Mechanical behavior cyclic deformations

Polyurethanes based on the HDI cyclic trimer show a rubberlike mechanical behavior. In fact, the Z1030/1072 and Z1031-H films show a low elastic modulus E, no yielding, and an ultimate, widely reversible, deformation beyond 100-150%, likely underestimated owing to the difficulty in assessing the ultimate properties of self-supported thin films. 

In this example, the mechanical and electrochemical coupling effects between the a-and y-phase have been shown to be the key for understanding the cyclic deformation behavior of the duplex alloy under various electrochemical conditions. 

In order to determine the viscoelastic properties of silicone gel materials, vibrational measurement techniques are used. The principle of vibrational measurement techniques involves determining die viscoelastic quantities by observing the responding mechanical behavior when cyclic deformation is applied repeatedly. Dynamic viscoelastic property measurements are performed with a dynamic mechanical spectrometer such as the RDA-II from Rheometrics. This instrument measures the complex modulus G and loss factor tan S from the torque measured by the transducer equipped at the upper portion of the sample chamber when shear deformation is applied to the sample at a specified shear strain and frequency from the actuator at the bottom of the sample chamber. 

One of the most common methods utilized to characterize the phase behavior of polymer blends employs low amplitude cyclic deformation studies to obtain the elastic and viscoelastic properties. This method, termed dynamic mechanical characterization, yields high resolution of polymer transitions including secondary relaxation processes, crystalline melting transitions and of primary importance, the glass transition. This method maps the data over a broad temperature range to ascertain the phase behavior. 

Landgraf, R.W. Cyclic Deformation and Fracture of Hardened Steels. In International Conference on Mechanical Behavior of Materials, Kyoto, Japan (1972)... 

FEA material models are typically based on this tensile data but only one curve is allowed. The first cycle data is termed non-cyclic data since it does not take the cyclic softening into account. The fifth cycle data is termed the cyclic data as it represents the conditioned material in its equilibrium state. Clearly the first and the fifth cycle data exhibit different mechanical behavior and a single material model would not capture both responses. This cyclic material testing is repeated for compression and shear in order to have a complete database based on cyclic testing in all deformation modes. 

It is noted, however, that both of the above CaF2 and Ti02 nanoceramics had some amount of porosity. This may account for an apparent soft behavior related to the superplastic deformation at low temperature, which does not yet reveal the plastic deformation characteristics in nanoceramics. Localized superplastic deformation under cyclic tensile fatigue tests was observed by Yan et al. on 3Y-TZP nanoceramics at room temperature [25], The micromechanism behind this phenomenon is argued to be essentially governed by grain-boundary diffusion. The contribution of dislocation slip might be in operation as a parallel mechanism to develop slip band-like microfeatures. 

The case of mechanical vibration is less well defined in terms of the predominance of creep versus time-independent (plastic) deformation. Although the applied stresses are often quite low, the strain rates can be sufficiently high, so that a mixture of creep and plastic deformation modes define the fatigue response of the material. This sensitivity of fatigue behavior to cyclic loading frequency has been widely studied for tin-lead (Sn-Pb) solders and is recognized as an important variable in the fatigue response of lead-free alloys (Ref 1-3). 

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