PSI - Issue 37

A. Vshivkov et al. / Procedia Structural Integrity 37 (2022) 570–575 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

571

2

features of energy dissipation and acoustic emission (AE) can be used in promising systems for monitoring the current state of metal structures. Theory of fracture in metals on the base of the structure-energy considerations was proposed by Ivanova and Terentiev (1975). According to the energy criteria of fracture proposed by Fedorov (1979), the measure of damage is internal energy stored in material due to deformation. Formulation of fracture criterion based on the critical value of stored energy was also proposed by Meneghetti (2007) and Sih and Gdoutos (1992). The relationship between AE and fracture criteria was obtained by L.R. Botvina et al. (2016). The fundamental possibility to assess the damage degree by the energy of AE signals was shown. Carpinteri et al. (2016) established the correlation between the heat dissipation energy and the AE energy in concrete beams under three-point bending. These authors conclude that the energy releasing during the loading process is partially dissipated to create the fracture surfaces, and partially transformed into emitted energy detectable by the AE technique. Modern techniques make it possible to register both temperature changes (IRT, thermistors, heat flux sensors) near the crack tip, and acoustic emission signals (AE sensors). Processing and joint analysis of these signals makes it possible not only to identify the main mechanisms and stages of deformation, but also to predict the propagating crack rate and assess the viability of the material in real time. So that the main goal of this work is investigation the thermodynamic and AE features of fatigue crack propagation in titanium alloy Grade 2 under cyclic loading conditions.

Nomenclature a

crack length (m)

N number of cycles da/dN crack growth rate (m/cycle) K max stress intensity factor (MPa*m 1/2 ) t time (s) Q heat dissipation (J) E stored stored energy (J) W p work of plastic deformation (J) H(t)

heat dissipation rate registered by heat flux sensor (W)

load (N)

F(t) x(t)

displacement registered by contact extensometer (m)

2. Experimental setup The Charpy V-notch (CVN) specimens of titanium Grade 2 with a 30-degree radius notch were tested under maximum loading of 7.5 and 8 kN at 10 Hz frequency and stress ratio R=0.1 on a 100 kN servo-hydraulic machine Instron 8802. The figure 1 presents the geometry of studied samples. The crack length was measured by a potential drop method described by Bowler (2006). The quantitative measurements of the heat dissipation rate at the crack tip area were carried out using the original heat flux sensor. Displacements near the notch were recorded with a contact extensometer with a base of 12 mm. For continuous recording of acoustic emission during the testing, the Amsy-5 hardware complex (Vallen, Germany) was used.

Fig.1. Geometries of titanium alloy specimens studied under cyclic loading conditions.