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Bondable Strain Gauges

Waterproof 3 Axis Rosette Strain Gauge

Waterproof 2 Axis Rosette Strain Gauge

Typical electrical cubicle including Data logger, power source and signal conditioning circuits.

Single Axis Gauges in Full Bridge Configuration

Bondable strain gauges are small, single use items that are designed for measuring strain in a variety of materials without the need to be physically present at the point of measurement. Multiple points (up to 8) can be measured simultaneously at a rate of up to 100hz.

Application
There is a wide variety of strain gauges manufactured to suit nearly all applications. Generally each project will require their own type of strain gauge based on the project requirements.
​We however carry a large array of standard sizes for emergency projects and the more general applications.
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Typical applications include :

Concrete Structures:
 - Bridges
 - Silos
 - Jetty's
 - Pre cast Deck Units
 - Prestressed & Post tensioned deck units 
 - Walls
 - Piles
 - Beams
 - Columns

Steel Structures
 - Industrial Framework
 - Piles
 - Bridges
 - Tanks
 - beams 
 - Columns
 - Plates
 - Machinery, Axles, chain & industrial links)

Timber Structures
 - Framework
 - Piles
 - Beams
​ - Posts                                              

Quality & Accuracy:
We only use Japanese manufactured units from TML Japan.
​The accuracy and repeatability of bondable strain gauges is generally very good.

Preparation:
Typically, an area of concrete, steel or timber is ground and polished to allow strain gauges to be adhered using specialty adhesives. Waterproof gauges are also available for exterior measurement.

How it works:
Strain gauges are made from a long strand of aluminium foil.
A very small DC voltage ( less then a 9 v battery) is placed through the strain gauge. Very small changes in tension or compression leads to change in electrical resistance of the strain gauge.
Through a series of circuitry, the signal is converted from voltage into a digital data stream that can be recorded, viewed and replayed on a site laptop or directly through the strain data logger. Results are readily exported into Excel for further data interpretation and to convert strain to stress.

Measurement Periods:
We typically remain on site for the duration of measurement to provide video recordings of the load passes for short term measurement (<5 days) and ensure the data stream is providing good results.
For structures undergoing long term measurement ( 5-60 Days), a control cubicle is typically setup with all required equipment in order to allow the data logger to wake up, take measurements and then go back to sleep. 

Safety
As there is no need to be under or around part of the structure during measurement, normal operations can generally continue without the need for restricted operating conditions or interruption to production. This is invaluable in environments that require 24 hour operation, or cannot be closed for measurement activities (such as road and rail bridges).

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Example Strain Measurement:
During dynamic strain measurement of a rail bridge, the tensile and compressive loads in the top and bottom flanges of the girders are compared to FEA modelling. This provides an additional layer of checks in order to ensure that calculations align with real world measurement.

Benefit:
Strain measurement on steel and concrete structures are invaluable, as many times the measured results are lower then theoretical calculations. This leads to a quantifiable method to safely revise modelling in order to ensure the provided advice is both sensible and backed by real world measurement.

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Bolt on Strain Gauges

Bolt on strain gauge installed onto 250UC column. Area milled flat prior to installation using our portable milling machine.

Bolt on strain gauges are typically employed for long
term measurement and monitoring, as well
​as monitoring in harsh mechanical environments.

Application
Bolt on strain gauges are typically applied to silos, tanks and hoppers, or as part of logic control of industrial equipment in which certain loads require real time monitoring (signal output is generally 4-20mA).

Quality & Accuracy
Dependant on the supplier, installation and calibration, bolt on strain gauges are generally quite good at producing long term, repeatable measurements. The caveat to this is that the sensors do not come calibrated (known as gauge factor). A test load is required to be placed into the area in order to then sync the gauge to the load.

Preparation
Pyramid Engineering carries a variety of portable milling equipment in order to create a smooth, machined surface that does not induce torsional load into the bolt on strain gauge.
This step from our experience is critical for accurate measurement and is often not performed.

Limitations
The only real limitation on bolt on strain sensors is attaining a flat machined surface. Sometimes on smaller columns this is more difficult due to the footprint required by portable milling equipment. Generally speaking, columns with a web width of 200mm or wider are acceptable for machining operations.

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Data Validation

Validation of strain loading through use of loading assembly designed specifically for project requirements.

Often During Strain measurement, a test load or other method of  of setup validation is performed. On bridges this typically means rolling a locomotive or suitably weighted flat bed truck over the area of interest.
The purpose of a test load is assist in calibration of FEA modelling.

At times, where loads are unknown or a test load is not able to be placed on the area of interest, a specific loading assembly is developed, fabricated and used as part of site activities in order to provide a dataset with known values. 

Test loading  is particularly important for bolt on strain sensors. Simply filling a silo or hopper with material is a poor method of calibration as each column is under different amount of loads. You will get a result, but as each column was compressed differently, each sensor is now calibrated to different levels of strain, often leading to poor results during part full conditions where material is unevenly loaded in the hopper.
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Dual 80 Tonne Calibration Assemblies

Pre Validation of strain gauge before site installation on same steel section