Building a strain tester?

This is a well-developed area of testing. Suggest you read about what has already been done in this area (try a metallurgy textbook), and then either apply that to your project, or just buy a strain tester (new or used), depending on the intent, focus, budget, and whatever else of your project.

What's the purpose of doing this? Is it for a school physics experiment? If so you could either invest in a small tensometer such as a Hounsfield, or use a length of wire as your test piece.

The commercial machines usually use dumbell-shaped samples, or have jaws which squeeze the sample on the sides to grip it. I suspect that drilling holes in your sample will not give you useful results due to the stress concentration around the hole.

Best wishes,

Chris

AIR, from my school days a standard test piece is a called a 505 test sample, 3/4-10 thread at each end and 0.505" dia at the middle for a length of about 1.5", the area of a .505 d piece is .2 square inches and multiplying your result by 5 gives you tensile in lbs/sq in.

I think the "nuts" were hard steel plates maybe 2" thick that the sample threaded into.

No tooling marks are allowed on the 505 part.

If one is using A36 steel for his test sample, the force needed would be 1/5 of 36,000 lbs. (A36 steel is 36,000 PSI tensile strength) or

OOP's scratch that would be yield strength, to get it to break you need ultimate strength which would be up around 60-75K lbs.

Thank You, Randy

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There are reasons for using standard design of specimens, if precise data is desired. (There may also be valid reasons for deviating due to circumstances.) Standard tensile test specimen configuration are published in ASTM standards, a multi-volume set published by the American Society for Testing of Materials. If you have a large library nearby (ideally at a university with an engineering department) you may find them on the shelves. Next problem is to determine which volume contains the standard you need.

This is for doing weld certifications. I've found out most of what I want and now an trying to get more info on grippers. They cost a fortune and I can't see why.

That's because they want you to build your own. You already asked in the right group. :-)

Nick