8.2 Ultrasonic Testing (UT) Procedures, Calibration & Tables 8.2 / 8.3

Ultrasonic Testing (UT) Procedures, Calibration & Tables 8.2 / 8.3

Ultrasonic Testing (UT) is the primary volumetric examination method utilized under AWS D1.1/D1.1M:2025 for detecting both planar (cracks, lack of fusion, incomplete penetration) and volumetric (slag inclusions, porosity) discontinuities in structural welds. Governed by Clause 8 Part D (Clause 8.19 through 8.28), AWS D1.1 specifies a standardized shear-wave (angle-beam) pulse-echo contact method.

Unlike radiographic examination, which produces a 2D projected shadowgraph on film, ultrasonic testing yields precise three-dimensional spatial coordinates (depth, sound path, lateral offset) and acoustic reflectivity data. Masterful interpretation of the AWS D1.1 ultrasonic rating equation (d = a - b - c) and the severity classifications in Table 8.2 and Table 8.3 is fundamental for structural welding inspectors.

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UT Equipment & Transducer Requirements (Clause 8.20 & 8.21)

AWS D1.1 mandates rigorous hardware specifications for ultrasonic instruments and search units to guarantee repeatable, calibrated inspections across structural fabrication shops and jobsites.

+-------------------------------------------------------------------------+
|                    ULTRASONIC PULSE-ECHO A-SCAN SETUP                   |
+-------------------------------------------------------------------------+
|                                                                         |
|    [ A-SCAN INSTRUMENT ] <------- Coaxial Cable -------> [ SEARCH UNIT ]|
|    - Calibrated Gain (dB)                                  Transducer + |
|    - Screen Height Linearity                               Lucite Wedge |
|    - Distance Sweep (Inches)                               (45/60/70 deg|
|             |                                                    |      |
|             v                                                    v      |
|    +------------------+                              /|      Couplant   |
|    |  ECHO DISPLAY    |                             / |       Layer     |
|    |  |    80% FSH    |                            /  |        ||       |
|    |  |   |\          |                           /   +--------++----+  |
|    |  |   | \         |                          /     Wedge   ||Weld|  |
|    |  +---+--\--------|                         +--------------++----+  |
|    |  0   2   4   6 in|                                Sound Beam /     |
|    +------------------+                               Angle Theta       |
+-------------------------------------------------------------------------+

1. Instrument Requirements (Clause 8.20)

• Type: Pulse-echo A-scan instrument with rectified video display.
• Gain Control: Must feature a calibrated gain attenuator adjustable in increments of 1 or 2 dB over an operating range of at least 60 dB, with an accuracy of ±1 dB.
• Linearity: Horizontal sweep linearity and vertical amplitude linearity must be verified every 40 hours of instrument operation per Clause 8.20.1.

2. Search Units & Transducer Crystals (Clause 8.21)

• Frequency Range: Transducers must operate at nominal frequencies between 2.0 MHz and 2.5 MHz (nominally 2.25 MHz). This frequency band provides the ideal compromise between steel penetration and flaw sensitivity.
• Crystal Dimensions: Angle-beam crystal dimensions shall be square or rectangular, measuring from 0.625 in. × 0.625 in. to 1.00 in. × 1.00 in. [16 mm to 25 mm] (or round transducers 0.5 to 1.0 in. in diameter for straight beam).
• Angle Wedges: Lucite wedges producing refracted shear waves in structural carbon steel at nominal angles of 45°, 60°, or 70° (±2° tolerance).

3. Angle Wedge Selection Chart (Table 8.7 Summary)

Under AWS D1.1 Table 8.7, search unit angle selection depends strictly on base metal thickness and joint geometry:

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Reference Standards & Calibration (Clause 8.22 & 8.23)

Standardization of the ultrasonic testing system is executed prior to every shift using the International Institute of Welding (IIW) Reference Block (Type 1 or Type 2) or the DSC Block (Distance/Sensitivity Calibration).

