Difference: Charpy "V" & DWTT 5

[zeitung]

Hi everybody!

What is the difference between the Charpy "V" Impact Test and the DWTT (Drop-Weight-Tear-Test) or rather the field of use?
When is required the one or the other ?
What do I have to consider using the one or another?

What does that mean?:
E.g.: ship plate D32,
temperature of test (°C):
20/0 respectively -20/-40
=> Akv (J): 31 - longitudinal
=> Akv (J): 22 - transverse

Thanks a lot for your help!

 

[ijzer]

The values indicated by you are minimum required Charpy V impact energies.
Drop weight tear test values are specified in percentage shear. See ASTM E 436. Drop weight tear tests are more used in material developments , impact testing are more used in Codes like ASME, TRD , BS and other pressere vessel or piping Codes

 

[grampi1]

DWTT is required by API 5L/B31.8 for linepipe after reaching a certain diameter. It assures resistance to ductile running fracture if shear is 85% or better at the minimum design temperature of the line, which a Charpy bar does not. This is important in gas transmission line service where stored energy can be very high.

 

[sayee1]

"temperature of test (°C):"
"20/0 respectively -20/-40" Since only °C indicated for the temperature of test, I am not sure on why two sets of temperatures 20/0 and -20/-40 given.
"=> Akv (J): 31 - longitudinal" In addition to what Ijzer metioned, the longitudinal implies that the test specimen is in the longitudinal direction or direction of rolling of the plate.
"=> Akv (J): 22 - transverse" In addition to what Ijzer metioned, the longitudinal implies that the test specimen is in the transverse direction or perpendicular to the direction of rolling of the plate with the notch generally perpendicular to the plate surface.

Typically the impact properties expected are higher in the longitudinal direction but the tensile strengths are lower in the longitudinal direction while the yield is higher in the longitudinal direction. It is all to do with the grain size and grain boundary area. Remeber by rolling the plates the grains are getting elongated in the rolling direction.

Impact tests Static tension tests of unnotched specimens do not always reveal the susceptibilty of metals to brittle fracture. Impact tests help do that by studying the simultaneous effect of stress concentration and a high velocity load application.Probably the biggest deficiency of the Charpy impact tests is that a small specimen is not a realistic model of the actual situatuion. Not only does the small specimen lead to considerable scatter, but a specimen with 10mm thk cannot provide same constarint as in a specimen of much higher thk.The DWT(Drop weight test) was developed specifically for the detremination of Nil Ductility temperature(NDT) on full thickness plate.

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
End of all knowledge is the attainment of immortality!

 

[priyasachin]

Priyasachin,

dear friend,
one more reason of DWTT is:
whereas the standard V notch impact test is conducted on fully machined bar of 10x10mm or smaller, it is certain that the surface condition of the mother plate or product does not get reflected in the test. in DWTT the sample is much wider, a crack starter is introduced, and the surface defects/effect interact in propogation of crack. this test is considered important for line pipes which run in populated areas and any chance fall of an object, or missile, or even running over some heavy earth moving machinery when the operating temperature is high and the plate has adequate ductility, even if a crack does form would not propogate to failure.
Reagrds,
MRCN