Nature’s Renewable Composite

Pound for pound, it is stronger than steel, environmentally friendly, renewable, recyclable and aesthetically pleasing. Man has used wood as a structural material for centuries and it continues today as a cost-effective, green alternative to concrete and steel in many applications.

As with most structural materials, load testing is an invaluable tool to ensure reliability and safety when using wood. So, can you use foil strain gage sensors on wood? Of course!  As with nearly all structural materials, strain measurement product selection, surface preparation and strain sensor installation techniques are the key to accurate strain measurement on wood.

Similar to composite materials, fiber orientation and size influences material strength and strain gage sensor selection for wood. Think of wood as “nature’s composite”. Gage size selection is based upon the same principles as with any other non-homogenous material.  The grid length should span 3-5 repetitions of the grain structure to take advantage of the strain gage sensor “strain averaging” properties.  Additionally, higher gage resistance values will improve zero stability by reducing current flow through the Wheatstone bridge circuit which in turn reduces the chance of self-heating.

Surface preparation is similar to other composite materials.

Refer to Instruction Bulletin B-129-8 

Adhesive selection is also similar to other composite materials.  Irregularities in the surface may be filled with one of our Micro-Measurements® epoxy systems such as M-Bond AE-10 room temperature curing epoxy. Once applied and cured, the surface can be sanded back to the peaks in the original material, leaving a surface condition appropriate for gage bonding with little or no change to the material properties.

At this point, the strain gage sensors can be bonded to the prepared surface using standard bonding techniques and room temperature curing adhesives such as M-Bond 200 or M-Bond AE-10 adhesive.

Protective coatings should be selected based upon the test environment.  Visit:  http://www.micro-measurements.com/installation-accessories/protective-coatings/

The test material must also be considered when selecting the instrumentation/data-acquisition system. Wood is a fairly poor heat sink so instrumentation excitation levels must be adjusted to avoid zero drift due to strain gage sensor self-heating. Refer to Micro-Measurements Tech Note 502 Optimizing Strain Gage Excitation Levels.

Micro-Measurements offers a wide range of instrumentation with low or variable excitation levels to avoid this problem. 

Visit: http://www.micro-measurements.com/instruments/

You may also contact our Applications Engineering Department for assistance by calling +1 919-365-3800 Ext. 2.

The next time your strain gage application calls for testing nature’s composite, turn to Micro-Measurements, so you’ll be armed with the information and products necessary to perform your test successfully.

 

jjohnson's picture

Jim Johnson

Technical Sales Manager