Validity of Charpy tests on thick material 4

[4Pipes]

Are 10 mm charpy tests as valid for thick material ?

My understanding is that Charpy tests are an indirect measurement of fracture toughness. The tests are quick and cheap. If a 10 mm samples are taken from a 10 mm plate of low carbon steel and tested at progressively lower temperatures, a plot of the results will accurately indicate the temperature transition. These results would be a direct relationship to fracture toughness of the 10 mm steel plate.

Question - What happens when samples are taken from thicker plate ? Would a 10 mm charpy test have the same direct relationship to the low temperature fracture toughness of 50, 100, 150 mm thick plates ? My limited understanding is that 10 mm charpy testing does not accurately identify the low temperature transition of the thicker plate. DWTT testing or fracture toughness tests should be done to accurately represent the properties of thicker parent plate. The codes or at least the implementation appears a bit vague. Everybody asks for Charpy test for low temperature vessels but never DWTT or fracture toughness tests for very thick material - say + 100 mm. Am I missing something ?

 

[JoeTank]

The ASME Boiler and Presure Vessel Code (as well as other pressure equipment codes) use the CVN in order to qualify all thicknesses of plate for use at low temperature. A quick review of this Code and the API-650 (for oil storage tanks) will show you that as the thickness increases, so does the acceptance criteria for the thicker plates.

I do not have an ASME VIII in hand, so please accept this info from API-650 as illustrative of the point. For ASTM A516-Grade 70 the required CVN acceptance criteria is as follows. The test is to be conducted with 10mm x 10mm x 55mm specimens at the design metal temperature for the tank's location.

30 ft-lbs for thicknesses of up to 1.5 inches
35 ft-lbs for > 1.5 and up to 1.75 inches
40 ft-lbs for > 1.75 and up to 2.0 inches
35 ft-lbs for > 2.0 and up to 4.0 inches

I hope this helps.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[sayee1]

See thread330-67956

Thanks and regards
Sayee Prasad R CEng MWeldI MIOMMM


If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[metengr]

4Pipes;
Charpy V-notch impact testing is basically a method for evaluating ductile to brittle transition temperature fracture behavior of metals, and as such does not directly take into account the affects of material thickness. Indirectly, it is a qualitative approach for evaluating fracture toughness of materials. To compensate for increased thickness of components that could result in plane strain design conditions, Charpy V-notch impact energy requirements are increased at the minimum design metal temperature to assure "adequate" toughness (the previous thread on this matter provides good background information). The ASME Boiler and Pressure Vessel Code committees have over the years relied on Charpy impact testing because of the early research work performed in assessing ductile to brittle fracture behavior of materials, popularity in using this test, volumes of impact test data for materials and most importantly, reliable experience in pressure vessel design using the Charpy impact test.

The DWTT (drop weight tear test)also evaluates ductile to brittle transition temperature fracture behavior . A more direct measure of fracture toughness for materials requires specific test methods recommended by ASTM (for determination of K, CTOD or J-integral) using either single edged notched bend specimen or compact test specimens. However, these tests can be quite expensive, and require more material in comparison to Charpy impact testing. As a side note, you certainly could request fracture toughness testing to provide a measured value for your specific application.

 

[stanweld]

SteveBraune,
The CVN values reported for SA-516-70 are considereably in excess of those mandated in ASME VIII, Div.1 Fig. UG-84.1 and Div.2 Fig AM-211. Per the Tables for materials having specified minimum Y.S of less than or equal to 38 Ksi (minimum for SA-516-70), 15 ft-lb avg is required for T less than or equal to about 2 3/8" with a linear scale upwards to 18 ft-lbs minimum for T greater than or equal to 3.0". The requirements for increased absorbed energy are based on both thickness and specified minimum yield strength.

 

[JoeTank]

stanweld,
API-650 material toughness requirements date back to the mid-1960s, far earlier than the ASME VIII toughness rules. It is a long standing requirement. I have frequently pointed this out in regards to the excellent record of properly built API tanks. Yes, the API requirements are far greater than ASME's. Perhaps it's because...
- API-650 tanks operate at much stresses much higher than ASME VIII pressure vessels.
- API-650 hydrotesting does not develop the level of "overload" vs. the service condition as seen in pressure vessels.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com