RT - general charactistics + comparison RT and UT

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.

Sensitivity to flaws / defects and detection of flaws / defects

Sensitivity

• RT - about 2% of thickness (proven by IQI sensitivity)
• UT - typically about 15% of thickness is smallest defect detected

• RT - limited ability to detect planar defects
• UT - usually good ability in detecting planar defects

trying to better explain defect detection ability - UT and RT

planar defects

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:

Defect	RT	UT	Better?	Comments
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.

volumetric defects

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:

• welds with only isolated small volumetric flaws (if any at all) almost certainly do not contain planar flaws of any size, small or large (*)
• good welding conditions produce welds with good engineering properties (strength, toughness)

(*) 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)