Time-dependent material functions of engineering plastics within the exploitation range of temperatures extend over several decades of time. For this reason material characterization is carried out at different temperatures and/or pressures within a certain experimental window. Using the timetemperature and/or time-pressure superposition principle, these response function segments can be shifted along the logarithmic time scale to obtain a master curve at selected reference conditions. This shifting is commonly performed manually (by hand) and requires some experience. Unfortunately, manual shifting is not based on a commonly agreed mathematical procedure which would, for a given set of experimental data, yield always exactly the same master curve, independent of a person who executes the shifting process. Thus, starting from the same set of experimental data two different researchers could, and very likely will, construct two different master curves. In this paper, we propose a closed form mathematical methodology (CFS), which completely removes ambiguity related to the manual shifting procedures. This paper presents the derivation of the shifting algorithm and its validation using several simulated and real experimental data. It has been shown that error caused by shifting performed with CFS is at least 10-50 times smaller then the underlying experimental error.
COBISS.SI-ID: 11702043
Recently by our group developed experimental-numerical-analytical (ENA) methodology, based on a simple non-standard falling weight experiment, was used for mechanical characterization of dry and wet climbing ropes. Analysis of the maximum impact force; the visco-plastic component of rope deformation; the amount of stored, retrieved and dissipated energy; the stiffness of the rope; and the maximum value of the first derivative of the deacceleration (jolt) showed that moisture significantly affects the functionality and durability of ropes. Wet ropes create larger maximum force, dissipate less energy, and generate larger retrieved energy that propels climbers in the opposite vertical direction. Properties of wet ropes are also more sensitive to number of repeated drops. Major changes of all physical quantities are, as a rule, observed during the first three to four drops. It has been shown that for the safety of climbers the most indicative properties are dissipated energy and jolt (first derivative of climber deacceleration). The ratio of dissipated and retrieved energy, psi=Wdys IWret, could be used as a criterion for evaluation of the quality of climbing ropes.
COBISS.SI-ID: 4044977
Low density polyethylene (LDPE) was exposed to one hundred (100) consecutive extensive extrusion cycles to simulate mechanical recycling. Collected samples were characterized by means of small amplitude oscillatory measurements to investigate rheological properties, by gel permeation chromatography (GPC) to measure molecular weight, and with differential scanning calorimetry (DSC) to study thermal properties. Finally, solid time-dependent mechanical properties were characterized by measuring creep compliance. The results show that simulated recycling did not significantly change the melting and crystallization temperatures of LDPE. However, results from rheological measurement, crystallinity, creep measurements and GPC suggest that thermal degradation and gelation of LDPE occur after extensive extrusion, which leads to simultaneous chain scission and crosslinking of the polymer chains. It can be concluded that processability, measured by rheological parameters at high frequency and durability of LDPE measured by creep compliance, are only affected after the 40th extrusion cycle. These observations correspond to the molecular changes of LDPE measured through GPC, MFI and crystallinity calculations obtained from DSC measurements.
COBISS.SI-ID: 12490267
To predict durability of polymeric structures information on polymers long term properties in the form of relaxation modulus and/or creep compliance is required. It is well known that determination of relaxation or creep properties from experimental data is an inverse problem, which, due to presence of experimental errors in input data, becomes ill-posed. To find a stable solution using standard integration schemes is practically impossible. In this paper we propose a hands on methodology which bypasses the solution of ill-posed integral equation and allows finding longterm relaxation or creep properties from simple constant strain rate or constant stress rate experiments performed at different temperatures. The proposed approach can be applied not only for characterization of viscoelastic materials in solid state but can also be used for prediction of time-dependent properties of polymer melts. The paper presents the detailed steps of the proposed method as well as its validation on several simulated and real experimental data. It has been shown that the proposed approach can accurately reconstruct the desired long-term time-dependent properties obtained in traditional way (i.e., from step loading).
COBISS.SI-ID: 13309211
This book presents the analysis of time-dependent behaviour of polymeric (viscoelastic) materials under cyclic loading conditions that polymeric products, such as transmission belts, are exposed to during their operation. Time dependent behaviour of cyclically loaded transmission belts is studied through the newly developed methodology for analyzing strain accumulation process that may be one of the mechanisms responsible for the hardening of material, crack formation, and ultimately for the failure of polymeric products.
COBISS.SI-ID: 11702299