Energy harvesting from earthquake for vibration-powered wireless sensors

Wireless sensor networks can facilitate the acquisition of useful data for the assessment and retrofitting of existing structures and infrastructures. In this perspective, recent studies have presented numerical and experimental results about self-powered wireless nodes for structural monitoring applications in the event of earthquake, wherein the energy is scavenged from seismic accelerations. A general computational approach for the analysis and design of energy harvesters under seismic loading, however, has not yet been presented. Therefore, this paper proposes a rational method that relies on the random vibrations theory for the electromechanical analysis of piezoelectric energy harvesters under seismic ground motion. In doing so, the ground acceleration is simulated by means of the Clough-Penzien filter. The considered piezoelectric harvester is a cantilever bimorph modeled as Euler-Bernoulli beam with concentrated mass at the free-end, and its global behavior is approximated by the dynamic response of the fundamental vibration mode only (which is tuned with the dominant frequency of the site soil). Once the Lyapunov equation of the coupled electromechanical problem has been formulated, mean and standard deviation of the generated electric energy are calculated. Numerical results for a cantilever bimorph which piezoelectric layers made of electrospun PVDF nanofibers are discussed in order to understand issues and perspectives about the use of wireless sensor nodes powered by earthquakes. A smart monitoring strategy for the experimental assessment of structures in areas struck by seismic events is finally illustrated

The strengths and failures of incentive mechanisms in notional defined contribution pension systems

Public pension systems based on the Notional Defined Contribution (NDC) principle were introduced during the ‘90s in Italy, Sweden and Poland, among other countries. They mimic private savings, in that individuals get back, as pensioners, what they contributed to social security during working life, plus returns. As such, NDC systems should realize actuarial equity and incentive neutrality. However, when one considers the presence of NDC pensions together with minimum and social assistance pensions, this is no longer true. Indeed, in all the three countries considered, the NDC system shows a regressive feature, which disincentivizes contributions, particularly from low earners, who would be better off entering, or staying in, the shadow economy. In order to reduce the extent of this phenomenon, we examine the effects of introducing, or increasing, the possibility of accumulation of social assistance and NDC pensions, which would also improve pension adequacy. A complete accumulation of the two would solve the incentive problem, but would be costly and would require a structural reform of the pension system financing mechanism, altering the current balance between social contributions and general fiscal revenues. We show the effects of a change in the cumulation rules for social assistance and NDC pensions in Italy using CAPP_DYN, a population-based dynamic microsimulation model, which allows assessment of the evolution of the pension system in the coming decades and the distributional implications of such reform.

Optimization of force-limiting seismic devices connecting structural subsystems

This paper is focused on the optimum design of an original force-limiting floor anchorage system for the seismic protection of reinforced concrete (RC) dual wall-frame buildings. This protection strategy is based on the interposition of elasto-plastic links between two structural subsystems, namely the lateral force resisting system (LFRS) and the gravity load resisting system (GLRS). The most efficient configuration accounting for the optimal position and mechanical characteristics of the nonlinear devices is obtained numerically by means of a modified constrained differential evolution algorithm. A 12-storey prototype RC dual wall-frame building is considered to demonstrate the effectiveness of the seismic protection strategy

Transfusion thresholds and beyond

Comment on Liberal transfusion strategy improves survival in perioperative but not in critically ill patients. A meta-analysis of randomised trials. [Br J Anaesth. 2015

Chloride Penetration in Circular Concrete Columns

Most of the diffusion models of chloride ions in reinforced concrete (RC) elements proposed in literature are related to an isotropic homogeneous semi-infinite medium. This assumption reduces the mathematical complexity, but it is correct only for plane RC elements. This work proposes a comparison between the diffusion model of chloride ions in RC circular columns and in RC slab elements. The durability of RC cylindric elements estimated with the circular model instead of the plane model is shown to be shorter. Finally, a guideline is formulated to properly use the standard and more simple plane model instead of the circular one to estimate the time to corrosion initiation of cylindrical RC elements

Strain rate, temperature and deformation state effect on Ecoflex 00-50 silicone mechanical behaviour

Silicone elastomers are extremely attractive materials due to their wide range of possible applications, from biomedical engineering to soft robotics. In this work, an extensive thermo-mechanical characterization of Ecoflex Shore hardness 00–50, a commercially available silicone elastomer, has been carried out to compensate for the lack of relevant literature. The mechanical behaviour of the material has been characterized by performing monotonic and cyclic loading tests. These tests were performed in different deformation states, i.e. uniaxial tension, pure shear and biaxial tension, at different strain rates and temperatures. Experimental findings allowed to highlight the material time-dependent response and quantify the contribution of dissipative deformation phenomena to the overall strain energy. Uniaxial tensile tests performed at different temperatures (between −40 °C and 140 °C) showed that the material mechanical behaviour is sensitive to temperature in this range: a decrease of the ultimate stress and strain has been observed with increasing temperature. Finally, the data obtained from the latter tests have been used to define a failure envelope, applied for the first time to Ecoflex silicones, and valuable to describe the material ultimate stress and strain at any temperature and strain rate

