What Daniel said, plus I might use a weight dropped from a height plus a plate for it to impact on to give an impact/impulse type load.
Some dial indicators can be set to stop and hold at max deflection, though momentum in the indicator itself might send it a smidge further than the actual. But that in turn would probably be in line with the true deflection under impact/impulse load, so that it’d be a reliable scalar quantity.
hope that’s of use - please do keep up the good work. This is something this business has very badly needed for a very long time. Please don’t hesitate to let me know if there is any way I can be of assistance.
I would like to measure data points and graph the deflection. I think if I anylize each frame of film I can see the weight on the scale and the reading off the caliper. I dont know if I can get a high enough frame rate though…
If surfboard components are anything like concrete, rate of loading will have a significant effect on the results. Too fast = poor strength. But as mentioned earlier, maybe a faster load rate would apply more to surfing.
Any Ideas of how to load a material quickly and still produce accurate measurments?? Video camera maybe?? then analyzing the video
I uploaded a copy of the spread sheet. NOTE: there are 11 work sheats in the spread sheet, One for each material I tested.
As best I recall, repetitive testing jigs that I have seen pictured have supports at the sides of the tested panel and a plunger (padded in this case probably) in the center. Some sort of reciprocating mechanism drives the plunger up and down at a predetermined speed and depth. I believe the most common test is to destruction, and would record the panel dimensions, depth, speed, and number of cycles. A video would certainly compliment that (especially one shot at a high enough frame rate to slo-mo later), as you could see how the failure occured and progressed. I bet you could improvise something with an electric motor and some kind of rotary to reciprocal converter, maybe a crankshaft from a very small IC engine? You might be able to set this up on your drill press as well (depending on vertical capacity and the actual set-up) and control the depth of flex that way.
Uhmm- well, therein lies the fun. The problem with doing impacts via video ( or film ) is you have to have a very, very fast frame rate, like the Edgerton strobe-lit pictures of the bullet going through an egg …and I don’t have any idea how you’d do that with an easily obtainable video camera.
The way I would play it is to figure out how to measure the deflection electronicly, then record that. And it’d have to be done repetitively over a time interval… bear with me, I’m thinking this through as I go -
I was involved in a project building a counting machine that used light and phototransistors to increment a count when something broke the light path - the design and setup were easy enough for an electronics imbecile like me to figure out… You could use a few of those ( phototransistors are quite cheap, or there are interesting possibilities with a single photoresistor and a Vee-shaped opening to allow light through ) in a vertical array with the light source on the other side of your test piece, poll them every few microseconds to see what was blocked ( by your test section under impact) and spit the data to the spreadsheet. Couple that with http://www.parallax.com/detail.asp?product_id=30056 and you have a nice little setup, though you might have to use a lever to exert heavier forces on the test piece.
Another way to go about it might be to run some low power, low voltage current through a circuit- it’d make contact with a fairly high resistance vertical strip so that your current at a given deflection was a function of the deflection… but the more I think about it, the more I like the photoresistor setup.
Another type - http://www.phidgets.com/index.php , with USB interface, so you can leave the microcontroller/microprocessor out entirely, and these are fairly cheap and easy to do. Stumbled across this site while looking for photoresistor info just now, and some of 'em might be tailor-made for this application.
Doc, I like your Ideas. I did something like that in physics lab while measuring the speed of a falling object. I dont know If I will go through all that now. For now I think that I have all the data I need.
Has anyone seen a food saver?? Iv’e heard that they are great for vac bagging on a small scale. I might get one on ebay.
Off the scene for a couple weeks and look what happens!
Very nice.
Personally I think series 4 looks pretty promising.
And don’t pay too much attention to anyone who tries to tell you that the static tests mean nothing. True, they aren’t the whole picture BUT they can provide some very nice indications for further exploration. And it’s a handy, controlled way to compare apples with apples. Instead of with mangoes, pineapples, oranges, pears and grapefruit. Which is what we usually see.
