Charpy testing question - what if...

[robsalv]

Materialists - a question about charpy testing.

We all know that a known amount of energy is applied to a known specimen size at various temperatures and this tells us something about the material property at that temperature.

WHAT IF at a constant temperature, you:
- increased the hammer load instead?
- increased the hammer speed instead?
- increased BOTH the hammer load and speed?

Over the years I've seen bolts and structures fail brittly due the sudden and rapid application of load at temperatures that would otherwise have suggested a ductile failure... well I've attributed the failure to a brittle mode... I could be wrong... fracture surfaces did look like cleavage as opposed to tears.

This whole post probably belies my lack of material knowledge, but in simplistic terms, could a charpy type test device be used to study material performance under rapid loading scenarios?? I've always thought the rate of strain had a bearing on whether a material would fail brittly or not... has fracture mechanics moved beyond this simplistic understanding?

Thanks in advance for your thoughts.

Cheers

Rob


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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[HappyViking]

Yes! Fracture mechnaics has come along miles since the Charpy test. High strain rate fracture mechanics testing is the subject of British Standard BS7448 Part 3 (1995) "Fracture mechanics toughness tests - Part 3: Method for determination of fracture toughness of metallic materials at rates of increase in stress intensity factor greater than 3.0MPa.m^0.5s^-1"

Okay, it's not a snappy title, but this standard uses notched and pre-cracked bend specimens to determine the fracture toughness in a similar way to standard CTOD fracture mechanics tests.

With respect to how an adapted Charpy test might work:
With a bigger load on the end of a hammer with the same arm length (bigger load, same speed), the specimen will absorb the same amount of energy, but with the hammer starting with more potential energy, it will subsequently swing higher than a smaller hammer would. However, a machine calibrated to the larger hammer would give the same reading of absorbed energy for the specimen.
With a longer hammer arm (faster impact speed), the specimen may absorb less energy, as a higher loading rate gives an apprently lower toughness in a specimen when compared to a normal loading rate.

 

[CoryPad]

There are instrumented impact tests that provide force vs. displacement, which can be used for fracture mechanics calculations. Drop weight testers are used more frequently. Here are some example machines:

http://www.instron.us/wa/products/impact/default.aspx

Regards,

Cory

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

 

[metengr]

It is a well known fact increasing strain rate will increase susceptibility to brittle fracture behavior in carbon and low alloy steels. There are plenty of technical articles on this specific subject.

 

[robsalv]

Thanks for those replies fellers. I see I wasn't a total nufty on the materials question!

Don't those instron machine look the business!


One more specific question - what about non ferrous materials and their susceptibility to increasing strain rate? Would Aluminium be susceptible to brittle failure under increasing strain rate for example? My conventional wisdom says that Aluminium doesn't have a nil ductility temperature, well, except for perhaps -273K.

Appreciate some more of your thoughts.


Thanks




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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[metengr]

robsalv;
Here is a suggestion, since you show an interest in this subject;

http://www.key-to-steel.com/default.aspx?ID=Articles

There are excellent feature articles on this very subject. Have a cup of coffee and read them.

 

[arunmrao]

Another must have and read book is " Mechanical Metallurgy" by G.E. Dieter published by McGraw Hill.

" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke

 

[robsalv]

LOL Metengr... a cup of coffee... the size of a bucket perhaps!

Ok, the link is book marked - and I've dived down to section 10 on fracture mechanics...

...interestingly though, the few other people I've asked about nonferrous brittle failure have all tended to look sideways too. This has piqued my curiousity all the more... I'm getting the impression it's not as straightforward as the CS situation.

Googling has brought up examples of Aluminium Alloy structures brittly failing under load due to weld related defects... or precipitation hardening/ageing resulting in embrittling... but no easy answer on the rate of strain question.


Anyway... once again, thanks for the contris.


Caffiene at the ready...!



- - - - - - - - - - - - - - -
"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[CoryPad]

Aluminium alloys generally are not rate sensitive (over several orders of magnitude). Also, they tend to tolerate cryogenic temperatures well, with strength, toughness, and ductility increasing below room temperature. One of the advantages of the FCC crystal structure.

Regards,

Cory

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

 

[TVP]

Just one example of aluminum used for a low temperature structure: the external fuel tank (hydrogen) for the NASA Space Shuttle. I forget what the temperature of the liquid hydrogen is, but it is minus a lot...

 

[robsalv]

Ok, it might be because my mechanical blinkers are on but I can't see anywhere in the key to steel articles that discusses non ferrous ductile materials and their performance under rapid strain loads. Plenty of stuff about CS and SS with some passing non-ferrous references... but nothing specific. I'm not smart enough to infer from the ferrous how the non ferrous would be expected to perform. :-(


I have a metallurgist mate who is swearing black and blue that it wouldn't matter how fast you drove the drop tester or Charpy testers at an Aluminium sample piece, in his mind Aluminium could never be made to demonstrate brittle failure on a test rig.


I referred him to an article about brittle failure of an aluminium alloy structure initiating from a weld defect and he mumbled stuff about compromised material and the like.


Now I'm reaaaaaallly interested.



Especially in regards to Aluminium, on a purely academic point of view, could Aluminium fail in a brittle manner if the strain rate was rapid enough? Surely Aluminium's cyrstalline structure includes slip planes and dislocations????



- - - - - - - - - - - - - - -
"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[metengr]

Especially in regards to Aluminium, on a purely academic point of view, could Aluminium fail in a brittle manner if the strain rate was rapid enough? Surely Aluminium's cyrstalline structure includes slip planes and dislocations????

No. Listen to your metallurgist mate. Also, review the above response from CoryPad. The key is the FCC crystal structure of Al that has more slip systems in comparison to other ferrous materials (like BCC and HCP) to avoid brittle fracture.

 

[robsalv]

Gotcha. Thanks for that Metengr


And thanks to all who indulged this query.


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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."