Influence of curing thermal history on cross-linking degree of a polydimethylsiloxane: Swelling and mechanical analyses

In this work, the change of the elastic properties induced by a change in cross-linking conditions of polydimethylsiloxane is investigated by measuring its shear modulus by dynamic mechanical analysis and correlating it to that predicted from swelling measurements. Polymer cross-linking is performed at different curing temperatures reached with ramps at different heating rates. From both mechanical and swelling measurements, the molecular weight between cross-links, MC, is determined, and its dependency on the applied thermal history is analyzed. The main results are: (i) the elastic modulus of the cured material is not significantly affected by the heating rate adopted, while (ii) the curing temperature has a significant influence on the polydimethylsiloxane mechanical properties. In addition, (iii) MC evaluation from swelling measurements is in good agreement with that estimated from mechanical measurements when appropriate theories are considered. This last result suggests that swelling experiments can be considered as a reliable tool to predict the elastic modulus of the polydimethylsiloxane studied. The quantitative information reported in this paper, also obtainable by the suggested method if other thermal curing histories are applied, is extremely useful for the proper design of devices based on polydimethylsiloxane

Exploratory habitation vehicles with trim intrinsic control

Considering the space environment and its critical issues and consequent risks, the challenge is to define the way and tools with which future astronauts will be able to act, live and work in space and, in particular on the Moon and Mars, exploiting, at the state of art, knowledge of innovative science, engineering and technology. On the Moon and Mars, the most obvious environmental factors are extreme temperature fluctuations, low gravity and the virtual absence of atmosphere and magnetosphere. The health of a human body can be damaged by reduced values of gravity. Due to the reduced gravity on the Moon and Mars, human bones and muscles are unloaded and begin to weaken. It increases the risk of bone fractures and atrophied muscles for astronauts returning to Earth from prolonged missions. The magnetosphere and atmosphere on Earth shield from much of the dangerous solar and cosmic radiation. Radiation with extremely high energies can damage even living tissue. The surface of the Moon and Mars has been crushed by millions of impacts of celestial bodies such as asteroids, leaving a layer of regolith that could be very deep depending on the areas of the planets. The habitation module, described in this paper, is carried by a vehicle equipped with two pairs of compass shaped legs that act as supports for the habitation module capable of maintaining a certain controlled height with respect to the ground as well as a horizontal attitude, during the movement of the compass. A system of ropes wound on pulleys allows to control the height of the habitat with respect to the ground, control the structure in movement, descent and ascent. The habitat can also be lowered to the ground. The geometry of the shape of the pulleys, around which the ropes are wound, is determined in such a way that the habitation module remains at a certain height during the movement defined by the two compass-shaped advancement supports. The paper describes and analyzes the movement of the pulleys during the entire phase of the movement of the habitation module and their geometric shape is discussed

Optimal preliminary design of variable section beams criterion

The present paper discusses about optimal shape solution for a non-prismatic planar beam. The proposed model is based on the standard Timoshenko kinematics hypothesis (i.e., planar cross-section remains planar in consequence of a deformation, but it is able to rotate with respect to the beam center-line). The analytical solution for this type of beam is thus used to obtain deformations and stresses of the beam, under different constraints, when load is assumed as the sum of a generic external variable vertical one and the self-weight. The solution is obtained by numerical integration of the beam equation and constraints are posed both on deflection and maximum stress under the hypothesis of an ideal material. The section variability is, thus, described assuming a rectangular cross section with constant base and variable height which can be described in general with a trigonometric series. Other types of empty functions could also be analyzed in order to find the best strategy to get the optimal solution. Optimization is thus performed by minimizing the beam volume considering the effects of non-prismatic geometry on the beam behavior. Finally, several analytical and numerical solutions are compared with results existing in literature, evaluating the solutions’ sensibility to some key parameters like beam span, material density, maximum allowable stress and load distribution. In conclusion, the study finds a critical threshold in terms of emptying function beyond which it is not possible to neglect the arch effect and the curvature of the actual axis for every different case study described in this work. In order to achieve this goal, the relevance of beam span, emptying function level and maximum allowable stress are investigated