See this? (Top: tested: Bottom: untested)
These beams are manufactured from two slabs of 4mm thick high strength aluminum beams bonded together with an automotive-grade structural adhesive. The glue thickness is only about 0.4mm and has a crack starter at the loading end (i.e. a flaw introduced to promote crack propagation).
The top beam in the picture is bent simply because when the top and bottom arms on either side of the glue-line was pulled apart in the testing machine, it takes more energy to make the pre-existing crack grow in the glue than to plastically deform the metallic arms!!
Check out the aluminum clad cars on the road. You’ll rarely find an aluminum part more than 4mm thick (unless it’s structurally critical) because it’ll make the car too heavy to be fuel efficient. Which means that, the metallic parts of the cars you ride in are more likely to bend than those two beams shown above!!
Well, the adhesive showcased above is actually used to bond some sports car first marketed about 2 years ago. I heard that they use about 5 kilos of these stuffs in each car and yes, many people actually pay loads of money for a car that is glued together. Not difficult to see why now huh?
Still, there some drawbacks of using adhesives in structural applications. For one, we can predict but wouldn't know for sure how long they will last in the service environment. Take your household superglue for example. They are indeed very-strong moments after cure but they lose their strength within 2 years. That is because acrylics degrade quickly when they come in contact with water/humidity. Jump around in a puddle and the newly glued sole of your shoe will come off again. Also, it is difficult to make a defect-free bond. Dust, oily surfaces, inappropriate surface preparation to raise the surface energy, trapped air-bubbles, these contaminants and defects can all cause the joint to fail early. Some glues are very strong if you try to pull them apart but subject them to compression or impact and they’ll crack easily. Many do not perform well in fatigue (cyclic) loading tests at all.
I guess this is why I’m still skeptical of traveling in a composite plane (which presumably uses quite a bit of adhesive for joining parts to avoid drilling holes through the reinforcing fibres of the composite materials). It will need a great leap of faith for me to get on one.
By the way, i realised that this is my 100th entry. Hopefully, I will see one more zero added in my lifetime. *grins*
No comments:
Post a Comment