Welding & NDT

TOFD

Excellent at detecting and sizing volumetric type indications (i.e. slag, porosity, etc) Good at detecting and sizing planer indications but is not always good at detecting and worst at sizing close to the surfaces. Sizing of defects with TOFD is problematic due to nature for which an indication is created by the sound wave. It can be used where there is a wall thickness transition at the weld.

PAUT or shear wave UT

OK at detecting and sizing volumetric indications. Typically the sizing is less accurate than TOFD unless the indication is large or a cluster and at an optimum orientation such that the round nature of volumetric indications does not deflection the UT wave. Excellent at detecting and sizing planer indications such as cracks, lack of fusion, incomplete penetration.

PAUT can also be set to target the surface region to detect indications that TOFD can not see and size. Typically not possible to use when there is a wall thickness transition at or close to the weld. The transition needs to be several inches from the weld.

There are advantages and disadvantages to each inspection method.  The method needs to be selected based on what type of indications are expected and targeted in the acceptance criteria.  Usually, both TOFD and shear wave UT (PAUT or multiprobe) are used together and both signals are analyzed to determine the type and size of the indications.  

That is a very short summary of the UT methods you asked about.  There are a lot more that needs to be considered and understood when applying either or both method.  That is especially the case regarding the method of which they are calibrated.

In either case, neither is better, they are just different.  One may be better for one situation and the other better in another situation.

Geoff Rogers

Welding Engineer

Houston, Texas

Ultrasonic phased arrays use a multiple element probe whereby the output pulse from each element is time delayed in such a way so as produce constructive interference at a specific angle and a specific depth. These time delays can be incremented over a range of angles to sweep the beam over the desired angular range. For example, 40 to 75 degree beam sweep would be produced by calculating the time delays to produce constructive interference at 40, 41, 42 ...75 degs. This NDT technology is also referred as Swept Beam Ultrasonic testing. The main advantages of phased array in NDE are:

1. Ability to sweep a range of angles

2. Ability to display the image in real time for the swept angles

3. Ability to focus

The above figure displays the concept of phased arrays. Time delays to the eight elements control focusing and beam sweep. Focal spot size (shown by the shaded orange area) is controlled by beam spread.