For impact and dynamic you could make a fixed strike distance with a set strike head and incrementally increase the weight behind the strike. Then measure the result. A lot more to record and more time consuming but it should be quite workable. If you really want to measure max deflection that’s really easy. A dial-indicator that doesn’t have a back spring will do the job nicely. Bought one for AU$20 years ago to make a spine-tester for arrows.
At work I do similar tests to determine flexural strength of concrete (you too ryan??). I’ve considered bringing surfboard materials to work and breaking them, but haven’t gotten around to it yet.
Francescutti, Are you kidding Me? Are you somebody I know? All I said was I use to break stuff. It could’ve been many things. You hit it right on the head. I use to work for a lab that tested construction materials among other things. I’ve broken countless concrete flex beams. It’s been a few years. Let me think…ASTM C78 comes to mind. Third point or center point? I hate that test. The beams are heavy and specimen prep’ and maintenence is critical.
I was reading through this thread, I’m sure that fact alone is some kind of symptom, and when I was reading yours I couldn’t help but chuckle. Therein lies the fun indeed. Adjectives come to mind…fun, passion, intellect, nerd, techy, smart, obsession, obscure. Whatever you call it, it’s good stuff.
It’s not everyday you find a bunch of guys talking about how to build stress/strain curves on the weekend.
To borrow a term from the kids today, I’m feelin’ you dawg.
Well, hey, I am enjoying this - and seeing some of the USB measurement stuff has given me some ideas for a few upcoming projects of my own: some counting/measuring stuff with motor controls, etc, easier to program and all kinds of good things.
Now, the possibilities for doing this semi-automated are considerable. Mebbe a pneumatic cylinder for force with pressure sensor, or who knows… displacement and pressure gauges, and so on. And that’s just with the goodies from one company, haven’t even looked at more of 'em.
Lets see - the cryovac rigs like the food savers, well, they don’t crank out a whole lot of vaccum, but they do pull some, no doubt about it. I just wonder if they are consistent enough, y’know? I doubt they have to be, just to vaccum pack some pork chops, y’know?
I’ve read the threads, albiet in haste, and I’m not sure of the objective at this point. What I get from it is that you’re looking for deflection under impact force. Is that right?
Or, are you talking about cyclical loading which is quite a bit stickier. At least it is if you want to measure force and deflection continuously. If you want to do that, you’ll be hard pressed to do it without a computer, data aquisition software/hardware, and some Not-Home-Depot-Cheap instruments. It would be nice to see a couple of sine waves plotted on a chart, one for stress and one for strain.
However, if you want deflection under impact load, that can be accomplished is crude but practical way. Drop something and use a clay telltale. You have a simple span, a specimen, an underneath you have a few small columns or cones of modeling clay topped with wax paper. Control the speed with the height of the drop. Contol the force with the mass of the object and the height. Then you’ve got to have good hand eye coordination. Remember Jax? Drop, bounce, catch. Fiberglass is lively. You’ll get enough rebound to catch the object on the first bounce. Now it’s drop, bounce, catch, measure.
Make the clay slightly higher than the anticipated deflection. Make them small enough not to provide significant resistence. Measure the difference betweent the span supports and the height of the clay.
Crude, yes. Requires some fine tuning, yes. But, it could work.
Doc, I’m not from New England but does this qualify as Yankee Ingenuity?
Aha! Right, that’d work quite nicely, kind of a poor man’s Plastigauge. I like it!
Only problem might be under very light impacts, when the force needed to deform the clay was a significant part of the force exerted - but that’d be curable with thin enough clay cones or scaling up the section and thus the overall force.
The only problem, leastwise for me, is the ‘good hand-eye coordination’ bit.
Right, stress and strain, I’d also be interested in what permanent deformation/weakening came after X cycles, akin to work-hardening. That would need quite a lot of repetion and almost certainly automation, I’ll bet.
Those USB goodies I stumbled across would do a nice job, I think, and you can run a bunch of them together. Not all that expensive either, compared to some of the workarounds you’d have to go with in some of the other setups.
Yankee ingenuity - yep, that kinda pops up everywhere. Plus, the rest of the world calls everybody in this country Yankees. Funny thing, the term ‘yankee’ is descended from the Dutch ‘janqui’.
leastwise, funny to me. Then again, there are those who’d say I badly need a life.
