Construção e testes de máquina de ensaio de torção plástica para levantamento do comportamento plástico de metais

AUTOR(ES)
DATA DE PUBLICAÇÃO

2004

RESUMO

A computerized prototype machine for cold torsion testing has been developed, constructed and tested in the Laboratory of Metal Forming at UDESC (University the State the Santa Catarina) - Joinville. The equipment consists of a horizontal torsion machine, employing an electric motor, a wheel, a horizontal axle and a control and acquisition data system. The goal of this torsion test machine is to simulate the conditions of industrial processing of metals as cold upsetting and rolling. Experimental tests were carried out at a constant angular speed which imposed a constant shear strain rate to the specimen. Setting the rotation speed to 60 rpm, it was possible to simulate the initial operation stage of sheet metal rolling process which utilizes the strain rate of about 2/s, and by increasing it up to the strain rate of 10/s , the final sheet calibration stage could be simulated (in cold rolling). To attain good repetibility of results in the torsion tests, the angular velocity must be controlled carefully to provide a sound bases for understanding the plotted curves and its features. The torsion test has been carried out on materials such as annealed steel 1020, brass, pure copper and pure aluminium. The mechanical behavior of the metallic materials until fracture can be studied considering the rupture type (how for example is the features of ductile and brittle fracture). This test has been used to determine the mechanical properties of those metals after the plastic flow has taken place (as for example τesc, τmáx, τrup e n workhardening) that are obtained from the torque (M) versus twist angle (θ) curves. Thus, from measurements of the torque (M - Nm) and the angle of twist (θ - degree), it was possible to construct the shear stress (τ) versus shear strain (γ), equivalent stress ( σ) versus equivalent strain ( ε) curves and obtain the work-hardening equations. The obtained results from the cold torsion tests has shown that the machine of plastic torsion is adequate for laboratory simulation of material in cold rolling and forging studies and also for the determination of mechanical properties of metallic materials. The observed equivalent stress and equivalent strain curves have shown that the total elongation or strain to fracture εf has not shown a meaningful increase with the strain rate for 1020 steel performed in simple tensile (strain rate 1,7x10-3/s) and plastic torsion (strain rate 2/s) : 16,5% in tensile test and 13,5% in torsion test, see figure 5.9. However, in the case of brass (εf 35% in tensile and 15% in torsion, figure 5.10 ); for copper (18% in tensile and 47% in torsion, see figure 5.11) and for aluminium (10% in tensile and 53% in torsion, see figure 5.12), all material were annealed. That is, for the same strain rate conditions as steel, these materials have shown a relevant increase in elongation for torsion test, or these materials have larger plastic strain before arriving fracture, although they have been tested in torsion with a higher strain rate compared with the simple tensile tests. According to the torque (M) versus twist angle (θ) curves for annealed 1020 steel, annealed brass, annealed copper and annealed aluminium, it was observed that tubular specimens have ruptured almost immediately after arriving the maximum torque or the plastic instability point. That is, rupture occurred immediately after the instability point has been attained, suggesting that rupture mechanisms are associated to the mechanics of instability. However, there is a subtile difference of material behavior: copper and aluminium have presented an extra small strain after the point of maximum. In view of this, it can be asserted that in torsion test of metals there is also the occurrence of the instability phenomena and the plastic strain localization in the narrow rupture band. Although, local necking has not been noted as in simple tension test.

ASSUNTO(S)

plasticidade deformação ensaio de torção plástica cisalhamento metais metalurgia de transformacao

ACESSO AO ARTIGO

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