Failure mechanisms in timber

1   Engineers are most familiar with metals, and when metals start to yield the stress is the same in tension as in compression. Timber is not like this.

2   Beams are structural members which take bending moment, and oars are in this sense beams.

3   All beams bend as they are loaded. At first they deflect by an amount proportional to the load. This is called elastic deformation and if the beam is unloaded it springs back to where it started with no permanent bend or damage.

4    At a certain point as the load is increased, the deflection increases faster than the load. This is called the Proportional Limit. Beyond this point there will be permanent bend when the load has been taken off, and some degree of damage. For oars to last, we need to stay below this point.

5   Beams resist bending by the material on the outside of the bend (aft face of an oar) being stretched and developing tension force, and the material on the inside of the bend (forward face of an oar) being squashed and developing compression force.

5     Wood is, under the microscope, a bundle of more or less parallel hollow tubes of cellulose glued together with a natural glue called lignin. It has been described as "a fibrous foam" and "a natural composite of fibre and resin". Unlike metal, its properties are very different parallel to and perpendicular to the grain. Unlike metal it has a lot of air spaces within the tubes.
[You must be registered and logged in to see this link.]


6   In tension all the cellulose tubes are being stretched, and when wood fails in tension there is a loud bang, a crack appears and the two halves move apart.

7   In compression the wood fails by the tubes crumpling or buckling, sometimes in a line at 45 degrees to the force, sometimes right across the piece at right angles to the force. The failure is almost invisible as the wood is pressed together. In a beam, compression failure means the wood locally is not developing compression force and throws the load on to the wood nearer the centre of the member, which fails and so on. At the end the undamaged section breaks on the tension side which is what most people notice.
[You must be registered and logged in to see this link.]

8   Oars which have had a compression failure often go un-noticed because the failures are often only visible as a fine line. The oar may feel "softer" in bending, as the compression crumples open and shut. Since these crumpled zones have almost no tension strength the oar may well fail when loaded in reverse as when stopping or reversing the boat. Such a "soft" oar is really a dead oar walking, and should not be relied on.

9   Most timbers are at least twice as strong in tension as they are in compression. Spruce is about four times stronger in tension. This is not well known, and it means that any symmetrical section for an oar where the front of the oar shaft looks like the back, is doomed to always fail in compression on the forward face before the aft face fails in tension. In the days when racing oars were made in timber for sliding seat boats, they had a sitka spruce aft and centre section with a hard ash strip on the front to deal with the compression. They were also hollow.

10   For an engineer the most efficient structure does the job without failing, but only just. So defining the mechanism of failure is essential to designing a structure which will not fail.

10  For designers of wooden oars the above facts mean that weight can be saved by:

• using light, weak timber for most of the oar but use a thin strip of harder stronger heavier wood like ash on the front face.
  

• making the aft face of the oar half as wide as the forward face, making the cross section a trapezoid. John Murray of Gaco in Australia first suggested this.
  

• making the oar bigger in the fore and aft direction than in the up and down direction (rectangular)
  

• making the oar hollow, since the material near the centre does very little for the oar's strength or stiffness. It may make unacceptable demands on the skills of community boatbuilders.
  

• all or some of the above.
  

Regardless of how much progress is made towards a more standardized oar design for the St Ayles, this sort of simple and readable briefing is an invaluable resource for a club about to start making their own oars. Well done Topher for writing it, and I hope that this sort of information finds its way onto the SCR web site. WRT the last bullet point, any group that can build a St Ayles skiff can build a hollow oar!