Materials Science Forum Vols. 587-588

Abstract: Incorporating phase change materials (PCM) in plasters is an effective solution both for more efficient use of energy and its consumption reduction, because it allows the use of free energy in the environment, by latent heat storage. As PCM has not binder properties, it is expected to decrease mechanical properties of the final mortar. Also the density of the mortar decreases due to PCM lighter specific weight. In order to be used as a final coat for wall plastering, it must verify the requirements of the respective European Standard. This paper presents the results of the composition development and mechanical properties (flexural, compressive and adhesive strengths) testing and validation of three different gypsum mortars, in order to select the appropriate one for thermal testing in test cells. The results show that the effective decrease in the studied characteristics of the mortar does not inhibit the use for plastering, as requirements are achieved by the three studied compositions.

Abstract: FeTi intermetallic powders are very promising media for reversible hydrogen storage. However, difficult activation treatments including annealing at elevated temperatures in high pressure H2 gas atmosphere are mandatory. In the present work nanostructured FeTi powders were produced and activated in situ at room temperature using mechanical alloying/milling (MA/MM) of pure metallic constituents, Fe and Ti, added with sodium borohydride. The resultant powders, FeTiHx, already H2 pre-charged, absorbed a significant amount of H2 but require optimization for reversible absorption/desorption. This system has one of the highest volumetric storage capacities and can be produced at low cost. Several parameters of the as-milled powders were controlled. The phase constitution of the reaction products was characterized by X-ray diffraction and scanning electron microscopy and the absorption isotherms of the activated powders were determined.

Abstract: Electrochemical impedance experiments were carried out in order to study the influence of the ZrO2 inorganic incorporation on the proton conductivity of sulfonated poly(ether ether ketone) (sPEEK) membranes. The impedance data was fitted to an extension of Randles’ circuit, within the inorganic content and temperature ranges considered. The model fits quite well for ZrO2 loads up to 10 wt.%. Such a model allows for characterizing the diffusion phenomena (Warburg) of the membrane electrode assembly (MEA), membrane and electrodes resistances, capacitive and inductive behavior. Proton conductivity was obtained from the impedance spectra and it was observed that it increases with temperature and decreases with the inorganic content. As a general trend, the Warburg parameter decreases slightly with the temperature, except for the 5 wt. % ZrO2 membrane that suffers a more pronounced influence. The Warburg parameter also decreases with the ZrO2 content.

Abstract: The production of La0.95Sr0.05Ga0.90Mg0.10O3-δ powders was achieved at room temperature by a mechanosynthesis route in a high energy planetary ball mill starting from a mixture of lanthanum, strontium, gallium and magnesium oxides. The milling was carried out in nylon containers, using zirconia balls and a balls:powder mass ratio of 10:1. The planetary rotation was kept constant at 650 rotations per minute (rpm), and the container at 1300 rpm, in the opposite direction. The formation of the perovskite phase was detected from the early milling stages and nearly completed after milling for 360 min, as shown by powder X-ray diffraction. Transmission electron microscopy results revealed that powders consist of agglomerates of homogeneous, crystalline particles with an average equivalent diameter of about 16-17 nm, in excellent agreement with average crystallite size estimates obtained from X-ray diffraction.

Abstract: The aim of the present paper is to investigate the metallurgical behaviour of AISI P20 tool steel frequently used in injection moulds, after welding. For this purpose Tig and Laser welding tests were conducted to determine the influence of the welding parameters, materials and preheating. The welding quality was assessed through the microstructural properties and microhardness evolution. The results achieved with the different processes were compared. This work allows to obtain a deeper knowledge concerning tool steels behaviour after welding.

Abstract: This work presents results of tensile testing of H400 stainless steel, DP600 and TRIP600 at different strain rates. Mechanical properties were determined from tensile test using flat sheet specimens and recurring to different test techniques: servo-hydraulic machine and a tensile-loading Hopkinson bar. The test results were used to compare different mechanical properties of the tested steels and to validate constitutive equations intended to provide a mathematical description of strain rate dependence, namely the Cowper-Symonds equation. Following previous research work in dynamic material proprieties of multiphase and stainless steel grades, the energy absorption in quasi-static crushing of thin walled section made of the tested materials was subsequently investigated. Crush tests were performed in top-hat and hexagonal section tubes manufactured using laser welding. The experimental results were compared in order to assess the efficiency of the different steel grades for energy absorption.

Abstract: Some exhaust systems of naval gas turbines have been periodically repaired due to thermal-fatigue crack propagation after entering into service. Those structures were made of austenitic stainless steel grade AISI 316L in thin wall plates, which were bent in rolling machines and welded with longitudinal and circumferential joints by means of shielded metal arc, TIG or MIG/MAG welding processes. The plate thickness is about 3.7 mm and the temperature on the exhaust system is approximately 500°C and 350°C in the critical zones, which are located in the lower and intermediate regions of the exhaust system.Several cracks were detected at the critical regions, near the weld toe of butt and T-welded joints. The stress concentration factors induced by the weld angle, toe radius and rolled surface finishing diminishes the fatigue life strength. Some cracked material samples were taken out from the exhaust system structure and were analysed with a Scanning Electron Microscope (SEM/EDS), in order to determine the failure mechanisms involved in the crack propagation process. Those results are presented in the paper. Several high temperature fatigue and creep tests were performed with CT specimens. The mechanisms of crack propagation on the CT specimens were studied by SEM and compared with the fracture surfaces obtained from the samples taken out from the structure. The carbide precipitation on the grain boundaries was also studied.

Abstract: The aim of present work is to study the influence of the plate’s size on low velocity impact on carbon-fibre-reinforced epoxy laminates. Experimental tests were performed on [04,904]s laminates, using a drop weight-testing machine. Circular, square and rectangular plates were tested under low velocity impacts using a hemispherical impactor with 20 mm diameter and 3 J impact energies. The impacted plates were inspected by X-radiography. Numerical simulations were also performed considering interface finite elements compatible with three-dimensional solid elements, which allows to model delamination onset and growth between layers. The results showed that the plate’s size has influence on the delaminated area. Good agreement between experimental and numerical analysis for shape, orientation and size of the delaminations was obtained.

Abstract: Safety is nowadays an increasingly important issue for automotive manufacturers. Plastic components, asides from its aesthetic function in the car interior, are required to act as passive safety components. In this work, the impact of an anthropomorphic mass with a given mass and velocity in a plastic pillar cover is simulated by a finite element code (ABAQUS/Explicit). The pillar is modelled as a solid discretised by 3D solid elements. The material’s properties (polypropylene) used in the pillar are obtained at high strain-rates and described by an elasto-plastic model, being adopted a maximum allowable strain failure criterion. The contact between the mass, the plastic component and the steel chassis are considered. A complete deceleration-time plot is obtained, being calculated the values of the Head Injury Criteria (HIC(d)) and maximum deceleration. The deformed geometry, resultant stress distribution and damaged zones are also predicted. The properties of the material (elastic modulus, yield stress and allowable strain level) are optimised, making extensive use of numerical simulations and a design of experiments approach, in order to meet the envisaged standards requirements and thus mitigating occupant injuries.

Abstract: The mechanical behaviour of homogeneous and inhomogeneous FSW aluminium tailored blanks is analysed in this paper. The heterogeneity in mechanical properties across the different weld zones is discussed based on hardness testing results. Tensile and formability test results are also shown and the mechanical behaviour of the welds is discussed in relation to the base materials. Despite the hardness tests have indicated very small differences in hardness, between the welds and the base materials, and the tensile test results also showed similarities in mechanical behaviour, the formability tests revealed additional difficulties in forming the welded sheets.