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ESAB Knowledge center.

Breaking Down CTOD and CVN Testing

Q: We're a job shop and much of our work is customized or unique. Because of this, we have to work with multiple welding codes and sometimes we have to verify welding procedures. We have been bidding on some new projects that have welding crack tip opening displacement (CTOD) requirements. How is CTOD different from Charpy V-notch (CVN) testing, and is it better?

A: That's a great question, so let's look at what each test is typically used for, how the tests differ, and what the results mean.

Neither test is better than the other, as each provides different information concerning the material or weld. Simply put, they are similar because they are used to measure the material's resistance to failure in the presence of a known flaw. CVN tests are commonly used for shipbuilding, bridge and structural, pressure vessel, and construction and mining equipment, whereas CTOD tests are typically used for offshore oil and gas platform building among other applications.

The most common toughness test used is the CVN test. It is performed by taking several small machined samples from the test piece and breaking them with impact at the minimum operating temperature for the part. CVN testing is a relatively inexpensive and straightforward test that measures qualitative values.

Results from a CVN test verify that the operating temperature did not fall below the material's ductile-brittle transition temperature. Why is this important? Because when a material is utilized below this temperature, it is prone to brittle failure. A brittle failure means there will be little or no warning before the material catastrophically fails. This is similar to a piece of glass or ice breaking when force is applied. This type of failure mechanism must be avoided at all costs.

CTOD testing is performed on a sample that is representative of an actual part thickness (or scaled appropriately) and is subjected to in-service loading conditions and temperatures. The test part has an opening machined into it along with a fatigue crack of known dimension. The piece is then placed under a gradual load, reflecting service conditions until the crack sufficiently opens or cleaves, which may be either partial or complete failure.

The information that is of interest is the load at failure, displacement of the opening (width) at the time the crack propagated. This information, along with load/displacement data collected throughout the test loading, is then used to determine the material's toughness. There is considerably more analysis of the testing variables, which are simulating conditions that are required for accurate CTOD tests, but that is not the focus of this column.

The results from a CTOD test can help determine or predict when necessary repairs are needed to a structure. In many cases, a small crack in a structure doesn't mean it will fail or that it requires immediate repair. Being able to schedule repairs when deemed necessary can save time and potentially tens of thousands of dollars or more in lost production. The CTOD test saves downtime and unnecessary repairs, and it reduces the overall costs to maintain a structure. 

 

This article originally appeared in The WELDER magazine.
It is reprinted here with permission of the Fabricators & Manufacturers Association, Intl.

Posted in Filler Metals , Tagged with Testing

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