PSI - Issue 5

O. Plekhov et al. / Procedia Structural Integrity 5 (2017) 438–445 A. Vshivkov et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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failure is a key issue in solid mechanics. The heat generation process depends on both the thermo elastic effect and plastic energy dissipation. The measurement of heat flux near the crack tip allows one to calculate the energy balance under crack propagation and to obtain a new equation for crack propagation. For a long time, infrared thermography is regarded as the most effective method for estimating the power of the heat sources in the process of mechanical testing. The principal solution of the problem of energy dissipation measurement under deformation and failure can be reach by the development of additional system for direct monitor of heat flow. This idea was affectively used for investigation of energy dissipation in hydrodynamics tasks (Pradere C., 2006). Many authors proposed dependencies linking the rate of crack growth and such quantities as the J-integral, the work of plastic deformation, the size of the zone of plastic deformation, the amount of dissipated energy and other (Matvienko Yu.G., 2004; Rosakis P., 2000; Oliferuk W., 2004; Izyumova A., 2014). The classical assumption of an almost complete dissipation of the energy of deformation into heat (Farren W.S., 1925) turns out to be correct only in a limited number of cases. Any real engineering construction contains stress concentrators, welded joints and other potential sources of defects. The analysis of the kinetics of damage accumulation, the process of crack nucleation and kinetics of the crack development allows specialists to predict the time of structure failure and to perform in proper time a partial replacement or repair of deteriorated units of complex structures. Moreover, the repair or replacement of the worn out parts on a timely basis is more effective than their complete replacement after mechanical damage. It is therefore very important to know the time during which the defects in the ill-behaved areas are reaching critical values. The previous authors ’ investigations were focused on crack growth problems under an opening or mode I mechanism ( Vshivkov А., 2016). However, most structures are failed due to mixed mode loading. Many uniaxial loaded materials, structures and components often contain randomly oriented defects and cracks which develop a mixed mode state by rotation of their orientation with respect to the loading axis. This work is devoted to the investigation of the dissipated energy in the process of crack propagation under mixed mode loading. For this purpose, the original contact heat flux sensor was developed to detect energy dissipation value in the process of crack propagation and verify the data of infrared thermography. This device is based on the Seebeck effect and includes two Peltier elements and temperature controlling feedback. This sensor allows us to study in details a dissipated energy evolution in metal samples (AISI304) with uniaxial and multi axial loadings and propose relations between heat dissipation and fatigue crack rate. A series of samples made from stainless steel AISE 304 were tested. The geometry of the samples is shown in Figure 1. The experimental study was carry out in University of the Federal Armed Forces Munich, Institute for Materials Science, Neubiberg, Germany. During tests the samples were subjected to cyclic loading of 20 Hz with constant stress intensity factor and ratio R = -1. The crack length in the course of the experiment was measured by the potential drop method (Nayeb-Hashemi H., 2004; Hartman G.A., 1987). The electrical potential drop method is accepted as being capable of monitoring the fatigue crack propagation in steel structures. The size of a crack in a steel sample is predicted by applying a constant d.c. (direct current) or a.c. (alternating current) to the sample and by measuring an increase in electrical resistance due to the crack. In this case, the potential method is capable of a sensitivity as fine as 0.02 mm for a d.c. 5 A. 2. Experimental setup

Fig. 1. Geometry of samples.