Sanford S. Sternstein

Viscoelastic properties of wet cortical bone—I. Torsional and biaxial studies

Abstract The dynamic moduli for frequencies between 2 × 10−3 and 100 Hz and the relaxation modulus between 1 and 105 sec have been measured in torsion for human and bovine cortical bone kept wet in Ringers solution, as a function of temperature, strain-level and a superposed axial load. At body temperature, the dynamic loss tangent increased from 0.009 at 100 Hz, to 0.013 at 1 Hz, to 0.025 below 0.1 Hz. The total change in shear modulus over 8 decades of time-scale was 15–35%, most of this change occurring at long times in relaxation. Bovine bone, although stiffer, exhibited viscoelastic behavior similar to that of human bone. Nonlinear viscoelastic response, in the form of the isochronous shear modulus increasing with strain level, became apparent for strains above 10−4, and was observed to be less pronounced in dynamic tests than in relaxation. Recovery at long times occurred more slowly than relaxation, but always approached completion asymptotically. This effect, small in human bone and negligible in bovine bone, is accentuated by the superposition of an axial stress on the torsion sample. Biaxial experiments were performed, since bones in the body are subjected to stresses which are more complex than uniaxial tension or shear. An axial tensile stress of 17.2 MN/m2 increased the high frequency loss tangent of human bone by ⋍20% and changed the shear modulus by ⋍1.5%; for bovine bone, the shear modulus was changed by 0.6% by an axial stress of 22.1 MN/m2. The temperature dependence of the viscoelastic response was found to be thermorheologically complex. This implies that bone experiments must be done at body temperature to be relevant to the in vivo situation, and that time-temperature superposition is of questionable validity for bone.

Nonlinear viscoelasticity of nanofilled polymers: interfaces, chain statistics and properties recovery kinetics

The mechanisms by which nano-scale fillers promote both reinforcement and nonlinear viscoelastic behavior in polymer melts are explored. Three fumed silica fillers having the same specific surface area but different surface treatments were used to prepare composites with a poly(vinyl acetate) matrix of various molecular weights. Filler concentrations were limited to a maximum of 12.5 vol.% to minimize filler agglomeration and networking. Dynamic storage and loss moduli at about 45 °C above the glass transition temperature were obtained over a range of shear strain amplitudes. The shape of the loss factor vs. strain function is found to have a strong characteristic dependence on the filler surface treatment. The recovery of properties following their degradation by a large strain perturbation was investigated using a timed sequence of dynamic moduli measurements at low strain amplitudes. Trapped entanglements and their effects on matrix moduli due to Langevin chain statistics is shown to be a viable mechanism for the initial high reinforcement, and its subsequent reduction with strain. Consistent with this, recovery kinetics suggests that the basic mechanism is controlled by chain diffusion. For simple filler (non-polymeric) surfaces, the self-diffusion of matrix chains is the dominant mechanism for recovery. In the case of a filler with surface-tethered polymer chains, the self-diffusion process is supplemented by the inter-diffusion of the tethered and matrix chains. Relative reinforcement is found to be the highest for the lowest molecular weight matrix, regardless of filler type. The proposed mechanism is based entirely on the behavior of the filler-matrix interface and the physics of polymeric melts, and no theories of filler agglomeration or network formation are invoked.

Fabrication and mechanical characterization of glass fiber reinforced UV‐cured composites from epoxidized vegetable oils

Novel fiberglass-reinforced composites were fabricated by the ultraviolet and visible (solar) irradiation of epoxidized vegetable oils in the presence of onium salt cationic photoinitiators. A variety of layup techniques and experimental conditions were explored to optimize composite fabrication. It was demonstrated that composites prepared by wet layup techniques containing up to five plies of glass cloth could be cured by a direct, 25-min exposure to solar irradiation. A series of composite samples were prepared using mixtures of epoxidized vegetable oils and synthetic epoxy resins, and their mechanical properties were evaluated. Based on these measurements, it may be concluded that photochemical routes to the fabrication of composites derived from epoxidized vegetable oils provide a simple, direct, and inexpensive route to the fabrication of composites with many potential low-performance applications.

Yielding of glassy polymers in the second quadrant of principal stress space

Abstract Second quadrant crazing and shear yielding studies were performed on glassy poly(methyl methacrylate) by means of combined torsion-tension loading. The results are in quantitative agreement with the shear and normal stress yielding criteria proposed by Sternstein and Ongchin. It is shown that four distinct regions of material response exist in the second quadrant and, depending on the stress state, 1) no crazing and no shear yielding, 2) crazing alone, 3) shear yielding alone, or 4) crazing and shear yielding can occur. An analysis of stress field induced brittle-ductile transitions is presented which is in agreement with other studies of high-pressure yielding.

