Specifying a tensile testing machine: sizing the frame and the load cell to the specimens you actually break, why grips and a tensile tester's extensometer decide the modulus more than the frame does, and the standard that has to be named before any number means anything

Specifying the system

1. Size the load cell to the specimens, not to the frame. Accuracy is a percentage of reading over a working portion of the cell's range, so a small specimen broken on a large cell is measured badly. A frame with interchangeable load cells is usually a better purchase than one big cell.
2. Choose grips for the material, because slip looks like extension. Wedge grips for metals, pneumatic grips for films and elastomers, threaded or shouldered fixtures where a specimen must not be crushed. A specimen slipping in the grips produces a curve that looks like yielding and is not, and this is the single commonest error in tensile data.
3. Measure strain on the specimen with an extensometer. Crosshead travel includes the compliance of the frame, the grips and the load train, so a modulus calculated from it is wrong and consistently too low. A contact or optical extensometer measuring the gauge length itself is what makes modulus meaningful.
4. Name the standard, and follow its specimen geometry. The standard fixes specimen shape, gauge length, preparation, conditioning, strain rate and how properties are calculated. Two laboratories testing the same material to different standards will disagree, and neither is wrong.
5. Condition the specimens and record the environment. Polymers in particular are sensitive to temperature and humidity, and standards specify a conditioning period for that reason. A laboratory that does not record its test environment cannot explain a seasonal drift in its own results.

Specimen preparation is half the measurement

Machining marks, a burr at the edge of a polymer specimen or a misaligned gauge section all start a crack in the wrong place. Specimens that fail outside the gauge length are discarded under most standards, and a laboratory discarding many has a preparation problem rather than a material one.

Measure every specimen's cross section rather than assuming the nominal dimension. Stress is force divided by area, and an assumed area propagates straight into every number reported.

Calibration and verification

Force calibration against traceable standards, verification of the extensometer, and a check of the speed accuracy. Ask for as-found readings on the force calibration for the same reason as any other instrument: they tell you whether past results were in tolerance.

Verify with a reference material where one exists for your test. A known material giving a known answer is the check that catches an error no calibration certificate would.

Buying against sending specimens out

A testing laboratory brings accreditation, calibrated equipment and experienced technicians, and is the right answer for occasional work or for anything that may be challenged. Owning makes sense at volume and for development work with fast iteration.

If you own one, get it into a proficiency testing scheme. Comparing your numbers against other laboratories on the same material is the only external check most mechanical testing laboratories ever get.

Common questions

How do I size a tensile testing machine?

From the highest force your specimens require, with a frame that accepts interchangeable load cells so small specimens are measured on a small cell. Accuracy is a percentage of reading, so results should sit in the middle of the cell's range.

Why is my modulus lower than the published value?

Almost always strain measured from crosshead travel rather than from the specimen. That includes the compliance of the frame and grips, which inflates apparent extension and depresses modulus. Use an extensometer.

What causes slipping in the grips?

Wrong grip type for the material, insufficient clamping pressure, or a specimen surface that is too smooth. Slip shows as a curve with a false yield and it invalidates the result, which is why specimens should be examined after a break.

Does the strain rate matter?

Substantially for polymers and for anything rate sensitive, which is why standards specify it. A faster test gives a higher apparent strength, so comparing results run at different rates compares the rates.

Browse by service class

• Compare Gene synthesis on advertised prices