William S. Slaughter

Biaxial Mechanical Response of Bioprosthetic Heart Valve Biomaterials to High In-plane Shear

Utilization of novel biologically-derived biomaterials in bioprosthetic heart valves (BHV) requires robust constitutive models to predict the mechanical behavior under generalized loading states. Thus, it is necessary to perform rigorous experimentation involving all functional deformations to obtain both the form and material constants of a strain-energy density function. In this study, we generated a comprehensive experimental biaxial mechanical dataset that included high in-plane shear stresses using glutaraldehyde treated bovine pericardium (GLBP) as the representative BHV biomaterial. Compared to our previous study (Sacks, JBME, v.121, pp. 551-555, 1999), GLBP demonstrated a substantially different response under high shear strains. This finding was underscored by the inability of the standard Fung model, applied successfully in our previous GLBP study, to fit the high-shear data. To develop an appropriate constitutive model, we utilized an interpolation technique for the pseudo-elastic response to guide modification of the final model form. An eight parameter modified Fung model utilizing additional quartic terms was developed, which fitted the complete dataset well. Model parameters were also constrained to satisfy physical plausibility of the strain energy function. The results of this study underscore the limited predictive ability of current soft tissue models, and the need to collect experimental data for soft tissue simulations over the complete functional range.

Material-length-scale-controlled nanoindentation size effects due to strain-gradient plasticity

Abstract This paper proposes a concept of strain-gradient plasticity that is based on characteristic material length scale. Experimental results for indentation size effect are correlated with the predictions from a dislocation-based strain-gradient plasticity model. Nanoindentations on single crystals of Al, Ag, Ni, polycrystalline Cu and poly-synthetically twinned (PST) lamellar α2- and γ-TiAl are conducted with an atomic force microscope with an add-on force transducer from Hysitron, Inc. It is found that the indentation size effect is controlled by a characteristic material length scale l , which is a function of the Burger’s vector, the shear modulus, and a material reference stress. The material length scales are found to be in the order l Ag > l Ni > l Al > l Cu > l α 2 -TiAl > l γ-TiAl , which corresponds to the indentation size effect.

Investigation of the morphology of internal defects in cross wedge rolling

Abstract Since internal defects in the cross wedge rolling (CWR) process can weaken the integrity of the final product and may ultimately lead to catastrophic failure, it is necessary to investigate the mechanisms of their generation and growth. Using a specially designed CWR experimental apparatus, experiments were performed at more than 50 different operating conditions. The cross-sectional profiles of the workpiece specimens were examined and compared at each condition. Based on the experiments, the influence of three primary parameters in CWR process—the forming angle α , the stretching angle β , and the area reduction Δ A were determined. From the experimental results, the morphology of void generation and growth in CWR is ascertained and discussed. Through the definition of a non-dimensional deformation coefficient e , a method for predicting the likelihood of void formation is also established and discussed with respect to optimizing CWR tooling design.

Length scale effect on mechanical behavior due to strain gradient plasticity

Abstract The characteristic material length scale l as introduced in the phenomenological theory of strain gradient plasticity by Fleck and Hutchinson [1] is crucial for the application of the theory. Three dislocation models are proposed in this paper for the derivation of the material length scales as there is a rational connection between dislocation theory and phenomenological theory. The length scale is determined as a function of material fundamental parameters and is independent of how the ‘overall effective strain’ is measured, or how the dislocations interact. The length scale is generally about 1.5 μm or less, agreeing with the observation that the size effect appears approximately at 1 μm and sub-micrometer levels for indentation deformations. The dislocation interaction is also discussed in the framework of strain gradient plasticity.

Microbuckling of fiber composites with random initial fiber waviness

Abstract A fiber microbuckling calculation is presented for the effects of fiber misalignment and material nonlinearity on the compressive strength of fiber composites. The role of fiber bending stiffness is included by using a particular form of couple stress theory. In order to examine the effect of a distribution of fiber waviness, the fiber misalignment angle is assumed to vary along the fiber length but is taken to be uniform in the transverse direction. Thus, the effects of wavelength as well as amplitude of fiber waviness are taken into account. A consideration of sinusoidal initial waviness reveals that short wavelength imperfections are much less deleterious than long wavelengths. A statistical analysis is presented for the effect of random fiber waviness on compressive strength, using a Monte Carlo simulation technique. Compressive strength is found to be particularly sensitive to the area under the spectral density curve and to the minimum fiber wavelength.

