Tensile testing of Hard thin steel....

[NickE]

Does any one have any suggestions as to a method for fixturing .010"-.030" ~HRc56 1095/420ss ASTM E8 tensile bars. The lab I usually use has brand new Instron grip faces (~HRc60-62) and these are slipping.

thanks.
Nick

 

[TVP]

What size are the specimens? Can you use subsize?

 

[NickE]

We're using the 2" gauge length (2"x.50"x thick) for our thickness and max UTS the forces are around: 4500lbs,

The lab has cutting templates for the JIS#5 and ASTM E8 2" only new templates are bout $2800.

I was thinking of having grip faces made up out of carbide, the knurling pattern would be a bear to cut but at least they'd be much harder than the steel. The only thing I worry about w/ carbide is the shock from the energy release when the samples fracture.

nick

 

[CoryPad]

NickE,

Why are you focusing on hardness? If you view this problem from a free body diagram perspective, you need to increase the normal force provided by the grips so that the friction force is larger and can prevent the applied shear force (due to tension in the specimen). Are you concerned that you cannot increase the clamp force on the specimen ends because it will plastically deform the machine's grips? If the grips are much wider than the specimen, then it is unlikely the grips will deform. The surface pressure required to indent a material is 3 times the yield stress, so you should have a large margin. If there are constraints not being considered, please provide them.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strokersix]

You could try applying an abrasive slurry such as diamond lapping compound to the specimen. This will increase the friction between the grips and specimen.

 

[TVP]

Nick,

I don't think I fully understand your response to my question. Since you are currently using standard size specimens, is it possible to use the subsize specimens, in order to reduce the tensile force, and hence, the normal force required on the grips?

Are you saying that changing from standard size to subsize would cost $2800? Perhaps you can obtain a few samples of the subsize dimensions without having to obtain a new template, in order to see if this is an improvement or solves the problem.

 

[NickE]

CoryPad -- The grip style is the wedge acting 100kN grip from Instron. I have personally seen samples have 18,000Lbs exerted on them by these grips. The problem is that the file grip (face) is too soft to bite the specimen.

Strokersix -- will an abrasive slurry increase the friction btw. a fine polished HRc 56 steel and an HRc 60 knurled (file w/ against the pull pointed teeth) surface? I think that the abrasive would just fill the areas btw the teeth.

TVP -- My customer likes the standard size specimen and unless there is no other means I'd rather not have to go to the sub-size.. Oh and then the lab would have to be re-certified (A2LA) to cut and pull the sub-size specimen. Yes, a new jig for the tensulKut machine is roughly $2800.

Thanks for the help.

nick

 

[funnelguy]

Could carbide gripper inserts as used on chuck jaws and fixturing be fitted? These could be replaced if/when the carbide gripper becomes damaged.

Perhaps try grit blasting the samples ONLY in the area of the fixturing? Might provide a bit of "tooth". Won't cost anything to try.

 

[GRoberts]

Since the jaws are not gripping by mechanically deforming the test piece, you are basically relying on friction. If you were able to build a bolt on frame that added to the clamping force, perhaps the friction force alone would be enought to test the sample.

 

[NickE]

We've thought about the carbide inserts, but I'm worried about stiffness of an insert in an insert in the wedge. (the faces are a replaceable part).

I'm gonna try the grit blasting Idea, seems that I only need a bit more friction, since the sample will break but the elong measurement is always off.

Instron is quoting a hardfaced grip file insert.

nick

Nick
I love materials science!

 

[gthelen]

NickE

Try this, it is similar to the diamond slurry but not as messy. Try Crocus cloth, it is used in polishing and is normally about 2 inches wide. The carbide on the cloth will help get a better grip on the material.
YOu need more surface area to hold onto the sample.
You can also increase the tab width from the standard 1 inch or 3/4 inch to 1-1/2 or 2 inch tab width.
About the only other thing you can do is buy the $17,000 hydraulic grips.
You can also try a grip with a higher angle on the wedge, normally they use a 15 degree wedge, you can get a 8-1/2 degree which adds more normal force to the sample.

Good Luck