Shape memory alloys (SMAs) present a rate-dependent behavior, which means that the thermomechanical response depends on the loading rate. Therefore, although martensitic transformation can be considered as a non-diffusive process, the phase transformation critical stresses are temperature dependent and, since heat transfer process is time dependent, it affects the thermomechanical behavior of SMAs. This article deals with the rate dependence of SMAs, proposing a 1D constitutive model to describe this effect. The proposed model is formulated within the framework of continuum mechanics and thermomechanical coupling terms of the energy equation are incorporated in the formulation in order to describe the rate-dependent behavior. Numerical simulations are carried out comparing results with experimental data available in literature for different loading rates and environmental media, presenting a close agreement. Afterwards, numerical tests are performed in order to evaluate the model capabilities showing that it is capable to capture the general thermomechanical behavior of SMAs.

Piezoelectric vibration-based energy harvesting systems have been used as an interesting alternative power source for actuators and portable devices. These systems have an inherent disadvantage when operating in linear conditions, presenting a maximum power output by matching their resonance frequencies with the ambient source frequencies. Based on that, there is a significant reduction of the output power due to small frequency deviations, resulting in a narrowband harvester system. Nonlinearities have been shown to play an important role in enhancing the harvesting capacity. This work deals with the use of nonsmooth nonlinearities to obtain a broadband harvesting system. A numerical investigation is undertaken considering a single-degree-of-freedom model with a mechanical end-stop. The results show that impacts can strongly modify the system dynamics, resulting in an increased broadband output power harvesting performance and introducing nonlinear effects as dynamical jumps. Nonsmoothness can increase the bandwidth of the harvesting system but, on the other hand, limits the energy capacity due to displacement constraints. A parametric analysis is carried out monitoring the energy capacity, and two main end-stop characteristics are explored: end-stop stiffness and gap. Dynamical analysis using proper nonlinear tools such as Poincaré maps, bifurcation diagrams, and phase spaces is performed together with the analysis of the device output power and efficiency. This offers a deep comprehension of the energy harvesting system, evaluating different possibilities related to complex behaviors such as dynamical jumps, bifurcations, and chaos.

Vibration-based energy harvesting with piezoelectric elements has an increasing importance nowadays being related to numerous potential applications. A wide range of nonlinear effects is observed in energy harvesting devices and the analysis of the power generated suggests that they have considerable influence on the results. Linear constitutive models for piezoelectric materials can provide inconsistencies on the prediction of the power output of the energy harvester, mainly close to resonant conditions. This paper investigates the effect of the nonlinear behavior of the piezoelectric coupling. A one-degree of freedom mechanical system is coupled to an electrical circuit by a piezoelectric element and different coupling models are investigated. Experimental tests available in the literature are employed as a reference establishing the best matches of the models. Subsequently, numerical simulations are carried out showing different responses of the system indicating that nonlinear piezoelectric couplings can strongly modify the system dynamics.

Shape memory alloys are attractive engineering materials due to their potential application as actuators using the ability to memorize shapes through a thermomechanical loading. This article develops a numerical investigation of different shape memory alloy actuator configurations considering bias and antagonistic arrangements. Numerical simulations are carried out using the finite element method together with a constitutive model for shape memory alloys. Parametric analysis is carried out evaluating the performance of each actuator configuration based on stress and strain. Basically, four representative configurations of general actuators are treated: shape memory alloy wire, linear spring connected to a shape memory alloy wire, two elastic springs connected by a shape memory alloy wire, and two shape memory alloy wires connected by a spring.

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