Effects of ageing on mechanical durability of round clinched steel/aluminium joints

The clinching is one of the most common metal joining processes in the manufacturing of metal plate based products (similar or dissimilar, pre-coated or galvanized), especially when the assembly without adding major joining elements is required. When the clinched joints work in an aggressive environment, particular attention would be placed on the electrochemical stability and corrosion resistance of the metal constituents (Mizukoshi and Okada 1997). In joining design, an appropriate material selection reduces the electrochemical potential differences and prevents significant galvanic currents (Kruger and Mandel 2011; Calabrese et al. 2014). The durability of the metal joints could be heavily influenced in a corrosive environment, whereas the less noble material will tend to increase its corrosion rate; instead the more noble one will reduce its electrochemical dissolution (He et al. 2008; Bardal 2004). Accelerated ageing tests (i.e. salt fog test) were carried out to evaluate the durability of the joints in highly aggressive environments (Calabrese et al. 2013; LeBozec et al. 2012). Although the durability for a long time of the clinched joint in a corrosion environment is a known problem, few works focus the attention on the relationship between durability of joints and electrochemical behaviour of the metal constituents. The aim of the present work is to evaluate the durability at long ageing time in salt spray test (according to ASTM B117) of carbon steel/aluminium alloy joints, obtained by clinching. The investigation has been conducted on one total thickness (2.5 mm) of unsymmetrical joints (i.e. thickness sheets of 1.5 mm and 1 mm) to inquire about the effect of corrosion on the two different unsymmetrical configurations (St1.5/Al1 and St1/Al.5). The joint resistance has been determined, by means of shear tests of single-lap joints in according to ISO/CD 12996. The samples were exposed to critical environmental conditions following the ASTM B 117 standard. To inquire the damage evolution of the samples, 0, 1, 2,3, 5, 7, 10 and 15 weeks of ageing time have been chosen. Seven samples for each combination and for each ageing time were realized. A Design of Experiment has been performed, followed by the ANOVA of the results to analyse the influence of the two factors, thickness combinations and ageing time, on the mechanical properties of the joints. The two sets of joints show a different behaviour at increasing ageing time: the St1.5/Al1 batch shows a constant decay of the load values, instead the St1/Al1.5 set maintains acceptable values of resistance for several weeks of ageing, at tenth week the mechanical stability is strongly impaired. In the latter case the presence of the thin oxide layer at the overlapping interface, which behaves as an adhesive interlayer, and the larger thickness of the aluminium plate improve the resistance of the St1/Al1.5 joints. Statistical analysis confirms that the two thickness combinations and ageing time are the significant factors. At zero weeks, neglecting the effect of ageing, the maximum load values of all samples belong to the same population. This means that the resistance of the clinched joints is the same regardless the combination of thicknesses, but by considering both the ageing and thickness, the analysis of variance shows that both thickness and weeks are significant parameters distinguishing two different populations in the distribution of loads. The experimental results evidenced that the corrosion degradation phenomena influence significantly both the performance and the failure of the joints. This is also confirmed by statistical analysis according to which the two thickness combinations and ageing time are the significant factors

Macro-porous permeability aspects of MgSO4 salt hydrate foams for energy storage applications

In the present work a macroporous silicone foam, able to contain the magnesium sulfate, was chosen as matrix for the reversible hydration/dehydration process of the salt hydrate. The aim of the article was addressed towards the assessment of the relationship among microstructure, permeability and mass diffusion of the composite foam. This aspect represents an essential step for the future industrial development of this composite material. The results show that the filler content influences the foam morphology where a transition from closed to mixed and then closed cell again was observed with increasing filler content. Consequently, depending on the distribution and interconnection of the structural channels, a different effectiveness in guaranteeing mass diffusion phenomena was identified. In particular, permeability tests show that foams with 50 wt% of salt hydrates have a highly interconnected microstructure allowing a permeability over three times higher than a closed cell structure making it suitable for thermochemical energy storage applications.This work was partially funded by the Ministerio de Ciencia, Innovaci on y Universidades de España (MCIU/AEI/ FEDER, UE) (RTI2018-093849-B-C31) and by the Ministerio de Ciencia, Innovaci on y Universidades - Agencia Estatal de Investigaci on (AEI) (RED2018-102431-T). Dr. Luisa F. Cabeza would like to thank the Catalan Government for the quality accreditation given to her research group (2017 SGR 1537). GREiA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia program. Furthermore, special thanks to Dr. Angela Caprì who made the composite foam samples used for this experimental campaign. Open Access Funding provided by Universita degl

Morphological and Structural Evaluation of Hydration/Dehydration Stages of MgSO4 Filled Composite Silicone Foam for Thermal Energy Storage Applications

Salt hydrates, such as MgSO4·7H2O, are considered attractive materials for thermal energy storage, thanks to their high theoretical storage density. However, pure salt hydrates present some challenges in real application due to agglomeration, corrosion and swelling problems during hydration/dehydration cycles. In order to overcome these limitations, a composite material based on silicone vapor-permeable foam filled with the salt hydrate is here presented. For its characterization, a real-time in situ environmental scanning electron microscopy (ESEM) investigation was carried out in controlled temperature and humidity conditions. The specific set-up was proposed as an innovative method in order to evaluate the morphological evolution of the composite material during the hydrating and dehydrating stages of the salt. The results evidenced an effective micro-thermal stability of the material. Furthermore, dehydration thermogravimetric/differential scanning calorimetric (TG/DSC) analysis confirmed the improved reactivity of the realized composite foam compared to pure MgSO4·7H2O.This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31). This work was partially supported by ICREA under the ICREA Academia program

A soft robot structure with limbless resonant, stick and slip locomotion

We present a smart robot structure that exploits anisotropic friction to achieve stick-slip locomotion. The robot is made out of three components: a plastic beam, a planar dielectric elastomer actuator and four bristle pads with asymmetric rigid metallic bristles. We show that when the robot is electronically activated at increasing frequency, its structure exploits the resonance condition to reach the maximum locomotion speed. The fundamental frequency of the structure is estimated both analytically and numerically, allowing the range of frequencies in which the top locomotion speed was observed during the experiments to be identified. The locomotion speed of the robot as a function of the actuation frequency is estimated with a frequency response analysis performed on a discretised model of the structure, revealing good agreement with the experimental evidence