Identification of complex moduli and Poisson's ratio from measured strains on an impacted bar

Abstract

The complex extension modulus, the complex shear modulus and the complex Poisson's ratio at room temperature have been identified from strain measurements on test bars made of polymethyl methacrylate (PMMA) and polypropylene (PP) which were subjected to axial and torsional impacts. Bars of each material with diameters 10 and 20 mm were tested at low and high levels of excitation for each mode of impact. The complex moduli were identified from the normal and shear strain histories at five non-uniformly distributed bar sections, while the complex Poisson's ratio was determined from the circumferential and axial normal strain histories at a single bar section. With the 10 mm test bars, satisfactory results were obtained from about 1 to 40 kHz for PMMA and from about 1 to 15 kHz for PP. Under the conditions of the tests, the responses of both materials were found to be very close to linear and nearly isotropic. The deviation from isotropy was larger for PP than that for PMMA, especially for the 10 mm test bars. A certain influence of the test bar dimension on the complex moduli was observed, especially for PP at low frequencies. The influence of test bar dimension on the observed deviation from isotropy and on the complex moduli for PP is believed to be partly due to the extrusion and cooling processes used for fabrication of the PP test bars.