Arthur E. Nicholls

Machine vision photogrammetry: a technique for measurement of microstructural strain in cortical bone

Understanding local microstructural deformations and strains in cortical bone may lead to a better understanding of cortical bone damage development, fracture, and remodeling. Traditional experimental techniques for measuring deformation and strain do not allow characterization of these quantities at the microstructural level in cortical bone. This study describes a technique based on digital stereoimaging used to measure the microstructural strain fields in cortical bone. The technique allows the measurement of material surface displacements and strains by comparing images acquired from a specimen at two distinct stress states. The accuracy of the system is investigated by analyzing an undeformed image set; the test image is identical to the reference image but translated by a known pixel amount. An increase in the correlation sub-image train parameter results in an increase in displacement measurement accuracy from 0.049 to 0.012 pixels. Errors in strain calculated from the measured displacement field were between 39 and 564 microstrain depending upon the sub-image train size and applied image displacement. The presence of a microcrack in cortical bone results in local strain at the crack tip reaching 0.030 (30,000 microstrain) and 0.010 (10,000 microstrain) near osteocyte lacunae. It is expected that the use of this technique will allow a greater understanding of bone strength and fracture as well as bone mechanotransduction.

Analytical Model of the Confined Compression Test Used to Characterize Brittle Materials

Numerical and analytical simulations of projectiles penetrating brittle materials such as ceramics and glasses are a very challenging problem. The difficulty comes from the fact that the yield surface of brittle materials is not well characterized (or even defined), and the failure process may change the material properties. Recently, some works have shown that it is possible to characterize and find the constitutive equation for brittle materials using a confined compression test, i.e., a test where a cylindrical specimen, surrounded by a confining sleeve, is being compressed axially by a mechanical testing machine. This paper focuses on understanding the confined compression test by presenting an analytical model that explicitly solves for the stresses and strains in the sample and the sleeve, assuming the sleeve is elastic and the specimen is elastoplastic with a Drucker-Prager plasticity model. The first part of the paper briefly explains the experimental technique and how the stress-strain curves obtained during the test are interpreted. A simple and straightforward approach to obtain the constitutive model of the material is then presented. Finally, a full analytical model with explicit solution for displacements, strains, and stresses in the specimen and the sleeve is described. The advantage of the analytical model is that it gives a full understanding of the test, as well as information that can be useful when designing the test (e.g., displacements of the outer radius of the specimen).

Taylor Anvil Impact

G. I. Taylor showed that dynamic material properties could be deduced from the impact of a projectile against a rigid boundary. The Taylor anvil test became very useful with the advent of numerical simulations and has been used to infer and/or to validate material constitutive constants. A new experimental facility has been developed to conduct Taylor anvil impacts to support validation of constitutive constants used in numerical simulations. A 37‐mm diameter Hopkinson bar apparatus was adapted to conduct the Taylor anvil experiments. An adaptor was designed to ensure impact planarity of the projectile onto the anvil, which is made from VascoMax steel, backed by a 1.82‐m steel bar to provide inertial mass to the anvil and ensure deceleration of the projectile solely from elastic waves within the projectile. A digital imaging system was adapted to determine radial deformation as a function of length. Details of the experimental techniques, along with examples of experiments using 6061‐T6, are discussed. Nu...

Testing boron carbide under triaxial compression

This article focuses on the pressure dependence and summarizes the characterization work conducted on intact and predamaged specimens of boron carbide under confinement in a pressure vessel and in a thick steel sleeve. The failure curves obtained are presented, and the data compared to experimental data from the literature.

Evaluation of Welded Tensile Specimens in the Hopkinson Bar

The high strain rate behavior of a welded interface was evaluated using a split Hopkinson pressure bar (SHPB). The welds of interest are under-matched welds between identical aluminum alloys; the welds were processed using metal inert gas (MIG) welding. A direct tension bar setup was employed for the high strain rate testing. To accommodate both the weld and the heat affected zone in the gage length of the tensile specimen, it was necessary to use a longer specimen than is typically used for SHPB tensile testing. Limitations on specimen geometry and maintaining the weld bead intact were imposed to provide a specimen that was most representative of the material and application. Challenges associated with specimen design and testing in the pressure bar are discussed. Numerical simulations were employed to assist with specimen design and interpretation of the wave response. The experimental results obtained to date will be presented at the conference.

Dynamic Compression of Aluminum Foam Processed by a Freeform Fabrication Technique

The compressive deformation behavior of a new type of aluminum foam was assessed under static and dynamic loading conditions. The aluminum foam investigated was processed by Advanced Ceramics Research using an extrusion freeform fabrication technique. The foam contained approximately 50 to 60 % porosity. The dynamic compression response was evaluated in air using a split Hopkinson pressure bar (SHPB) system with aluminum bars, and strain rates ranging from 600 s−1 to 2000 s−1. Compression tests were also conducted at lower strain rates (10−3 s−1 to 4 s−1) to determine the extent of strain rate strengthening. The low strain rate tests were performed with a servo‐controlled hydraulic test machine. The results were analyzed as a function of foam density, structure, and process conditions.