B31.3 _ Temperature reduction for sub-size charpy impact specimens

[hung7601]

Hi Every one
We have been discussing about para 323.3.4 ASME B31.3 2008, we all understand that it is required to reduce the test temperature for sub-size impact test specimens as per table Table 323.3.4. But none of us could explain why.
Please share your thought.
Thanks
MIKE

 

[weldstan]

It is understood that the subsize coupon must be less than 80% of the minimum size specimen obtainable. In short not all subsize specimens need to have the temperature reduction; for example, the minimum coupon size of a .25" thick test plate is .20" before the reduction is mandated. The reduction was implemented to reflect the beneficial effect of geometry on the absorbed energy values and shear fracture appearance of ferritic steels.

 

[hung7601]

Hi Weldstan
I do not clearly understand the sentence "The reduction was implemented to reflect the beneficial effect of geometry on the absorbed energy values and shear fracture appearance of ferritic steels".
Could you please elaborate? I understand the absorbed energy for subsize specimens will be multiplied by the ratio of actual specimen width to that of a full-size specimen (10 mm), so reduction of temperature is not necessary. The asme book could be right but I can't find the explaination.

Thanks

 

[weldstan]

The thinner the Charpy coupon, the more likely it will fail in ductile shear. Lowering the test temperature was done to compensate.

 

[RoboCop16]

hung7601,

The requirement to reduce the test temperature for sub-size impact test specimens is compensate for the fact, that thinner test specimens tend to have better impact test properties.

To quote Peng & Peng's 'Pipe Stress Engineering', page 12: 'This thickness correlation is mainly because a thicker wall creates a higher uneven stress distribution and higher probability of containing bigger size defects'.