Determining the power spectral density of the waviness of unidirectional glass fibres in polymer composites

A pilot study has been completed into the accurate measurement of 3D fibre waviness in high packing fraction, unidirectional, glass fibre reinforced polymer epoxy. It has been shown that the confocal laser scanning microscope (CLSM) can determine fibre waviness amplitudes,A⩽40 µm andsimultaneously fibre wavelengths, λ⩽4 mm. Knowing the fibre-centre coordinates in 3D with sub-micron precision, the fibre waviness may be characterised in terms of the power spectral densitiesSu andSw orthogonal to the fibre direction (taken to be in they direction) and also in terms of the power spectral densities of fibre slopes,Su′ andSw′. In future studies, these characterisation parameters will enable models linking random fibre waviness to compressive strength to be evaluated.

Viscoelastic Microbuckling of Fiber Composites

A theoretical study is given of viscoelastic microbuckling of fiber composites. The analysis is formulated in terms of general linear viscoelastic behavior within the kink band. Material outside the kink band is assumed to behave elastically. Two specific forms of linear viscoelastic behavior are considered: a standard linear viscoelastic model and a logarithmically creeping model. Results are provided as deformation versus time histories and failure life versus applied stress. Failure is due to either the attainment of a critical failure strain in the kink band or to the intervention of a different failure mechanism such as plastic microbuckling.

Super hardening and deformability in epitaxially grown W/NbN nanolayers under shallow and deep nanoindentations

Epitaxially grown W/NbN nanolayers are superlattice materials that exhibit a hardness at small bilayer repeat periods which exceed the hardness predicted by the rule of mixtures for normal composites. Interfaces in the bilayered superlattice play a critical role in the superhardening process. The objective of this investigation was to examine the behavior of the superlattice material, W/NbN. Nanoindentations and in situ surface imaging were conducted over a range of applied loads on samples of W/NbN with two different bilayer periods (Λ=5.6 and 10.4 nm), and monolithic samples of the niobium nitride (NbN) ceramic and the tungsten (W) metal which comprise the superlattice material. Shallow nanoindentations were made to a depth equal to the individual layer thicknesses of the epitaxially grown W/NbN nanolayers in order to investigate the individual interface effect. The mechanical properties were determined using the Oliver and Pharr method and compared for all the samples. The energies of indentation are calculated. The characteristics of the material pileup resulting from the nanoindentations are determined from the scanned surface images. An increase in hardness is observed in the superlattice materials at deeper indentation depths. The results indicate that this increase in hardness is related to the nature of the interface between the layers in the superlattice materials.Epitaxially grown W/NbN nanolayers are superlattice materials that exhibit a hardness at small bilayer repeat periods which exceed the hardness predicted by the rule of mixtures for normal composites. Interfaces in the bilayered superlattice play a critical role in the superhardening process. The objective of this investigation was to examine the behavior of the superlattice material, W/NbN. Nanoindentations and in situ surface imaging were conducted over a range of applied loads on samples of W/NbN with two different bilayer periods (Λ=5.6 and 10.4 nm), and monolithic samples of the niobium nitride (NbN) ceramic and the tungsten (W) metal which comprise the superlattice material. Shallow nanoindentations were made to a depth equal to the individual layer thicknesses of the epitaxially grown W/NbN nanolayers in order to investigate the individual interface effect. The mechanical properties were determined using the Oliver and Pharr method and compared for all the samples. The energies of indentation are c...

A quantitative analysis of the effect of geometric assumptions in sintering models

Abstract Simple geometric models of the three stages of sintering are used to predict the path of microstructural change—the evolution of the microstructure as a function of relative density—in terms of stereological parameters. The geometric assumptions in the models are based on those frequently used in other sintering models. Two nondimensional parameters, the ratio of the mean grain and mean void intercepts and the ratio of the solid/solid and solid/void surface areas, are introduced so that the model predictions can be compared quantitatively with experimental results, independent of the characteristic length scale. The comparison is made with two alumina powders. The results suggest that an important contributor to the inaccuracy of sintering models based on simple geometric assumptions is the failure to account for nonuniformity of particle packing.

Dynamic compressive failure of fiber composites

A model is presented for the dynamic compressive response of polymer matrix fiber composites. The model includes the effects of fiber misalignment and material nonlinearity as well as material inertia. The role of fiber bending stiffness is included via a couple stress formulation. The response of fiber composites to suddenly applied, constant compressive axial load is examined. It is found that under constant load, inertial effects contribute to a reduction in the critical stress for composite failure. This reduction is greatest for composites with long initial fiber imperfection wavelengths. For a given load, there is a range of initial fiber imperfection wavelengths that will result in composite failure. Within this range, there is a preferred wavelength, which results in the shortest failure time.