17 replies to this topic

[NeilR]

I need to attach 6061 alloy shear panels to a mild steel space frame. The panels primary method of attachment will be via a structural acrylic adhesive and primer often used for metal bonding (plexus ma300). However I have a desire to use a belt and braces approach, which is typical in such things, by adding structural break-stem rivets. Large head 'Q' rivets are not available in small quantities, so 5/32 SS rivets will be used with washers and spaced every 100mm. My concern is localised stress from the rivets creating a cleave force adjacent to the clamping area. Has anyone seen any good information on the net about this?

[carlt]

I need to attach 6061 alloy shear panels to a mild steel space frame. The panels primary method of attachment will be via a structural acrylic adhesive and primer often used for metal bonding (plexus ma300). However I have a desire to use a belt and braces approach, which is typical in such things, by adding structural break-stem rivets. Large head 'Q' rivets are not available in small quantities, so 5/32 SS rivets will be used with washers and spaced every 100mm. My concern is localised stress from the rivets creating a cleave force adjacent to the clamping area. Has anyone seen any good information on the net about this?

Cant give you any tech sheer data , but -
I just used 4mm [5/32] pop rivets , no washers , with 2 pack metal adhesive .

Had a major shunt which involved having to replace a section of the space frame - the method held up extreemly well to my crash test methods
We had to drill out the rivets and peel off the ally skin with a hammer and old 1" wood chisel it was held so firm - even on buckled steel tube !

[mariner]

You probably already know this but long, long ago Mike Costin pointed out that in a typical lozenge situation of a sheet panel rivetted to a space frame only the rivets at the corners did any work since that is the area of greatest relative movement.

So you might want to think about varying the rivet spacing with the rivets near the corners being closer ( within good practice) to reduce stress.

[NeilR]

Thanks for the replies. I suspect that the rivets may not be required, but I wanted to be sure that peel was controlled for. I had read similar advice to Costin's, but the issues of stress was raised with me. I had hoped to simply reduce the number of holes in the tubes (for structural and effort reasons), though they are substantial is section. The irony is that having them closer is better than further apart!

Edited by NeilR, 03 October 2011 - 06:30.

[carlt]

You probably already know this but long, long ago Mike Costin pointed out that in a typical lozenge situation of a sheet panel rivetted to a space frame only the rivets at the corners did any work since that is the area of greatest relative movement.

So you might want to think about varying the rivet spacing with the rivets near the corners being closer ( within good practice) to reduce stress.

was this bonded sheet - or just rivets

[MclarenF1]

For efficient load transfer, a shear joint is typically double row, transition fit holes, with a pitch of 3-5D. You can find some good guidance in NACA Tech Note 2661/2662 which describes the analysis method for intermediate diagonal tension of shear webs. This method is used in stringer/frame stiffened structures typically used in aerospace. There is also guidance in Bruhn (Analysis and design of flight vehicle structures) and Niu (Airframe Stress Analysis & Sizing) that you might be able to reference.

[NeilR]

Thanks, I should be able to get most of that.

[bigleagueslider]

Thanks for the replies. I suspect that the rivets may not be required, but I wanted to be sure that peel was controlled for. I had read similar advice to Costin's, but the issues of stress was raised with me. I had hoped to simply reduce the number of holes in the tubes (for structural and effort reasons), though they are substantial is section. The irony is that having them closer is better than further apart!

NeilR,

Your comment about peel in adhesive bond joints is very important. Bonded joints can take tension and some shear, but fail catastrophically in peel. However, if your shear panels are functioning as stiffeners, and the failure of the panel's adhesive bond joint will not result in structural failure of the primary steel tube space frame, then rivets would not be absolutely necessary.

A rule of thumb with riveted joints is 2D min edge distance and 4D min spacing with thin sheets.

Also, watch for galvanic corrosion when bonding dissimilar metals.

