Nathan K. Brown

Environmentally Assisted Cracking Measurements in Structural Aluminum Alloys Under Accelerated Test Conditions

Environmentally assisted cracking (EAC) of aluminum alloys in corrosive atmospheres is an important maintenance and safety issue for U.S. Department of Defense assets. EAC initiation and propagation of cracks is influenced by the complex interactions of load, environment, and alloy properties. Traditional environmental fracture testing conducted under immersion or constant humidity conditions may produce results that are different than measurements collected under thin electrolyte layers or droplets formed during atmospheric exposure. In addition, most standard methods do not provide instantaneous measures of crack velocity that can be used to identify specific environmental conditions that promote cracking. Improved assessment of EAC susceptibility and the conditions that promote cracking of aluminum alloys has been accomplished with an autonomous, in situ measurement system that can be used in accelerated corrosion test chambers and outdoor exposure sites. Continuous measurements of crack length through...

Identification of material damage precursors using nonlinear ultrasonics

The primary goal of this research effort is to develop nondestructive evaluation techniques capable of detecting material damage precursors mainly for turbine engine materials under low and high-cycle fatigue testing. The experimental results presented in this paper show a significant increase of the relative acoustic nonlinearity, βr, in aluminum and Ni-based superalloy fatigued specimens. While in agreement with the prior research, the main advantage of the current technique over the previous methods is that the ultrasonic beam may be focused to inspect the presence of damage precursors at localized stress concentrator site. For example, when the ultrasonic beam travelled through the root of the round-notched specimens, the acoustic nonlinearity exhibited an increase of approximately 450% as compared to the pristine specimens. This procedure will be further developed to detect damage precursors in propulsion components undergoing thermo-mechanically fatigue to quantify their remaining useful life.

Relationship between electrochemical processes and environment-assisted crack growth under static and dynamic atmospheric conditions

Abstract Environment-assisted cracking (EAC) of aluminum alloys in corrosive atmospheres is a significant maintenance and safety issue. EAC is influenced by the interaction of stress, environment, and microstructure. Atmospheric conditions and corrosion kinetics are dynamic due to diurnal cycles and changing operating conditions. Temperature, relative humidity, and surface contaminants interact to control thin-film electrolyte properties. Within a crack, the separation of the anode and cathode may occur due to concentration gradients between the crack tip, mouth, and external surface. Conventional immersion testing is not well suited to study factors and interactions leading to atmospheric EAC because the bulk electrolyte conditions for immersion testing are different from the thin-film properties. Additionally, standard three-electrode immersion measurements are not well suited to directly investigate the variation and distribution of cathodic and anodic currents on a sample surface and within an EAC crack. In this work, atmospheric electrochemical tests have been conducted using a segmented, multielectrode sensor with an artificial crevice to quantify local, dynamic anodic and cathodic current distributions. These tests are compared to EAC growth rate measurements. Maximum EAC growth rates are observed when high cathodic current is measured at the tip of artificial crevices, suggesting a hydrogen embrittlement mechanism.

Energy harvesting to power embedded condition monitoring hardware

The shift toward condition-based monitoring is a key area of research for many military, industrial, and commercial customers who want to lower the overall operating costs of capital equipment and general facilities. Assessing the health of rotating systems such as gearboxes, bearings, pumps and other actuation systems often rely on the need for continuous monitoring to capture transient signals that are evidence of events that could cause (i.e. cavitation), or be the result of (i.e. spalling), damage within a system. In some applications this can be accomplished using line powered analyzers, however for wide-spread monitoring, the use of small-scale embedded electronic systems are more desirable. In such cases the method for powering the electronics becomes a significant design factor. This work presents a multi-source energy harvesting approach meant to provide a robust power source for embedded electronics, capturing energy from vibration, thermal and light sources to operate a low-power sensor node. This paper presents the general design philosophy behind the multi-source harvesting circuit, and how it can be extended from powering electronics developed for periodic monitoring to sensing equipment capable of providing continuous condition-based monitoring.