Charpy Impacts 2

[mikem123]

I understand that charpy impact values give you an understanding of "material toughness".

If a matl has a charpy value of 32 ft*lbf at -50F and the test coupon was 1in X 1in X 1in, could I say that 1 cubic inch of this matl could sustain 32 ft*lbf of kinetic energy before failing?

With this same logic could I rationalize that a speciman 4 times as big could sustain 4 times the kinetic energy than the original coupon at the same conditions?

 

[metengr]

mikem123
No. The CVN impact test in of itself is used as a screening tool to evaluate notch sensitivity and fracture behavior of materials exposed to various service temperatures. The correlation with fracture toughness should not be used with certainty.

Instrumented CVN impact testing would establish a better correlation with fracture toughness, which is the resistance of a material to crack propagation. Increasing specimen size has more to do with the stress state at a notch or pre-existing crack. Fracture toughness is a material parameter and is obtained thru specific test methods.

 

[redpicker]

Like Metengr has said, your logic is wrong.

I suspect the 1" by 1" test coupon only refers to the size of the test piece from which the Charpy specimens were obtained. The standard Charpy specimen is 10mm by 10mm (0.394" by 0.394"). Further, as mentioned by Metengr, while the Charpy test results are given as Ft-Lbs, the results do not represent a fundamental material property, but only a relative ranking of materials when using identically sized specimens. It is possible that merely by changing the specimen size (say from a 10mm by 10mm to 10mm by 5mm) can change this relative ranking between different materials.

rp