|NDT.net June 2005 Vol. 10 No.6|
TOFD Dead Zone CalculatorEd Ginzel Materials Research Institute, Waterloo
Corresponding Author Contact:
Email: firstname.lastname@example.org, Internet: www.mri.on.ca
Introduction:Most users of the TOFD (time of flight diffraction) technique are familiar with the loss of information that occurs immediately under the test surface. This is sometimes referred to as the lateral wave dead zone. It results due to the fact that the pulse ring-time limits the resolution of flaws immediately below the surface for a time approximately equal to the depth equivalent to that ring time for the probe configuration used.
However, some users of TOFD seem to be unaware that the same effect occurs at every diffraction interface below the surface and a similar effect is had from the backwall reflection boundary.
These three "dead zones" are identified and approximated in the EN standard EN-583-6 in paragraphs 10.1.5 and 10.2. Although no special corrections are made for wedge angle variations with depth and the assumption is made that the indication is at the midpoint of the Probe centre Spacing (PCS), the approximations are generally adequate for most applications.
Estimations of resolution limits of TOFD are, in the writers' opinion, one of the most important considerations when using TOFD with fracture mechanics based acceptance criteria. When using TOFD to estimate flaw size (height) the smallest resolvable flaw is a function of the PCS, probe frequency and damping quality and the depth of the flaw below the surface. Failure to understand this could lead to grossly overestimating the capabilities of a system. We recently read a specification developed by a consultant that required extreme sizing capabilities on the inside surface of a putatively critical component nearly 40mm thick. Small (<0.5mm high) surface breaking flaws were considered critical and TOFD was identified as the means to both "detect" and "size" any flaws on that surface. This seems to have been the result of an assumption that the improved time resolution as one approaches the far wall would allow sizing and detection on this scale of things. This assumption optimistically ignored the ring-time limits. Even when using a 10MHz probe with a 45° L wave, detection and sizing capability of <0.5mm would be unlikely. In fact the 0.5mm calibration notch on the far wall was apparently not detected (therefore not sizeable).
Template for CalculationsThe calculations of the three dead zones are derived from relatively simple trigonometric equations.
The authors have developed a free downloadable (and share-able) software that calculates these three TOFD dead zones. This is an Excel® compatible worksheet so requires Excel be present on the user's computer. Users enter a set of parameters typical of a TOFD setup and in addition to a numeric value of the three ring-times a graphic presentation is provided.
For the most part TOFD probes are relatively broadband, i.e. have short pulse durations on the order of a single cycle or 1.5 cycles. The software has fixed the ring to 1.5 cycles. Since these are approximations for ideal conditions they are to be used as a guide only.
Figure 1 illustrates the parameter entry (yellow) and calculated values (green) for a test setup on a 38mm wall thickness. The user must decide the depth at which the flaw is located to determine the depth resolution. In the example in Figure 1 this is 16mm.
As an aid to visualizing the limits a dynamic graph is provided. This is illustrated in Figure 2 for the setup defined in Figure 1. The three ring times are denoted by the coloured lines extending back to the scale on the left. The red line indicates the lateral wave dead zone, the yellow lines indicate the resolution that may be expected for a flaw at the specified depth (i.e. the minimum flaw height to see a separate upper and lower tip signal) and the blue line indicates the height above the opposite surface that a flaw must exceed before it is reasonably detected as separate from the backwall echo.