I purchased some glass cloth (3/4 oz) from Thayer. It is a 56 X 56 weave. I ASSUME it should make no difference which way it is laid on the surface. It should be as strong in both directions. Correct????
That is style #106. It has the same thread count AND
has the thread used in both directions. That would make it balanced which allows you to use it in +/-45 applications without warping. Style 2113 (2.3 ounce) on the Thayercraft is NOT balanced. The thread count is close (60x64) but the yarn used in the fill in half the size of the yard used in fill (ECE 225 1/0 x ECD 450 1/0). This is a highly unbalanced fabric that has a near balance thread count.
In the case of most glass. carbon, or other reinforcing fibers, there is usually very slight differences in strength fo a given direction of weave.
The subtle differences in the breaking strength of balance fabrics has to the stress that was applied to the yarn during weaving. The fine fill yarns usually get "tweeked" when they are woven.
In general, if you are going to use glass cloth (or carbon, kevlar or other) as a reinforcement or in a composite structure, you would lay the cloth on bias. The reason is simple geometry and physics; hopefully you've had highschool trig where you studied right triangles. You get greater strength in torsion which resists twisting. You have both sets of strands working for you at the same time rather than just one set in one direction and the other set in the other direction
In your case, whatever strength the 3/4 oz cloth adds (isn't much), still works to produce better torsional strength of your structure. Bias layup is particularly useful on wings if you want to maximize resistance to twisting over time. Overall strength addition is low tho because 3/4 oz cloth is light but not particulary strong. Nevertheless, bias laydown maximizes the benefit of whatever little strength you add. Point is if you are adding the weight anyway, why not get the max possible out of the lay-up?. BTW- bias laydown is lay down at 45 degree angle
I'd say it really depends on the application. Bias cut fabric works great on flying surfaces because they need the torsional strength and they usually have a spar to handle the bending loads. The misundstanding is that +/-45 material is stronger in bending because you have twice the amount of fibers being active. Unfortunatly, this isn't the case. Using half the fiber but aligning them with the load is stronger and stiffer than aligning twice the fibers on 45*. Some tests have shown that +/-45 fibers to the loading axis are 1/20 of the strength of those that are parrallel with the forces. The moral to the story is that you need to aligne the fibers with the primary loading axis.