An Experimental Study On Ratcheting Of Polycarbonate Polymer At Room Temperature

Polymer materials in recent years are widely used, especially in the field of electronics, civil structures, mechanical parts, and aerospace components and so on. Ratcheting is a cyclic accumulation of inelastic strain occurred under asymmetric stress-controlled cyclic loading, and fatigue failure is a main failure mode of such components. It’s necessary to research the cyclic deformation of polymer material due to its importance on the assessment of safety and life of engineering structures. In recent years, even though a large number of studies were performed for ratcheting of polymer materials, there are still two shortcomings: (1) Ratcheting deformation of polymer materials with different molecular weight has not been reported; (2) The ratcheting of polymer materials under pure shear and multiaxial cyclic loading has not been reported yet.Therefore, in this paper, a series of ratcheting tests for the polycarbonate(PC) polymer are conducted at room temperature, including uniaxial, pure shear and multiaxial ratcheting. Uniaxial tension and torsion creeps are also tested, but main attention is focused on the effect factors of the ratcheting, such as the stress level, stress rate, peak stress hold time, loading history and non-proportional loading paths (including the square, butterfly, hourglass, rhombic types)and so on. In addition, uniaxial ratcheting of the PCs with different molecular weights is discussed. The results show that the monotonic tensile and torsion deformations of PC show a significant rate-dependence; the higher loading rate, the greater corresponding yield strength. And the PC presents obvious viscosity in the axial and shear directions, even at room temperature. For the PC, the ratcheting occurs during the asymmetrical uniaxial, asymmetrical pure shear and non-proportional multiaxial stress-controlled cyclic loadings, and the ratcheting strain increases as the applied equivalent stress amplitude and equivalent mean stress increase. The ratcheting of the PC also is significantly time-dependent, the longer peak hold time and the smaller loading rate, the lager ratcheting strain. In the non-proportional multiaxial stress-controlled cyclic loading the ratcheting is also dependent on the non-proportional loading paths. Finally, during the asymmetrical uniaxial stress-controlled cyclic loading, ratcheting behavior of the PC apparently depends on its molecular weight.