8.1 Radiographic Testing (RT) Requirements, IQI Placement & Acceptance
Key Takeaways
- Radiographic testing under AWS D1.1 Clause 8 Part D requires calibrated radiation sources (X-ray or Gamma isotopes Ir-192 / Co-60) and strict optical density control between 1.8 and 4.0 for X-ray, and 2.0 to 4.0 for gamma ray.
- Image Quality Indicators (IQIs) must be placed on the source side of the joint, but film-side placement is allowed for inaccessible hollow or pipe sections when accompanied by a lead letter 'F'.
- Wire-type IQIs (ASTM E747) require the essential wire to span across the weld perpendicular to the weld axis, while hole-type IQIs (ASTM E1025) require clear visibility of the specified hole (e.g., 2T hole for 2% sensitivity).
- Discontinuity acceptance criteria differ fundamentally between statically loaded (Clause 8.12.1 / Figure 8.1) and cyclically loaded structures (Clause 8.12.2 / Figure 8.2), with tension-loaded cyclical members enforcing the most severe size and clearance restrictions.
- Cracks, lack of fusion (LOF), and incomplete penetration (in CJP welds) are non-negotiable rejectable defects under all AWS D1.1 radiographic criteria.
Radiographic Testing (RT) Requirements, IQI Placement & Acceptance
In structural steel fabrication under AWS D1.1/D1.1M:2025, Radiographic Testing (RT) is the premier volumetric nondestructive examination method for evaluating the internal soundness of Complete Joint Penetration (CJP) groove welds. Governed by Clause 8 (Inspection), specifically Part C (Acceptance Criteria) and Part D (NDT Procedures), RT utilizes penetrating ionizing radiation to produce permanent photographic or digital records of internal weld quality.
Certified Welding Inspectors (CWIs) and NDT Level II/III technicians must master the exact procedural controls for radiation source selection, geometric setup, Image Quality Indicator (IQI) placement, Hurter & Driffield (H&D) optical film density, and the differentiated acceptance criteria between statically loaded and cyclically loaded structures.
Fundamentals of Industrial Radiography (Clause 8.16 & Clause 8.17)
Industrial radiography operates on the principle of differential absorption. As high-energy electromagnetic radiation passes through a welded joint, radiation is absorbed in proportion to the density and thickness of the material. Internal discontinuities (such as slag inclusions, porosity, or cracks) represent voids of lower density, allowing greater radiation transmission and producing darker (higher optical density) features on radiographic film.
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| INDUSTRIAL RADIOGRAPHY SETUP |
+-------------------------------------------------------------------------+
| |
| [ RADIATION SOURCE ] |
| (X-Ray Tube or Isotope) |
| | |
| | Radiation Cone |
| v |
| +-----------------------+ |
| | [ IQI / Penetrameter]| <-- Source-Side IQI |
| =============+=======================+============= |
| BASE METAL | WELD REINFORCEMENT | BASE METAL |
| | (Weld Joint) | |
| =============+=======================+============= |
| | [ LEAD 'F' ] | <-- (If Film-Side IQI) |
| +-----------------------+ |
| | [ RADIOGRAPHIC FILM ] | |
| +-----------------------+ |
| | [ LEAD BACKING FOIL ] | <-- Eliminates Back- |
| +-----------------------+ scatter radiation |
+-------------------------------------------------------------------------+
1. Radiation Sources & Isotope Selection (Clause 8.16.1)
AWS D1.1 permits both industrial X-ray machines and gamma-ray radioactive isotopes:
- X-Ray Tubes: Electrically generated radiation where kilovoltage (kV) controls penetrating energy (beam hardness/contrast) and milliamperes (mA) controls radiation intensity. X-rays provide superior radiographic contrast and adjustable energy levels ideal for structural plates.
- Gamma-Ray Isotopes: Radioisotopes undergo continuous radioactive decay, emitting discrete, unalterable gamma energy spectra:
- Iridium-192 (192-Ir): Emits average energy of approximately 0.38 MeV with a half-life of 73.8 days. It is the standard isotope for structural steel thicknesses ranging from 0.25 in. to 2.5 in. [6 mm to 65 mm].
- Cobalt-60 (60-Co): Emits high-energy gamma photons (1.17 MeV and 1.33 MeV) with a half-life of 5.27 years. Due to its high penetrating power and lower contrast, it is reserved for heavy structural steel sections from 1.5 in. to 6.0 in. [38 mm to 150 mm].
