fracture toughness 1

[kerntek]

I have the results of compact tension tests and charpy v-notch tests for some steels.

I need to know how to calculate the fracture toughness
corresponding to each of these measurements?

I understand that the results of these tests are affected by the temperature of the test specimens.

I also have tensile test results for some of the steels.

 

[arto]

J.F. Harvey's "Pressure Component Construction,"(NY: Van Nostrand Reinhold),1980, Has a lot of good info on this subject in Chapters 5.22-5.27

A while back[1978?], the French CODAP Code gave a Charpy conversion for different temperatures of 0.66J/degC, to covert to different test temps.

 

[desertfox]

Hi kerntek

I am no expert in this field, however my understanding of your problem is that in order to get the fracture toughness
values you require you need to relate the energy values you obtained from the Charpy impact tests in the following formula:-


Kc=(ExGc)^0.5

where Gc = toughness measured in kJ/m^2 (these are the values you should have from your impact test)

E= Modulus of Elasticity of the material which you can get from your tensile tests.

Finally Kc = fracture toughness (which is the thing your after)

hope this helps

 

[SHG]

There are many thing to know than select the acceptance limits for fracture toughness requirment that main is the material specification that have been used for example in the ASTM A20 you can find the requirments for many common material that are used in pressure vessel construction.
the several formula also are for estimation sauch as Buttelle Columbus :
CVN=0.0108 S^2 R^(1/2) t^(1/3)
S= Hoop Stress (ksi) R=Cylinderical Radius (in) t=thickness (in)

 

[kerntek]

the minimum fracture toughness of the steel in a load bearing part can be used to decide whether a defect which has been found and measured in the part needs to be repaired or not. this is one of the important ways in which ndt results are used at the present time. unfortunately the fracture toughness cannot be measured directly. therefore the fracture toughness has to be calculated from tests of samples of the steel. the charpy v-notch energy test is univeral and has been in used for may many years. the compact tension test is more difficult and expensive and may not yet be properly justified. there are other tests such as the pellini drop weight test or nil ductility test.
perhaps no way of calculating the fracture toughness from practical test results exists? or if there is then it is not public knowledge? surely such important information must be made public for us all to see and judge?

 

[Bushbaby]

There are many methods of estimating fracture toughness from Charpy impact data (but you need to check their limitations). The main standards which cover this in their appendicies are BS 7910 (replaced BS PD 6493) and API RP 579.

It should be noted that these methods generally give you a lower bound result. If you wish to produce a more accurate figure you need to conduct actual fracture toughness tests.

Another issue if you are examining flaws in welds you will need the HAZ weld toughness properties.

 

[davefitz]

It is my understanding that most failures occur in the weld material or weld HAZ, so the fracture toughness of the weld properties may govern , in terms or predicting actual failure. But the US codes seem to be written based only on consideration of the parent material, with safety factors applied to account for weld properties. See the new european code prEN 13445-3 clause 18 for detailed discussion on weld cracking.

Another consideration that affects fracture toughness is heat treatment applied after fabrication ( bending, welding), and if these were not applied in a uniform manner ( eg, temperature variations throughout the stress relieving furnace)then the fracture toughness might not be uniform thru the vessel. And of course, if the hydrotest is conducted at too low a temperature, some cracking may occur during this testing that is not predicted based on normal operations.

 

[Metalguy]

The fracture toughness of many carbon & low alloy steel welds is higher than many base metals, to the point that welds (not HAZs) can be considered to be "crack-stoppers" of HAZ cracking.