Strength of glue bonds

[Packard]

Has anyone come across a chart that lists the strength of glue bonds based on the square inches of glue area?

Not to get into discussions on various joinery methods, I would like to know if I just pressed two pieces of face grain to face grain lumber together, how much strength I would get from 1 square inch of glue area.  1/2” and 2”, etc. 

All the tests on joint strength are basically a test showing which joint has the most glue area.  But where is the test that shows the glue area to strength relationship?

 

[woodferret]

TLDR: It's in the TDS.

But for comparison sake, Lap Shear Strength for various
https://www.staff.ncl.ac.uk/s.j.bull/mmm373/ADHESIVE/sld006.htm

Testing procedure


There's also tensile strength, but loading is typically not that fashion other than 'crazy glue' commercials where you stick it to your hat and I-beam.  Although TDS will also list it, just in case you're loading it that way.

Peel (bell) strength is also sometimes specified, but I haven't seen that outside of industry.  3M adhesives will have it in their TDSs.

 

[smorgasbord]

Has anyone come across a chart that lists the strength of glue bonds based on the square inches of glue area?

Species of wood is also a factor.

All the tests on joint strength are basically a test showing which joint has the most glue area.

That's not the summary view I take from those tests. For instance, the DowelMax comparisons opportunistically chose a stock size where they could fit 5 dowels and have 2 of them close to the edges, while the Domino has only two dominos, each set further in from the edges, so even they have equal pulling resistance, the lever geometry factor favors the DowelMax:

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[Mini Me]

My experience with construction adhesive may be helpful in this discussion. I fitted out a basement about 35 years ago and used construction adhesive in the build for fixing panelling, battens etc. We recently stripped all that out and found that it had crystallised and just fell apart in some areas which is something I recall reading about prior to the demolition and we saw the proof so to speak.

 

[Packard]

I used to work for a company that made wire forms.  We sold several million S-hooks a year to the makers of rubber straps for holding down truck tarpaulins. 

We originally produced these hooks in 6 gage (0.192”).  The design specs changed several years later to 3/16” (0.187”).  And then a few years later to 7 gage (0.177”).  And then the lawsuits came pouring in.

When installing a rubber strap as a hold down, the strap is invariably pulled towards your face.  When the spec came out to 0.177” diameter, people were damaging or losing an eye.  Each lawsuit was settled for one million dollars. 

Because it was impossible to determine after the fact which manufacturer was responsible, and since all were using the same wire diameter hooks, they simply spread the financial burden across all the manufacturers (including our company).

I asked my customers how much weight the hooks were required to hold (250 pounds static load).  I then had a testing laboratory test all three gages of wire.  Only the 0.192” diameter would hold 250 pounds (but with no safety factor included).

We shared that data with all our customers.

Our biggest customer, Radiator Specialty, asked for a letter certifying that the 0.177” diameter hook would hold 250 pounds.  I told the customer that it would not hold 250 pounds and he said, “I don’t care.  All I care about is getting that letter certifying that it will hold 250 pounds.”

We dropped the customer. 

I don’t know why I am telling this story.  I seemed related when I started but not when I finished.  But I am not going to delete it.  It is just another story of corporate malfeasance.  I think Radiator Specialty is now out of business.  Their main business was making rubber washers for the plumbing industry.  They were also the largest manufacture of rubber straps in the USA.

 

[woodferret]

Probably because you don't trust random letters of conformity.... and you shouldn't.  That's why ASTM exists, along with the various third-party certification labs associated with it.

 

[Steve1]

I would like to know if I just pressed two pieces of face grain to face grain lumber together, how much strength I would get from 1 square inch of glue area.  1/2” and 2”, etc. 

Depends on many factors, but the short answer is that you can assume the joint is as strong as the wood you are gluing.

 

[Packard]

I would like to know if I just pressed two pieces of face grain to face grain lumber together, how much strength I would get from 1 square inch of glue area.  1/2” and 2”, etc. 

Depends on many factors, but the short answer is that you can assume the joint is as strong as the wood you are gluing.

That is not really meaningful information though.  I was looking for “per square inch of glue area” type of data.

1” x 1” = 1 square inch glue area. Shear strength = XXXX, Tensile strength = XXXX
2” x 2” = 4 square inches of glue area. Shear strength = XXXX; Tensile =XXXX

And of course, the type of wood used would be another variable.

 

[squall_line]

I would like to know if I just pressed two pieces of face grain to face grain lumber together, how much strength I would get from 1 square inch of glue area.  1/2” and 2”, etc. 

Depends on many factors, but the short answer is that you can assume the joint is as strong as the wood you are gluing.

That is not really meaningful information though.  I was looking for “per square inch of glue area” type of data.

1” x 1” = 1 square inch glue area. Shear strength = XXXX, Tensile strength = XXXX
2” x 2” = 4 square inches of glue area. Shear strength = XXXX; Tensile =XXXX

And of course, the type of wood used would be another variable.

There are a number of reasons why "strength per square inch" is not constant, and therefore not a very useful measure.

One prime example is that a 2"x2" square of glue will have different shear and tensile strength than a 1"x4" or 1/2"x8" rectangle of glue. The strength will also depend on how much wood is in play and how the forces are applied (is this a square in the middle of a large flat plane, or on the edge of two boards, etc).

 

[woodferret]

But that's why our tech specs are in pressure units.  PSI, MPa, etc.  That bit about location, is reflected in the roll test.

 

[Packard]

A while back I demoed a bathroom that was done in 1953 when the house was built.

The vanity and cabinets were site built using butt joints and hammered nails.  The lumber was probably sawn with a hand saw. 

It was particularly stubborn about being demolished.  That lead me to believe that my cabinets are over-built. 

But in the absence of any data, I am building cabinets that are way stronger than they need to be.

I recently built a medicine cabinet using butt joints, glue and a nail gun.  Even before I mounted it on the wall (with the subsequent added structure from the wall), the cabinet seemed way stronger than required. 

But after that, I reverted to my over-build-is-my-standard attitude.  No data to fall back on. 

 

[Tom the Remodeler]

The Rosenheim Institue (and I'm sure others) sorted out the engineering on wood joints decades ago. As Woodferret, smorgasbord, Steve1 and other point out, it really depends on a spectrum of factors including (but not limited to):
-- Wood species
-- Joint type (butt, miter, etc...)
-- Joinery method (dowels, bridle joint, mortise/tenon, etc...)
-- Overall assembly size/shape
-- Loading axis
-- And about three dozen other factors that play into "how strong is a joint/glue bond"....

I recently discovered that one of my industry colleagues has access to the Rosenheim Institute's data resources, and can provide me with an engineered solution for dowel joint joinery in wood doors, including the specific dowel sizing, "peppering" pattern, and lengths to use, along with the predicted joint strength/performance.