Articles in The Wool Press describing work on the strength of wool have constantly reported that the normal commercial measurement–staple strength–has very little to do with the strength of individual fibres. On the face of it, this seems odd because staples are, after all, just bundles of individual fibres. David Scobie of AgResearch New Zealand visited the CRC for Premium Quality Wool for three weeks in June. Most of that time was spent working with Andrew Peterson of the Wool CRC’s program 2. Both Andrew and David are very interested in the strength of individual wool fibres and explain why the strengths of fibres and staples are so different.

If the protein substance, keratin, that makes up the wool fibre were highly variable in strength, then weak fibres would lead to weak staples and the explanation would be easy. Sadly, both AgResearch New Zealand and the Western Australian Department of Agriculture have found that, except in certain circumstances, there is almost no variation in the strength of the keratin itself. Whether the wool comes from a well fed hogget, or a half starved ewe carrying twins, the tensile strength of the keratin material is the same. This is true for both Romneys and Merinos. Both produce keratin that has a strength equivalent to around 200 N/ktex, whereas we know that the strongest staples measured fail to reach half that strength!

Figure 1:
A staple of 3 fibres of identical relaxed fibre length, held between clamps that extend and break it.
Note that all of the fibres will come under tension at the same time and will all contribute to the measured staple strength.

On the other hand, the length of the relaxed fibre is a major contributor to the tensile strength of a staple. The easiest way to explain this is by means of Figures 1 and 2. The first staple (Figure 1) has three very uniform fibres, that extend to the same degree under tension and tend to break at the same time. So, the strength of this first staple is the sum total of the strengths of the fibres breaking when peak force is recorded ie. the sum of the strength of the three fibres. The second staple (Figure 2) has fibres that are of different relaxed lengths. When the staple is extended under tension, the straight fibre comes under tension sooner than the other two, and breaks first. The second and third fibres will also break at different times. The tensile strength of this staple will be the force to break the strongest of the three fibres much lower than that to break the first staple. Of course a staple of wool is made up of many more than three fibres, but the staple strength is only the sum of the forces to break those fibres which break at the point of peak force (the “N” in N/ ktex), during the entire extension process. All the fibres contribute to the cross-sectional area (the “ktex” in N/ktex), and since all the fibres do not break at the same time, the potential maximum of 200 N/ktex is never achieved. It is important to note that if shed fibres are present in the staple, then they also contribute to the cross-sectional area of the staple but contribute nothing to the force required to extend the staple and thus result in an even weaker staple.

Figure 2:
A staple of 3 fibres of different relaxed fibre length, held between clamps that extend and break it.
Each of the fibres will break in sequence so that only the strongest fibre will contribute to the measured staple strength.

In this issue of The Wool Press: