High-temperature Moduli Research Articles

An approach to calculate master curve for glass moulding press based on finite element simulation

A new approach is proposed to calculate the master curve (MC) of glass. It can solve the problem: when the temperature is higher than Tt,l (defined later), the measured shear relaxation modulus cannot describe the mechanical response of the annealing stage during glass moulding press (GMP) through the thermal rheological simple (TRS) theory. Based on the temperature dependent Yang's modulus and TRS theory, the MC with a reference temperature less than Tt,l is calculated by an iterative process. In order to verify the suitability of MC at high temperature, the MC and shear relaxation moduli are used in the finite element (FE) simulation at the isothermal compression stage. Similar von Mises stresses are shown and the usability of the MC at moulding temperature is proved by the simulation results. In order to predict residual stress within the formed glass lens, the MC is used to conduct FE simulation for the whole GMP process.

An approach to calculate master curve for glass moulding press based on finite element simulation

A new approach is proposed to calculate the master curve (MC) of glass. It can solve the problem: when the temperature is higher than Tt,l (defined later), the measured shear relaxation modulus cannot describe the mechanical response of the annealing stage during glass moulding press (GMP) through the thermal rheological simple (TRS) theory. Based on the temperature dependent Yang's modulus and TRS theory, the MC with a reference temperature less than Tt,l is calculated by an iterative process. In order to verify the suitability of MC at high temperature, the MC and shear relaxation moduli are used in the finite element (FE) simulation at the isothermal compression stage. Similar von Mises stresses are shown and the usability of the MC at moulding temperature is proved by the simulation results. In order to predict residual stress within the formed glass lens, the MC is used to conduct FE simulation for the whole GMP process.

Evaluating high temperature elastic modulus of ceramic coatings by relative method

The accurate evaluation of the elastic modulus of ceramic coatings at high temperature (HT) is of high significance for industrial application, yet it is not easy to get the practical modulus at HT due to the difficulty of the deformation measurement and coating separation from the composite samples. This work presented a simple approach in which relative method was used twice to solve this problem indirectly. Given a single-face or double-face coated beam sample, the relative method was firstly used to determine the real mid-span deflection of the three-point bending piece at HT, and secondly to derive the analytical relation among the HT moduli of the coating, the coated and uncoated samples. Thus the HT modulus of the coatings on beam samples is determined uniquely via the measured HT moduli of the samples with and without coatings. For a ring sample (from tube with outer-side, inner-side, and double-side coating), the relative method was used firstly to determine the real compression deformation of a split ring sample at HT, secondly to derive the relationship among the slope of load-deformation curve of the coated ring, the HT modulus of the coating and substrate. Thus, the HT modulus of ceramic coatings can be evaluated by the substrate modulus and the load-deformation data of coated rings. Mathematic expressions of those calculations were derived for the beam and ring samples. CVD-SiC coatings on graphite substrate were selected as the testing samples, of which the measured modulus ranging from room temperature to 2100 °C demonstrated the validity and convenience of the relative method.

Synthesis of epoxy‐functionalized hyperbranched poly(phenylene oxide) and its modification of cyanate ester resin

AbstractHigh curing temperature is the key drawback of present heat resistant thermosetting resins. A novel epoxy‐functionalized hyperbranched poly(phenylene oxide), coded as eHBPPO, was synthesized, and used to modify 2,2′‐bis (4‐cyanatophenyl) isopropylidene (CE). Compared with CE, CE/eHBPPO system has significantly decreased curing temperature owing to the different curing mechanism. Based on this results, cured CE/eHBPPO resins without postcuring process, and cured CE resin postcured at 230°C were prepared, their dynamic mechanical and dielectric properties were systematically investigated. Results show that cured CE/eHBPPO resins not only have excellent stability in dielectric properties over a wide frequency range (1–109Hz), but also show attractively lower dielectric constant and loss than CE resin. In addition, cured CE/eHBPPO resins also have high glass transition temperature and storage moduli in glassy state. These attractive integrated performance of CE/eHBPPO suggest a new method to develop high performance resins. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Reinforcement of Poly(ethylene terephthalate) Fibers with Polyhedral Oligomeric Silsesquioxanes (POSS)

Poly(ethylene terephthalate) (PET)-based composite fibers were prepared by melt spinning three types of PET/polyhedral oligomeric silsesquioxane (POSS) composites. These composites were made by either melt blending POSS with PET at 5 wt% loading level (non-reactive POSS and silanol POSS) or by in-situ polymerization with 2.5 wt% reactive POSS. Significant increases in tensile modulus and tensile strengths were achieved in PET fibers with non-reactive POSS at room temperature. The hightemperature modulus retention was found to be much better for PET/silanol POSS fiber when compared to that of control PET. Although other PET/POSS nancomposite fibers tested did not show this high retention of modulus at elevated temperatures, PET/isooctylPOSS nanocomposite fibers did show increased modulus at elevated temperature compared to that of PET. Higher compressive strengths, compared to PET fibers, were observed for all three nanocomposite fibers. Gel permeation chromatography measurement suggested that there is no significant change in molecular weight during preparation of PET/POSS nanocomposites. SEM observations suggest that there is no obvious phase separation in any of the three PET/POSS systems. Crystallization behavior and thermal stability of the composite were also studied. The fiber spinning and mechanical performance with 10 and 20 wt% of trisilanolisooctyl POSS2 were also investigated1 the composites with higher concentrations of this nanofiller can be spun without any difficulty. At room temperature, the fiber tensile modulus increased steadily with the POSS concentration while fiber tensile strength showed no significant change. The elongation at break decreased significantly with increasing of POSS concentration. The high-temperature moduli of PET/POSS nanocomposite fibers were found to be rather variable, likely due to the modest compatibility between filler and polymers, which can lead to structural anisotropy within the composite.

