To predict durability of polymeric structures an information on polymer's 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. 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 be also used for prediction 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 chapter in the Encyclopedia of Thermal Stresses provides an 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. Proper characterization of the time-dependent properties in shear and bulk is necessary for reliable prediction of mechanical response of the material in linear viscoelastic domain as well as in the non-linear viscoelastic domain.
In this paper, the equivalence of pressure and temperature effect on the free volume of polyamide (PA6) is discussed. The degree of time dependence of polymers is strongly influenced by temperature and pressure conditions to which the material is exposed. As reported in the review of Tschoegl et al., the two effects may be modelled through the so-called time-temperature(-pressure) superposition principle. This principle was explained by the “free volume concept”, in which the volume, and consequently mechanical properties, can be modelled with respect to temperature and pressure changes. In this paper it was shown that equivalency of pressure and temperature on material free volume for PA6 is not valid. Therefore, this indicates that the existing models based on the free volume concept need to be re-examined.
COBISS.SI-ID: 12944923
Powder injection moulding (PIM) is a versatile technology for manufacturing small metal or ceramic parts withcomplex geometry. Invariably, PIM consists of 4 stages: feedstock preparation, injection moulding, debindingand sintering. Debinding is the most time consuming step and in an effort to reduce debinding times, catalyticdebinding was introduced, which rely on the sublimation ability of Polyoxymethylene (POM). Besides fastdebinding, POM provides excellent mechanical strength to the moulded part. One major problem of POM-basedbinders is their high viscosity that can complicate the injection moulding process. This paper examines thepossibility of lowering the viscosity of POM without affecting its mechanical strength by changing its averagemolecular weight (Mw). It was observed that POMs viscosity increases with MW at a faster rate than impacttoughness and it is suggested that a Mw of around 24000 g/mol provides the most appropriate combination ofstrength and fluidity.
COBISS.SI-ID: 13138459
Mechanically strong crosslinked silica aerogel with ordered mesopores structure were synthesized. This paper describes the use of polymer-crosslinked aerogels and how concentration of ingredients from which they are made, can have an effect on the morphology including the shape and size of the nano/micro pores can be varied, resulting in monoliths with variable morphostructural characteristics and different mechanical and physical properties. Drying process using pentane under ambient pressure enables the elimination of the sample size limitation imposed by autoclaves and allowed fabrication of larger samples suitable for testing in practical applications. From that perspective, bulk properties of interest include mechanical strength under high strain rates, tensile tests and thermal conductivity. The morphostructural properties of CTSAs were characterized before and after compression testing using a battery of methods including SEM, TEM and small angle X-ray scattering (SAXS).