Testing glue joints for my sons science project

[PartsMutt]

Hello all,

I'm helping my 3rd grader with his science project. He's decided to test the strength of several glues. We're using a simple joint just one piece pressed in place with the other at a 90 degree angle. Although that can change to a simple 180 degree butt joint, or pretty much anything else if needed. Then we want to measure the amount of effort it takes to break the joint. That's the rub. How do a make a measurement of this? Anyone have an idea how I can pull or push on the wood to break the joint in a measurable fashion? I'm stumped for a good idea at the moment.

Thanks

[smorris]

My first thought is to use plate weights from a barbell set and hang them until it lets go, not super accurate, but...

[cabinetman]

Sounds like an interesting and fascinating project for your son. What comes to mind for a simple home test would be to clamp one piece so it is fixed to the bench. Attach a fishing scale that has a poundage marking. Use the kind that has a hook or loop at each end and is spring mechanized. Attach to the other piece and pull until something gives. It could be the wood or the joint. You might have to find a scale that goes to a fairly high reading. It's also possible you may have to use a come-along, for the graduated and consistent pulling leverage.

[Mr__Bill]

With the 90 angle how about putting one end on a bathroom scale and pressing the other end. Scale should read one half the amount of force necessary to break the joint.
If you make up a bunch of test pieces then people can test for them selves at the fair.
You could add another dimension to the test and do both clamped joints and ones that are just laid together.
This is a great project for a science fair, hope you and your son have fun doing it.

Bill

[gerti]

The problem with the scales is that they do not freeze at the breaking point weight, unless you have some special ones.

How about a pulley or lever system to get the force at a reasonable level, then just use a bucket and add water?

[LCHIEN]

The problem with the scales is that they do not freeze at the breaking point weight, unless you have some special ones.

How about a pulley or lever system to get the force at a reasonable level, then just use a bucket and add water?

Gerti is right, the shock load of a weigh falling will 1) lose the value, and 2) get damaged by the shock load.

Like Gerti suggests, I halped my son with a similar project, testing fishing line knots. We loaded the knots with a bucket and added water until the line broke. If you add it slowly, you get very fine control of when it breaks.

We added a rope "catch line" to keep the buket fro hitting the ground and spilling contents. Then we had a fishing scale to pick up and weigh the bucket after it broke the line.

The loads in our case were around 10-15 pounds. A five gallon bucket can hold 40+ pounds plus the bucket weight.

Depending upon the size of the wood you might take a lot of force to break it.

Also you will need to think about how you can make the joints precisely the same. If you want consistent results, you need to use consistent and quality joints. Consider measuring the amount of glue and using a consistent technique to spread it. Also consistent cutting and sanding techniques.

[LCHIEN]

The problem with the scales is that they do not freeze at the breaking point weight, unless you have some special ones.

How about a pulley or lever system to get the force at a reasonable level, then just use a bucket and add water?

Gerti is right, the shock load of a weight falling will cause any measuring device without a special peak load capture 1) lose the value, and 2) get damaged by the shock load.

Adding weights by hand is hard to do accurately without inducing some variations of deceleration unless you have a very steady hand (hard for a boy).

Like Gerti suggests, I helped my son with a similar project, testing fishing line knots. We loaded the knots with a bucket and added water (pouring slowly from a pitcher) until the line broke. If you add it slowly, you get very fine control of when it breaks.

We added a rope "catch line" to keep the buket fro hitting the ground and spilling contents. Then we had a fishing scale to pick up and weigh the bucket after it broke the line.

The loads in our case were around 10-15 pounds. A five gallon bucket can hold 40+ pounds plus the bucket weight.

Depending upon the size of the wood you might take a lot of force to break it. you might use a longer arm to keep the forces lower. Don't forget to wear safety glasses and shields - teach you son safe working techniques.

Also you will need to think about how you can make the joints precisely the same. If you want consistent results, you need to use consistent and quality joints. Consider measuring the amount of glue and using a consistent technique to spread it. Also consistent cutting and sanding techniques.

[mpc]

Another thing to keep consistent is how far the weight/stress is applied from the joint. If the force is "bending" the joint apart, you must keep the distance consistent. With two pieces of wood glued on the ends, I imagine most of the stress is being applied offset from the joint which is actually creating a torque. Torque = force * distance
Just like using a long handled wrench to bust free that stubborn lug nut.

Hanging a bucket of water, or sand, is a simple method and it's easy to track "how much" is in there. Larger fishing weights (they come in sizes from half a pound to several pounds) work well too. If you "hang" the bucket, you can eliminate the torque by cutting a notch in a vertical piece of wood; that vertical piece is glued to a horizontal support. Then the force applied is dead-vertical, no torque at all. That's the most "direct" and easily visualized example too; torque is probably somewhat foreign to 3rd graders. In fact, you could just screw in eyelet hooks into the stick and hang a fish scale on that, and the bucket hangs from the fish scale. Direct reading of the force - so you don't have to measure water or sand. Just pour it in slowly as the kids watch. I've seen rotary (dial) force guages with a tattle-tale needle too that captures the highest reading. Just can't remember where... it's been too long.

mpc

[cabinetman]

I kept thinking about the actual test. Although it not being a critical exercise, it should be accurate. Testing the glue joint would entail a more shear stress on the actual glue joint than waiting for something to break loose around the joint. IOW, the surrounding wood in a "break off" test would not be testing the actual joint, but rather the integrity or lack thereof, of the wood next to the joint.

