Viscoelastic Energy Dissipation Research Articles

Viscoelastic effects in cutting of elastomers by a sharp object

The cutting behavior of elastomers by a sharp object was investigated using various elastomers such as acrylonitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), and natural rubber (NR). The effects of crosslinking density, cutting rate, and temperature on the cutting energy of elastomers were investigated. The cutting behavior of swollen elastomers was also investigated. It was found that the cutting energy increased as the molecular weight between crosslinks increased. It was also found that the cutting energies of various elastomers did not yield a single line. Moreover, even in the threshold condition of cutting process, the cutting energy was much higher than the threshold fracture energy. These results suggest that the cutting behavior cannot be explained by only a C-C bond rupture process, but it includes other energy dissipation processes. The curves for cutting energies obtained at different cutting rates and temperatures were well superimposed on a single master curve when they were shifted using the WLF (Williams, Landel, and Ferry) equation. Therefore, it is supposed that the cutting of elastomers by a sharp object includes viscoelastic energy dissipation process and is the viscoelastic behavior. It was also found that the variation of cutting energy over a considerable range of effective rates was smaller than that of the tear energy. It is attributed to the fact that the change of the crack tip diameter, i.e., roughening or reduction, was restricted by the diameter of razor blade.

Viscoelastic Behavior of Polymer Surface during Wetting and Dewetting Processes

The contact angle of water was measured with the Wilhelmy plate method on apolar polymers: soft ethylene-propylene copolymer (EP), rigid polypropylene (PP), and their blends PP/EP (80 wt % PP) and EP/PP (20 wt % PP). Upon liquid displacement, the contact angles of the soft materials, EP and EP/PP, depended on viscoelastic energy dissipation due to solid deformation. This was observed in different situations: wetting and dewetting, liquid displacement due to the relative motion of the plain liquid with respect to the solid, and meniscus relaxation when that motion was stopped. The wetting hysteresis of polymer blends may be understood by considering viscoelastic energy dissipation and the static contact angles. The former depends on the elastic modulus while the latter is determined by the more hydrophobic phase and the less hydrophobic phase, for advancing and receding contact angles, respectively.

Molecular mechanics modelling of interfacial energy and flexibility on cellulose

Molecular mechanics modelling is used to calculate the energies of interaction, hence the molecular level energy of adhesion at the interface with crystalline cellulose I of three different photopolymerizable primers and of a polyester varnish at the interface with the primer/cellulose assembly. The energy of interactions for just one of the primers with the statistically most common conformation of amorphous cellulose has also been obtained for comparison. Experimental results of adhesion by a standard peel test and by thermomechanical analysis, in which the effect of viscoelastic energy dissipation by crack tip propagation has been respectively minimized or is not present, hence in which the energy of interfacial interaction is nothing else but the work of adhesion, correlated well with the energies of interaction calculated by molecular mechanics. An equation correlating the energy of interaction at each finish/cellulose interface with the deflection derived by thermomechanical analysis, and with the number of internal bond rotational degrees of freedom as well as the degree of networking of the finish, has been derived and is presented. A relationship between the intrinsic fracture energy Go and the molecular mechanics-derived energy of interaction at the interface equating this to the square of the work of adhesion is obtained and is presented.

Visco-elastic energy dissipation in a sialon ceramic: Quantification and implications for fatigue resistance

In this letter a method to estimate the visco-elastic response of monolithic ceramics to cyclic loading conditions at high temperatures is proposed. A relation is observed between the visco-elastic energy dissipation measured for two silicon nitride materials, and the structural characteristics of their respective intergranular phases. Some consequences for the fatigue resistance of the tested materials, and of non-transforming monolithic ceramics in general, are discussed. Two batches (G for glassy and C for crystalline) of SiAlON have been studied. The G-batch is obtained by pressureless sintering of silicon nitride powder with Y{sub 2}O{sub 3} (6 wt%) and 6AlN-SiO{sub 2} (5 wt%) as sintering additives. The main phase after sintering is {beta}-sialon. Upon cooling from the sintering temperature the amorphous intergranular residues of the sintering additives and of SiO{sub 2}, which is unavoidably present as a thin layer on the silicon nitride powder, crystallize partially into YAM (Y{sub 2}Al{sub 4}O{sub 9}), which is detected by both XRD and TEM.

The effect of different storage methods on the mechanical properties of trabecular bone

The effect of different storage methods on the elastic and viscoelastic properties of trabecular bone was examined using non-destructive compression tests to 0.45% specimen strain. There was a 10% decrease in stiffness during the first 24 h post mortem. Storage by freezing or in ethanol for 100 d did not change the stiffness, and neither did several thawing, testing and refreezing sequences. The viscoelastic properties were more sensitive to storage and showed significant although small changes during 100 d of storage. The effect of defatting the specimens was a 30% increase in stiffness and a 50% decrease in viscoelastic energy dissipation.