+-------------------------------------------------------------------------+
|                   IIW TYPE 1 REFERENCE BLOCK CALIBRATION                |
+-------------------------------------------------------------------------+
|                                                                         |
|            +-------------------------------------------+                |
|            |         Index Point Mark / Angle Scale    |                |
|            |    70    60    45                         |                |
|            |     \     |    /                          |                |
|     +------+      \    |   /                           |                |
|    /               \   |  /                            |                |
|   |  4 in. [100 mm] \  | /                             |  1 in. [25 mm] |
|   |     Radius Arc   \ |/      (O) 0.060 in. [1.5 mm]  |   Radius Arc   |
|   |                   X        Side-Drilled Hole (SDH) |       |        |
|    \                 / \       (Sensitivity Reference) |      /         |
|     +---------------+---+------------------------------+-----+          |
|                                                                         |
+-------------------------------------------------------------------------+

Mandatory Four-Step Calibration Sequence

1. Index Point Verification: Position the angle wedge on the IIW block facing the 4 in. [100 mm] radius arc. Maximize the echo signal to locate the exact sound exit point on the wedge.
2. Angle Verification: Align the verified index point over the angle scale markings on the IIW block. Maximize the reflection from the steel cylinder or side-drilled hole to verify that the refracted angle is within ±2° of nominal (45°, 60°, 70°).
3. Distance (Sweep) Calibration: Adjust the horizontal sweep display to represent 5 in. or 10 in. of true sound path by setting screen peaks from the 4 in. radius arc and its multiple reflections.
4. Sensitivity (Reference Level b) Standardization: Position the search unit to peak the reflection from the 0.060 in. [1.5 mm] diameter side-drilled hole (SDH) at a sound path distance of approximately 1.4 in. (70°), 1.7 in. (60°), or 2.0 in. (45°). Adjust the gain control until the echo reaches the calibrated Reference Screen Height (typically 50% or 80% Full Screen Height, FSH). The resulting gain setting in decibels is recorded as the Reference Level (b).

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The Ultrasonic Decibel (dB) Rating Formula (Clause 8.24.6.4)

In AWS D1.1, the severity of any reflector is quantified by its Decibel Indication Rating (d), calculated using the fundamental equation:

d = a - b - c

Where:

• a = Indication Level (in dB): The instrument gain required to bring the peak reflection from the discontinuity to the calibrated reference screen height (e.g., 50% FSH).
• b = Reference Level (in dB): The instrument gain established during sensitivity calibration on the 0.060 in. side-drilled hole in the IIW block.
• c = Attenuation Factor (in dB): The decibel compensation factor that accounts for sound energy absorption and beam divergence as sound travels through steel. Under Clause 8.24.6.4, it is calculated as: c = 2 × (Sound Path in inches - 1.0) (Note: Round c to the nearest whole dB. For sound paths ≤ 1.0 in., c = 0 dB.)
• d = Decibel Rating (in dB): The calculated severity rating used to enter Table 8.2 or Table 8.3.

CRITICAL CWI PRINCIPLE: The Decibel Inversion Rule In ultrasonic decibel rating calculations, LOWER OR NEGATIVE d-VALUES REPRESENT MORE SEVERE DEFECTS. A large, highly reflective crack requires less instrument gain (a), yielding a smaller or negative d-rating. Conversely, a tiny pore requires immense gain (a), yielding a large positive d-rating.

+-------------------------------------------------------------------------+
|                   DECIBEL RATING RELATIONSHIP                           |
+-------------------------------------------------------------------------+
|                                                                         |
|  Very Large Defect / Crack  --> Low Gain (a)  --> Negative / Low d (SEV A)|
|  Medium Slag / Inclusion    --> Moderate Gain --> Moderate d      (SEV B)|
|  Small Reflector / Pore     --> High Gain (a) --> Large Positive d(SEV D)|
|                                                                         |
+-------------------------------------------------------------------------+

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Scanning Levels & Testing Procedures (Clause 8.24.7)

During manual scanning of production welds, human inspectors cannot maintain perfect transducer coupling and may miss small off-axis flaws if scanning at reference sensitivity. Therefore, AWS D1.1 mandates adding Scanning Gain (b + ΔdB) during the physical search:

When an indication appears on screen during scanning, the inspector switches the gain back to measure the exact Indication Level (a) at reference screen height (50% FSH) to calculate d.

