Experimental Determination of the Specific Strain Energy of 65G Steel Under Cyclic Loading
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EXPERIMENTAL DETERMINATION OF THE SPECIFIC STRAIN ENERGY OF 65G STEEL UNDER CYCLIC LOADING Yu. V. Mol’kov We develop an experimental procedure for the construction of cyclic stress-strain diagrams of the material in front of the tip of a fatigue crack by using the method of digital image correlation and an algorithm for the calculation of the specific strain energy of the material subjected to cyclic loading. We compute the strain energy for three values of the fatigue crack-growth rate corresponding to the Paris section of the kinetic diagram of fatigue fracture. It is shown that the total energy spent for the damage to the material under cyclic loading is comparable with the critical value of the specific strain energy under quasistatic loading. Keywords: cyclic loading, specific elastoplastic strain energy, cyclic stress-strain diagram, digital image correlation, local strains.
The fracture strength of materials under cyclic loading is, as a rule, estimated according to the force approach based on finding the characteristics of cyclic crack resistance with the help of the experimentally constructed kinetic diagrams of fatigue fracture. The obtained characteristics enable us to range materials according to their cyclic crack resistance and to substantiate their choice for the production of structural elements. However, it is difficult to estimate the load-carrying ability of structural elements via the stress intensity factors, especially under the conditions of low-cycle fatigue. In these cases, it is reasonable to apply the energy fracture criterion by taking the specific strain energy as a characteristic of the investigated material [1–4]. The contemporary digital optical methods, in particular, the method of digital image correlation (DIC) [5–7], enable one to determine the stress-strain state in volumes of materials several µm3 in size and, hence, to compute the specific strain energy in the immediate vicinity near the tip of the stress concentrator. As shown earlier [8], this energy does not depend on the geometry of the deformed body and the force scheme of its loading. We consider the case of cyclic loading of a plate weakened by a central crack under the conditions of plane stressed state. Further, we consider an elementary volume of the material (Fig. 1) located, from the onset of loading, at a certain distance from the crack tip equal to the length of plastic zone. As soon as the volume is located outside the plastic zone, the material inside this volume is deformed only elastically. At the same time, if this volume stays within the limits of the static plastic zone, its material is plastically deformed but the residual strains are practically absent after each loading cycle. Finally, if the analyzed volume is located inside the cyclic plastic zone, then its plastic strains become reversible [9] and residual strains are formed after each loading cycle. On attainment of the critical density of defects of the crystal lattice [10] in the elementary volume as a result of multiple events of plastic deformati
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