Materials with negative Poisson's ratio (auxetic) exhibit an unusual property of expanding when stretched in the direction normal to an applied uniaxial tension and vice versa when compressed, a phenomenon which is known to result in many beneficial effects in the performance of materials in various practical situations. In particular, it has long been suggested that spherical shells made from isotropic materials having Poisson's ratio less than −0.5 exhibit enhanced resistance to buckling as a result of externally applied normal pressure. This work uses finite elements (FEs) modelling to assess the behaviour of various spherical, torispherical and ellipsoidal shells when they are subjected to external (in all cases) or internal (in the case of torispherical and ellipsoidal shells) pressures. We find that to a first approximation, the critical buckling pressures scale linearly with (1 − ν2)−1/2 thus suggesting that the critical buckling pressures tends to infinity as ν tends to −1, this being in accordance to what was known for spherical shells. We also find that the Poisson's ratio has an effect on the amplitude and the number of buckling wavelengths that occur when the shells buckle.
This paper considers the learning and assessment process of a mechanical engineering undergraduate student and applies it to designing a set of laboratory activities in the field of mechanics of materials. An informal survey was carried out among third-year mechanical engineering students on a four-year bachelor course at the University of Malta in order to find out about their preferred learning style. Thirty-one students were surveyed, which represented 12.5% of students following the BEng (Hons) course in mechanical engineering. The survey indicated that 59.7% of engineering students prefer learning through ‘feeling or concrete experience’, 9.7% prefer learning through ‘watching or reflective observation’, 12.9% prefer learning through ‘thinking or abstract conceptualisation’ and 17.7% prefer learning through ‘doing or active experimentation’. The laboratory activities were designed in such a way as to entice students to use hands-on learning to complement the theory explained during lectures. The four-stage Kolb learning cycle was used as a model on which to design the set of laboratory activities. An example of a topic in mechanics of materials is used in this study to assess the students' response in terms of Kolb's proposal for effective learning. The topic selected (combined bending and torsion) is part of the mechanical engineering degree curriculum.
This study presents an experimental testing regime conducted on filament wound composite pressure vessels (CPVs) made up of an asymmetric and unbalanced layup (chopped strand mat (CSM)/−82.7 deg/±54.3 deg) and subject to an internal pressure. Polyester reinforced with e-glass CSM and direct roving was used. The mechanical properties of the different lamina used in the test specimens were identified through a series of standardized tests. The evolution of strain and volume changes with respect to the applied pressure loading were recorded. The results also present a characterization of identifiable first-ply failure (FPF) loads based on strain evolution and volume changes, and monitors when ultimate failure occurs.
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