Science Fair

[Larry Klaaren]

My son is doing a science fair project about the relative strength of woods.

His plan is to suspend strips of wood between sawhorses and then hang a bucket from them. Then he wants to put weights in the buckets. We have two hundred pounds of weights, and he can do it in two and a half pound increments. The object of the project appears to be the research, standardizing and charting. He's already done his research.

Right now, my bandsaw has a broken part, so I cut strips for him on a tablesaw. They are one quarter inch thick, an inch wide and three feet long. (He was supposed to do it, but I got real nervous about cutting strips that thin, where he was standing as he did it, and his sister running around while he did it.)

His woods are partly scraps I have and partly scraps I purchased at a woodworking store for a buck (for four foot long one by ones). We have mdf, pine, bass, red oak, white oak, cedar, poplar, walnut, maple, and cherry. They are all half-sawn so the grain is the same on all of them, except the mdf of course.

I'm sure that we'll reach the breaking point on all the strips, although I did stand on a one foor long sample strip of red oak setting on two 2x2s and it didn't break or bend that much, but I'm not close to two hundred pounds. I'm betting the mdf breaks first, and I think that is a safe bet!

Just wondering if any one has done anything like this and has any wisdom for us.

Thanks,
Larry

[Steve Wargo]

Larry the only variable in your experiment will be the grain of the wood. You'll need to make sure that you select material that is very straight grained for all samples in order to keep this close to a legit experiment. Obviously anything with funky grain, curl, knots, or the grain running diagonally should try to be avoided. Please post result of the experiment. Will be a good project.

[Lars Thomas]

As a control, you'll need to make sure the saw horses remain a constant distance from each other. You will also need to make sure the bucket is suspended from the center. Depending on the species, splinters may go flying - protect your eyes.

Now, I'm not sure of this, but would it make sense to clamp each end of the test piece to the saw horse? I'm thinking this would limit the amount of bending each piece can do. Afterall, you are measuring the point at which they break, not how much they can bend.

[Larry Klaaren]

Go od thoughts.

We do have a safety factor to consider. If the boards do bend and not break, we'll have to mark when the bucket hits the floor. We'll be allowing a six-inch or so drop.

Not sure about clamping. Could cause the sawhorses to tilt if the boards bend. We have a few trial pieces.

[Larry Klaaren]

Good thoughts.

We do have a safety factor to consider. If the boards do bend and not break, we'll have to mark when the bucket hits the floor. We'll be allowing a six-inch or so drop. That's quite a bit for a three foot piece.

Not sure about clamping. Could cause the sawhorses to tilt if the boards bend. We have a few trial pieces. I think the important factor will be consistency. Whatever we do, prepare each piece the same way, subject each piece to the same factors, measure with the same method.

Still listening.

Larry

[Dan Stuewe]

Depending on how crazy you want to get there are a few other things to concider (others might give more information)...

Even with much more homogeneous material, like metals, tests like this are done with at least a few samples of each and the results are statisticly combined. Also, strength is usually looked at more as a curve, something like load vs. deflection (the deflection being the hard part to measure). The point on the curve where it goes non-linear is the limit load and where it breaks would be the ultimate load.

I don't run this type of test, but I know that here at work they are done quite often (I belive they're called "two-point bending tests"). The scary thing is everytime they test something it seems like we prove that things that work, shouldn't :eek: !

Good luck.

[Ed Lang]

After you setup the saw horses, take two 2X4's and screw then to the top ends of the horses to keep them the same distance apart and keep them from tilting in. Then use your C clamps to clamp the test wood to the outside edge of the top of the horse. To measure the deflection, have a straight edge running next to the test wood and then measure the deflection from the bottom of the straight edge to the top of the test wood. Chart the weight vs deflection. I would think 6" would be more than what is needed to break the wood. If not, recut the wood thinner.

My now 20 year old son did some fun things for Science Projects. One I remember best was where he found out what material insulated the best.

He used remote thermocouples inside of little boxes made from all sorts of materials like paper, wood, glass, foam, etc... All on the counter in the kitchen overnight then placed in the freezer and charted the temp drop for a few hours. He did most all of the work and it was his idea.