Effect of compatibilizer on the structure‐property relationships of kenaf‐fiber/polypropylene composites

AbstractAlthough lignocellulosic, fiber‐thermoplastics composites have been used for several decades, recent economic and environmental advantages have resulted in significant commercial interest in the use of these fibers for several applications. Kenaf is a fast growing annual growth plant that is harvested for its bast fibers. These fibers have excellent specific properties and have potential to be outstanding reinforcing fillers in plastics. This paper reports the structure‐property relationships of kenaf fiber reinforced polypropylene (PP) and its impact copolymers. The use of maleated polypropylenes (MAPP) is important to improve the compatibility between the fiber and matrix. A significant improvement in impact strengths was observed when the MAPP was used in the composites. Results also indicate that the impact copolymer blends with coupling agent have better high temperature moduli and lower creep compliance than the uncoupled systems. The coupling agent also changes the crystallization and melting behavior of these blends. Because of the better adhesion between the polymer molecules and kenaf fibers, the coupled samples have more restricted molecules than the uncoupled blends. As a result, the crystallization of the coupled high molecular weight blends is slower than the uncoupled blends, resulting in a lower crystallization temperature (Tc) and reduced crystallinity. For the lower molecular weight blends, the coupling agent enhances the crystallization of polymer matrix and results in a higher crystallization temperature and increased crystallinity of the coupled blend. The coupled blends also have more defects in the polymer crystals, and the crystallinity of coupled blends is also lower than the uncoupled blends. This could explain the lower melting temperatures of the coupled samples as compared to uncoupled samples.

High temperature young's modulus of a fine-grained nuclear graphite oxidized or prestressed to various levels

Effects of oxidation and compressive prestress on high temperature Young's modulus were examined on a fine-grained isotropic graphite, IG-110. Oxidation was performed in air at 500°C up to a burn-off level of 10%. Prestress level was either 60% or 80% of the mean compressive strength. Both oxidation and prestressing caused decreases both in the room temperature and high temperature moduli, which were explained by a crack opening and growth model. An empirical equation by which the high temperature modulus of oxidized specimens can be estimated from the room temperature modulus of the unoxidized specimen was obtained as E (T, d) E(RT, d o ) = ƒ(T)( d d o ) n(T) . Here, E( T, d) and E( RT, d o ) are the high temperature modulus of the oxidized specimen with apparent density d, and the room temperature modulus of the unoxidized specimen with apparent density d o , ƒ(T) is a polynomial in the third order of T, and n( T), a linear function of T. Annealing treatment or heat-up and cool-down process caused a partial recovery of the modulus of the oxidized specimen. This recovery was explained with the aid of a model: The internal stress generated as a result of the anisotropy of the thermal expansion of crystallites becomes smaller for the oxidized material because of the preferential oxidation of the binder region.

Fracture toughness and thermal shock of tool and turbine ceramics

The behaviour of some commercial tool carbides and turbine ceramics has been investigated in regard to resistance to crack initiation, crack propagation and retained strength after thermal shock. New data are provided, particularly measurements of the fracture toughness of these materials at actual operating temperatures (up to 1200° C). Many of the materials did not follow the generally accepted Hasselman theory for thermal shock in ceramics, and instead of showing a discontinuity in retained strength at some critical quenching temperature difference, their residual strengths fell gradually at temperatures lower than their supposed critical quenching temperature. This behaviour is explicable when high temperature toughnesses, strengths and moduli are used in the damage resistance parameter (ER/gs f 2 ). It seems that materials not following the Hasselman model suffer cumulative damage with increasing number of shocks. Sub-critical crack growth occurs even if (KIC/gsf)2 values are constant, and such damage, which reduces the room temperature retained strength, is enhanced by (KIC/gsf)2 decreasing at temperatures below ΔTc. In contrast, materials obeying Hasselman's model appear to have a constant (KIC/gsf)2 below ΔTc and for some temperature range above. Only then are “one-shock“ characterizations of materials possible, otherwise, the retained strength depends upon the number of prior shocks. Experiments are also reported which describe the effects of rate of testing on the unshocked and shocked mechanical properties of ceramics. Oxidation is shown to influence the results in a manner not obvious from single shock tests.