Balsa - Is it strong?

The Wood Technology Society
,
5 Mar 2019

One of the outreach activities that WTS Board Member Gervais Sawyer does is to attend the Brighton Science Festival events.

Last month they held a 'Pocket Science' event at the i360 Observation Tower. Parents bring their children (all ages up to 11 years old) to take part in various scientific activities.

Gervais has a static display of 84kg of concrete blocks suspended from a 3mm x 3mm piece of wood. Usually he would use a piece of pine and then ask children to stack the blocks and bet, with chocolate money, when it will break.

However, on this occasion, the main display was to have children looking down microscopes to see the amazing microstructure of wood, so Gervais substituted the pine for a piece of ash, for which there was no chance of it breaking. He was distracted for just 5 minutes and then found to his horror that some children had unloaded the blocks and replaced the ash test piece with a piece of balsa wood - and then reloaded all the concrete blocks. He had used the balsa wood for levelling a table leg at the display.

Amazingly, the balsa wood took the load even though it was crushed from 3mm thick to 1mm thick by the grips of the test rig.  The cam grips are actually made for testing paper and pull tighter to the wood the greater the tension force.

Could there be a better demonstration of the extraordinary tensile strength of wood parallel to the grain. Small wonder, that balsa - the lightest commercial timber - was used for the Mosquito bomber.

 

Photo 1.  Shows the test rig loaded with 84kg of concrete paving blocks. The load platform is freely swinging about 50mm above the ground.

Photo 2.  A close-up of the balsa wood tensile test sample, which is 3mm x 9mm. The grippers are on a cam that grip tighter the more the specimen is pulled.