Charpy testing question - what if...
Charpy testing question - what if...
(OP)
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
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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RE: Charpy testing question - what if...
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.
RE: Charpy testing question - what if...
htt
Regards,
Cory
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Charpy testing question - what if...
RE: Charpy testing question - what if...
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..."
RE: Charpy testing question - what if...
Here is a suggestion, since you show an interest in this subject;
ht
There are excellent feature articles on this very subject. Have a cup of coffee and read them.
RE: Charpy testing question - what if...
" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke
RE: Charpy testing question - what if...
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...!
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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."
RE: Charpy testing question - what if...
Regards,
Cory
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Charpy testing question - what if...
RE: Charpy testing question - what if...
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????
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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."
RE: Charpy testing question - what if...
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.
RE: Charpy testing question - what if...
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..."