A few notes mainly comparing RT and UT - with some comments specific to the 3rd Bosphorus Bridge / Yavuz Sultan Selim Bridge.
Abbreviations - see RT solution main page.
Planar defects ("crack-like defects") have the biggest effect in reducing the fitness of the structure. Therefore, comparative abilities of UT and RT are considered for planar weld and weld-zone defects:
|Laminations||No (never!)||Yes - but only if use a "Normal / 90degree" probe||UT||UT - pre-scan plate with "Normal" probe before angle probe to scan weld|
|Lack of Side-Wall Fusion (loswf)||Sometimes (associated oxides can reveal loswf)||Yes, if use correct scan with probe with correct angle||UT better, but RT can be effective||Good UT operator applying appropriate technique will detect important loswf - however RT may detect smaller loswf|
|Lack of Inter-Run Fusion (loirf)||Often - sometimes directly, sometimes images associated oxides||Likely to detect||~||Advantage is probably with UT|
|Through-thickness cracks||Yes - very good at detecting this most deleterious defect||Often fails to detect||RT||For UT, because of low plate thickness mainly 14mm, 70degree probes used may detect this defect, at a cost of low ability to detect other defects unless they are large|
The overall "picture" is not simple. Where both UT and RT are applied, it can be RT which finds the planar defects. This has been observed in this project. Even the low-resolution RT with Ir192 has found planar defects at seam intersections which UT has not detected.
RT is very good at detecting volumetric defects. Volumetric defects which are seen with complete clarity on RT are often too small to detect by UT given UT's limited sensitivity.
That isolated volumetric defects clearly detected on RT, like isolated pores and isolated slag inclusions, are almost entirely harmless to engineering capability of the weldment is not the issue being considered here. That is the engineering field of engineering critical assessment, otherwise known as engineering critical analysis, which is an applied branch of the engineering study of fracture mechanics.
The purpose of this section regarding the 3rd Bosphorus Bridge is:
sample RT can be good for diagnosing whether welding conditions are good. Good welding conditions produce good welds:
(*) Only planar defects caused by deficient welding technique. "Metallurgical" cracks are a different issue.
Poor welding conditions and / or poor welding technique can systematically produce numerous small volumetric flaws. RT will sensitively detect and image these.
Therefore: RT is an additional tool in ensuring that welding conditions applied in manufacturing weldments are good and remain good.
Note that welding is a "special process" (definition: ISO9000:2005 3.4.1 ... NOTE 3 A process where the conformity (3.6.1) of the resulting product (3.4.2) cannot be readily or economically verified is frequently referred to as a "special process".). As noted in the well-known welding Standards ISO15607 (qualification of welding procedures), ISO3834 (weld quality management) and ISO 14731 (welding coordination).
Hence ensuring good welding conditions are maintained, which frequent sample RT can diagnose, is extremely desirable.
(R. Smith, 02Jun2015 22Jun2015)