Abstract
A new oscillatory-type testing machine was developed for determining the mechanical properties of materials under alternating torsional stress. The damping capacity and dynamic modulus of rigidity of both plastics and metals were evaluated by the use of this machine and studies are reported of how these properties are affected by sustained cyclic stress below the endurance limit and also at impending fatigue failure. A similar dynamic testing machine was built for applying alternating direct stress, and parallel studies were made under axial loading conditions. The mechanical properties of selected materials in static tension, compression, and torsion were also determined to supplement the dynamic tests. The wide deviations observed between the static and dynamic moduli of elasticity for plastics are analyzed and are associated with the damping capacity of the material. The significance of these deviations, as related to the repeated constant-deflection type of fatigue test on plastics, is discussed. Experimental data on the damping capacities, dynamic moduli of elasticity, and some static mechanical properties are presented for mild steel, Duralumin, grade X laminated bakelite, laminated-canvas phenolic, and methyl-methacrylate plastic.
