In these calculations all references to "strength of thread" mean the strength at rupture. This is warranted in the case of linen because stress-stretch curves show that the strength at the elastic limit is at least 95 percent of the strength at rupture. With silk this is not the case as shown below.
Strength at Elastic Limit | Strength at Rupture | Percent Stretch at Elastic Limit | Percent Stretch at Rupture | |
Linen | 10.5 lb | 11.1 lb | 2.8 | 2.9 |
Silk | 6.0 lb | 14.8 lb | 2.6 | 11. |
This property of linen to develop its high rupturing strength without acquiring a permanent elongation makes it a pre-eminent material for bowstrings.
In an article Dr. R. P. Elmer described an "Oriental knot used for attaching a string to the bow. I have made measurements and can find no detectable reduction of strength due to this knot providing that sufficiently large tennis gut is used for attaching the linen string to the bow. A number 12 gut (diameter 0.07 inches) seems to be sufficient. Use of this "Oriental" knot simplifies string making greatly—all that is necessary is to wind the right amount of thread around two pegs in the wall, fasten the ends under the serving, twist slightly and tie to the bow.
Moisture increases the strength of linen very greatly. By saturating the linen with water the strength is increased about 50 percent over its strength in air dry condition. Its strength seems to be proportional to the moisture content as is shown in the following table:
EFFECT OF MOISTURE | |||
---|---|---|---|
Pull | Strength Coefficient | ||
Treatment using 40/3 button thread | (P) | W/f | (S.C.) |
Wet and then dried to air dry condition | 10.1 | .0400 | 49800 |
Wet and then dried to 36.5% water | 14.4 | .0630 | 45000 |
Wet and then dried to 32.2% water | 14.4 | .0590 | 48100 |
Wet and then dried to 27.8% water | 12.4 | .0554 | 44100 |
Wet and then dried to 15.4% water | 11.9 | .0473 | 49600 |
Wet and then dried to 9.5% water | 11.9 | .0442 | 53000 |
Wet and then dried to 5.4% water | 10.9 | .0423 | 50800 |