Henry R. Piehler

Pathological considerations in replacement cardiac valves

The long-term outcome following cardiac valve replacement is primarily determined by three factors: irreversible cardiac and pulmonary pathology secondary to the valvular disease (especially left ventricular myocardial hypertrophy and degeneration, and pulmonary vascular disease), pre-existing cardiac disease, including congenital lesions and coronary arterial atherosclerotic occlusions, and prosthesis-host interactions. For most of the approximately 40,000 patients who undergo valve replacement each year in the United States, prosthesis-associated pathology is a a major determinant of prognosis. In this article, the significance, morphology, and pathogenesis of the major complications and other alterations during function of mechanical, bioprosthetic, and allograft valves are reviewed. Other reviews of pathologic considerations in cardiac valve replacement are available (l-7).

Formability of sandwich sheet materials in plane strain compression and rolling

The stress states developed during room temperature, plane strain compression modes of deformation of stainless steel clad aluminum and aluminum clad strainless steel sheets have been investigated in order to gain insight into the formability of bonded ductile sandwich sheet materials in primary metalworking processes. Assuming uniform, isostrain deformation in the component layers, sandwich compression stress-strain curves were predicted to be rule of mixtures averages of component compression stress-strain curves. These predictions showed good agreement with experimental data when friction and in-homogeneous deformation were taken into account. Since the through-thickness applied pressure can be assumed to be the same in both components of thin sandwich sheet materials, in-plane stresses which are tensile in the harder component and compressive in the softer component of a clad sheet are developed in order to satisfy the yield conditions. The nature of these in-plane stresses was confirmed by measurements of residual stress distributions in rolled clad sheet specimens, and it was shown how the tensile stress in the harder component may lead to unstable flow and failure of this component during forming. The observed failures were similar in both plane-strain indentation and rolling tests. Although the initiation of instability in symmetric clad sheet metals appears to be independent of the arrangement of the component layers, the process of final localization leading to fracture was observed to depend heavily on the layer arrangement.

Engineering careers, job rotation, and gatekeepers in Japan and the United States

Abstract Japans employment system is generally characterized as involving infrequent moves between firms, but regular and systematic transfers within the firm. This system might be expected to result in Japanese engineers having smaller interfirm networks of professional contacts than Americans. This could impede the efficient interfirm diffusion of new technology. On the other hand Japanese engineers might be expected to have larger intrafirm networks of professional contacts. This should facilitate the intrafirm transfer of technical information. The research reported here tests the applicability of our images of Japanese career practices to engineering careers, replaces the simple dichotomous characterizations of differences between the U.S. and Japan with data suggesting the degrees of difference, and explores linkages between personnel practices and the transfer of information by engineers in both countries.

An analysis of constrained deformation by slip and twinning in hexagonal close packed metals and alloys

A procedure for predicting the isostrain behavior of hcp metals and alloys which deform by basal, prism, and pyramidal slip as well as twinning is developed. This procedure assumes that deformation will occur on four slip systems and one or more twinning system(s). Stress states are derived for the simultaneous operation of all combinations of four slip systems for allc/a ratios as well as all values of the three critical-resolved shear stresses for slip. A numerical procedure for obtaining the remaining kinemati-cally required deformation by twinning is outlined for the case of impure titanium.

Forming limits of sandwich sheet materials

Failure of sandwich sheet materials by tensile instability and localized necking was studied by performing punch-forming experiments on stainless steel clad aluminum. By using narrow blanks and no lubrication, lateral contraction was possible, and failures could be produced in the drawing area of the forming limit diagram. For this deformation regime, diffuse instability led to localized necking. As in monolithic materials, the development of the localized neck in stainless steel clad aluminum determined the forming limit, and predictions of the strain levels for the onset of local instability correlated well with the observed forming limit strains. By preventing lateral contraction, failures in stretching were produced. The forming limit strains in this case depended on the strains at the onset of diffuse instability in much the same manner as is observed for monolithic materials. The strains at the onset of diffuse instability were predicted using a generalized rule of mixtures, and agreement between measured values and values predicted from component properties was good when the strain-path dependence of the instability strain for the individual components was taken into account. The diffuse necking process in stretching of stainless steel clad aluminum led to local thinning when deformations involved small degrees of biaxiallity. On the other hand, nonuniform through thickness straining of the component layers in specimens strained close to balanced biaxial stretching appeared to control the localization process and gave rise to forming limit strains lower than expected from observations of punch formed monolithic sheet materials. For all deformation modes, localized flow culminated in delamination and fracture.