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Acceptance Tables: Table 8.2 (Static) vs Table 8.3 (Cyclic)

AWS D1.1 establishes four standardized Discontinuity Severity Classes across base metal thicknesses from 5/16 in. to 8 in. [8 to 200 mm]:

Discontinuity Severity Classes

• Class A (Large Discontinuity): Rejectable regardless of length. All Class A indications must be repaired.
• Class B (Medium Discontinuity): Rejectable if length exceeds 3/4 in. [20 mm]. Acceptable if L ≤ 3/4 in. and separated from adjacent Class A or B indications by at least 2L.
• Class C (Small Discontinuity): Length limits depend on where in the weld thickness the indication lies: acceptable up to 2 in. [50 mm] in the middle half of the weld thickness, but only up to 3/4 in. [20 mm] in the top or bottom quarter. This depth-zone distinction is a favourite exam discriminator — always establish the through-thickness position of the indication before applying the length limit.
• Class D (Minor Discontinuity): Acceptable without regard to length or location, provided adjacent Class D flaws are separated by at least 2L.

Use these excerpts to learn the pattern, not as your answer key. Tables 8.2 and 8.3 are large matrices indexed by weld thickness and search-unit angle (70°, 60°, 45°), and the exam is open book precisely so you look the value up. Learn where the tables sit, how the bands step, and which table applies to which loading case; read the exact dB boundary off the codebook on exam day.

Table 8.2 (Statically Loaded Nontubular Connections - Excerpt for 70° Search Unit)

Table 8.3 (Cyclically Loaded Nontubular Connections - Excerpt for 70° Search Unit)

Crucial Comparison: Notice that in cyclically loaded structures (Table 8.3), the Class A cutoff occurs at significantly higher positive dB levels (e.g., +8 dB vs +2 dB for 1 in. plate). This means much smaller reflectors will cross the threshold into Class A rejection on cyclic structures due to fatigue vulnerability!

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Other Examination Methods (Clause 8 Part G: 8.32 through 8.35)

The dB-rating method in Clause 8 Part F is the default ultrasonic technique, not the only permitted one. Part G opens the door to modern equipment, and each route carries its own approval burden.

Two points are worth memorising:

1. Annex H is normative, not informative. When PAUT is invoked, Annex H is mandatory — its essential variables and its own acceptance criteria apply, and you do not fall back on the Table 8.2/8.3 dB ratings.
2. Annex N is informative. UT Examination of Welds by Alternative Techniques offers an alternative amplitude-based scheme with its own acceptance table (Table N.1), but it only becomes binding when the contract documents or the Engineer invoke it. A question that describes an alternative UT technique "in accordance with the contract documents" is pointing at Annex N; one that just says "phased array" is pointing at Annex H.

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Worked Example: Ultrasonic Rating Calculation & Evaluation

Problem Statement

An ultrasonic technician is inspecting a CJP groove weld in a 1.25 in. [32 mm] thick flange on a statically loaded building column using a 70° transducer. During calibration on the IIW block 0.060 in. SDH, the Reference Level was established as b = 48 dB. During scanning, a flaw is discovered at a sound path distance of 3.5 in. [89 mm]. The Indication Level required to bring this flaw echo to 50% FSH is measured as a = 55 dB. The technician traverses the flaw and measures its physical length as 0.50 in. [13 mm]. Evaluate the decibel rating and code acceptance under Table 8.2.

Step-by-Step Mathematical & Code Resolution

1. Calculate Attenuation Factor (c):
   • Sound Path (SP) = 3.5 in.
   • Formula: c = 2 × (SP - 1.0) = 2 × (3.5 - 1.0) = 2 × 2.5 = 5.0 dB
2. Calculate Decibel Rating (d):
   • Formula: d = a - b - c
   • Substitute values: d = 55 dB - 48 dB - 5.0 dB = 7 dB - 5 dB = +2 dB
3. Consult Table 8.2 for Statically Loaded Structures:
   • Base metal thickness = 1.25 in. (falls in the > 3/4 to 1-1/2 in. category).
   • Search unit angle = 70°.
   • Look up d = +2 dB in Table 8.2:
     • Class A (Reject): +2 dB & lower.
     • Class B: +3 dB
     • Class C: +4 dB
     • Class D: +5 dB & higher.
   • Since d = +2 dB matches the Class A threshold (+2 dB and lower), this discontinuity is categorized as Severity Class A.
4. Determine Final Disposition:
   • Under Clause 8.13.1, Class A discontinuities are REJECTABLE regardless of length.
   • Even though the physical length is only 0.50 in., the high acoustic reflectivity mandates excavation and weld repair.