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Immersible 4-point bend rig for stress corrosion cracking tests

Joe Kelleher's picture

Does anyone please know of any clever designs for 4-point bend rigs that can hold the load themselves (without a tensile test machine) and can uniformly distribute the load between all pins, even if small manfacturing misalignments are present? The samples will be reasonably small, about 70 mm long and up to 5 mm thick, and displacement would need to controllable at the 10-100 micrometre level. I'm envisioning some sort of system where the sample loading pins and some other levers and pivots create a mechanical linkage that constrains the rig to bend symmetrically when a bolt is turned. Or, if the rig is not actually constrained to bend symmetrically, it will have its lowest energy configuration when it does deform symmetrically.

Mechanical linkage designs often impress me with how simply they perform a specific task - is there one that is designed to do 4-point bending?

For info, I want to perform some stress corrosion cracking tests on strips of 316L stainless steel, that have been plastically strained to various levels. The uniform plastic straining would not in itself create bulk residual stress, so I need to elastically bend the samples to create tensile stress on one side. If the samples are held in 4-point bending and immersed in a suitable chemical environment (hot magnesium chloride works well), stress corrosion cracks will appear on the tensile face. Of course, I need a bending rig to accomodate the samples that will not itself suffer corrosion too. Alumina pins and Hastelloy C-276 other parts seem like a reasonable choice.

Comments on the rig design or any other aspect of this work would be gratefully received.

 Thanks,

Joe 

a frame with spring loading serves your purpose.

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