James A. Nemes

Load shift of the intervertebral disc after a vertebroplasty: a finite-element study.

Infiltrating osteoporotic cancellous bone with bone cement (vertebroplasty) is a novel surgical procedure to stabilize and prevent osteoporotic vertebral fractures. Short-term clinical and biomechanical results are encouraging; however, so far no reports on long-term results have been published. Our clinical observations suggest that vertebroplasty may induce subsequent fractures in the vertebrae adjacent to the ones augmented. At this point, there is only a limited understanding of what causes these fractures. We have previously hypothesized that adjacent fractures may result from a shift in stiffness and load following rigid augmentation. The purpose of this study is to determine the load shift in a lumbar motion segment following vertebroplasty. A finite-element (FE) model of a lumbar motion segment (L4-L5) was used to quantify and compare the pre- and post-augmentation stiffness and loading (load shift) of the intervertebral (IV) disc adjacent to the augmented vertebra in response to quasi-static compression. The results showed that the rigid cement augmentation underneath the endplates acted as an upright pillar that severely reduced the inward bulge of the endplates of the augmented vertebra. The bulge of the augmented endplate was reduced to 7% of its value before the augmentation, resulting in a stiffening of the IV joint by approximately 17%, and of the whole motion segment by approximately 11%. The IV pressure accordingly increased by approximately 19%, and the inward bulge of the endplate adjacent to the one augmented (L4 inferior) increased considerably, by approximately 17%. This increase of up to 17% in the inward bulge of the endplate adjacent to the one augmented may be the cause of the adjacent fractures.

Material changes in osteoporotic human cancellous bone following infiltration with acrylic bone cement for a vertebral cement augmentation

Bone cement infiltration can be effective at mechanically augmenting osteoporotic vertebrae. While most published literature describes the gain in mechanical strength of augmented vertebrae, we report the first measurements of viscoelastic material changes of cancellous bone due to cement infiltration. We infiltrated cancellous core specimen harvested from osteoporotic cadaveric spines with acrylic bone cement. Bone specimen before and after cement infiltration were subjected to identical quasi-static and relaxation loading in confined and free compression. Testing data were fitted to a linear viscoelastic model of compressible material and the model parameters for cement, native cancellous bone, and cancellous bone infiltrated (composite) with cement were identified. The fitting demonstrated that the linear viscoelastic model presented in this paper accurately describes the mechanical behaviour of cement and bone, before and after infiltration. Although the composite specimen did not completely adopt the properties of bulk bone cement, the stiffening of cancellous bone due to cement infiltration is considerable. The composite was, for example, 8.5 times stiffer than native bone. The local stiffening of cancellous bone in patients may alter the load transfer of the augmented motion segment and may be the cause of subsequent fractures in the vertebrae adjacent to the ones infiltrated with cement. The material model and parameters in this paper, together with an adequate finite-element model, can be helpful to investigate the load shift, the mechanism for subsequent fractures, and filling patterns for ideal cement infiltration.

Time Domain Finite Element Simulations of Damped Multilayered Beams Using a Prony Series Representation

The dynamic response of steel beams treated with variousconfigurations of constrained viscoelastic layers is consideredexperimentally and analytically. The cantilevered sandwich beams aresubjected to spike inputs and large finite rotations. Frequency domainviscoelastic material properties provided by the manufacturer areconverted to the Prony series time domain expression of the shearrelaxation modulus. A finite element model of the cantilevered beams isimplemented in ABAQUS and numerical predictions are obtained. Naturalfrequencies and damping factors obtained from linear and nonlinear, timedomain finite element analyses are found to be in good agreement withexperimental results.

Predicting the Ductile Failure of DP-steels Using Micromechanical Modeling of Cells

Thus far, micromechanical modeling of cells has been used successfully to capture the deformation behavior of dual phase (DP) steels, which display impressive mechanical properties, especially for the automotive industry. However, the prediction of ductile failure, which is essential in the manufacture and design of parts, needs to be modeled in order to develop a model, which can fully characterize DP-steels. The Gurson—Tvergaard (GT) damage model is coupled with a micromechanical model developed in earlier works, which captures the deformation behavior of DP-steels well, making a complete material model. A procedure that accounts for damage in terms of the void volume fraction, stress triaxiality and the mechanics of failure in DP-steels as major damage factors, is developed in this work to determine the calibrating parameters in the GT yield function. When these parameters are determined, they are employed in numerical simulations of a tensile bar test to compare the experimental and numerical fracture parameters. The results show good agreement between the numerical predictions using the GT parameters obtained by the procedure developed in the current work and the experimental findings at different levels of volume fraction of martensite (Vm). It is also shown that the GT parameters obtained using a calibrating procedure, which ignores the local deformation behavior of the material, does not produce the appropriate parameter values.

