Dynamic Deformation with Static Load
We conduct a series of tensile analyses on metal-plate specimens to investigate the relation between the fatigue level and the material’s response to external loads. We use Electronic Speckle-Pattern Interferometery (ESPI) to measures the in-plane displac
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Dynamic Deformation with Static Load S. Yoshida, H. Ono, T. Sasaki, and M. Usui
Abstract We conduct a series of tensile analyses on metal-plate specimens to investigate the relation between the fatigue level and the material’s response to external loads. We use Electronic Speckle-Pattern Interferometery (ESPI) to measures the in-plane displacement, and an acoustic transducer to assess the elastic modulus of the specimen via acoustic velocity measurement at various stress levels. We apply a tensile load at a constant pulling rate up to a certain stress level substantially lower than the yield stress, and analyze the strain field obtained with the ESPI setup at each time step. At the same time, we measure the acoustic velocity at various tensile stress levels. We have found repeatedly in the experiment on an aluminum-alloy specimen that (a) the strain field changes over several seconds after the tensile machine stops pulling, and (b) the acoustic velocity at the same point of the specimen considerably varies from measurement to measurement at the same stress level. These observations indicate that the specimen is deformed even if the crosshead of the tensile machine is stationary. This mysterious phenomenon is consistent with the observation made by Pappalettera et al. in their fatigue analysis that the acoustic emissions, which normally stops when the fatigue reaches a certain level, resumes if the specimen is dismounted from the test machine and remounted for continuation of the test. Keywords Fatigue • Strain energy damping • Plastic deformation wave
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Introduction
Fatigue of metals is a complex problem and its mechanism has not been fully understood. Fatigue tests [1, 2] utilize low level cyclic loading to the specimen until it breaks and evaluate the fatigue life. It is usually a long test, requiring the total cycle of the order of million. Often cyclic frequency is raised to save the time of experiment. There is always a question regarding the influence of the cyclic load frequency on the fatigue life. In polymer engineering, on the other hand, fatigue life is often discussed in conjunction with dynamic modulus [3, 4]. Dynamic modulus consists of the real part known as the storage modulus and imaginary part known as the loss modulus. The storage modulus represents elastic energy (conserved energy) and the loss modulus represents energy dissipation. In the context of metals, the conserved energy is elastic strain energy and the energy dissipation is irreversible (plastic) deformation. A recent theory of deformation and fracture [5] indicates that plastic deformation of metals can be described as loss modulus. We have a series of studies on fatigue of metal specimens using electronic speckle-pattern interferometry [6, 7]. In these studies, we apply a cyclic load to metal plate specimens of the same material and dimension at various levels, and observe their response to a low level tensile load. Here we apply the tensile load at a constant pulling rate and observe the resultant in-plane strain at each time s
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