T-88 bond line thickness

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autoreply

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It did take some time, but as the question about bond line thickness surfaced again, I did recall where I had read something about it. The datasheet for Huntsman on Araldite 2011 epoxy glue. A search and I quote:



I do interpret this as a recommendation to get the adhesive layer between 0.05 to 0.10mm thick for maximum shear strength of the bond. To consider excessive clamping pressure a possibility causing a to thin adhesive layer would not be far fetched when reading this datasheet.
Interesting find. The earlier testing I referred to in the other thread (post #11) were with Araldite, 2011, 2014 and 2020 to be exact. Did lots of testing with varying clamping pressures, cure cycles etc (surface prep is simple, mechanicallly break it and and keep it contamination-free. Simple at least in testing...)

Bottom line is that even with utterly ridiculous clamping pressure and very smooth surfaces, you won't be able to get even near 50 microns of bond thickness. Tried plenty of materials and all the possible combinations between them (steel, alu, TSA, VE, VE with a perpendicular PE fiber, PE, epoxy, phenolic, all blasted, sanded and untreated) and even at 10 MPa (that's 100 times atmospheric pressure or about 1000 tonnes per square metre), you won't get to too thin bond layers. Test strength obviously increases with decreasing bond strength, though that effect is much more prominent with materials with different thickness*stiffness figures.

Though much of that testing is proprietary, I can probably answer most specific questions.

Below a picture of a test sample I did today. Mostly put in to scare the **** out of people who want to glue unclamped.


2015-12-09 16.47.59.jpg

On the right the original surface of the core, on the left the broken out core. Surrounding it is the area (VE+fibers) that were clamped under pressure against a metal surface.

All surrounding surface is clamped under a pressure of .16 MPa and that glue joint is extremely strong. The core is (obviously) unpressured. All those air inclusions will make your glue joint to go hell and it shows in testing. Orders of magnitude less strength...

Adhesive is Araldite 2014 cured under a heat cycle.
 

RUSTY

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In their respective literature both System Three and West Systems cautions against excessive clamping pressure. System Three says if large clamping pressures are required to keep the pieces to be bonded in place that mating surfaces should be coated with epoxy. The epoxy should be allowed to cure thoroughly, sanded flat and the cleaned thoroughly. Clamping pressure should then not be an issue. This, of course, applies to porous surfaces. Maybe this is the difference between the "don't clamp too much" and the "can't clamp too much" camps.

I would post links but I'm on a mobile device and don't know how.
 

amv8vol

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To be honest, I would not account for glue thickness and just make the part to whole or calculated numbers per plan. Moisture content, sanding, ruler and machine accuracy will all require a certain of hand fitting anyway. To get good glue bond I coated both part and didnt rush the assembly, waiting anywhere from 1- 5 minutes (sometimes longer) before sticking the parts together. This gives the glue a chance to soak in and prevent a starved joint. Put the T-88 on, let is soak a bit and stick it together, maybe clamps, maybe weight the assembly, nothing scientific. Make up test pieces occasionally and test the glue joints. The ony failures I had was with some 2 year old Resorcinol glue, wound up using T-88 on that lamination and all was good (made a test joint and broke it apart).
 

cholla

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Bonding without any continuous clamping pressure on it is a risky proposition. All structural adhesives need some clamping force to reach full strength. Not clamping might yield a slightly lower final strength, it might also be a useless weak joint. Point is, you don't know until you load-test it.
Below a picture of a test sample I did today. Mostly put in to scare the **** out of people who want to glue unclamped.
My statement about gluing unclamped was simply a regurgitation of the info the System Three tech gave me via email. I have no intention of gluing without applying pressure in some way whether it is clamps, cam blocks, or sand bags etc. In order to achieve the 1/64 thickness that I am targeting clamping will be necessary since the applied glue will be thicker than 1/64 and the parts will need to be sqeezed together to get them oriented correctly. This will inevitably result in "squeeze out" which will ensure there are no voids in the bond.

To be honest, I would not account for glue thickness and just make the part to whole or calculated numbers per plan. Moisture content, sanding, ruler and machine accuracy will all require a certain of hand fitting anyway. To get good glue bond I coated both part and didnt rush the assembly, waiting anywhere from 1- 5 minutes (sometimes longer) before sticking the parts together. This gives the glue a chance to soak in and prevent a starved joint. Put the T-88 on, let is soak a bit and stick it together, maybe clamps, maybe weight the assembly, nothing scientific. Make up test pieces occasionally and test the glue joints. The ony failures I had was with some 2 year old Resorcinol glue, wound up using T-88 on that lamination and all was good (made a test joint and broke it apart).
I disagree about disregarding glue thickness. The whole intent is to not need to sand, file, or do any other time consuming custom fitting work. With the accuracy a cnc router is capable of, not accounting for glue thickness would result in inaccuracies. The glue is going to take up space - there is no avoiding this. Not accounting for this would result in pushing pieces out of position. For example, the longerons angle towards eachother as you move towards the tail. If I was to install a crossmember sized exactly to the distance between longerons, in the correct location, the extra length taken up by glue (however minute) would bend the longerons outward. Multiply this by all the cross members, uprights, and diagonals installed and you can end up with a warp or twist in your fuselage when you remove it from the construction frames and these forces equalize themselves.

Normally one accounts for this during construction by sanding, filing, planing etc in order to make the piece fit without needing to apply any force. In the case of CAD and CNC, the piece will be cut exactly as it is modeled. If I account for glue thickness, it will come off the router table ready to be glued. If I don't, then sanding or other fitting work will be required which defeats the whole point of 3D modeling and machining the parts in the first place.
 

amv8vol

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I'm not sure I understand how the CNC router is going to be used. One of the cabinet makers we use has a 5'X10' cnc panel router and is is terrific for cutting plywood and MDF sheet but it is never used to cut lumber to length. . Do you intend to cut the spruce longerons and struts to length with the router? I can't imagine a router makes a good cut off machine. Looking at the construction photos on the Osprey website it does look like the CNC router would work very well for cutting the rib plywood and maybe the templates for the laminated turtle deck formers.
Maybe its just me, but I cherish the time & skill spent fitting and shaping the individual pieces together. we are working with wood here, after all.
 

cholla

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I'm not sure I understand how the CNC router is going to be used. One of the cabinet makers we use has a 5'X10' cnc panel router and is is terrific for cutting plywood and MDF sheet but it is never used to cut lumber to length. . Do you intend to cut the spruce longerons and struts to length with the router? I can't imagine a router makes a good cut off machine.
I intend to cut nearly everything CNC. For most parts I will use my own 3-axis. For more complex pieces, if necessary, I have access to a 5-axis mill. Though plywood and other laminates are the most commonly cut materials, it is possible to cut just about anything cleanly and accurately. It's all about using the correct end mill, spindle speed, feed rate, and cut depth.

Maybe its just me, but I cherish the time & skill spent fitting and shaping the individual pieces together. we are working with wood here, after all.
To each their own. I don't see what the material being wood has to do with it though. I think of wood as the "natural composite" and feel that it is highly underrated by today's builders. It can be cut and machined as accurately as aluminum or any other material out there. For me, I'm excited about blending the latest and most accurate fabrication technology with the original aircraft building material.
 
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