Weldtest NZ Limited

The difference between surface methods of non-destructive testing and volumetric testing

This fortnight we look at the difference between surface methods of non-destructive testing (NDT) and volumetric testing.
A surface method means just that: It is a test of the outside of the weld or component, to establish whether or not cracks or other defects exist on the outer surface, which would otherwise not be detected by the naked eye. Essentially, this method is a visual aid.

In the case of welds on carbon steel, the usual surface method test is a Magnetic Particle Test.
Magnetic particle testing is normally carried out on fillet welds where volumetric testing is not practical, such as small leg fillets, 10mm and less.
This is to ensure that good fusion at the weld toe with no hydrogen cracking or small surface cracks have not occurred. The internal integrity of the weld is thus relied upon by the application of a qualified Weld Procedure Specification. The importance of the weld procedure specification has been discussed in last weeks update and here again, shows its role in ensuring quality for areas unable to have non-destructive testing.

Volumetric Testing or ultrasonic testing and radiographic testing are both used to examine the internal or volumetric integrity of welds or materials where the joint quality is usually critical. In the case of structural steel the usual test method is ultrasonic testing.

This is normally carried out on full penetration welds on joints such as end plates to beams, corner joints, splice connections etc. The advantage Ultrasonic testing has over radiographic testing, is that the results are instantaneous, there is no quarantining required from radiation hazards, equipment is portable and only requires a one technician operation.

Ultrasonic testing should always be selected for use at the beginning of a job to check the technical skill of welding personnel, verification of weld procedure specification and joint fit up etc. This way should any issues be present, these can be picked up early on and corrected before significant fabrication has taken place. Sporadic testing should be carried out to ensure quality levels don’t drop off later.

Regardless of what method of testing is carried out however, there are many examples of rework that has run into hundreds of thousands of dollars from not carrying out testing until a project is coming to completion. Remember, that specifications do not always mandate what and how much testing is to be carried out for the particular job. It is therefore essential that consideration be given to the recommendations of the applicable standard.

In short – don’t wait till the tenants are in the building before you read the standard! If your unsure – call us, we are happy to help.

Have a great week!