A pipe carries thousands of wave modes. Guided wave testing uses one of them. This module shows why, using a comparison you can see rather than an argument you have to accept.
The test is simple. Put a weld and a defect close together on the same pipe and look at the returning signals.
With T(0,1), the two signals stay separate. Two reflectors, two arrivals, readable even far from the probe.
With a dispersive mode, the same two reflectors blend into one long, smeared pulse. Even though it is a strong signal that is higher than threshold. One signal where there are two, and nothing on the screen tells you that has happened.
The reason is what “dispersive” means.
In a dispersive mode the velocity depends on frequency.
A pulse contains a band of frequencies, so its components travel at different speeds and the pulse spreads as it goes.
The further it travels, the wider it gets. Spatial resolution therefore decays with distance,
and it decays exactly where you need it most — at the far end of your range.
T(0,1) does not do this. Its velocity is the shear velocity of the steel or the material,
and it is the same at every frequency and every wall thickness. One number, valid everywhere.
That is also why distance calibration works.
With a dispersive mode the velocity depends on frequency
and on wall thickness, so a time-to-distance conversion has no single correct answer.
There is a second non-dispersive candidate, L(0,2), and the module explains why we do not use it.
Three reasons.
- Mode purity: For T-mode operation below the cutoff frequency of T(0,2), only one axisymmetric mode, T(0,1), exists. In contrast, during L-mode operation, both L(0,1) and L(0,2) propagate throughout the operating frequency range at different velocities.
- Contents: The T(0,1) mode involves purely circumferential particle motion, which does not significantly couple to the fluid because liquids cannot sustain shear stress. As a result, the fluid inside the pipe has little effect on wave propagation. In contrast, L modes involve radial and axial particle motion, which can couple to the fluid, making their propagation sensitive to the fluid inside the pipe.
- Wavelength: At 128 kHz, the T(0,1) mode has a wavelength of 1 inch, while the L(0,2) mode has a wavelength of 1.7 inches. The shorter wavelength provides better spatial resolution, and makes the T(0,1) mode more sensitive to smaller features.
This is the same reasoning conventional UT settled on decades ago. Angle beam weld inspection uses shear, not longitudinal, and for the same kind of reason: past the first critical angle only the shear beam exists in the steel, so there is one beam at one angle with one velocity, and no second arrival to confuse the reading.
Guided wave testing is ultrasonic testing. The mode selection follows the same logic of conventional UT.