Metal to Metal Bonding - Lap Shear Tests 1

[Obsidian]

Hi All ,

I've got a major problem bonding anodised aluminium plates together to form a tensile lap shear specimen .

The film adhesive used is Redux 308 NA and my problem is that the failed bond looks exceptionally porous . There are large voids within the glue line , possibly up to a couple of millimetres in diameter ,
and these are significantly reducing the surface area over which the film adhesive acts .

The film adhesive itself is cured at 170 Deg c for 60 minutes under a residual vacuum of 6 in Hg and an autoclave pressure of 22 psi ( the actual component is a honeycomb sandwich component ) .

We've tried various things such as varying the pressure , bagging the test piece differently etc. but nothing appears to work to reduce the " perceived " porosity in the bond . The film adhesive has a volatile content of < 1% .

Has anyone seen this effect before and , more importantly , how was it rectified ?

Any help much appreciated , thanks .

 

[Demon3]

Hello,

Well firstly I am no expert on adhesives. Nevertheless I will try to make some suggestions. Some chemical curing reactions give off volatiles like water, in that case you will not avoid having gas given off and risking bubbles. Without knowing the chemistry of the adhesive I can't be sure. I will assume the reaction does not give off gas. 1% volatiles may not seem a lot, but that is a huge amount of gas, easily enough to give bubbles (18g water gives 22 litres of water vapor!).

I just checked Google and found that the adhesive is epoxy so it doesn't give off water. Instructions for using it can be found here if you don't have them

http://www.hexcel.com/NR/rdonlyres/874DE313-5CAD-4A3C-863D-A3B92AE49D31/0/Redux308_us.pdf

My advice is to ask the supplier. It's their job and in their interest to help you.

 

[Compositepro]

I suspect it is primarily your bagging technique causing the problem. If you are bagging to to a rigid caulplate you have to shim every thing carefully so the bondlines stay flat and uniform. The adhesive becomes fluid and flows during cure. I sounds like you are dishing your adherends and the adhesive is pooling. This kind of adhesive flow will result in all trapped air coellescing into one big bubble. Air entrapment in bondlines is very common with tacky film adhesives. Normally it is seen as uniformly distributed small bubbles that don't affect strength too much. There are several techniques to totally eliminate air entrapment but it requires some change in film manufacturing process or in the application process. Manufacturers and users of film adhesives are very conservative about any intentional changes, but sometimes get burned by unintended changes.

Regardless of voids check if your cured bondlines are uniform in thickness. I'll bet they are close to zero at the edges and thicker than nominal in the center. Istead of shims you can also envelope bag your specimens so that there is a flexible bag on both sides. But, pay attension to where the bag bridges over the edges of your adherends. Bridging puts extra pressure on those points where the exiting air must pass and will cause "pinch-off". The more vacuum you pull, the tighter the seal, so air will not be removed.

 

[Obsidian]

Thank you for your advice gentlemen .

I do suspect the bagging of the job .

I will investigate this further with your comments in mind .

Cheers