A closed-form solution for stresses at curved free edges in composite laminates: A variational approach

An analytical method developed for determining the interlaminar stresses at straight free boundaries is extended to predict the free-edge stresses at curved boundaries of symmetric composite laminates under inplane loading. The three-dimensional (3D) stress distribution in laminates with curved boundaries is approximately described on the basis of a zero-order approximation of the boundary-layer theory. The related stress functions are found by minimization of complementary energy and the variational principle, and satisfy zero-order equilibrium equations, boundary conditions and traction continuity at interfaces between plies. Results obtained from the present method are compared with those in the literature for laminates with both straight edges and circular holes. This method provides a computational technique with the advantage of simplicity and versatility for analyzing the 3D stresses in composite laminates of general ply orientations with curved boundaries.

A New Approach for Single Crystal Materials Analysis: Theory and Application to Initial Yielding

A new approach used for single crystal (SC) materials analysis is described. Its principle is based on the extension of predictive models for isotropic material behavior to anisotropic materials such as SC nickel base superalloys. A viscoplastic model describes the material in the macroscopic level while a factor, based on the crystallographic approach, accounts for the global state of micro slip on the crystal. The combination of both elements defines the so-called combined approach (CA). This paper presents the development of the theory and its applications to the determination of initial yielding and tension-compression asymmetry.

Cutting load capacity of end mills with complex geometry

Cutting load capacity of cemented carbide end mills with high length-to-diameter ratios is determined from critical geometric and loading parameters, including a stress concentration factor (SCF) to account for serrated edges, which is determined by finite element analysis. Tensile strengths are characterised using a statistical Weibull analysis from 4-point bend tests of cemented carbide blanks of two different diameters. The approach is used to predict probability of survival for cutters under different loading conditions. Results are compared to measured failure cutting loads under service conditions as well as to those measured in static three point bend tests.

Influence of strain distribution on microstructure evolution during rod-rolling

A thermo-viscoplastic constitutive model is used in an explicit finite element analysis to determine the strain distribution that develops during rod rolling. The computed strains are then used to predict the resulting grain size of the material microstructure. Predictions are compared to experiments conducted on rolling of square billets. There is good agreement between the measured and predicted roll loads and torques as well as on the deformed geometry. The ability of the analysis to predict the resulting heterogeneous microstructure is also demonstrated.

On the propagation of elastic-viscoplastic waves in damage-softened polycrystalline materials

Abstract Theories for uniaxial wave propagation as, for example, along the longitudinal axis of slender rods composed of materials that behave elastically or plastically with hardening, encounter difficulty when confronted with softening material. For such theories, onset of softening causes the value of the wave speed to become complex thereby transforming the governing partial differential equations from hyperbolic to elliptic, implying no further possibility for wave-like motion in the softened material. The purpose of this paper is to show how an elastic-viscoplastic-damage type of constitutive theory together with the equation of motion produce a system of governing partial differential equations that can be shown to be hyperbolic. As an outgrowth of the calculation for the characteristics of the system, an expression relating the elastic dilatational wave speed with material damage and softening can be derived, demonstrating positive value for all phases of the material deformation including material softening that terminates in fracture. The paper also shows how experimental data from plate impact spall fracture tests can illustrate the reality of wave motion through damage-softened polycrystalline material.

Investigation of Structural and Material Effects on Crashworthiness of Advanced High Strength Steel Columns

An experimental and numerical study was performed to evaluate the crashworthiness of several advanced high strength steels. The behavior of two Dual Phase (DP) steels and an HSLA steel are compared by examining the crush response of longeron column specimens, experimentally and computationally. The closed section columns, fabricated by spot welding formed channel sections, in both single hat and double hat configurations were exposed to 182 kg and 454 kg axial impacts at different velocities. Final column height and impact force history were recorded and compared with results of finite element simulation of the columns. Good agreement was found between experiments and computations.Copyright