Effect of Pressure on the Infrared Spectra of Some Hydrogen‐Bonded Solids

By using a diamond‐anvil pressure cell, the infrared spectra of oxalic acid, polyvinyl alcohol, nylon 6–6, and a number of other materials at various pressures up to 25 000 atm were obtained. Studies on two of the pressure‐induced changes, (1) the shift to lower frequencies of the hydrogen‐bonded —OH and —NH stretching bands, and (2) the intensity changes of the two —CH2 stretching bands, are reported.

Inhomogeneous swelling in filled elastomers

Abstract An exact formulation is given for the inhomogeneous swelling of an elastomeric matrix containing a spherical inclusion, and is presented in a form amenable to solution for arbitrary free energy of mixing and network strain energy functions, rigid or soft inclusions, and finite or infinite matrix size. Specific examples of the deformation field, stress field, and composition variation are given as functions of distance from a rigid inclusion imbedded in an infinite matrix displaying Flory-Huggins/Gaussian swelling behavior. Interfacial stresses and extension ratios are given as functions of cross-link density and Flory-Huggins X parameter. In a related study published elsewhere, Kotani and Sternstein have obtained experimental verification of the theory using birefringence techniques on model-filled elastomers. Stein and co-workers have found similar agreement of the theory with their light scattering measurements.

Thermoelastic analysis of composite CVD SiC fibers

Abstract A thermoelastic model has been developed for the prediction of the state of stress of composite fibers when subjected to thermomechanical loads. The model is based on a system of four infinitely long and perfectly bonded concentric cylinders. Predictions are presented for composite fibers produced by chemical vapor deposition (CVD), and in particular for the SCS-6 fiber, for which it is found that neglecting the strains that the substrate experiences during CVD in the calculations, can lead to severe underestimates of the magnitude of the predicted residual stresses. Relationships are established between the predicted state of stress and experimentally observed changes of strength with temperature, stress thresholds in creep and structural defects such as debonding at the interfaces for the SCS-6 filter.

High temperature structural stability of chemically vapor-deposited silicon carbide filaments

Abstract The structural stability of SCS-6 filament was investigated in the temperature interval 1000–1600 °C by differential scanning calorimetry (DSC) and in situ X-ray diffraction techniques. It was found that this filament is composed mostly of the cubic β polytype of SiC and of graphite, and that these phases are stable in the temperature interval studied. The lattice parameter for β-SiC was determined between room temperature and 1600°C, and the calculated average linear thermal expansion of the lattice in this temperature interval (5.24 × 10 −6 °C −1 is in agreement with macroscopic thermal expansion measurements on single filaments. Thermal events observed by DSC occur at 1380 °C on heating and at 1300°C on cooling, and are suggested as being related to the melting and solidification, respectively, of excess silicon. These results are analysed in relation to the anomalous thermal expansion behavior exhibited by this filament.

Nanofiller-Polymer Interactions At and Above the Glass Transition Temperature

Rheological data are reported for a series of fumed silica filled PVAc samples, using fillers of different specific surface areas and surface treatments. Data at the glass transition temperature and 45 C above Tg are presented. The addition of filler systematically increases Tg, and all samples obey time-temperature superposition. However, temperature normalized and frequency normalized plots of loss modulus indicate that there is no change in the dispersion of the glass transition, with the only exception being a surface modified with covalently bonded polymer chains. Thus, contrary to expectations, an increase in filler content or change in surface treatment has no effect on the relative shape of the relaxation time spectrum at the glass transition. At 45 C above Tg, different behavior is observed. The filler concentration has a major effect on the nonlinearity of dynamic moduli vs. strain amplitude, with higher filler content reducing the strain amplitude at which nonlinear behavior is observed. Specific filler surface treatments result in major changes in the shape of the loss factor versus strain amplitude relationship. These results suggest that interfacial interactions strongly modify the far-field polymer behavior with respect to chain entanglement slippage at large strains.

High-temperature dynamic mechanical testing of ceramic fibers: Apparatus and preliminary results

A test apparatus is described for the dynamic testing of single ceramic fibers from room temperature to 1600°C using forced vibration sine wave displacements from 0.1 to 25 Hz. Both storage (elastic, real or in-phase) and loss (dissipative, imaginary or out-of-phase) components of the dynamic modulus are obtained. The deconvolution equations required to extract isothermal data from a non-uniform (parabolic) temperature distribution are presented. Preliminary data on sapphire filaments and CVD silicon carbide fibers are discussed. The results show the validity of the testing method and data, which are in good agreement with available literature data on similar materials.