Ricardo Alexandre Amar de Aguiar

Shape Memory Alloy Helical Springs Performance: Modeling and Experimental Analysis

Shape memory alloys (SMAs) are metallic materials that have the capability to recover its original shape eliminating residual strains when subjected to adequate thermal process. This behavior is related to phase transformation induced either by stress or by. During the phase transformation process of an SMA component, large loads and/or displacements can be generated in a relatively short period of time making this component an interesting mechanical actuator. Because of such remarkable properties, SMAs have found a number of applications in different areas. The present contribution deals with the modeling, simulation and experimental analysis of SMA helical springs. Basically, it is assumed a one-dimensional constitutive model to describe its thermomechanical shear behavior and, afterwards, helical springs are modeled by considering classical approach. SMA helical spring thermomechanical behavior is investigated through experimental tests performed at different loads. Numerical results show that the model is in close agreement with experimental data. Since the thermal process has an essential importance in the performance of an SMA actuator, different cooling medium conditions are investigated, evaluating the actuators performance.

Nonlinear geometric influence on the mechanical behavior of shape memory alloy helical springs

This paper investigates the nonlinear geometric effect on the mechanical behavior of shape memory alloy (SMA) helical springs. First, the SMA wires are characterized, and then the design and fabrication of SMA helical springs are discussed. Experimental tensile tests are carried out to show the nonlinear geometric influence. Results show a coupling between constitutive and geometric nonlinearities that defines the spring stiffness. Two springs with different geometries are built from SMA wires to define springs with both weak and strong nonlinear geometric influence. Numerical analyses are developed, using the finite element method to confirm the general conclusions shown in our experimental observations.

Influence of Welding Parameters in Substrate/Coating of Galvanized Sheets Using Resistance Spot Welding

The main objective of this work is to propose an optimization of welding parameters through the process of resistance spot welding, applied to Interstitial Free (IF) steel sheets in order to increase the life of conventional copper electrodes. The methodology consisted of using a weld tester in order to ensure the accuracy of values. The main parameters selected through a survey of the weldability were: force, time and welding current. To evaluate the resistance of the weld and the electrode wear, mechanical tests were performed like tensile-shear, cross-tension, peel test and microhardness. A microstructural evaluation by optical microscopy (OM) and scanning electron microscopy (SEM) was made. The results show reducing the thickness of the coating in the weld region and macro and microstructures were observed. Through the tensile-shear and cross-tension tests were verified using electrodes caps up to 1250 points with acceptable welds. The microhardness test results indicated significant hardness increase in weld nugget elucidated by differences in microconstituents evaluated by OM and SEM. The methodology used for parameter selection has highlighted an optimal combination with 200kgf electrodes force, 9cy welding time and 7.8 kA welding current, in overlap IF steel sheets with 0.75mm of thickness, thereby increasing the lifetime of the electrode and ensures a better quality of welds and the consequent reduction of energy applied to the weld process.