TECHNOLOGY

—by Richard Mandel

Fatigue rosettes. Lockheed Martin Aeronautics Company and Direct Measurements Inc. (DMI), Columbia, SC, have completed phase-one tests of a new strain/fatigue gage technology. DMI uses laser bonded SSR (Symbolic Strain Rosette) fatigue gages and a handheld, field-ready instrument to measure fatigue damage on a part’s surface. The SSR fatigue gage is a highly scalable, 2D pattern (or compressed symbol) that, when read and analyzed by DMI instruments, provides fatigue damage readings throughout a part’s operational life. Applied wherever fatigue monitoring is needed, SSRs require no wiring or electrical connections, and can endure harsh operating and environmental conditions. Simulated maintenance conditions using 2024-T3 aircraft-aluminum test specimens provided a number of conclusive observations regarding the DMI technology. Static tensile tests demonstrated effectiveness in making on-board strain measurement in operational (non-laboratory) applications. In a single-cycle-to-failure test, the DMI instrument measured strain magnitudes greater than 100,000 micro-strain, far exceeding the capabilities of conventional foil gages. A third test involved fatigue testing near a hole to simulate the stress concentrations at a rivet or fastener. DMI’s fatigue gage technology measured fixed plastic strains (i.e. fatigue damage) in the vicinity of the hole prior to the formation of visible cracks. “The ability to measure structural strains under ground testing or flight conditions, without the need for bulky, intrusive cabling or costly sensor installations, would be an enormous benefit,” says Marc Wood, Lockheed Martin Aeronautics’ principal engineer for structural testing. A second phase of tests will include fatigue specimens designed to simulate riveted lap-joints. Additional early planning is underway for possible in-flight testing of the DMI technology on select components in Lockheed Martin aircraft. DMI is targeting a Q2-05 release of a benchtop instrument product for use in testing and laboratory environments. In addition, DMI is finalizing a bundled solution that integrates their fatigue detection and monitoring technology with finite element analysis. “By combining the two technologies, DMI can feed actual product lifecycle management data back into the product-development process for cradle-to-grave model verification,” says William Ranson, President of DMI.

Direct Measurements Inc.,
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Lab-on-a-chip update. A diagnostic device being developed at the National Nuclear Security Administration’s Sandia National Laboratories promises better healthcare diagnostics for millions of Americans, replacing off-site labs for analysis and waiting days to obtain the vital information. Much of the research is centered on detection of gum disease from a patient’s saliva and gingival crevicular fluid (the fluid between the tooth and gum). Additionally, Sandia researchers are also developing tools to recognize cardiovascular disease markers such as C-Reactive protein. “We have taken technology that we’ve worked on for several years — Sandia’s lab-on-a-chip devices — and are adapting them for use in medical diagnostics,” says project lead Anup Singh. “We’ve tested saliva samples from healthy patients for gum disease, and within the next few months we will begin using the diagnostic tool to test diseased samples.” Expanding on established microchip-based separation technologies, the research team adapted a method known as immunoassay to a chip. The combination allows for fast and sensitive analysis of biomarkers specific to certain diseases. As part of the immunoassay process, antibodies specific for biomarkers of interest, such as gum or heart disease, are tagged with a fluorescent dye and then mixed with a patient’s saliva or blood. Biomarkers present in the sample attach themselves to the fluorescent antibody. The mixture is injected into a microchip using a syringe. An applied electric field forces the sample to flow through a microchannel that is 2 to 5 cm long, tens of microns deep, and a few hundred microns wide. As the sample moves through the channel, cast-in-place porous polymers in the microchannel sort molecules based on their sizes and electrical charges. If biomarkers for the disease are present in the patient’s sample, the lab-on-a-chip analysis will separate fluorescent antibodies bound to the biomarker from unbound antibodies. A photomultiplier tube then detects the fluorescence emission with extreme sensitivity. After quantifying the relative fluorescence of the two species — bound and unbound antibodies — researchers can determine the amount of biomarker present in the patient’s sample. The entire device, including the channeled glass chips, photomultiplier, and electronics, will fit into a hand-held package that weighs less than 5 lbs. “The beauty of this device is that it has everything required to make it useful — sensitivity, portability, and the ability to run tests quickly,” Singh says. “It is small and can be carried with ease almost everywhere. It’s also very sensitive and works fast. Within a few minutes you can tell if you have a diseased sample.”

Sandia National Labs,
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