Not really, no, if you are looking for relative strengths and such of different materials or techniques. Say, kevlar/epoxy/eps foam vs some other combination, or different laminating methods or such. Which is stronger, or stiffer, or rebounds faster.
It all scales up. Or down. And the maths for scaling 'em up and down are standard engineering stuff.
Add to that that differently shaped full-sized panels will be stiffer or flexier, ultimately weaker or stronger, and that all boards are shaped somewhat different…well, kinda comes down to relative strengths are the best you are gonna get.
Doc wrote: “Only problem might be under very light impacts, when the force needed to deform the clay was a significant part of the force exerted - but that’d be curable with thin enough clay cones or scaling up the section and thus the overall force.”
I used to buy modelling media for my kids in an art supply store that needed to be baked to harden and was much less dense than the standard modeling clay. My suspicion is that it was cellulose-based and heavily air-entrained. Unfortunately I can’t rermember what it was called, but I bet a search at a specialty art supply site would turn it up. That substance should have a near zero effect on deflection.
stuff it into a lightly oiled drilled hole, nip it off to whatever length you needed, so that your ‘before’ and ‘after’ lengths would be easily dealt with.
Hmm- at the risk of starting another 'how do I make my own ___ ’ thread, I’d guess that something like very soft surf wax would also work, and it could be molded very consistently…
The main thing is you want something with no rebound. That’s why clay comes to mind. Playdough, Floam, and other things I can think of have too much resilience. They’ll rebound giving a false indication of bottom. Also, with trial and error you can find the ideal clay height and shape. Probably going to want a cone with a taper something like the drill in Doc’s link. Thinner is better, but you don’t want it so thin it just bends over or falls.
Here’s a little ding resistence device I worked up for someone. We never built it. I can think of some upgrades but something like this could support the striking aspect. You can use large metal spheres for other things for the falling weight. Probably would need a plate across the sample to evenly distribute the strike.
In the latest TSJ there is a photo of Clarks plugs at the concrete recycling plant, now you guys are trying to use concrete testing machines (or similar types) to test flexure on boards. Remember that concrete and steel can bend and flex back to their original shape. (If you have ever seen a tilt up panel go up, or watch 20 pipe booms set a steel gas pipe). The ‘max flexural strength’ is when the material will not return to its original shape - read fail.
Floam? Ah- not having kids, I wasn’t real familiar with the stuff.
It kinda strikes me that between gauging material rebound, the force needed to squash it and squash it straight-line, the problems of measuring it accurately both initially and at end-state, it might be easier ( overall) to just make a gadget, kind of a fulcrum arm/seesaw as below:
If the arm was built fairly light, the inertia of it wouldn’t suck up that much of the energy involved. It could be zeroed fairly easy relative to the test piece and the deflection could be read out directly. Friction could be as simple as a machine screw, a wingnut and a couple of rubber washers. You might need to calibrate the scale some relative to actual movement of the short arm, but again this isn’t awfully difficult.
In fact, I have seen very similar gizmos someplace or other…
I got a foodsaver and did some vac-bagging. I don’t know what kind of vacuum pressure this thing pulls but it is awesome. I did a test section and it worked out really well. Im gonna pull it out of the bag tomorrow. The foodsaver has a built in bag sealer so it traps the vacuum pressure in the bag so the pump doesn’t have to stay on all night like a conventional vacuum bagging system. Also you can make custom sized bags by using the sealer to make a seam between two pieces of plastic of any size. You just have to make sure to run a cotton cord around the perimeter for breathing and taper the bag down to 12" at the end so it can fit in the machine. As I stated at the beginning of the thread, my purpose for making my materials tester has nothing to do with surfboards… I am making an airplane for aerial combat flying. I wanted to find the best material to make it out of. It must be able to withstand prop strikes and midair collisions. I am going to make an EPS foam core fusalage, wing and tail with a balsa shear webs in each, then sheet it all with 1/16" balsa and glass it with carbon tow and 5 oz kevlar. The whole plane will be vac-bagged with the food saver. It should be kick ass!! check out the pics of the food saver.
