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Re: Can TOFD replace conventional UT on welding?
Posted by: H. Wuestenberg , E-mail: Hermann.Wuestenberg@bam-berlin.de , on May 27, 1997 at 08:10:54:In Reply to: Can TOFD replace conventional UT on welding? posted by : Rolf Diederichs on May 20, 1997 at 16:55:43:
The debate about advantages and drawbacks of the application of the TOFD approach for ultrasonic weld inspection should not forgot the original reasons for the introduction of the ultrasonic weld inspection during the 60th of this century.
The mayor advantage at that time had been the better crack detection potential of the ultrasonic method against x-ray techniques in view of an increased use of steels and welding technologies with a remarquarble risc of diverse cracking phenonema (e.g. cold cracking, transverse cracks etc. ). An ultrasonic technique based on the matching between the orientation of the beam and possible crack surfaces and/or based on the corner effect together with a suitable choice of the amplitude related sensitivity had been the back bone for a successful application of the ultrasonic weld inspection. The listed interactions are using a reflection rather than a diffraction at the defect that means the zero-th order diffraction instead of higher order diffractions. This is an important fact because the reflection (=zero-th order diffraction) can be reduced to fairly simple basic physical laws enabling us to predict the response from cracklike or other defects with reliable assumptions. Especially worst case conditions that means the influences reducing the echo answers from cracks can be taken into account (e.g. misorientation, mode conversions etc.). This is a distinct advantage against all other techniques using weaker interactions based on higher order diffractions, because in most cases they can only be calculated for idealized conditions like e.g. the response from a crack tip. Residual stress conditions, corrosion products between the crack boundaries or crack branching having a tremendous influence on the crack tip diffraction can only be estimated from experimental data but not be predicted and therefore not taken into account during a sensitivity setting procedure or the choice of suitable angles of incidence. This is until today the mayor reason why TOFD approaches will have difficulties to guarantee under worst case assumptions the reliable detection of cracks in the before mentioned steel and welding technology combination with a certain risc of cracking. In addition also for the TOFD the dependency of an indication from the possible misorientation of linear diffraction sources have to be considered.
Fortunately modern welding procedures have in the past strongly reduced the probability of cracking bringing into the foreground again conventional quality deficiencies in welds like slags and porosity. For this situation the TOFD methods may be valuable tools because they can easely detect corresponding defects. This explains partly the obvious success of NDE vendors applying TOFD and should be considered during the application of TOFD. But one should not oversee the limitations for a reliable crack detection, which could result in severe riscs, if this technique will be used for welds with a somehow increased potential of cracks. Such a situation can nowadays not be excluded given the fact that a global economy orders welded products every where, even on places, where the experience with the cracking potential is not always present.
The possibility of TOFD methods to size crack dimensions once they are detected is not discussed here. This is of course an important advantage of all crack tip based sizing approaches and is not specific to the TOFD technique.H. Wuestenberg