Interface fracture and viscoelastic deformation in finite size specimens

In the small scale yielding limit, the viscoelastic energy dissipation and the predicted fracture toughness of the interface between two viscoelastic polymers are monotonically increasing functions of the crack growth rate. For a finite size specimen however such prediction are no longer strictly valid. We demonstrate here that there is a maximum viscoelastic energy dissipation (and thus fracture toughness) at a characteristic interface crack velocity which depends not only on the micromechanical properties of the interface and the viscoelastic properties of the bulk polymers but also upon the specimen dimensions. Our analytical results allow us to deduce G=Gad[1+φ(aTȧ,c)] which relates the experimental fracture toughness to the intrinsic fracture toughness of the interface Gad, the WLF shift factor aT and c, a dimensionless parameter which is a function of the micromechanical properties of the interface, the mechanical properties of the polymers and the specimen dimensions. This result is similar to an empirical relation developed to fit experimental results; this relation however was assumed to be independent of c (specimen geometry). We demonstrate that the dependence of φ on c is substantially weaker than its dependence on aTȧ, which accounts for the success of the empirical relation in correlating the experimental data.

A fracture model for a weak interface in a viscoelastic material (small scale yielding analysis)

The problem of adhesion at the interface between two polymers is investigated using a fracture mechanics approach. The effect of viscoelastic deformation of the bulk polymers and the micromechanical behavior of the interface on the fracture toughness of the interface was investigated by analyzing the problem of a steadily growing interface crack under the condition of small scale yielding. This condition assumes that the region of displacement discontinuity across the interface (the adhesive zone) due to loss of adhesion is small compared with typical specimen dimensions. It is found that the amount of inelastic or viscous energy dissipation is strongly coupled to the interface behavior and that the measured interface fracture toughness can be much higher than the intrinsic interface fracture toughness. For sufficiently high crack growth rates, a simple expression is obtained which relates the rate of viscous energy dissipation and the intrinsic interface fracture toughness to the viscoelastic properties of the bulk polymer. The dependence of the stress field inside the adhesive zone and the length of the adhesive zone on the viscoelastic properties of the polymer, the applied loading, the interface micromechanical model and the crack growth rate ȧ is obtained by numerically solving an integral equation. The size of the region of the viscoelastic energy dissipation is found to scale with ȧτr , where τr is the retardation time of the bulk polymer.

Three-axial strain controlled testing applied to bone specimens from the proximal tibial epiphysis

Reproducibility of the determination of Young's modulus and energy absorption along the three axes of trabecular bone cubes was analysed by non-destructive compression to 0.5% strain using different testing protocols. These protocols included testing with and without pre-conditioning to a viscoelastic steady state, and different orders of test directions. Reproducibility of conditioned tests was generally better than that of non-conditioned tests. No major effect of changing the order of the test direction was found. Three-axial conditioned testing of cubes from the proximal tibial epiphysis of five humans revealed a global transverse isotrophy while most cubes showed orthotropy. The ratio between stiffness along the long axis of the tibia and the stiffness in the transverse plane was 3.7 ± 0.4 (mean ± 2 SE). The corresponding ratios for elastic energy storage and viscoelastic energy dissipation were 2.5 ± 0.2. There was no difference between the relative energy loss during a testing cycle (loss tangent) in the three axes.

Dynamic mechanical and impact properties of PP/SEBS blend

AbstractStudies on impact behaviour of the blend of isotactic polypropylene (PP) with styrene‐b‐ethylene‐co‐butylene–b‐styrene triblock copolymer (SEBS) in the composition range 0–25 wt % SEBS at three temperatures, viz., ambient, −30°C, and −190°C, are presented. Dynamic mechanical properties on a torsion pendulum in the temperature range −100−100°C are also studied for this blend at various compositions. Scanning electron microscopic studies of the impact‐fractured surfaces are presented to illustrate the differences in the mode of fracture at the three temperatures of impact tests. Choice of the three temperatures for impact tests was such that the effect of shear yielding mechanism of toughening of PP at ambient temperature remains suppressed at −30°C, whereas at the lowest temperature (i.e., −190°C) the elastomeric role of the inclusion SEBS is suppressed. The observed considerably large difference in impact toughening at ambient temperature and at −;30°C seems not entirely accountable by the prevalence of shear yielding or crazing mechanisms in the respective temperature regions. A third mechanism, viz., viscoelastic energy dissipation, is invoked to account for the observed large difference of impact toughening at these two upper temperatures. Correlation of peak area of dynamic mechanical loss peaks occurring below the impact test temperature with the impact strength is also shown. This suggests greater significance of viscoelastic energy dissipation mechanism in the toughening of this blend at ambient temperature than at −30°C.

Energy dissipation and pulse wave attenuation in the canine carotid artery

This paper presents methods for calculating, for a segment of artery in vivo, (1) viscous and viscoelastic energy dissipation as a function of time, and (2) the viscous and viscoelastic components of the frequency-dependent attenuation coefficient. The calculations require measurement of arterial diameter and of intra-arterial pressure and flow-rate at two sites along the vessel. Viscous energy dissipation is calculated from the radius-dependent velocity shear in the lumen given by linear theory from the pressure measurements. The attenuation coefficient for a given harmonic of heart rate is calculated as half the sum of the viscous and viscoelastic components at that frequency of the energy dissipated per unit length by forwardtravelling waves, divided by the forward-wave flow work input to the segment at that frequency. Measurements in canine carotid arteries indicate that wall viscoelasticity contributes relatively little to energy dissipation per cardiac cycle and pulse wave attenuation.