Root-Region Signal in a Backed Single-V Weld

Convert both readings using depth = sound path x cos of the beam angle, and surface distance = sound path x sin of the beam angle:

• 45 degree probe: depth = 35 x cos45 = 24.7 mm; surface distance = 35 x sin45 = 24.7 mm
• 60 degree probe: depth = 50 x cos60 = 25.0 mm; surface distance = 50 x sin60 = 43.3 mm

Both place the reflector at about 25 mm depth, which equals the plate thickness - the root/backwall region. The agreement between two quite different beam angles is good evidence that this is a single real feature at a consistent depth, not a spurious echo.

But that same arithmetic is the problem. A plain backwall echo appears at exactly 35 mm sound path for a 45 degree beam in 25 mm plate, and at exactly 50 mm for a 60 degree beam. So a legitimate geometric reflection would land on precisely the same points on the time base.

To prove a defect, you must show the signal is not the normal backwall response: compare its amplitude against the parent-metal backwall echo at the same path, and traverse the probe along and off the weld while watching whether the signal follows the root contour or simply tracks the backwall everywhere.

The DAC explains path and reference-hole response only. It corrects for beam spread, attenuation and the response of a 1.5 mm side-drilled hole at a given sound path. It does not correct for reflector orientation, planarity or type. A planar, smooth reflector is specular: its echo amplitude depends sharply on the angle between the beam and its face. A volumetric reflector is far less angle-sensitive.

That is why the same feature reads DAC + 4 dB at 45 degrees and a different value at 60 degrees: the angle of incidence has changed, not the reflector.

The one-sided response is the stronger clue. A volumetric inclusion, or a rounded root-geometry feature such as the backing bead or root reinforcement, produces a broadly similar response from either side of the weld. A planar, oriented discontinuity - lack of root fusion, or lack of sidewall fusion on one side - is specular and may return almost nothing when approached from the other side. A 14 dB drop between sides is consistent with a planar, oriented reflector, and should not be dismissed as attenuation.

Note also that the ceramic backing changes the root reflection: the backing and the root reinforcement are themselves geometry reflectors, and a backing-line signal must be identified and separated before any defect call is made.

Next action: the signal is above the reference level and has measurable length, so it is a reportable indication that requires characterisation, not an acceptance decision on the spot.

1. Confirm it is discrete by traversing along the weld and off the weld: if the signal tracks the backwall across the whole plate away from the weld, it is geometry and should be removed from the report.
2. Map and size it properly. The 6 dB drop is acceptable for a quick length, but on a planar reflector it overestimates size because the beam edges still hit the flat face well away from the true tips. Add a 20 dB drop check, and look for a tip diffraction signal from the upper and lower extremities - tip timing gives a truer through-wall height for a planar flaw.
3. Cross-check with a second angle and skew, and with a 70 degree beam for the root region, since the 45/60 pair shows strong angular dependence.
4. Consider RT as an independent method: it is well suited to volumetric root defects and will either corroborate or contradict the planar-reflector interpretation.

Limits to state in the report: amplitude relative to a side-drilled-hole DAC is a reference-level comparison, not a size; the 6 dB length is technique-dependent; and the intact root reinforcement plus ceramic backing means the root region is acoustically cluttered, so the call must rest on corroborating evidence rather than one amplitude reading.