ASTM D1415 vs. D2240 Type M micro-hardness testing 3

[PatAero]

Hi all,

I am looking to better understand the differences between ASTM D1415 and ASTM D2240 for type M micro-hardness testing. I have had some O-rings tested at two different labs. One lab used D2240 and the other used both D1415 & D2240. The D2240 results from both labs matched however the D1415 test results varied. After reading the ASTM sections the only differences I can come up with are the time dependency and the actual test apparatus. Could anyone better explain the differences between the two? Material is NBR rubber. I'm new to the rubber scene so any help is appreciated!

Thank you for any and all tips!

Pat

 

[tom1953]

One difference (I'm sure there are others) seems to be that D1415 (IRHD) uses a ball indentor while D2240 Type M uses a conical indentor.

 

[PatAero]

Thanks for pointing those out Tom, I found those specs under the "Apparatus" section of the ASTM book.

Something I should have included before:

The test results using D2240 Type M were found to be a 70 Durometer however the D1415 Type M test results yielded between 50 and 60 Durometer. In this case which test should one believe? Are the results even comparable in the same scale?

Thanks Again
Pat

 

[tom1953]

"...which test should one believe?"
- Is there any reason not to believe both? The compound has a D2240-M hardness of 70 and D1415-M hardness of ~55. I think it depends on what what specification or requirements you're trying to meet, or what the customer wants. If the specification that needs to be met is an ASTM D2000 callout, for example, then D2240 hardness is specified.

I think it's like tear strength; there are several different test methods (e.g., angle, crescent, nicked/unnicked, trouser) and none of the results necessarily relate to each other. So, if somebody wants 200 lb/in tear strength, it has to be clarified what method (D624 Type C or B or T, for example) is used.

"Are the results even comparable in the same scale?"
- I wouldn't think so.

Good luck!

 

[PatAero]

Thanks Tom for the insight!

 

[stancom]

I agree with much of tom1953's comments but I am surprised that the difference between the two instruments is 10 numerical units or even greater. In my experience, admittedly with the normal test, the difference ought not to exceed 5 units or so even when we recognise the underlying difference in durometer and International Rubber Hardness tests.(Do not regsrd both as durometer). My understanding is that the difference can be larger for a lossy rubber such as NBR than for a more resilient rubber such as NR, but again not as high as reported here.

Have a look at the paper 'Comparison of results and test methods using the Micro IRHD and the Micro Shore rubber hardness measurement instruments' by R. Morgans (University of Greenwich). and S. Lackovic and P. Cobbold (Both H W Wallace). It is easy to find on the web. Interestingly they report higher numerical values for IRHD than for Shore A.

If something is amiss I think it may well be with the quality of the D 1415 test

 

[PatAero]

First off, thank you for your input! Secondly, Since method D2240 shows both samples to be 70 durometer could the varying results using method D1415 be caused by a difference in compression set, since the method is dependent on time?

Note: Material 1 and Material 2 should be the same.

 

[stancom]

Hi, Pat. The time dependent property involved will be creep/stress relaxation rather than compression set and, yes, this will be responsible for some of the drop in hardness over the 30 seconds. Obviously an extreme case will be the rapid fall when unvulcanised rubber is tested. However, relaxation is not the only factors at play. The tip of the indentor takes some time to settle into the depression it makes, eg overcoming drag forces - which one reason for the application of a solid lubricant, usually talc.

i would try to get a measure of how the IRHD reading changes throughout the 30 secs and compare this will the same test using the durometer. If the IRHD reading is always well below the Shore A one I would start to get a little suspicious.

 

[PatAero]

Thank you stancom for the suggestion!