4.4 Radiographic Testing (RT) and Ultrasonic Testing (UT)

4.4 Radiographic Testing (RT) and Ultrasonic Testing (UT)

RT and UT are volumetric NDE methods — they can detect discontinuities inside the weld (subsurface) as well as on the surface. They complement each other: RT excels at detecting volumetric discontinuities (porosity, slag), while UT excels at detecting planar discontinuities (cracks, lack of fusion).

Radiographic Testing (RT)

RT uses X-rays (from an X-ray tube) or gamma rays (from a radioactive isotope such as Ir-192 or Co-60) to create an image of the internal structure of a weld on film or a digital detector.

How RT Works:

1. Radiation passes through the weld
2. Discontinuities absorb or transmit radiation differently than sound metal
3. A film or digital detector on the opposite side records the image
4. Discontinuities appear as darker areas on film (more radiation passed through the thinner/less dense discontinuity)

RT Terminology:

RT Advantages:

• Permanent record (film or digital image) for future review
• Excellent for volumetric discontinuities (porosity, slag)
• Detects internal discontinuities through full thickness
• Widely accepted by codes for groove weld inspection

RT Limitations:

• Radiation hazard — requires safety protocols, restricted areas, trained personnel
• Poor at detecting planar discontinuities oriented parallel to the beam (cracks parallel to beam may be missed)
• Not effective on thick materials (limited by source energy and exposure time)
• Requires access to both sides of the weld
• Interpretation requires trained, certified personnel (ASNT Level II or III)

Ultrasonic Testing (UT)

UT uses high-frequency sound waves (typically 1–6 MHz) to detect internal and surface discontinuities. A transducer sends sound pulses into the material; discontinuities reflect the sound back to the transducer, producing an echo on the display.

How UT Works:

1. A transducer generates sound waves and sends them into the material through a couplant (gel or liquid)
2. Sound travels through the material until it hits a reflector (discontinuity or back wall)
3. The reflected sound returns to the transducer and appears as a signal on the display
4. The time of the signal indicates the depth; the amplitude indicates the size of the reflector

UT Techniques for Welds:

UT Advantages:

• Excellent for detecting planar discontinuities (cracks, lack of fusion)
• Immediate results — no processing time
• Can determine depth and size of discontinuities
• Requires access to only one side of the weld
• Portable and fast
• No radiation hazard

UT Limitations:

• No permanent record (unless PAUT or TOFD with electronic data storage)
• Requires skilled, trained operators (ASNT Level II or III)
• Less effective on thin materials (< 1/4")
• Surface must be reasonably smooth (couplant contact)
• Difficult on complex geometries
• Less effective for volumetric discontinuities (scattered porosity) than RT

RT vs. UT Comparison

For the Exam: Know that RT is best for volumetric discontinuities and UT is best for planar discontinuities. RT provides a permanent record but has radiation hazards. UT provides depth information and needs only one-side access but traditionally lacks a permanent record (PAUT addresses this).