Design of metal-matrix composite consolidation practices based on the foil/fiber/foil approach

Abstract The main issues that influence the processing of continuous fiber MMCs based on the foil/fiber/foil approach were examined in order to develop a quantitative method for optimizing such processes. Because of the form of the describing equations, simple graphical means to obtain design data were developed and applied for Ti-6A1-4V/SCS-6 fiber composites. For this particular system, a range of processing parameters that can produce fully dense composites with optimal reaction zone thickness, subject to certain temperature and time constraints, was identified.

On the nature and crystallographic orientation of subsurface cracks in high cycle fatigue of Ti-6Al-4V

Subsurface fatigue damage, in the form of cracking of the α phase, was observed in Ti-6A1-4V during high cycle fatigue of total hip prostheses tested in a simulated physiological test geometry and environment. The subsurface cracking was found only in the region of highest fatigue stresses and was present in a zone between 50 and 700 μm beneath the surface. The density of these cracks appeared to depend on the fabrication process used to form the part, where the direction of forging deformation strongly influenced the texture and grain morphology of the near-α bimodal microstructure. A novel scanning electron microscopy (SEM) technique, using selected area channeling patterns (SACPs) and electron channeling contrast imaging (ECCI), is described and was used to determine the crystallographic orientation of the fracture plane in the a phase. The texture resulting from the forming operation appeared to be such that the basal pole of the hcp lattice became oriented in the direction of flow. Also, the deformation substructure (in the form of dislocation subcells) influenced the formation of the subsurface cracks. Observations based on four independent fractured grains, using the channeling analysis techniques, indicated that the fracture plane for these subsurface fatigue cracks is the pyramidal plane of the hcp lattice.

Modeling of hot isostatic pressing and hot triaxial compaction of Ti-6Al-4V powder

Abstract The accuracy of a hybrid continuum–micromechanical model for predicting the rate-dependent consolidation of metal powder was established using measurements from hot isostatic pressing (HIP) and hot triaxial compaction (HTC) experiments. The experiments were performed with PREP® Ti–6Al–4V powder encapsulated in thin-walled containers and yielded relative density vs time and height vs relative density data for direct comparisons with model predictions. The model was found to underpredict the densification rate in HIP, primarily during the early stages of consolidation, and the degree of consolidation during HTC. Adjustments to the values of the strain-rate sensitivity or a state variable representing the geometric structure of powder compacts resulted in good agreement between hybrid model predictions and measurements. The physical basis for discrepancies between predictions and measurements and justification for the postulated/required parameter adjustments were attributed to two factors: (1) differences in microstructure between the unconsolidated powder and the fully consolidated material used in the compression testing to obtain the material coefficients for the model; and (2) interparticle bonding characteristics/particle sliding effects not taken into account in determining the material properties for the model.

Grain egression: A new mechanism of fatigue-crack initiation in Ti-6Al-4V

A new mechanism of fatigue-crack initiation (FCI), grain egression, was observed in the course of investigating corrosion-fatigue crack initiation in Ti-6A1-4V hip prostheses fabricated using three different processes. Extensive scanning electron microscopy (SEM) was used to document this new mechanism as well as the other FCI mechanisms operating. Grain egression entails the fracture and egression of primary α grains from the surface of the sample, resulting in a sharp pit that subsequently acts as the site of crack initiation. The different sizes and morphologies of the grain-egression sites observed are very similar to the sizes and morphologies of the pri-mary α grains resulting from the three different fabrication processes, providing further evidence for grain egression as an operative FCI mechanism.

Yielding anisotropy from the bauschinger effect and crystallographic texture in drawn HSLA steel sheet

The influence of prior deformation by in-plane compression, a laboratory technique used to simulate drawing in sheets, on the yield-strength anisotropy of an 80-ksi minimum yield-strength vanadium-nitrogen HSLA steel has been documented and the origin of this anisotropy has been investigated. A large tensile yield-strength anisotropy (40 to 50 ksi difference between orthogonal directions in the plane of the sheet) was observed for all prior effective strains investigated (from εe ≃ 0.1 to εe ≃ 0.5). Based on: (1) an analysis of Tresca-type yield loci obtained before and after in-plane compression, and (2) yield-strength anisotropy calculations performed with a Taylor model and a Sachs model using results of crystallographic texture measurements, it was concluded that the tensile yield-strength anisotropy originates primarily from the Bauschinger effect. The anisotropy due to texture is much smaller (15 pct of the total yield strength difference) and of opposite sign.