Blake Barnett

This study investigates the correlation between sinusoidal interface wave characteristics and the shear strength of explosion welded dissimilar metal joints. Interface geometries were extracted from backscatter electron (BSE) microscopy images of aluminum-stainless steel specimens using a custom Python code. Shear strength was measured by orienting specimens to load parallel to the interface plane using a hydraulic universal testing machine. Pearson correlation analysis and multiple linear regression were used to assess the influence of wave parameters on mechanical performance. Shear strength increased with both higher interface frequency and higher wavelength but decreased with increasing amplitude, with spatial frequency being the dominant factor. These findings provide new insights for optimizing explosion welding parameters to maximize joint performance.

Mechanical hardness is classically defined as force divided by indented area. Hardness (dynamic) is alternately sometimes defined based on indentation energy absorbed normalized by displaced volume. This elementary study compares static and dynamic hardness using standard definitions. Hardness is often observed to increase with strain rate and this is commonly interpreted as strain-rate hardening by mechanisms such as dislocation drag. This analysis considers the simplest situation – ballistic indentation of an elastic-perfectly plastic material without rate dependence. Results from both Coupled Eulerian-Lagrangian (CEL) and Smoothed Particle Hydrodynamics (SPH) show a linear correlation between indenter impact speed and dynamic hardness. Extrapolating to an impact speed of 0 m/s converges to static hardness. Neither approach demonstrates indenter size dependence. High impact speeds also can introduce shock. It is suggested that speed and shock hardening effects can account for the increase in dynamic hardness with increased indenter speed without strain-rate-hardening.

Commercial/military fixed-wing aircraft and rotorcraft engines often have to operate in significantly degraded environments consisting of sand, dust, ash, and other particulates. Marine gas turbine engines are subjected to salt spray, while the coal-burning industrial power generation turbines are subjected to fly ash. The presence of solid particles in the working fluid medium has an adverse effect on the durability of these engines as well as performance. Typical turbine blade damages include blade coating wear, sand glazing, calcia–magnesia–alumina–silicate (CMAS) attack, oxidation, and plugged cooling holes, all of which can cause rapid performance deterioration including loss of aircraft. This research represents the complex thermochemomechanical fluid structure interaction problem of semimolten particulate impingement and infiltration onto ceramic thermal barrier coatings (TBCs) into its canonical forms. The objective of this research work is to understand the underpinning interface science of interspersed graded ceramic/metal and ceramic/ceramic composites at the grain structure level for robust coatings and bulk material components for vehicle propulsion systems. This research enhances our understanding of the fundamental relationship between interface properties and the thermomechanical behavior in dissimilar materials for materials by design systems, and creates the ability to develop and fabricate materials with targeted macroscale properties as a function of their interfacial behavior. This project creates a framework to enable the engineered design of solid–solid and liquid–solid interfaces in dissimilar functionalized materials to establish a paradigm shift toward science from the traditional empiricism in engineering TBCs and high temperature highly loaded bulk materials. An integrated approach of modeling and simulation, characterization, fabrication, and validation to solve the fundamental questions of interface mechanisms which affect the properties of novel materials will be validated to guide component material solutions to visionary 2040+ military vehicle propulsion systems.

Gas turbine engines for military/commercial fixed-wing and rotary wing aircraft use thermal barrier coatings in the high-temperature sections of the engine for improved efficiency and power. The desire to further make improvements in gas turbine engine efficiency and high power-density is driving the research and development of thermal barrier coatings with the goal of improving their tolerance to fine foreign particulates that may be contained in the intake air. Both commercial and military aircraft engines often are required to operate over sandy regions such as in the middle-east nations, as well as over volcanic zones. For rotorcraft gas turbine engines, the sand ingestion is adverse during take-off, hovering near ground, and landing conditions. Although most of the rotorcraft gas turbine engines are fitted with inlet particle separators, they are not 100% efficient in filtering fine sand particles of size 75 microns or below. The presence of these fine solid particles in the working fluid medium has an adverse effect on the durability of turbine blade thermal barrier coatings and overall performance of the engine. Typical turbine blade damage includes blade coating wear, sand glazing, Calcia-Magnesia-Alumina-Silicate (CMAS) attack, oxidation, and plugged cooling holes, all of which can cause rapid performance deterioration including loss of aircraft. The objective of this research is to understand the fine particle interactions with typical turbine blade ceramic coatings at the microstructure level. Finite-element based microstructure modeling and analysis has been performed to investigate particle-surface interactions, and restitution characteristics. Experimentally, a set of tailored thermal barrier coatings and surface treatments were down-selected through hot burner rig tests and then applied to first stage nozzle vanes of the gas generator turbine of a typical rotorcraft gas turbine engine. Laser Doppler velocity measurements were performed during hot burner rig testing to determine sand particle incoming velocities and their rebound characteristics upon impact on coated material targets. Further, engine sand ingestion tests were carried out to test the CMAS tolerance of the coated nozzle vanes. The findings from this on-going collaborative research to develop the next-gen sand tolerant coatings for turbine blades are presented in this paper.

Although highly desired because of their low density, magnesium (Mg) alloys have long been passed over for military applications in favor of aluminum alloys due to their low strength and poor corrosion resistance. However, continued advances in Mg alloy development have resulted in a renewed interest within the U.S. Army regarding their use in both structural and protection applications. As a result, in 2009, the U.S. Army Research Laboratory initiated a 5-year research effort to understand the fundamental scientific mechanisms governing the mechanical properties and corrosion performance behavior of Mg alloys. This report details the results of in-house and collaborative efforts in areas such as enhancing mechanical properties through grain refinement and minimizing texture, improving corrosion performance through coatings and elemental additions, and computational modeling to identify underlying mechanisms that govern ductility and processing pathways. The knowledge and insight gained from this multifaceted approach has advanced the state of the art for Mg alloys, thereby increasing their potential insertion into Army systems.