Impact values of steel and cast iron / low temperature 3

[chrisgibson]

Hi, can anybody help me. I am working an application which has to operate successfully at low temperatures (–40°c). As I see it, the main affect that the low temperature will have on the materials that we will be using (steel and cast iron) is a reduction in the impact value (Av measured in J). Although I have found references as to the minimum impact values of materials used for particular applications, I have not been able to find out how one can determine the minimum value necessary for a new application. The application is a hydraulic actuator where no impact or shocks other then normal pressure pulsation are to be expected.

 

[TVP]

You have identified the biggest problem with using a commonly performed test that yields no real design data, e.g., charpy impact testing (ASTM E 23 or A 370, BS EN ISO 14556, etc.). Fracture toughness is the material property of interest, which is usually measured in plain-strain conditions and called K_Ic. The formula relating stress and flaw size is as follows:

K = Y&[ignore]sigma[/ignore]; &[ignore]radic[/ignore];&[ignore]pi[/ignore];a

where Y is a constant depending on geometry
&[ignore]sigma[/ignore]; is applied stress
a is the allowable flaw size

Fracture occurs when K is greater than K_Ic. The are some approximations for relating fracture toughness and charpy impact energy (Barsom-Rolfe, etc.). ASM HANDBOOK Volume 19 Fatigue and Fracture and Deformation and Fracture Mechanics of Engineering Materials by R. W. Hertzberg are both excellent references on this subject. You can obtain more information on these references using the following links:

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=10494

http://www.wiley.com/cda/product/0,,0471012149,00.html

 

[smslab]

TVP is right, but you still need one more piece of information, namely your initial flaw size. Probably you don't know, right?? So you have to estimate. What is the largest possible crack you think you might have? Are you doing any NDT? What is the minimum detectable crack size? If you can't find it, you better assume it is there. Use this flaw size and your calculation of the stress to calculate what stress intensity factor K you are most likely applying in service. If this value is lower than K1c, you are in trouble! Alternatively, you can do the old 'proof' test. Load the part up to the expected maximum load. If it doesn't fall apart, you know that any cracks which are present are below the critical size.

 

[arunmrao]

Castiron samples will contain large flaws as the graphite flakes are considered to be discontinuities in the matrix. Also casting process will also lend itself to introducing defects. It would be advisable to use steel samples which are mechanically worked in that case only the chemistry and heat treatment will be influencing parameters for k1c studies.

 

[kenvlach]

There is an interesting post on another forum which you may not have seen:

Mechanical Engineers > Activities > Boiler and Pressure Vessel engineering Forum
fracture toughness
thread794-40581

As there are lots of Charpy test results as f(T) and most Kic values are for room temperaure, I found some very interesting equations given by arto and desertfox for relating Charpy results to fracture toughness & for converting Charpy results to different Ts.

Would someone please comment on their validity; I think maybe true for limited circumstances, but I am not an expert on mechanics.