Browsing by Author "Costa, J. D. M."
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- Cyclic creep response of adhesively bonded steel lap jointsPublication . Reis, P. N. B.; Pereira, António; Ferreira, J. A. M.; Costa, J. D. M.The viscoelastic nature of polymeric adhesives means that the effect of fatigue frequency has to be treated cautiously. However, this subject has received limited attention and very few studies can be found. Therefore, this work aims at investigating the cyclic creep response of adhesively bonded steel lap joints. Load-controlled fatigue tests were performed with shear stresses of 9.1, 7.4, and 6.3 MPa, which are typically low cycle fatigue stresses. Only during the last 20%of fatigue life canwe observe an increase in the cycle hysteresis area due to the decrease of the shear stiffness caused by the failure mechanisms. Under fatigue load, the maximum/ minimum strain curves exhibit a shape being similar to that of the steady creep curves, inwhich occurs a second stage with nearly onstant strain rate, independently of the number of cycles and increasing with the load range. A linear relationship between the log cyclic creep rate and the log of the number of cycles to failure was observed, indicating that fatigue behaviour is strictly related to cyclic creep.
- Effect of process parameters on the strength of resistance spot welds in 6082-T6 aluminium alloyPublication . Pereira, A. M.; Ferreira, J. M.; Loureiro, A.; Costa, J. D. M.; Bártolo, P. J.In this study the microstructural and mechanical behaviour of resistance spot welds (RSW) done on aluminium alloy 6082-T6 sheets, welded at different welding parameters, is examined. Microstructural examinations and hardness evaluations were carried out in order to determine the influence of welding parameters on the quality of the welds. The welded joints were subjected to static tensile-shear tests in order to determine their strength and failure mode. The increase in weld current and duration increased the nugget size and the weld strength. Beyond a critical nugget diameter the failure mode changed from interfacial to pullout. Taking into consideration the sheet thickness and the mechanical properties of the weld, a simple model is proposed to predict the critical nugget diameter required to produce pull-out failure mode in undermatched welds in heat-treatable aluminium alloys.
- Mechanical behaviour of PVC/CaCO3 Particulate Composites – Influence of TemperaturePublication . Costa, J. D. M.; Capela, C.; Ferreira, J. A. M.This paper is concerned with the study of temperature influence on Young’s modulus, ultimate strength and fracture toughness properties of PVC/CaCO3 particulate composites with different volume fractions. The tests were performed in three- and four-point bending. The resonant technique was also used to analyse the influence of both volume fraction and temperature on Young’s modulus. Significant decrease of ultimate strength, fracture toughness and Young’s modulus was observed with the increase of the temperature. Ultimate strength decreases with the increase of particle volume fraction at room temperature. For the other temperatures, this decreasing trend is less clear. PVC/CaCO3 flexural Young’s modulus calculated for a much lower loading segment increases with volume fraction. The same trend was obtained using the resonant technique. However, as the loading segment used to calculate the Young’s modulus was increased a significant decrease of Young’s modulus was obtained as a result of a progressive debonding at the particle-matrix interface. A 2D simplified FE simulation also confirms such trend. The dependence of Young’s modulus relatively to the loading segment increases as the volume fraction is increased, leading to composite Young’s modulus below matrix value for higher volume fractions and higher loading segments. Fracture toughness decreases with volume fraction.