responses (late at night down under)
Ok, I'll try to respond as rapidly as the messages come in, but soon I must snoo!
Firstly, it is not the wash fluid which are the problem. It is the persistence of humidity, combined with temperature. Remember that higher humidity simply accelerates the problem for a deficient interface. It is a moisture diffusion problem, not a moisture flooding problem. A durable interface is resistant to any hydration mechanism, be it wash fluids or humidity.
Secondly, the major difference between Redux bonding and current processes is that the Redux adhesives were phenolic based and they had a strong acid-base reaction which produced durable bonds. My understanding is that current epoxy adhesives do not exhibit as strong an acid-base reaction and hence they rely on the chemical activity generated by processing of the surfaces. They are therefore more susceptible to hydration, so treatment must address hydration reistance.
Why don't we change back to phenolic adhesives? Because they are condensation polymerisers which produce water as a by-product of the reaction. That water leads to micro-voiding as that water turns to steam during the cure cycle, which results in micro-voiding which reduces the bond strength. Don't be fooled by thinking that vacuum draws such volatiles out; in reality vacuum reduces the pressure and therefore makes the bubbles larger. It is the adhesive which is drawn out, not the volatiles. In contrast, epoxies are addition polymerisers, and they do not produce water as a by-product and hence they are less susceptible to micro-voiding. But the interfacial reactions are more dependent on the chemical activity of the surface and the resistance to hydration.
Now to my favourite subject: Quality. Almost every manufacturer currently uses Quality Assurance (QA) testing to verify structural integrity. Indeed, there are regulatory requirements that are directed towards QA. As already stated, strength tests only produce a snap-shot of current strength. So if the test is conducted before the interface has had time and humidity to hydrate, a false positive result occurs. If combined with NDI (which can not interrogate the interface) then one has a warm, fuzzy feeling that the product is structurally sound. In some cases, such products have an absolute certainty that they will fail in service. Again, I stress that such products can actually demonstrate compliance with the regulations (FAR, JAR, DEF STAN 00 970 etc.)
The problem with QA is that it is nothing more than a "leave pass". If you can meet the requirements of selected tests (usually strength tests based on coupon samples cured with the part) and if it passes NDI, then it is an approved part and you get the weekend off. IT DOES NOT MATTERWHAT YOU DO TO GET THAT LEAVE PASS.
In contrast, there is a far better way of managing quality, and that is Quality Management (QM). In this process, every step of the process is firstly validated using tests that guarantee bond integrity and importantly guarantee bond hydration resistance. Process specifications must match exactly those validated processeses. Then the easy bit. Just make sure it is correctly implemented by certification of compliance with validated process specifications by competent technicians using approved materials that have been correctly handled and exposed in only a controlled environment and using equipment that is appropriate to the task..
Recognise that you can inspect a product and test coupons all you like. You will NEVER change the quality of the product. Manage the quality and it will pass every test you throw at it. And it will actually be a quality product which should not fail in the manners described in this thread.
How can I be so confident? These principles reduced the repeat repair rate for bonded repairs from 43% in 1992 to three bond failures since then, and every one of those could be tracked to technician error or laziness.
We do not have an engineering problem; we have an education problem.
Regards
blakmax
PS If you contact me by PM, blakmax has no "c" in it. We convicts down under can't spell.
PPS SASless. You can take a weekend off after this! Put your feet up, old chap! :-)} (The additional } is because I have a beard. Never did understand why grown men scratch the hair off their face so they look like women!)
Last edited by blakmax; 2nd Sep 2009 at 13:35.
Reason: told you we convicts can't spell