Meccanica

Thermo-mechanical coupling is an intrinsic property of first order martensitic transformation. In this paper, we study the temperature evolution during phase transition at a wider strain rates from quasi static to impact loading to reveal the thermodynamic nature of the strain rate effect of phase transition materials. Based on the laws of thermodynamics and the principle of maximum dissipated energy, a thermal-mechanically coupled model was proposed. The model shows that, in the quasi static case, the temperature profile grades around the moving phase boundary, while for the dynamic case, thermal response of the specimen can be reached homogeneously due to random nucleation. The predicted results of the model are in good agreement with the experimental results, suggesting that the interaction between the self-heating effect and the temperature dependence of phase transition behavior plays a leading role in the process of the transformation deformation mechanism associated with the loading rate.

A mechanically driven centrifuge is considered with a baffle near the bottom cover which rotates at a slightly lower velocity than that of the centrifuhe. A feed gas is introduced into the centrifuge by means of a tube coaxial with it. The immission takes place at the middle of the centrifuge height. The effect of the feed flow is investigated. The partial differential equation system, which rules the flow field, has been linearized and the calculations have been carried out by means of a numerical code using the finite difference method. The field in general is obtained by linearly combining the three following flow fields: Several flow patterns in different conditions of operation are presented.

Mode solutions in rigid-plastic structures subjected to fixed external loads are dynamic solutions which are products of separate functions of space and time. If the small displacement assumptions are adopted, there will exist at least one mode shape for any structure subjected to given fixed external load, and possibly a multiplicity of modes. The time function is a simple linear function, and is easily determined once the mode shape is known. The paper puts forward, a simple in physical terms, algorithm for the determination of mode shapes. An iterative procedure is established in which each iteration is equivalent to the solution of a static limit analysis problem. Convergence is proved.

The demand of air bearings is increasing for those applications that require precision linear movements or high-speed rotations. In particular in this paper air pads for air motion technology are studied. The paper analyses the effect of a circumferential groove machined on the pad surface on pressure distribution, air flow consumption and stiffness. Two geometries are investigated and compared: one with three supply orifices and the other with a circumferential groove as well. The static characteristics of the pads are experimentally determined with also the pressure distributions under the pads along the radial and circumferential directions. The experimental pressure distributions are compared with the simulated ones, obtained with a numerical program at the purpose developed. The numerical model considers a general formulation of the supply holes discharge coefficient that can be used also in presence of a circumferential groove.

The paper is mainly motivated by the observation that, in creep tests, a secondary stage occurs where the strain rate is relatively small and roughly constant. The paper first examines the well-known models based on connections of springs and dashpots (Maxwell, Kelvin–Voigt, standard linear solid, Wiechert) and finds that they are far from showing the constant strain rate of the secondary stage. Next a simple model is considered where a Maxwell element is connected in parallel with a dashpot. The resulting stress–strain relation is established and the secondary stage is investigated thus showing that the nearly constant strain rate occurs. Moreover, the stress–strain relation is shown to be compatibile with the second law of thermodynamics the stress being the sum of two terms. One of them is purely dissipative and the other emerges from a Graffi–Volterra free energy potential.

We present the use of piezoelectric disk buzzers, usual in stringed musical instruments to acquire sound as a voltage signal, for experimental modal analysis. These transducers helped in extracting natural frequencies and mode shapes of an aluminium beam and a steel arch in the laboratory. The results are compared with theoretical predictions and experimental values obtained by accelerometers and a laser displacement transducer. High accuracy, small dimensions, low weight, easy usage, and low cost, make piezoelectric pickups an attractive tool for the experimental modal analysis of engineering structures.

The aim of this article is to study the consequences of the active stiffening of a compliant mechanism on the workspace created by the deformation of its structure. In connection with recent soft robotics research integrating shape-memory alloys (SMAs), the variation in stiffness over time is here obtained by the thermal activation of a nickel–titanium SMA spring. The workspace is created by the deformation (in the strength of materials sense) controlled by two rotary actuators acting on a structure comprising two angled flexible beams. In addition to a natural variation in the elasticity modulus of the SMA component during its thermal activation, its shape reconfiguration adds a structural deformation modifying the workspace. The existence of a common area between the workspaces of the mechanism corresponding to the non-activated and activated modes of the SMA is preserved. Several compliance maps are determined from measurements using a laser tracker targeting a given position of the loaded structure. The impact of SMA pre-stretch on stiffness variability is compared to that of a change in Young’s modulus. Variations in the stiffness distributions between the two modes reveal interesting properties (stiffness sign inversion, anisotropy) for the future optimal design of compliant mechanisms with high versatility, associating the spatial positions of the effector with variable stiffness values.