2. Geometric Unsharpness (Ug) & Source-to-Film Distance
Geometric unsharpness (Ug) defines the penumbral shadow (blurriness) at discontinuity boundaries, governed by focal spot size (F), source-to-object distance (D0), and object-to-film distance (d):
Ug = (F × d) / D0
To ensure crisp defect resolution, AWS D1.1 mandates that the radiation source shall be positioned as far from the weldment as practicable, maintaining a perpendicular alignment to the joint surface unless special geometry dictates an angled exposure.
Image Quality Indicators (IQIs / Penetrameters) (Clause 8.17.1)
An Image Quality Indicator (IQI), historically termed a penetrameter, proves that the radiographic technique achieves the mandatory sensitivity and resolution. AWS D1.1 recognizes both hole-type IQIs (ASTM E1025) and wire-type IQIs (ASTM E747).
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| HOLE-TYPE VS. WIRE-TYPE IQIs |
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| |
| ASTM E1025 (Hole-Type) ASTM E747 (Wire-Type) |
| +-----------------------+ +-----------------------+ |
| | AWS-20 (4T) | | SET B | |
| | +--+ O | | | | | | | | | |
| | | | O o | | | | | | | | | |
| | +--+ (2T) (1T) | | | | | | | | | |
| +-----------------------+ +-----------------------+ |
| Plaque thickness T Encapsulated Wires |
| 2T hole must be visible Essential wire visible |
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1. Hole-Type IQI Specifications (ASTM E1025)
Hole-type IQIs consist of a rectangular metal plaque of radiographically similar material (carbon/low-alloy steel for structural welding) having a thickness equal to a specified percentage of the joint thickness (typically 2%). The plaque contains three true-drilled holes:
- 1T Hole: Diameter equals 1 × plaque thickness T.
- 2T Hole: Diameter equals 2 × plaque thickness T (Essential Hole under standard AWS D1.1 2-2T quality level).
- 4T Hole: Diameter equals 4 × plaque thickness T.
2. Wire-Type IQI Specifications (ASTM E747)
Wire-type IQIs consist of a series of six parallel, encapsulated wires of increasing diameter mounted in clear plastic. Four standard sets exist: Set A (wires 1–6, thin), Set B (wires 7–12, medium), Set C (wires 13–18, thick), and Set D (wires 19–24, heavy). Under Clause 8.17.1, the essential wire designated for the nominal steel thickness must be clearly discernible across its full length across the weld image.
3. IQI Selection Table (Table 8.4 / ASTM E747 & E1025)
| Nominal Material Thickness Range (Tn) | Hole-Type Plaque Designation | Hole-Type Essential Hole | Wire-Type Set Designation | Wire-Type Essential Wire No. (Diameter) |
|---|---|---|---|---|
| Up to 0.25 in. [6 mm] | 10 | 2T | Set A | Wire 4 (0.008 in. [0.20 mm]) |
| > 0.25 to 0.375 in. [6 to 10 mm] | 12 | 2T | Set A | Wire 5 (0.010 in. [0.25 mm]) |
| > 0.375 to 0.50 in. [10 to 12 mm] | 15 | 2T | Set A | Wire 6 (0.013 in. [0.33 mm]) |
| > 0.50 to 0.75 in. [12 to 20 mm] | 17 | 2T | Set B | Wire 7 (0.016 in. [0.40 mm]) |
| > 0.75 to 1.00 in. [20 to 25 mm] | 20 | 2T | Set B | Wire 8 (0.020 in. [0.51 mm]) |
| > 1.00 to 1.50 in. [25 to 38 mm] | 25 | 2T | Set B | Wire 9 (0.025 in. [0.64 mm]) |
| > 1.50 to 2.00 in. [38 to 50 mm] | 30 | 2T | Set B | Wire 10 (0.032 in. [0.81 mm]) |
| > 2.00 to 2.50 in. [50 to 65 mm] | 35 | 2T | Set B | Wire 11 (0.040 in. [1.02 mm]) |
| > 2.50 to 4.00 in. [65 to 100 mm] | 40 | 2T | Set C | Wire 12 (0.050 in. [1.27 mm]) |
4. IQI Placement Rules & The Film-Side Lead "F" (Clause 8.17.1.1)
- Source-Side Placement (Standard): IQIs shall normally be placed on the source side of the member, adjacent to the weld joint. For wire-type IQIs, the wires must be placed perpendicular to the longitudinal axis of the weld so that the essential wire spans both base metal and weld reinforcement.
- Film-Side Placement (Exception): When the source side is physically inaccessible (e.g., closed box girders, pipe interiors, or structural hollow sections), the IQI is placed on the film side. In every such case, a lead letter "F" at least 1/4 in. [6 mm] high must be placed immediately adjacent to or on the IQI to permanently document film-side placement on the radiograph.
