Bradley Martin

Compressive behavior of fine sand.

The compressive mechanical response of fine sand is experimentally investigated. The strain rate, initial density, stress state, and moisture level are systematically varied. A Kolsky bar was modified to obtain uniaxial and triaxial compressive response at high strain rates. A controlled loading pulse allows the specimen to acquire stress equilibrium and constant strain-rates. The results show that the compressive response of the fine sand is not sensitive to strain rate under the loading conditions in this study, but significantly dependent on the moisture content, initial density and lateral confinement. Partially saturated sand is more compliant than dry sand. Similar trends were reported in the quasi-static regime for experiments conducted at comparable specimen conditions. The sand becomes stiffer as initial density and/or confinement pressure increases. The sand particle size become smaller after hydrostatic pressure and further smaller after dynamic axial loading.

Dynamic Tensile Experimental Techniques for Geomaterials: A Comprehensive Review

This review article is dedicated to the Dynamic Behavior of Materials Technical Division for celebrating the 75th anniversary of the Society for Experimental Mechanics (SEM). Understanding dynamic behavior of geomaterials is critical for analyzing and solving engineering problems of various applications related to underground explosions, seismic, airblast, and penetration events. Determining the dynamic tensile response of geomaterials has been a great challenge in experiments due to the nature of relatively low tensile strength and high brittleness. Various experimental approaches have been made in the past century, especially in the most recent half century, to understand the dynamic behavior of geomaterials in tension. In this review paper, we summarized the dynamic tensile experimental techniques for geomaterials that have been developed. The major dynamic tensile experimental techniques include dynamic direct tension, dynamic split tension, and spall tension. All three of the experimental techniques are based on Hopkinson or split Hopkinson (also known as Kolsky) bar techniques and principles. Uniqueness and limitations for each experimental technique are also discussed.

A New Kolsky Bar Dynamic Spall Technique for Brittle Materials

For many years there have been controversial opinions on the value of dynamic tensile data obtained through the Kolsky bar spall tension technique. This is primarily due to the experimental conditions (i.e., specimen stress state and strain rate) not being well defined for these types of tests, thus making data interpretation and comparisons difficult. In this paper, a new spall theory is presented that ensures constant strain rate deformation while maintaining a uniform tensile stress within a large portion of the specimen. In light of this theoretical framework, pulse shaping was carefully designed to experimentally obtain this solution in Kolsky bar experiments. The incident wave generated under the guideline of the new theory has shown promising results for better refined Kolsky bar spall tension experiments on brittle materials.

Analysis of a Soft Catch for Conventional Warheads

A “soft catch” is a device with which an explosively formed projectile can be decelerated to zero velocity without sustaining significant damage. The recovered projectile provides data, via metallurgical analysis, on the deformation conditions found within the explosively formed projectile. At Eglin AFB, FL, the soft catch consists of a sequence of sections (Figures 1–3), each roughly one meter long, filled with various soft media. Velocity screens are placed at the entrance and exit of each section. This enables investigators to experimentally determine the time at which the projectile passes each station in the catch. Based on these experimental measurements, average velocity estimates for each section of the soft catch can be made. The purpose of this paper is to support the soft catch design process with a one-dimensional analysis. The mathematical modeling is based on observations presented in studies by Allen, Mayfield, and Morrison [1,2]. Their work addresses the penetration of sand, but their modeling is appropriate for materials in the soft catch. The current paper describes application of their model to interpreting three soft catch experiments where Tantalum projectiles with initial velocities of approximately 1400 m/s were successfully recovered.Copyright

Mechanical Response and Damage Evolution of High-Strength Concrete Under Triaxial Loading

Current weapons effects modeling efforts rely heavily on quasi-static triaxial data sets. However, there are fundamental knowledge gaps in the current continuum modeling approach due to limited experimental data in the areas of dynamic effects and damage evolution. Arbitrary scalar values used for damage parameters have experimentally unverified mathematical forms that often do not scale to different geometries, stress states, or strain rates. Although some preliminary tests have been performed through dynamic triaxial compression experiments, the results are difficult to interpret due to changes in specimen diameter and length-to-diameter ratio. In this study, a high-strength concrete (f’c ∼130 MPa) was investigated under triaxial loading conditions at confining pressures up to 300 MPa. Three cylindrical specimen sizes were used to determine size effects, including 50 × 114 mm, 25 × 50 mm, and 25 × 13 mm. For a limited number of specimens, X-Ray Computed Microtomography (XCMT) scans were conducted. It was noted that size and length-to-diameter ratio have substantial effects on the experimental results that must be understood to determine dynamic effects based on specimen geometries used in dynamic triaxial compression experiments. Additionally, by quantifying pore crushing and crack development under a variety of triaxial loading conditions, future multi-scale modeling efforts will be able to incorporate systematically defined damage parameters that are founded on experimental results.

Experimental Investigation on Dynamic Behavior of Ultra High Performance Composite Concrete with Kolsky Bar

Cor-Tuf is ultra high performance composite concrete consisting of pozzolanic powder, fine aggregates and steel fibers. Its compressive strength is more than 200 MPa, five times larger than conventional concrete. We conducted compressive Kolsky bar experiments on Cor-Tuf to investigate the behavior of Cor-Tuf under dynamic loading. Considering the inhomogeneity of concrete, we used two types of samples with 19 and large 76 mm diameters. To see the effect of steel fibers to bulk properties of concrete, both samples with and without steel fibers were used. We found that the strength was affected by strain rate.

Dynamic Strength Estimates for a High Strength, Experimental Steel

For several years, the Air Force has been engaged in the development of high velocity air to surface missiles. The objective is to replace larger, high mass weapons with smaller, more versatile projectiles that can achieve the same goals. The reduction of mass requires that the impact velocity be increased to meet the performance requirements. This has presented researchers with several challenges. First, the steel must be such that it survives the initial shock at impact. Second, because the tunnel is long, the material must resist friction and wear, which could erode the projectile nose, thereby degrading performance. The purpose of this paper is to present the results of dynamic testing of an experimental, high-strength steel. Using a one-dimensional model for the Taylor cylinder test, the constitutive behavior of the steel as a function of strain and strain-rate can be assessed through a strain-rate of roughly 105 /second. This behavior is consistent with that required for successful modeling of the response of a penetrator casing in the ultra-ordinance velocity range.Copyright