Material Property: Fracture Toughness

[interface222]

This may be the more appropriate location for this question.

Could anyone point me to a formula or other relation that specifically relates Notched Izod Impact Strength [J/cm] to Fracture Toughness [MPA-m^1/2]. If it also relates Charpy I'd consider it a bonus.

Is this a direct relation? Is fracture Toughness derived from the Izod or Charpy testing methods?

I've done some online and offline searches but no formulas are presenting themselves.

Thanks in advance.

 

[metengr]

interface222;
There is no direct relationship between V or U-notch impact testing and fracture toughness. There are some upper shelf empirical correlations of impact energy to fracture toughness, such as the Barsom and Rolfe or Sailors and Corten equation, and these correlations would be applicable to only a very limited range of steels. I would not make any flaw calculations based on these empirical correlations.

I would say that the one method that could get you to a closer fracture toughness value is to use an instrumented impact test.

 

[RPstress]

The ASM handbook volume on fatigue and fracture (I think vol 19) gives some empirical relationships for low alloy steel for KIc and Charpy V-notch (I've never found anything on Izod). I've located three formulas:

1. (K/sigma)^2 = 0.64 ((CVN/sigma)-0.01)
K = plane strain fracture toughness in MPa m^0.5
sigma = yield strength in MPa
CVN = Charpy V notch value in J

2. KIc^2/E = 2*(CVN)^(3/2)
CVN ft.lb, E psi, KI psi.rt".

3. (KIc/sigma)^2 = 5*(CVN/sigma - 0.05)
CVN ft.lb, sigma yield in ksi, KI ksi.rt".

Threads thread330-65956, thread330-60518 and thread330-24020 are relevant.

Note the cautions - these formulas are NOT very accurate, even for the limited range of steels they supposedly apply to. I believe these sorts of equation are essentially a curve fitting exercise; there is no theoretical relationship between CVN or Izod and toughness.

Also recommended by AJohnson in thread330-24020 is "Fracture and Fatigue Control in Structures: Applications in Fracture Mechanics" by John M. Barsom and Stanley T. Rolfe.

To answer your second question, "No". Stress intensity toughness is not derived from Charpy or Izod tests. To get the plain strain KIc compact tensile specimens are usually used.

-RP.

 

[Maui]

This subject is discussed in some detail in thread330-128281

Maui