[quote author=Paul J link=topic=59433.msg474476#msg474476 date=1309040802]
I would look into the construction of the wing, rather than look to measure the forces. It should be possible to make a very strong wing light enough to carry your glider safety. It is possible that flutter is the cause of the failure - a torsionally stiff wing would prevent that from happening. If your wing is polystyrene foam, a skin of thin balsa (1/32") would have a big impact on the stiffness of the wing without adding weight. How big is your model?
I agree with this. Add some carbon fiber rods to your wings. Use is extremely common for R/C aircraft. If you show pictures of your wing, I can help here.
There are many strategies which can be employed to significantly strengthen your wings without adding significant weight. An extremely common approach is to grab some fabric, or more preferably, some fiberglass. Epoxy it over your wing halve joints. If you can spare the weight, insert several carbon fiber rods between the two halves, before you laminate the joint. Next, make some small routes the span of your wings just deep enough to embed/glue some carbon rods the span of the wings. If you expect very high stresses, two should be considered; one near the leading edge and one about mid chord. If your wing has no dihedral, considering spanning the entire wing with single pieces of carbon fiber rod.
Also, if you have control surfaces, flutter might also be a significant event here. Pictures and performance window expectations would really help shed some light on things. Control surface flutter can easily induce control surface failure. It can also cause the entire airframe and wings to suffer violent shearing forces as it rolls, pitches.
Also, if you are creating the aircraft, rather than buying outright, I can help you with some cheap, light, readily available materials which are extremely strong.