- Use of Steel Shims: When weld reinforcement or backing prevents placing the plaque IQI flush against the base metal at equivalent thickness, steel shims of radiographically similar material must be placed beneath the IQI. The shim thickness must equal the average weld reinforcement profile so the radiation penetrates equal steel thickness.
Radiographic Density & Quality Control (Clause 8.17.5 & 8.17.6)
Radiographic film density represents the quantitative measure of film blackening, expressed in Hurter & Driffield (H&D) optical density units:
D = log10(I0 / It)
Where I0 is the incident light intensity from a calibrated film illuminator and It is the transmitted light intensity measured by a calibrated diffuse-transmission densitometer.
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| H&D OPTICAL FILM DENSITY LIMITS |
+-------------------------------------------------------------------------+
| |
| X-RAY EXPOSURES: Min Density = 1.8 <------> Max Density = 4.0 |
| GAMMA RAY EXPOSURES: Min Density = 2.0 <------> Max Density = 4.0 |
| |
| DENSITY VARIATION: -15% to +30% from density through the IQI |
| DENSITOMETER CHECK: Calibrated every 90 days against a step tablet |
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Mandatory Density Boundaries
- X-Ray Radiographs: Transmitted film density through the weld metal and heat-affected zone must be between 1.8 minimum and 4.0 maximum (Clause 8.17.5.1).
- Gamma Ray Radiographs: Transmitted film density through the weld metal and heat-affected zone must be between 2.0 minimum and 4.0 maximum (Clause 8.17.5.2).
- Density Variation Limits: The optical density throughout the entire area of interest (weld and HAZ) shall not vary by more than -15% to +30% from the measured density through the body of the IQI or essential hole.
- Backscatter Radiation Check: A lead letter "B" with a minimum height of 1/2 in. [13 mm] and thickness of 1/16 in. [1.6 mm] must be attached to the back of the film cassette. If a light image of the letter "B" appears on a darker background of the processed radiograph, backscatter radiation is excessive, and the radiograph must be rejected and re-shot with additional lead shielding.
Radiographic Discontinuity Acceptance Criteria (Clause 8.12)
AWS D1.1 divides structural weldments into two distinct loading regimes, each with its own non-destructive acceptance criteria: Clause 8.12.1 (Statically Loaded Nontubular Connections) and Clause 8.12.2 (Cyclically Loaded Nontubular Connections).
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| RADIOGRAPHIC ACCEPTANCE DISCIPLINE |
+-------------------------------------------------------------------------+
| 1. ABSOLUTE PROHIBITIONS: Cracks, Lack of Fusion (LOF), Incomplete |
| Joint Penetration (CJP) are REJECTABLE. |
| 2. ISOLATED DISCONTINUITIES: Size limited by joint thickness (E). |
| 3. CLEARANCE RULES: Adjacent flaws must be separated by minimum |
| clearance distances (Sum of lengths or 6L). |
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1. Absolute Prohibitions (Clause 8.12.1.1 & 8.12.2.1)
Regardless of member thickness or loading category, the following planar discontinuities are strictly rejectable under AWS D1.1:
- Cracks: Zero tolerance. Any crack of any size, orientation, or location is rejectable.
- Lack of Fusion (LOF): Zero tolerance in CJP groove welds.
- Incomplete Joint Penetration (IJP): Prohibited in all CJP groove welds.
2. Statically Loaded Connections (Clause 8.12.1 & Figure 8.1)
For statically loaded buildings and stationary frames, elongated discontinuities (such as slag inclusions or piping porosity) are evaluated based on weld size (effective throat E):
- Maximum Individual Length: The maximum length of any individual elongated discontinuity shall not exceed E/3 or 3/4 in. [20 mm], whichever is smaller.
- Minimum Clearance Distance: The clearance between any two adjacent elongated discontinuities shall be at least the sum of their lengths.
- Distance to Member Edge: The minimum distance from the end of an elongated discontinuity to the edge of the member or intersection of welds shall be at least 6 × L (where L is the length of the discontinuity).
- Accumulation of Discontinuities: The total cumulative length of all discontinuities over any 12 in. [300 mm] length of weld shall not exceed E (the weld size).
3. Cyclically Loaded Connections (Clause 8.12.2 & Figures 8.2 and 8.3)
For bridges, crane girders, and dynamically loaded members, fatigue failure is the governing design limit state. Acceptance criteria are significantly stricter:
- Tension vs. Compression Stress Regimes: In welds subject to tensile stress or stress reversal, permissible discontinuity sizes are roughly half of those permitted in compression zones.
