The short version of a very long story is that I am sponsoring a group of students in a project that aims to determine the effect of the size and shape of (dumb bell / dog bone) shaped specimens on the ultimate tensile properties of highly extensible silicone elastomers.
I have no budget for this - I can spend about $300 AU of my own money on this - that’s it.
The only suitable and available tensile testing machine was made in 1987 and has an RS232 interface that probably only controls the machine motor and limited if any way to digitise load/displacement values.
While load cells are inexpensive and its almost trivial to to get the load/time data through an Arduino onto a pc, the elongation should ideally be obtained using non-contact means (video or laser) to get the true elongation of the specimen rather than the separation of the clamps (“jaws”) holding the specimen, which can produce quite different results.
My old workplace had a laser extensometer and I witnessed one lab using a video extensometer, but the cost of commercially available laser or video extensometers is eye watering, and building one is well beyond my DIY Arduino project expertise.
The working principle of such devices is that laser or video is used to measure the gradually changing separation of two markers on the specimen under test until it breaks. The stress vs strain curve should preferably be produced in real time, as this can reveal problems before the sample breaks, but if too hard and expensive then plotting acquired correlated values in a spreadsheet or the like would be acceptable.
Does any one have any ideas, links or other information on a DIY - (preferably Arduino based) laser or video extensometer that they can share with me?
I’ve searched the Internet and YouTube, etc. and I am aware of Arduino based range finders that measure distances between an object and the sensor but that’s not really what I need.
You can coat the part with a speckle pattern and record video. Mapping the distortion to the original image will give the extension.
Ideal speckles are circular so that centerpoints can be found to sub-pixel resolution. Size for that would be at least 10 pixels diameter. Lets you find center by fitting an ellipse to the 50% grey edges of each spot.
There are speckle pattern spray paints available at home improvement stores.
Alternately, a moire method can show distortion. Print a grid on the part, and view through a transparent, stationary grid of similar period. Rotate that grid slightly for improved contrast. You’ll get grid interference bands correlating to displacement that are porportional to the grid difference caused by extension.
Simple extension can be seen with stripes instead of grids.
Thank you Mark! I am constantly amazed by the generosity with time and knowledge displayed by members of groups such as this.
Yes I am now convinced that Video is the cheapest and most readily available means of having a contactless extensometer.
For the information of members who may read this post and be interested, my search of the internet had me stumble across a free video analysis program called Tracker, which will track the movement of features in a video and export the information to columns in a spreadsheet. Tracker Video Analysis and Modeling Tool for Physics Education. It’s almost as though it was made especially for the problem at hand.
Now I am wishing I was 20 something again doing these experiments in a lab… I will have to be content with watching the students do it.