To predict durability of polymeric structures an 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 long-term 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 pro-posed 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 entry provides a brief overview on experimental approaches commonly used to determine the thermal and the time-dependent mechanical properties in shear and bulk of materials along with the numerical procedures for obtaining the unique master curve of the selected material function. However, characterization of the frequency dependent properties is not discussed here. Many important materials, such as natural and synthetic polymers, can be considered as time-dependent materials, since their properties change with time and thus their functionality and applicability can change considerably after a certain period of time. The time dependency of materials is an inherent property of their structure, which further depends on the initial kinetics and applied thermo-mechanical boundary conditions. Therefore, material structure depends on its thermal properties, which are generally measured through differential scanning calorimetry. Time-dependent properties are experimentally determined in a segmental form at different temperatures and/or pressures, which can be then combined into master curves that provide a more complete description of the material behavior over a prolonged period of time. This book-chapter, among others, provides a review of the existing experimental techniques used for prediction of long-term behavior of polymeric materials.
COBISS.SI-ID: 513991543
This paper describes a novel apparatus for measuring dynamic bulk compliance B*([Omega]) of time-dependant materials. System can measure dynamic bulk compliance at room temperature, at pressures up to 100 +/- 1,5 bar and frequencies from 100 Hz to 1000 Hz. Functionality of the apparatus is demonstrated by performing measurements of dynamic bulk compliance for two different materials, i.e., polyvinyl acetate (PVAc) and thermoplastic polyurethane (TPU). Measurements were conducted at room temperature, atmospheric pressure and frequencies from 100 Hz to 1000 Hz.
COBISS.SI-ID: 13363739
Mechanical material properties of polymers may change significantly when they are exposed to high pressures and/or temperatures. The effect of hydro-static pressure on shear relaxation modulus of three thermoplastic polyurethanes has been presented in this paper. The results show that chemically identical materials may have significantly different hydro-static pressure sensitivity. At pressure of 300 MPa, mechanical properties of two materials change only for a factor of 10^5, while for the third material, mechanical properties change 10^9 times!
COBISS.SI-ID: 13741083