How can a DIY test machine enhance student learning in Materials Engineering?

[Gersty]

Hello there. I'm a veteran science teacher at the middle and high school level. I've taught a few years of physics and am interested in moving towards engineering.

I have looked at a number of available curricula like Project Lead the Way which require a significant investment in time and money regarding training and curriculum materials. I'm still considering this approach but in the meantime wanted to get my feet wet. I am interested in building a quarter-long unit on Materials Engineering. For this unit I want to build a low cost universal testing machine based on a 10-20 kg load cell and Arduino.

The idea is to have kids do some of the same build projects they would do in an ordinary physics class (toothpick bridge, CO2 car, windmill, crane, trebuchet, etc...) but have them test a number of materials for strength and toughness in tension, compression, and bending. They would learn to build a stress strain curve and calculate Young's Modulus in order to decide which materials to use in their build project.
I'm looking for advice regarding...

1. Building the test machine. I've looked at 2-3 examples of DIY test machines on youtube already. I'm thinking about using threaded rod and rectangular steel tubing. No access to welding gear means I'll be bolting it together.

2. Is a 10kg load cell like this one https://www.sparkfun.com/products/13329 sufficient? Should I go with 20 kg? I'm new to the idea of stress and strain calculations and so I'm not sure what size my samples will need to be but I'm planning on using popsicle stick sized samples of wood, plastics, thin metal pipes and tubes, toothpicks, etc...Given that, what amount of force will I need to apply, and which load cell will be appropriate?

3. Arduino as a data collection device. I have researched the idea of using an Arduino Uno with a load cell amplifier HX711 board and have spent time online with Arduino tutorials and it seems do-able.

Thanks in advance for any input.

 

[jrmichler]

1) You just need a load cell frame that is rigid under the highest loads to it will be subjected. Threaded rod and bolted steel will work. So will bolted / screwed / glued wood or plywood.

2) I could easily build a popsicle stick truss that would hold my weight (66 kg). I suggest at least a 100 kg load cell. If you want to do tensile tests, even that may not be enough. I suspect that a good popsicle stick could hold more than 100kg in tension. You can run some simple tests by clamping a test piece to a ceiling beam, then finding if it will hold your weight. Be careful, an overloaded load cell is permanently damaged.

3) I'm not familiar with the Arduino, the other parts you listed should work well enough for your purposes.

 

[dlgoff]

... have them test a number of materials for strength and toughness ...

What ever you come up with, make dang sure it will be safe for the students to use.

 

[Gersty]

Safety will certainly be a top consideration.

The model I have in mind will be small. I understand that the mechanical properties of a material (toughness, strength, ductility, hardness) are the same irrespective of the size of the sample. So if I can successfully test a very small sample of a Popsicle the stress and strain curve should apply to a larger sample, correct?

If this is indeed the case, could I get away with a 20 kg load cell around which I could build a small apparatus that could be safely used in a classroom setting?