Small Specimen Fatigue Testing

Studying fatigue properties of small specimens can be challenging, yet is needed for research purposes as well as for product and verification testing in industries like the medical device and additive manufacturing.

Fatigue testing allows manufacturers and researchers to get a critical understanding of how a material or component will perform in real-world loading scenarios over the course of time. Common fatigue testing definitions and keywords include:

• Fatigue strength at N cycles: The stress value for failure at N cycles as determined from a S-N curve, which is the diagram of stress (S) against the number of cycles (N) to failure.

• Fatigue life: The number of cycles that a test specimen could sustain before failure.

• Fracture toughness: The measurement of resistance to extension of a crack.

A researcher at a major American university turned to ADMET for a materials testing solution that is able to run fatigue testing on very small specimens, the width of the specimen gauge section varied from 250microns, to 500 microns, to 1mm maximum, with specimen thickness matching the gauge width.

Standard fatigue testing grips would not have been a solution for such small specimens. As shown in the image below, small specimens would have been dwarfed in between the grip jaw faces.

Furthermore, with such small specimens, alignment becomes critical to a successful test. Our customer was looking to perform fatigue testing with R Values of -1, meaning they wanted to go from tension to compression and back to tension for every cycle. So fatigue grips couldn’t be simply designed for tension-tension fatigue. A stiff gripping configuration that wouldn’t exhibit any backlash or slippage when going from tension to compression was required.

Small Specimen Fatigue Testing Equipment Configuration

ASTM E466 and other fatigue type tests require that a test machine is perfectly aligned to ASTM E1012 standards in order to gather proper and accurate fatigue test data. ADMET proposed a solution that utilizes a unique grip design with an alignment fixture to connect to the two opposing grips to ensure perfect alignment. It also incorporated alignment “fingers” to ensure that the specimen would be perfectly vertical in the plane of the grip faces. Once the test specimen is inserted, aligned, and tightened down to the grip faces, the grip alignment fixture can be removed.

Fatigue Grips with Alignment Fingers for Small Specimens

Also, to ensure the specimens are perfectly positioned in the plane of the load cell and line of force, different grip inserts were provided that would correspond to a particular specimen thickness: for 250 microns, 500 microns, and 1mm.

The same grip design was used in two different testing machines:

1. eXpert 4902 with MTESTQuattro – for fatigue testing
2. eXpert 5602 with MTESTQuattro – for low speed tensile testing on order of 10^-5 mm/mm/sec strain rate

Click on link to read more about the test application and equipment: https://www.admet.com/small-specimen-fatigue-testing-and-alignment/