Sorry for typing too much.

Enjoy the project and be sure to take pictures and let us know how it goes.

[Bill Lewis]

Dan, this is called a three point test, but who's counting.

Forget clamping the ends, it'll only require more weight to break the specimens.
Provide "fulcrums" for the test specimens, metal preferably. You want a consistant and relatively thin surface that will not wear or change shape as the tests proceed. A piece of metal angle c-clamped or bolted to each sawhorse whould work quite well. Likewise your bucket/load applicator should have a consistant contact surface.
Attach a ruler or metal yardstick to the test fixture to read your deflections directly as you apply the load. If this intereferes with your load buck too much, be inventive and build a remote mecanical read out.

Yes I do make my living doing this sot of thing. But I would be using high speed photography and explosives instead.

[Tom Scott]

Larry,
My daughter and I did this for her science fair contest last year. As others said, use more than one sample of each. We used up to 5, throwing out any abnormally high or low values, and averaged the results. Also, don't clamp to the end supports as it just adds a negative bending moment into the equation, and you want to keep it simple.

Our testing set-up was a little more complicated. We used a weight lifting bench for testing, with the wood sitting on the bar supports, a digital shipping scale sitting on the bench, and a car jack sitting on the scale pushing up on the wood. The weight bench had hooks that flip up over the bar when not in use, that way we could push up with the jack. This set up allowed us to get more exact readings on applied loads. She would jack and I would read the load on the scale.

We used 3/4" samples of pine, cedar, cherry, oak, maple, and poplar because that is what I had on hand.

Take plenty of pictures of the set-up and testing procedures for the display. We also used some of the broken samples for the domonstration.

Deflections of the different species varies a lot, but we decided to leave that out of the equation since this was for a 4th grade fair.

Good luck,
Tom

[Tom Donalek]

Also, don't clamp to the end supports as it just adds a negative bending moment into the equation, and you want to keep it simple.
I was going to point out that clamping the ends would cause the required load for failure to be much higher, but Tom put it better.

In thinking about deflection and failure, and coming up with 'nice clean results', the idea of just cantilvering the samples off of a rigid 'table' made sense. This isn't the classic 'bridge testing' arrangement of spanning between two points, but it would reduce the amount of weight required for failure and potentially give more 'dramatic' results.

Also, I was thinking about how they do a lot of this seat-of-the-pants type testing on Mythbusters. They always have cool looking black-and-white striped do-dads in the backgrounds of their testing camera shots. These could be an easy, nice touch in the background of the photos of the test rig - having a visual reference to compare the deflection of different woods or different loads!

If your kid is far enough along in math to start dealing with the equations for bending and stress, I'm sure several of us here could explain a bit and point you in the direction of some books on the subject. It could be a great touch to compare the actual test results to the 'theoretical' expected failure load. (Can you tell that Science Fair was the highlight of the year for me in grammar school? :rolleyes: )

[john mclane]

Sounds like a fun experiment. A couple of simple things to add: make sure the wood thickness and other dimensionsis are the same. Could get the weight of the wood (density for the area or board foot) and plot that as a variable. If you have a moisture meter could add that in as a potential variable. (I don't think it will add much between different woods but in the same wood esp a soft wood, might give some interesting differences). I agree that 3 or 5 samples of each wood should be tryed if that is practable.

Have fun

[Frank Chaffee]

I agree that 3 or 5 samples of each wood should be tryed if that is practable.



…And perhaps eliminating the strongest and weakest samples.




If your kid is far enough along in math to start dealing with the equations for bending and stress, I'm sure several of us here could explain a bit and point you in the direction of some books on the subject. It could be a great touch to compare the actual test results to the 'theoretical' expected failure load. (Can you tell that Science Fair was the highlight of the year for me in grammar school? :rolleyes: )

Larry,
Many of us here as well as your son could benefit from the equations Tom offers.

Frank

[Larry Klaaren]

We did the project last week. It's my son's project, so he did it. I'll try to add pictures after I get done with this post. They are kind of amateurish.

