral crack under uniaxial tension at a frequency of 25 Hz. The length of crack on the specimen surface was determined using the binocular microscope with the precision not less than 0,02 mm. To calculate the crack depth b in the deepest point of the crack front, the crack shape b/a of the initial and final cracks was determined using the linear interpolation. The fatigue crack growth rate was determined using this value in the deepest point of the crack front after certain number of loading cycles. It was found out that, unlike the traditional concepts of fracture mechanics, the mechanical driving force of fatigue crack growth, that unambiguously describes the failure kinetics despite of R is not the stress range, but a maximal value of stress intensity factor. According to the microfractographic studies, the main mechanism of crack propagation is connected with the brittle failure of martensite alloy structure elements. Therefore, the strain austenite–martensite transformation at the crack tip is responsible for its kinetics. The intensity of such transformation is determined mostly by the stress levels, and not by its range, that determines the mechanical driving force of fatigue crack growth.

Author Biography

V. P. Yasnii, Ternopil Ivan Puluj National Technical

Dr. Sc. (Eng.), Associate Professor, Head of the Chair of Structural Mechanics

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Published

2020-09-25

How to Cite

[1]
V. P. Yasnii, “Peculiarities of Fatigue Crack Growth in Pseudoelastic NiTi Alloy”, Вісник ВПІ, no. 4, pp. 120–124, Sep. 2020.

Issue

No. 4 (2020)

Section

Mechanical engineering and transport

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