Universidade de Coimbra SLOW DYNAMICS EXPERIMENTS ON THIN SHEETS

Like many other materials, thin sheets having no bending rigidity exhibit a slow non-equilibrium dynamics. We performed experiments on paperboard (100 µm in thickness) and on a laminate made of paperboard and Low Density Polyethylene (25 µm in thickness), both used in food packaging industries. The investigated samples were rectangular strips clamped at two ends on a tensile test machine. They were loaded at two different strain levels and monitored under constant strain; resonance frequencies were measured for several conditioning cycles of loading. We found that the resonance frequency shifts downward in response to a conditioning strain and the resonant line shape changes with increasing number of cycles due. The results are equivalent to long-time slow dynamics relaxation experiments on solid materials of many types. The conditioning period in our experiment is about one million longer than in typical earlier acoustic conditioning measurement. But, this does not influence the result to any significant degree, as the recovery response is even slower. In addition, these thin sheets have an even larger effect in terms of percentage of resonance frequency shift than rocks, which are known to have a very high degree of slow dynamic recovery. This type of measurement is important for the fundamental understanding of material dynamics