Design and construction of a torsion testing machine

A low cost torsion testing machine is developed. The machine is capable of determining the shear properties of engineering materials like Steel, Aluminum, Copper and similar materials. In ASTM test, the solid or hollow round cross sectional specimen is twisted in the test machine and simultaneously the torsion angle and performed torque is recorded. The torque-twist diagram is derived from these data and elementary mechanics theory is then used to construct the shear stress-strain diagram. The process of design and construction was in accordance with ASTM standard. An incremental rotary encoder used to measure angle of twist with accuracy of 0.1 degree. For measuring performed torque with accuracy of 0.15 Nm, four strain gages (3.175 mm) is used. A rotational plate designed to prevent the appearance of axial load through the test and to make possibility of movement of jaw in order to insertion the specimens. An electromotor and two worm gearboxes with total reduction ratio 400, used to perform torque and to have the rotational speed of specimen within the range of 0 to 3.5 rpm. In order to acquiring data from sensors, processing them, displaying in LCD, and sending to the computer with RS232 protocol a 89c52 microcontroller was used. To recording data in computer, drawing online stress-strain curve, Delphi programming language was used. By understanding of applied torque and torsion angle in elastic limit, the shear elasticity modulus of the specimen recorded in the computer easily.

Keywords:

shear modulus of elasticity , rotary encoder , strain gage , torsion angle , torsion testing