13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Study of Stored Energy Evolution at Fatigue Crack Tip Based on Infrared Data Anastasia Fedorova*1, Michael Bannikov1, Oleg Plekhov1 1Institute of continuous media mechanics Russian academy of science, 614013, Perm, Russia *anastfed@mail.ru Abstract The presented work is devoted to the experimental study of heat dissipation process caused by fatigue crack propagation. To investigate a spatial and time temperature evolution at the crack tip, set of experiments has been carried out using plate titanium specimens with pre-grown centered fatigue crack. An original mathematical algorithm for experimental data treatment has been developed to obtain a power of heat dissipation caused by plastic deformation at the crack tip. The algorithm includes spatial-time filtration and relative motion compensation procedures. The time dependence of the stored energy was calculated as difference between work caused by plastic deformation near the crack tip and heat dissipation energy obtained from experimental data. As a result, it has been shown that the stored energy has to accumulated during the fatigue test and has to be equal to zero when the crack reaches the critical length corresponding to the sample failure. Keywords stored energy, plastic work, fatigue test, heat dissipation, infrared thermography 1. Introduction Infrared thermography is the simple way to measure the temperature of surface and to monitor of crack propagation during cycling test. It is well know that the changing of the temperature correlates to the physical processes of damage and failures in metals [1]. The application of infrared thermography as a non-destructive method to detect the damage accumulation and to investigate the fatigue process of materials has become popular and has been wildly investigated in literature in last 25 years. In materials under cyclic deformation, fatigue cracks are initiated in the area of plastic deformation localization and lead to an intensive heat dissipation [2]. Investigation of the heat dissipative and absorption laws can take information about dissipative ability of material and current state of structural evolution. This work is devoted to investigation the time evolution of the stored energy during fatigue test using infrared technique and specially developed methods of data processing. Proposed approach allows us to measure parts of plastic work that dissipated as heat and stored in metals at fatigue crack tip. Study of these tasks enables to obtain information about the evolution laws describing the irreversibility of the fatigue process. 2. Materials and conditions of experiments Experimental study of temperature evolution at the fatigue crack tip was carried out on the plane specimens of titanium Ti-4.2Al-1.6Mn. The specimens were manufactured from a commercial titanium sheet 3 mm thick. Mechanical properties of material are modulus of elasticity - 64 GPa, yield stress - 800 MPa, ultimate stress 900 MPa, fatigue limit (R=-0.051) – 460 MPa, fracture toughness – 75.6 МРа√m. The geometry of specimen is shown in Fig. 1. The specimens were weakened by holes to initiate fatigue crack at the specimen center. The fatigue crack (about 10 mm) was initiated at the initial stage of the experiment by high amplitude cyclic loading of the specimens at the average stress of 215MPa, stress amplitude of 238 MPa and loading frequency of 20 Hz. Then the load was decreased to slow down the rate of crack propagation, which allows a detailed analysis of the heat generation processes at the crack tip. The surface of the specimens was polished in several stages by
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