Maybe a butt joint with shear stress until the actual joint gives way.
.

[Charlie R]

About 50 years ago I helped build a plywood row boat with an oak frame. The joints were glued with Weld Wood glue. I took a cut off from a frame joint and put one piece of oak in a bench vise and smacked the other with a hammer thinking I'd see how strong the glue was. The oak broke, not the joint.

Good luck with your project.

[cabinetman]

About 50 years ago I helped build a plywood row boat with an oak frame. The joints were glued with Weld Wood glue. I took a cut off from a frame joint and put one piece of oak in a bench vise and smacked the other with a hammer thinking I'd see how strong the glue was. The oak broke, not the joint.

WELCOME TO THE FORUM CHARLIE

About the same time as you were building a boat, I was helping my dad put together an 8' pram, I think it was a kit...I can't remember. Anyway it was the kind with a flat stern and three cross seats. I remember thinking when we were gluing pieces together that they would never hold. I was very impatient as a kid, (and still am) and remember how put off I was to wait for it all to cure. Eventually it all did cure, and we put a 5HP outboard on it, and drilled a hole in the center seat to stick a patio umbrella in for the just sittin' around on the fish to show up.

Nothing ever came apart, and I'm sure my dad had more faith in it than I did.

[Mr__Bill]

Keep in mind that this is a 3rd grade science fair and should reflect what he can do and not what dad can do. As a dad who has helped through several fairs I know just how hard it is to keep it a rewarding learning experience for your son and not a winning experience for dad. While I don't think that any of the ideas presented here could not have been thought up by a third grader (no disrespect guys) implementing them could be beyond his abilities and it's in the implementing that we dads get sucked into doing things.

I have often thought that elementary school science fairs should have separate categories for: My idea dad built it. Dad's idea and dad built it. My idea I built it and mom cleaned up the mess.

Climbing down of his soapbox... hope you and your son have a great time, we all are expecting pictures and results of the tests.

Bill

[LCHIEN]

trust me, you don't want a load on a fishing scale or any mechanical poiinting scale to be released suddenly when there is structural failure.
The needle will swing rapidly back to zero and whack the stop and damage the scale and shift the zero point, if the indicator needle isn't bent all to heck.

DAMHIKT.

regarding how the experiment is done, its very discouraging to get results that don't show some clear result, or worse yet, give contradictory results. THe kids won't understand the reasons for all the experimental variation - its best to helop them eliminate it early on, also good practice for them to use consistent techniques even if they don't understand the reasons why.

[PartsMutt]

Thanks for all the replies fellas.

I think we hit upon a good idea. And one that will test the glue more than the wood. We're going to take popsickle sticks and overlap the last 8th inch or so. Hang them vertically and put a bucket on the other side. I used to do pistol reloading, so I have about 40 pounds of copper plated 40S&W rounds to use as weights.

And he'll be able to glue up popsickle sticks, add weights, etc etc on his own with me just supervising.

Thanks again
Dave

[guycox]

Another thing to keep consistent is how far the weight/stress is applied from the joint. If the force is "bending" the joint apart, you must keep the distance consistent. With two pieces of wood glued on the ends, I imagine most of the stress is being applied offset from the joint which is actually creating a torque. Torque = force * distance
Just like using a long handled wrench to bust free that stubborn lug nut.

Hanging a bucket of water, or sand, is a simple method and it's easy to track "how much" is in there. Larger fishing weights (they come in sizes from half a pound to several pounds) work well too. If you "hang" the bucket, you can eliminate the torque by cutting a notch in a vertical piece of wood; that vertical piece is glued to a horizontal support. Then the force applied is dead-vertical, no torque at all. That's the most "direct" and easily visualized example too; torque is probably somewhat foreign to 3rd graders. In fact, you could just screw in eyelet hooks into the stick and hang a fish scale on that, and the bucket hangs from the fish scale. Direct reading of the force - so you don't have to measure water or sand. Just pour it in slowly as the kids watch. I've seen rotary (dial) force guages with a tattle-tale needle too that captures the highest reading. Just can't remember where... it's been too long.

mpc

Probably the easiest way to maintain a consistent moment is to break the joint that is suspended between two end points rather than just one.

Set up a couple of bricks on the work bench and place the glued up stock between the bricks. Place a small block directly over the glue joint and place the bucket on the block ( cut a circle to size of the bucket and glue the block in center of the circle?).... Fill the bucket with water... If the bricks aren't tall enough substitute cinder blocks...