Good luck.
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[NeilR]

NeilR,

Your comment about peel in adhesive bond joints is very important. Bonded joints can take tension and some shear, but fail catastrophically in peel. However, if your shear panels are functioning as stiffeners, and the failure of the panel's adhesive bond joint will not result in structural failure of the primary steel tube space frame, then rivets would not be absolutely necessary.

A rule of thumb with riveted joints is 2D min edge distance and 4D min spacing with thin sheets.

Also, watch for galvanic corrosion when bonding dissimilar metals.

Good luck.
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Thanks for the information. I have chosen the structural acrylic and primer system in particular because it performs much better in peel than any easy to use epoxy system I could find, whilst retaining 85% of the shear performance...it is also much more forgiving to user errors.
The galvanic corrosion will be controlled for by using a small magnesium sacrificial anode attached to the chassis. Failing that the sheets themselves will be the anode (large area will limit corrosion), hence the use of SS rivets.

[bigleagueslider]

NeilR,

Even with cres rivets, it would be a good idea to install the rivets "wet" using a primer or sealant. The primer will seal the cres rivet/alum panel/carbon steel tube joint from any moisture intrusion. This approach is commonly used in aerospace.

Best of luck.
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[Greg Locock]

A colleague in a previous job was a test engineer at one of the Australian aviation companies, Hawker Siddeley from memory. He pointed out that the failures of their composite layouts was generally initiated at the locations where the mechanical fasteners were used, as belt and braces, and that the durability or strength of the a/c would be improved if they were not used.

Needless to say his practical experience was ignored.

[mariner]

Greg, it is welcome back isn't it?

[mariner]

Has anybody found a quick, effective way of deburring the rivet hole INSIDE the frame tube?

You are always warned to deburr all rivet holes or the rivetrs will potentially be of little help.

All I have ever been able to do is run a very fine hook file around the inside and hope. I have this feeling that somebody, somewhere in the aerospace industry has solved this one!

[MclarenF1]

A colleague in a previous job was a test engineer at one of the Australian aviation companies, Hawker Siddeley from memory. He pointed out that the failures of their composite layouts was generally initiated at the locations where the mechanical fasteners were used, as belt and braces, and that the durability or strength of the a/c would be improved if they were not used.

Needless to say his practical experience was ignored.

I suppose it depends on what design criteria is driving the need for the fasteners. The absolute ultimate strength of a bonded joint is absolutely effected by the inclusion of holes and fasteners. The effect of drilling hole thru these joints can cause small disbonds which propagate thru the joint in addition to the stress concentrations. BUT, on the other hand, bonded joints tend to be designed for damage tolerance since the ability to inspect for "weak" bonds is not commercially available. The regulations require a secondary "feature" which prevents damage (either manufacturing or service) from reaching a critical crack length where the structural strength is reduced below some threshold. Most companies simply add chicken fasteners as the simplest form of meet the regulations.
Other considerations are the environment, stress level, load type, cycles, etc etc etc. This is all of course assuming a good material and process engineer has the appropriate specifications/inspections in place to guarantee the quality of the joint to begin with.

Not knowing the use, or experience in this particular application, I would hesitate to recommend using a bonded joint as a the primary load path in a critical structure. But, again, that depends on the application, experience and methods used. Sometimes, something as simple as what type bagging material is used can be the difference between a good joint and disaster.

[NeilR]

I suppose it depends on what design criteria is driving the need for the fasteners. The absolute ultimate strength of a bonded joint is absolutely effected by the inclusion of holes and fasteners. The effect of drilling hole thru these joints can cause small disbonds which propagate thru the joint in addition to the stress concentrations. BUT, on the other hand, bonded joints tend to be designed for damage tolerance since the ability to inspect for "weak" bonds is not commercially available. The regulations require a secondary "feature" which prevents damage (either manufacturing or service) from reaching a critical crack length where the structural strength is reduced below some threshold. Most companies simply add chicken fasteners as the simplest form of meet the regulations.
Other considerations are the environment, stress level, load type, cycles, etc etc etc. This is all of course assuming a good material and process engineer has the appropriate specifications/inspections in place to guarantee the quality of the joint to begin with.

