If you have managed to get a compression mode velocity I assume that you have a sample of known thickness and you identified the pulse-echo transit time. This was perhaps done using a sample with parallel surfaces. Therefore from a known thickness you should also be able identify a reasonable approximation of the transverse mode velocity too. In my old copy of the Krautkramer Ultrasonic Testing of Materials (3rd English edition) there is a useful image in Figure 14.22 showing steel and aluminium plate signals at high gain. You can see the faint transverse mode signals that occur as a result of diffraction from the probe edges.
But an even better signal can be had for shear mode using an SH shearwave probe. For coupling you require a nonNewtonian viscous fluid (honey is the best option) and then you can make a direct reading of the pulse-echo arrival time.
This may help you. I asked my nephew at Chrismas if he new a formula that would measure my Skip Distance for curve hollow parts (ring). He's in his third year in mechanical engineering at university. This is what he gave me:
Arc Length = (3.1416 ro / 90) (90 θ cos-1 (ro / 90 ri Sin θ))
Theta is your Shearwave Angle, ro is the outer radius of your ring and ri is the inner radius. In this case, the Arc Length will be your Skip Distance. So once you made the calculation, you can calibrate your screen for one or two Skip Distance. At our shop, tone of the code we are working to allow us to put a punch mark in the Outside Diameter of the ring, so we can mark with a pencil our probe location and this way calibrate our screen for the material we are working with.
I hope this will be of help. The formula maybe a little funny, this is probably due to my inability to write formula with the software I have.
Yes, I got the longitudinal velocity by using a machined connecting flange, which was acutally in use along the piping - it was also at the same temperature (reasonably the same anyway). So that's how I worked it out. It was 5615m/s, not 5815m/s that I typed earlier.
SH shearwave probe? Is that one of the rarer ones, with the crystal cut in a different direction, so it only emits shear waves?
Nathan..yes, an SH shear wave probe is polarised to extend and contract its surface parallel to the plate surface, thereby inducing a strong true "shear" mode.
The other option I referred to, using the tip effects, is not as precise but may be adequate for your use. You can see the formation of the transverse mode off the tips of the probe in modelled beams and on some of my visualisations (like Figure 3 in http://www.ndt.net/article/wcndt2004/pdf/array_transducers/127_ginzel.pdf). With enough gain you can see this signal on the A-scan. I uploaded the image from Krautkramer to illustrate it. You will need to select the correct signal as it is possible that the mode conversion can occur off the backwall resulting in an earlier arriving TL mode. Both may be visible (as they are in the image of the steel plate). Or one may be obscured by the L mode multiple (as in the case of the aluminium plate on the image).
I have a couple of straight beam shear wave transducers just for measuring shear wave velocities. They aren't cheap until you compare them with having extra test blocks made. The beam direction is the same as for a longitudinal wave transducer placed on the same spot, so you get two values in the same direction and location. Honey is a good couplant for these transducers. I expect they are made by several transducer manufacturers. Mine came from Ultran. e-mail is email@example.com. They can be used look for anisotropy in a material that could cause shear wave velocity to vary with beam angle. Any type block with a circular segment end can be used with regular angle beam transducers on the reference line and the straight beam shear wave transducer on the circular surface as a receiver. This also determines the actual beam angle by locating the position on the circle of maximum signal amplitude. It's actually fun
Nathan...the images I pasted back on Feb 5 indicate how you can do this without the SH shearwave probe and monitor the late arriving transverse mode in the multiples.
However, if you prefer to use the SH shearwave probe the purpose of honey is to couple the shear mode! Honey is a nonNewtonian viscous fluid.
In a Newtonian fluid, the relation between the shear stress and the strain rate is linear the constant of proportionality being the coefficient of viscosity. In a non-Newtonian fluid, the relation between the shear stress and the strain rate is nonlinear, and can even be time-dependent. Honey is one of those time-dependent fluids. This accounts for one of the biggest reasons we cannot use SH shearwave probes for contact testing...it is not practical to try to slide along on honey.
I agree completely with everything Ed Ginzel has posted on this subject. You can buy a tube of special couplant for these transducers, but to me it seems to be no better than honey, or molasses. The only limitation to honey or molasses is the time-dependency. Using contact pressure to squeeze it down to a thin layer, it wll give a stable signal for a couple of minutes, then lose amplitude and disappear. You can clean the surface and repeat with fresh honey if necessary. The reason I suggested honey is that it is cheap and readily available and easy to clean up. It is also delicious if you bring your lunch to the job and include some bread and butter.
No, there is not. You still need a zero-degree shearwave 'ducer and your block. It always goes back to 'ask the part'. You have to 'ask the part' what its velocity is, by sending a compression wave or shearwave on a calibrated distance. The other option is to have an IIW block fabricated from 2205 and use a standard shearwave 'ducer. Neither option is cheap, sorry.
With your new step wedge, why not turn it on it's side and obtain a corner trap signal with a 45 deg, calibrate for sound path and you should have your velocity. Nathan, I have to ask though, as this is all being done for weld root erosion as per your original comments, why would you not perform 0 degree through the weld? Unless your welds were capped with horizontal stringer beads, you should be able to obtain quality signals without (or with minimal) grinding of the weld cap. When shear wave is done to evaluate this type of damage, it opens the door to many other possibilities, such as excessive penetration geometrical reflectors, lack of fusion (sidewall), root cracks, etc. You may also consider the "Creeping Wave" technique, which would allow for fast scanning to identify anomalous areas and evaluate with 0 degree.