Christopher U. Brown

Time-Dependent Circumferential Deformation of Cortical Bone Upon Internal Radial Loading

Short and long duration tests were conducted on hollow femoral bone cylinders to study the circumferential (hoop) creep response of cortical bone subjected to an intramedullary radial load. It was hypothesized that there is a stress threshold above which nonlinear creep effects dominate the mechanical response and below which the response is primarily determined by linear viscoelastic material properties. The results indicate that a hoop stress threshold exists for cortical bone, where creep strain, creep strain rate and residual strain exhibited linear behavior at low hoop stress and nonlinear behavior above the hoop stress threshold. A power-law relationship was used to describe creep strain as a function of hoop stress and time and damage morphology was assessed.

Recommended Protocol for Round Robin Studies in Additive Manufacturing

One way to improve confidence and encourage proliferation of additive manufacturing (AM) technologies and parts is by generating more high quality data describing the performance of AM processes and parts. Many in the AM community see round robin studies as a way to generate large data sets while distributing the cost among the participants, thereby reducing the cost to individual users. The National Institute of Standards and Technology (NIST) has conducted and participated in several of these AM round robin studies. While the results of these studies are interesting and informative, many of the lessons learned in conducting these studies concern the logistics and methods of the study and unique issues presented by AM. Existing standards for conducting interlaboratory studies of measurement methods, along with NISTs experience, form the basis for recommended protocols for conducting AM round robin studies. The role of round robin studies in AM qualification, some of the limitations of round robin studies, and the potential benefit of less formal collaborative experiments where multiple factors, AM machine being only one, are varied simultaneously are also discussed.

Interlaboratory Study for Nickel Alloy 625 Made by Laser Powder Bed Fusion to Quantify Mechanical Property Variability

Six different organizations participated in this interlaboratory study to quantify the variability in the tensile properties of Inconel 625 specimens manufactured using laser powder bed fusion-additive manufacturing machines. The tensile specimens were heat treated and tensile tests were conducted until failure. The properties measured were yield strength, ultimate tensile strength, elastic modulus, and elongation. Statistical analysis revealed that between-participant variability for yield strength, ultimate tensile strength, and elastic modulus values were significantly higher (up to four times) than typical within-participant variations. Only between-participant and within-participant variability were both similar for elongation. A scanning electron microscope was used to examine one tensile specimen for fractography. The fracture surface does not have many secondary cracks or other features that would reduce the mechanical properties. In fact, the features largely consist of microvoid coalescence and are entirely consistent with ductile failure.

A Case Study of 3D Imaging Productivity Needs to Support Infrastructure Construction

The national infrastructure system is at a crossroads with a need for renewal and expansion in the most efficient manner possible. Light railway construction requires the installation of embedments in reinforced concrete pavement along the length of elevated sections of the railway system. Conventionally, wooden dowels are manually placed into the reinforcing steel mat before concrete placement to form the slot for the embedments; however this is labor intensive and can yield inconsistent spacing. An alternative method is digitally mapping the locations of the reinforcing steel-free space prior to concrete placement to identify where holes could be drilled without hitting the steel reinforcement. The challenge is avoiding impacting production. Using field-based data, this study identifies the number of hours to create the map without impacting production for a typical railway section. Discrete event simulation (DES) modeling is utilized to conduct the analysis. To substitute the alternative method, scanning a typical railway section falls within the capabilities of most laser scanning technologies; however, the processing of images to create a useable model controls. This research demonstrates a case study of applying DES to analyze productivity impacts on a repetitive process and investigates the capabilities of 3D imaging technologies for effective field use. PROJECT BACKGROUND Traditionally, the resources required to support a construction crew included manual labor, equipment, and materials. Over time as information systems and sensing agents have advanced in their capabilities and durability, advantages are becoming more apparent in providing greater information and automation to crews as well. If the realm of construction resources is to be expanded to include information, then the pertinent restraints to providing the necessary information when required has to be planned as is the case with labor, equipment, and materials. The following case

The Relationships Between Femoral Cortex Geometry and Tissue Mechanical Properties

Bone tissue and geometry are constantly modified through modeling and remodeling at the periosteal, endosteal and intracortical envelopes. Results from several studies indicate that femoral bone geometry is a predictor of whole bone strength (e.g. femoral neck strength), however, it is not known whether there is a relationship between bone structural and material properties. Bone geometry can be determined from parameters based on plane X-ray radiogrammetry which are used to evaluate femoral bone quality for implant success. If there is a relationship between these parameters and tissue mechanical properties, this would have implications in the interpretation of such parameters for assessment of fracture risk and in further understanding of bone biology. Following measurement of radiogrammetric parameters from antero-posterior and medio-lateral X-rays (cortical thickness, bone diameter, bone area, moment of inertia, cortical index, Singh index), human femurs were machined into standard test specimens for assessment of tensile fracture toughness (GIc) of the tissue. Results indicated that tensile fracture toughness generally increased with increasing bone size. We also found that fracture toughness of the tissue was significantly related to radiogrammetric indices and that some of these indices explained a greater variability in toughness than porosity, age or gender.

A Survey of Standards for Product Lifecycle Management of Structural Composites

Composite materials are widely used in the aerospace, automotive, and sports industries. Designing a composite product requires highly concurrent processes including manufacturing and assembly considerations. This paper examines existing classification schemes for composite materials based on reinforcement and matrix type. Next we review various research, industry, and standards organization efforts to develop computer representation models of composite materials. We then present a survey of specification and verification standards for composite materials, highlighting their gaps and overlaps. Key findings of our analysis are that standard test methods are lacking for particle-reinforced composite materials, and that existing bonding inspection methods and computer-aided design presentation methods for layered composite materials need improvement.