Not knowing the use, or experience in this particular application, I would hesitate to recommend using a bonded joint as a the primary load path in a critical structure. But, again, that depends on the application, experience and methods used. Sometimes, something as simple as what type bagging material is used can be the difference between a good joint and disaster.

An interesting response, as too was Greg's.
The chassis is sufficiently stiff without panelling, so the skin is not a critical structure as such but simply regulations require some covering. It could of course be carbon/plastic appliqué or composite bodywork, but then it needs to be fastened in a manner suitable to pass technical scrutineering...and by the time I do this I may as well make the panels 'structural' as it would most likely be lighter overall. Having said that the manner of attachment of alloy panels also needs to pass scrutineers, who may or may not have sufficient knowledge to accept a bonded joint with no mechanical fasteners. One of the last things I want to happen is tow the car 1700km to an interstate championship only to have it knocked back at scrutineering (it is relatively unlikely but has been rumoured to happen for some in the past).
So I have 35mm x 1.6mm SHS 350mpa steel chassis and will be attaching 1.2mm 6061 alloy skins, with a structural adhesive intended for such metal bonding.
I have approached Textron and sought advice and they have provided me with a SS Advel breakstem fastener that they assure me is water tight, easy to use and has sufficient grip range. At the same time I have purchased a suitable gun to pull the rivets (FAR RAC180 - bless fleabay!) in one operation (recommended by Textron). I was planning to hold the sheets in place with Cleco's during bonding, so I'd better decide on the spacing in the next four weeks.

[mariner]

Aerospace practice is obviously the gold standard for thin sheet metal fabrication but, at the risk of heresy, maybe a lot of it is not relevant to amateur race car construction.

Why - well commercial planes have a 20 year operating life and can spend up to 16 hrs a day flying. that is over 100,000 hours and 15,000 pressurization cycles. A race car is liable to have a 10 year life and do maybe 30 events/yr. at one hour per event max so 300 hours life. So much of the corrosion and fatigue problems in aerospace simply don't apply.

However cars have a stress case that planes don't - crashing "safely" . The FAA tests are aimed at making sure a A320 or whatever does not hit the ground nose first at 150 mph not whether the passengers can survive that crash. So in that sense racing cars face a tougher test than planes do.

I take the point about starting stress raisers near rivets but there is the practical point that you often need rivets not to prevent peel but to pull the panels together for good bonding so the spacing is less a matter of theorectical stress loads in service and more a matter of using them to get good bonding of the adhesive.

I used Araldite 420A/B for my honeycomb and as clamping such a large assembly was not possible the rivets were critical to bonding. IIRC the sheer strength of Araldite 420 is about 2,000 lbs/sq inch so IF you can get it to bond the available strength is huge. For example most space frame bays might be 0.8m by 0.5m (30" by 20"). With 35mm tubes that is 952 sq. ins of bonding area assuming square tubes. At 2,000 lbs /sq ins the total adhesive max. strength is about 850 tons. Whilst that can't all be used for unidirectioanl loads it is so far above any likely load case that the most important practical question I would suggest is simply getting a good bond on all the underlying tubes, or panels , that is where close rivetting helps.

[bigleagueslider]

If you look at the composite primary structures in the Boeing 787, you'll see rivets/hi-locks used in some places, but not in others. I'd assume it depends upon the criticality of the particular structure. In this picture, fuselage skin longitudinal stringers are simply bonded/co-cured. While frame members are bonded/riveted.



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[NeilR]

Well blast from the past - time has flown past and I am just up to the bonding tests, which I'll report when they are complete. I have a much better understanding of some of the construction issues now and the mk2 will be much easier to make! All alloy sheets are cut and ready, just trying to convince the authorities that a honeycomb floor 25mm thick with 1.2mm 6061 skins will support by weight (500mm wide). Apparently this is a 'new' material to some.
I also have some solvent primer for the bonds and will also 'pickle' a test strip to see if acid etching will increase the bond strength, though it is not mentioned by the adhesive manufacturer.