i'm trying to write the analysis for a lab we've done at uni.
it's work hardening of materials and vickers hardness testing.

we seem to have an anomoly that i'm struggling to explain. from the raw sample to the first load cycle, the hardness increases as you'd expect. however, the next time it's loaded, the hardness drops back down to around the first hardness value before it's loaded. it's the same for an annealed stainless steel sample and a brass sample.

our lab instructor had no explanation for it, but i know there's a few people on here that are a damn sight more knowledgable than anyone at that uni!

confused i is!
help, please!
tom

what was the cross sectional area of the the test piece?

If it's over the machine test size it can have an impact on the results.

12.7mm x 0.8mm, 87mm long

apparently it has worked ok in the past, but it's recently started giving these spurious results.

tom

A couple of things spring to mind...

How are you working the material? If the strain is too great it can cause yielding in the material's structure. I think matals go through elastic/plastic/elastic phases so continue to increase the strain and test for re-hardening.

As the crystaline lattice grows it forms stress fields, pockets which you can hit or miss using the Vickers pyramid. The Brinell method is good for negating this in that it has an averaging effect on an area tested. Try taking multiple measurements across your test area and averaging your result.

I'd like to thow a few phrases about. Try Hypo-eutectic and perhaps hyper-eutectic. We'll have a few elastic limits for good measure.........
I have no idea what they relate to as it's a long time since I had to worry about such things.........
I do recall the Brinell been a better test though!

bloody hell owelly..............i've been trying to figure out your eutectic stuff and either it's too late in the day, or it's in a foreign language

the hardness readings were taken at 3 points along the length of the test pieces. all the tested areas exhibit the hardness being low, going up then back down to not much more than the initial value.

it seems too uniform to be anomolous......yet doesn't seem to match any established theory i can find
tom

Assuming uniform test pieces and a consistent test procedure with the same machine. If this anomalous behavior has only begun to occur recently and is exhibited across a number of test pieces can you rule out a problem with the test machine or a change in the test procedure?

[Edited on 14/3/08 by Puk]

I would buy the explanation that perhaps the mechanical properties aren't homogenous throughout the material, particularly if the Chrome and or nickle conent of the steel is high.

I'm currently working with Sheffield Uni on High Nickel Chrome Alloys, they are still discovering properties they didn't know existed.

Depends on your sample what test method to use whether Brinell or Vickers, but you have to start somewhere....

Good luck as most of this is over my head....

Steve

It means more work but try hardness testing whilst gradually loading the part and plotting the result. Prepare the surface (for hardness test indent clarity) before the test too.
You may find that it phases hard/soft/harder

Got a spectrometer there? (OES works well) Check the carbon content against ISO/Din

[Edited on 14/3/08 by Litemoth]