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RELIEVING STRESS

MEMS technology detects crack growth

Many engineering structures, such as pressure vessels, pipelines, bridges, boats and aircraft, are subjected to stress in a corrosive environment. Because these conditions can cause structural failures, experimental models have been developed to determine a structure's susceptibility to stress corrosion cracking (SCC) and to measure the rate of crack propagation. One limitation of these models is that crack propagation may occur at a rate slower than the current limit for measurement. Rates below that level are too small to accurately predict whether SCC may lead to component failure during the structure's expected life cycle, given current test methods. However, small, lightweight sensors installed in critical areas of an aircraft could alert maintenance crews to replace a part before failure occurs.

Using MEMS technology, a miniature SCC test beam can be fabricated out of the structural material of interest and mounted on the MEMS chip. Such a test beam can detect crack growth based on either a resistive or capacitative measurement as a static stress load is applied to the test beam prior to installation, and either a static or a cyclic load is imparted to the beam during testing.

Engineers at SwRI have built a system that can detect the small change in resistance that results from a MEMS-based SCC sensor made of brass, operating in a corrosive environment. The system's usefulness depends on the structural materials available for the MEMS fabrication process.

As a crack grows, the cross-sectional area of the test beam is decreased. This change in cross section affects the resistivity of the test beam, which can be detected by the sensor. Because the test beam is so small, a small crack has a relatively large effect. This method is useful provided that the structural material is electrically conductive.

To make a capacitive measurement, a conductive plate is attached to one end of the test beam. This plate overlaps a series of stationary conductive plates on the chip. After a constant load is applied, the conductance can be measured between the test beam and the stationary plates to determine the position of the end of the test beam. As a crack begins to form, the test beam weakens and the applied load causes it to move more than it did initially. This additional motion is correlated with the crack growth to obtain a growth rate. Again, because the test beam is so small, a small crack has a relatively large effect on the beam's motion under a given load.

The MEMS fabrication process used by SwRI engineers is derived from the semiconductor industry and is material-dependent. The established processes are designed for conductive materials that have a minimized corrosion risk. The fabrication processes would have to be improved for materials such as aluminum and stainless steel before the sensor would be commercially viable. SwRI is pursuing funding to achieve this goal.

Numerous cases of structural failures from stress corrosion cracking (SCC) have been reported in boilers, pressure vessels, oil and gas production and transmission piping, nuclear power generation components, bridges, sea craft, aircraft and more. SwRI developed a MEMS device using structural engineering materials for sensitive crack growth rate measurements that facilitate SCC sensing and monitoring. A U.S. patent is pending.

Published in the Winter 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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