Innovations in Measuring Structural Integrity

Herbert Chelner, CEO and Chief Scientist
Dr. Robert A. Mueller, President and General Manager

Abstract

Micron Instruments has developed high sensitivity, long life, high resistance homogenous semiconductor strain gages that makes the wireless transmission of stresses in structures and through cement and other materials possible. The high gage resistance means little power is required for the measurement of these stresses; therefore, the sensor can operate wirelessly with a small coin-cell battery for as long as 10 years, and passively (with no battery) if necessary. No maintenance, repair, replacement or recharging are required.

Semiconductor Strain Gages

Semiconductor strain gages were discovered during the transistor era and became commercially available early in the 1950's. These gages may be homogeneous or diffused or silicon on sapphire (SoS) deposited. Diffused gages have a variety of problems that limits useful life and affects performance. SoS deposited gages have a much lower gage factor and are less corrosion resistant.

There are two main elements from which semiconductor gages are made. These elements are Geranium and Silicon and they can be P or N doped. At Micron Instruments, the P doped (Boron) Silicon gage is selected for the basic strain sensor, and the N doped Silicon is used for the temperature sensor. Silicon gages have been proven to be more stable and more corrosion resistant than Geranium gages.

The Micron Instruments Miniature P-doped Silicon Semiconductor Strain Gage

Micron’s strain gage is manufactured from a Boron Doped Silicon ingot grown as a single crystal. The strain gage crystalline axis used is where the longitudinal over the transverse ratio is maximized. The reverse is true for the Silicon temperature sensor. This means that the finished gages will be unidirectional and transverse strains will have no significant effect on performance when properly manufactured.

Gage shape is application sensitive. Semiconductor strain gages are normally bar shaped; the length and resistivity varies but the width is nominally 0.005 inches1 and the thickness 0.0005 inches for a bar gage. Normally, a gold lead is bonded to ends of the gage for electrical connection. For miniature sensors, it can be important that the gold
electrical leads come out the same end, requiring U shaped gages. The U shaped gage also has twice the resistance over the same length, making it desirable for small areas of high strain or for wireless applications where higher resistances are important. There are also M shaped gages, which provide four times the resistance of the same length bar gage when even higher resistance is required.

Structural Integrity Measurement Applications

Measuring structural integrity in tunnels, bridges, buildings, roadways and etc. is not only possible, it has been accomplished and demonstrated with impressive results.

Semiconductor strain gages are a critical element of structural health monitoring solutions. Integrating semiconductor strain gages with wireless communication is a very powerful solution. With this approach, we have immediately available knowledge of a structure’s health, and will know whether there are maintenance requirements or safety concerns. As a result, there will be no reason to destroy solid structures prematurely, and early warning if they become dangerous.

When building new structures, early warning of problems can save lives and eliminate costly repairs. For old structures, retrofitting is possible but it is best to have the history of how the stress varies from full occupation to no occupation and some idea of what changes have occurred from earth quakes, cooling and heating and accidental structural damage which the gages will see and report. This history, when applied properly, will provide prognostic insight into when the structure will become unsafe.

Wireless Sensing Considerations

A major benefit of structural health monitoring is early failure mode warning and response. An additional benefit offered by wireless sensing in these operational conditions is real-time awareness and decision-making. Also, wireless sensors are non- intrusive and avoid the many problems with tethered wired sensors. And, it’s important to avoid power sources that have limited life when operating in environments where extended unattended operation is required. The good news is there are several wireless technologies, both active and passive and covered by international (ISO) standards, which meet these requirements. The transceivers are commercially available and relatively inexpensive.

Wireless sensor data can be acquired from both stationary and mobile readers. By using industry-standard wireless technologies, data can be read with commercially available smart phones, tablets, laptop computers, and mobile devices. Micron Instruments’ forthcoming wireless sensor options will all be based on industry standard technologies.

Instrumenting Concrete Tunnels with Wireless Semiconductor Strain Gages

Micron Instruments recently provided high impedance semiconductor strain gages that detect the strain on rebar embedded in concrete. Using Micron’s SS-080-050-10000PU- S4, 10,000Ω full-bridge sensor, with carefully matched temperature compensated gages, Micron Instruments gaged rebar using their proprietary adhesive curing process to insure high sensitivity and virtually no creep. Some images of the gaged part are shown below.

Two of the gages on the bridge measure longitudinal stress, and the other two measure transverse stress, as shown in the image.



The gaged rebar was then integrated with a wireless system to detect strain and monitor stress inside the concrete wall, both throughout the curing process and after the concrete is fully cured. The result is a non-intrusive health monitoring system for concrete structures. It has been installed and tested and is fully functional.

In addition to consistently delivering precise measurement data for this application, with proper sealing and installation, the sensors should last more than 100 years. These gages are homogeneous, P-doped single Silicon crystal, have no P/N junctions and the wires are gold. There are no materials that corrode easily or that change significantly with time.
Given the permanency of the installation, this is a key element of the business case for this application.

For the business-minded reader, think about expensing the installation in three years and use it for 100 years.

Contact Us.

Micron Instruments has already worked with innovative companies on optimal selection, placement, and processing of semiconductor strain gages for high frequency, high pressure, and high-temperature applications. If you’d like to discuss your application or design, please contact us for a free, confidential consultation either by email (Sensors@MicronInstruments.com) or phone (805-522-4676).
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1 All dimensions herein shall be in inches, unless specified otherwise.
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