The wood was 1/4 x 1 x 12 inches. The sticks were suspended between two pieces of wood ten inches apart. Some of the wood held an incredible amount of weight.

We hung the weights on a hook I made from two bike hooks screwed into a 2x2x3 piece of oak.

We had 50 washers and a lot of weights. It took 39 washers to equal 2.5 pounds, so we would add in washers until we ran out then take them off and add two and a half pounds and start again. We kept adding weight until the wood broke. There were three or four of each sample.

I made an Excel sheet with formulas so he could get the average weight for each one.

Here were the results.



<TABLE cellSpacing=0 cellPadding=12 width=282 border=1><TBODY><TR><TD vAlign=bottom width="33%" height=0>

</TD><TD vAlign=bottom width="33%" height=0>
Average Weight
to Breaking





</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>

</TD><TD vAlign=bottom width="33%" height=0>

</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Bass


</TD><TD vAlign=bottom width="33%" height=0>
19.2


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Cedar


</TD><TD vAlign=bottom width="33%" height=0>
11.8


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Cherry


</TD><TD vAlign=bottom width="33%" height=0>
32.5


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
MDF


</TD><TD vAlign=bottom width="33%" height=0>
4.8


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Maple


</TD><TD vAlign=bottom width="33%" height=0>
59.8


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Oak, Red


</TD><TD vAlign=bottom width="33%" height=0>
28.9


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Oak, White


</TD><TD vAlign=bottom width="33%" height=0>
60.9


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Pine


</TD><TD vAlign=bottom width="33%" height=0>
11.3


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Poplar


</TD><TD vAlign=bottom width="33%" height=0>
30.4


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR><TR><TD vAlign=bottom width="33%" height=0>
Walnut


</TD><TD vAlign=bottom width="33%" height=0>
35.4


</TD><TD vAlign=bottom width="33%" height=0>

</TD></TR></TBODY></TABLE>


Zach's comment was "And all these houses are made of Pine?"

[Larry Klaaren]

I couldn't get the pictures to upload. I will search the help files and try this weekend.

BTW . . . For anyone out there who is a Sacramento Kings fan. Remember, this is not an emergency, it is only Artest. You'll understand before too long.

Larry

[Tony Falotico]

Hi Larry, I've judged science fair projects for the local schools for several years. As we 'observe' each project, we discuss it with the student. To be honest, the 'majority' of the score is based upon the students enthusiasm, basic knowledge, and how much the student participated in the project. It's easy to tell which ones the parents did with little or no student participation. We don't count off if it's "amateurish", as long as some effort on the students part is evident and it's not downright sloppy. We try to consider the student's personal achievement, not so much professionalism of the final presentation.

Your son's project sounds good. I would suggest lots of photos of the actual loading and testing, and display of the resultant pieces along with the chart showing the results.

Have fun with this, your assistance and the memories your son will have far outweigh whatever the judges may say! Several of the engineer's in my office participate every year in judging, something we look forward to.

[Larry Klaaren]

Thanks Tony,

Unfortunately, Zach's school is very large, so they do the project on their own and write a paper about it. We did it last week. I think I have to reduce the size of the pictures before they can be uploaded.

Larry

[Pete Harbin]

It might be cool to show the effects of beam size. How much more weight can be loaded on the beam by changing the dimensions?

For example, say your sample size is 12" long and 1/4" X 1/4" and it supports 5 lbs before failure. Adding a "second beam" or making the beam another 1/4" wider would double the load the beam could carry, but doubling the height of the beam would increase load capacity by 4 times (or more depending on material.) Some cool compression and tension diagrams might be great to have here!

Comparing load capacity to the weight of the beam would be neat too. Be sure to include fir in your samples since it is the strongest wood for it's weight.

Now I'm getting excited! Boy do I love science fairs! Our company heavliy supports our local schools and I volunteer to judge and mentor every year. I absolutely love seeing those kids challenge themselves and really put something of themselves into thier work.

Pete

[Larry Klaaren]

These are some picture of the the last batch of pieces laid out, the weights, the "hook" and the apparatus.

Figured that my dimensions on the pictures were too large.

Thanks everybody for your suggestions

Larry