A UT transducer generates a pulse of high-frequency sound (typically 1–10 MHz for metals) that travels through the material. When the sound wave encounters an interface — the back wall of the plate, or any internal discontinuity — it reflects back to the transducer. The instrument measures the time of flight and amplitude of the returning signal. A discontinuity shows up as an echo arriving earlier than the back-wall reflection, at an amplitude that indicates its relative size.[1]
For plate inspection, immersion UT is the most common method: the plate is submerged in water (which serves as the coupling medium) and scanned in a raster pattern. The entire volume of the plate is checked, not just the surface. Modern automated systems can scan large plates quickly and produce C-scan maps that show a top-down view of any detected indications and their locations within the plate cross-section.
Surface inspection — visual, dye penetrant, eddy current — can only find flaws that reach or are very close to the surface. A hydrogen porosity void, a solidification shrinkage crack, or an inclusion buried 2 inches deep in a 5-inch plate is completely invisible to surface methods. Those internal flaws can act as stress concentration points, significantly reducing fatigue life and fracture toughness below the values the design assumes.[1][2]
This risk is particularly acute in thick aerospace structural members made from 7050 or 7075 plate. A primary structure bulkhead might start as a 200+ lb billet and finish as a 15 lb part — if a hidden void is revealed only after 90% of the material has been removed, the part is scrap and the machining investment is lost. UT on the incoming plate catches these problems before the first chip is cut.
AMS 4050 references AMS-STD-2154 as the governing UT procedure for wrought aluminum products. This standard defines the scanning procedure, transducer frequency, scanning increment, reference standard (the flat-bottom hole calibration standard), and acceptance criteria by class.[2]
The acceptance classes used in aerospace aluminum UT:
| Class | Max allowable discontinuity (FBH equivalent) | Application |
|---|---|---|
| Class A | 1/64 in (0.40 mm) flat-bottom hole equivalent | Most stringent — primary aerospace structure, fracture-critical parts |
| Class B | 2/64 in (0.79 mm) flat-bottom hole equivalent | Standard aerospace — secondary structure and most primary structure applications |
| Class C | 3/64 in (1.19 mm) flat-bottom hole equivalent | Less critical aerospace or high-performance non-aerospace applications |
| Class D | 4/64 in (1.59 mm) flat-bottom hole equivalent | Non-aerospace structural applications where UT is still specified |
When you receive 7050-T7451 plate ordered per AMS 4050, the material test report (MTR) should explicitly state that ultrasonic inspection was performed per AMS-STD-2154, that the material was accepted to the specified class, and that no rejectable indications were found. The MTR should also reference the lot and heat numbers, the AMS 4050 revision in effect, chemistry results, and three-direction mechanical test results.[1]
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