Let the sensor take the strain

Dave Wilson explains how a smart sensor measures strain without using electricity.

Loading on a structure can be applied in several different ways. By parking a heavy lorry in the middle of a bridge, for example, the bridge becomes ‘statically loaded’. If the same lorry is driven at high speed over the bridge, so that it bounces around, the bridge is ‘dynamically loaded’. If thousands of cars drive over the bridge over several years, it is subject to ‘fatigue loading’. All three types of loading combine in a complex way to strain and damage the structure of a bridge.

Now developers at the US-based Strain Monitor Systems have developed an innovative technique to measure such loading by using a small device that can be easily attached to bridges, or any other structure for that matter, where such measurements may need to be made. In operation, when a bridge is strained, the device is strained by a corresponding amount. The device itself is a small ‘sensing element’, which is nothing more than a tiny strip of a special TRIP (TRansformation Induced Plasticity) steel enclosed in a housing.

Certain steel alloys (among them, TRIP steels) change from an austenitic, non-magnetic state to a martensitic, ferromagnetic state as they undergo axial straining. The key to the SMS technology is the one-to-one correspondence between the peak amount of strain and the percentage of ferromagnetism produced in the TRIP alloy. Since the phase transition that accompanies the straining is irreversible, the gauges will indicate the maximum strain until that previous level of strain is subsequently exceeded. Ultimately, the gauge elements must be replaced or reset to zero, but at that point, the structure will require repairs anyway.

PASSIVE DEVICES

Although these devices need short bursts of low amperage DC to report the strain, the TRIP steel sensing elements require no electricity at all to measure the strain. Thus they are considered ‘passive’. They remember the peak strain they have seen and report it later when asked, so they are considered ‘smart’. They also report the combined effect of the static, dynamic and fatigue loads, which are called the ‘peak strain’.

Being independent of a stable electrical supply makes the sensors reliable. They are also robust, they can be left in very hot, corrosive, radioactive, or freezing environments and they continue to function, and are inexpensive to install.

TRIP steels are quite strong, typically stronger than structural steels. This means that they can also be used as actual structural load carrying elements (reinforcing bars, beams, cables.) which will become ferromagnetic when strained to specific levels. One roof bolt manufacturer in the mining industry is interested in developing bolt sensors to monitor rock deformations that cause bolt failures.

Strain itself is denoted in terms of percent physical deformation of the strained material. The range accurately measured by SMS gauges extends from 0.01% to over 100%, wide enough to cover many structural strain and deformation applications. The instrumentation for reading the strains need only be connected when the data is to be collected.

The self-contained, attachable gauges use Hall-effect sensors to measure the ferromagnetic response of the sensor elements which take the form of either wire or thin ribbon. The instrumentation hardware is ‘off-the-shelf’ technology, requiring only that systems be integrated properly for the purposes of detecting and monitoring changes in ferromagnetic response of the SMS gauges. Such sensors can be integrated onto structures in numerous ways.

EARTHQUAKE DAMAGE

SMS has already completed Phase I work on an SBIR (Small Business Innovation Research) grant from the National Science Foundation (NSF) in the US to evaluate the application of the SMS technology for earthquake damage assessment and has obtained encouraging test results.

The graph shows a comparison of the earthquake-induced displacement at a critical location on the column of a concrete bridge (on Interstate 5 at Gavin Canyon in Los Angeles, CA) with the output of an SMS gauge. The earthquake strength was computed and then used as input in a real-time laboratory testing of the gauge. At each time the peak displacement is exceeded, the gauge output correspondingly increases.

Phase II for that NSF program involved fabricating and installing sensor systems into six residential structures, into a Caltrans bridge in Los Angeles, and into the Marin County Hall of Justice building (in CA).

Investigations have also bee conducted in conjunction with Conductus, a manufacturer of custom magnetometers, to remotely detect and measure the sensor element ferromagnetic response using a SQUID (Superconducting Quantum Interference Device) magnetometer. These tests demonstrated that the sensors can measure through intermediate materials consisting of aluminium, stainless steel, concrete and air.

SMS has also completed a one-year research contract with the Air Force to develop and evaluate a crack detector for the engine mount trusses on the C-130 cargo plane. Two styles of prototype sensors have been built, installed on the airframe component and fatigue tested. The prototypes were able to detect structural failure about 4,500 cycles before its occurrence; there is a good chance that, with a follow-on program, the sensitivity can be greatly increased.

One of products manufactured by SMS using the technology is the SMSS010 Surface-Mountable Sensor Strip. This is a tensile, peak strain gauge for mounting onto any non-ferromagnetic surface. The unit is fastened onto the surface and the gauge measures the average planar strain (or deformation) between the anchor points.

SMS products are available in the UK from Graham and White Instruments.

SMS (Graham and White Instruments. Tel: 01727 859373