- Edge Distance Restrictions: In tension members, no discontinuity exceeding 1/16 in. [1.6 mm] is permitted within 1 in. [25 mm] of the plate edge or weld termination.
- Stricter Clearance Curves (Figure 8.2): Discontinuities must be separated by larger clear spacing distances to prevent cyclic stress concentration overlap.
Comparison: Static vs. Cyclic RT Acceptance Rules
| Parameter / Defect | Statically Loaded (Clause 8.12.1) | Cyclically Loaded - Tension (Clause 8.12.2) | Cyclically Loaded - Compression (Clause 8.12.2) |
|---|---|---|---|
| Cracks | Prohibited (0 tolerance) | Prohibited (0 tolerance) | Prohibited (0 tolerance) |
| Lack of Fusion | Prohibited | Prohibited | Prohibited |
| Max Slag Inclusion Size | ≤ E/3 or 3/4 in. max | ≤ E/3 or 3/8 in. max (Figure 8.2) | ≤ E/3 or 3/4 in. max (Figure 8.3) |
| Spacing Between Flaws | ≥ Sum of flaw lengths | Per Figure 8.2 curve (up to 6L) | Per Figure 8.3 curve |
| Clearance to Plate Edge | ≥ 6L | ≥ 1 in. or 6L (whichever is larger) | ≥ 6L |
| Aggregate Length in 12 in. | ≤ E (weld size) | ≤ 2E/3 | ≤ E |
Worked Example: Radiographic Interpretation Audit
Problem Statement
A CWI is reviewing a production radiograph of a 1.5 in. thick CJP butt joint on an ASTM A572 Grade 50 statically loaded column flange. The radiograph was exposed using an Iridium-192 source. The CWI notes the following on the film:
- An ASTM E1025 plaque IQI stamped '30' is located on the source side with the 2T hole clearly resolved. The optical density through the plaque is 2.40.
- The optical density in the weld metal area of interest varies between 2.15 and 2.90.
- An elongated slag inclusion measuring 3/8 in. [10 mm] in length is located 1/2 in. [13 mm] from a second elongated inclusion measuring 1/4 in. [6 mm] in length. Evaluate the compliance of this radiograph and weldment under AWS D1.1.
Step-by-Step Code Evaluation
- Verify IQI Selection:
- Member thickness Tn = 1.50 in.
- Under Table 8.4, for > 1.00 to 1.50 in., the required hole-type plaque is Designation 25.
- Plaque 30 corresponds to a 2.0 in. plate thickness (a thicker, less sensitive plaque). Under Clause 8.17.1, using a plaque thicker than required represents inferior sensitivity, making the radiographic technique non-compliant unless qualified otherwise.
- Evaluate Optical Density:
- Gamma-ray density range per Clause 8.17.5.2: 2.0 to 4.0.
- Measured weld density range = 2.15 to 2.90, which falls within the absolute 2.0 to 4.0 limits.
- Density variation check relative to IQI body (DIQI = 2.40):
- Minimum allowable: 2.40 - 15% = 2.40 × 0.85 = 2.04. (Actual min 2.15 > 2.04 -> Compliant).
- Maximum allowable: 2.40 + 30% = 2.40 × 1.30 = 3.12. (Actual max 2.90 < 3.12 -> Compliant).
- Evaluate Discontinuity Sizing and Clearance:
- Weld size E = 1.50 in.
- Maximum permitted individual length = E/3 = 1.50 / 3 = 0.50 in. [13 mm].
- Flaw 1 (3/8 in. = 0.375 in.) and Flaw 2 (1/4 in. = 0.250 in.) both individually satisfy L ≤ 0.50 in.
- Clearance between Flaws 1 and 2 = 1/2 in. = 0.50 in.
- Required minimum clearance = Length1 + Length2 = 0.375 in. + 0.250 in. = 0.625 in. [5/8 in.].
- Because actual clearance (0.500 in.) is less than the required clearance (0.625 in.), the weld is REJECTABLE under Clause 8.12.1.
Under AWS D1.1 Clause 8.17.1.1, when physical access prevents placing an Image Quality Indicator (IQI) on the radiation source side of a weld joint, which procedural requirement must be satisfied for film-side placement?
What are the minimum and maximum transmitted H&D optical film density limits specified in AWS D1.1 Clause 8.17.5 for radiographic examination using an X-ray source versus a Gamma-ray source?
In a 1.20 in. thick CJP groove weld on a statically loaded structure governed by Clause 8.12.1, two adjacent elongated slag inclusions are detected with lengths of 3/8 in. [9.5 mm] and 1/4 in. [6.4 mm]. What is the minimum clear spacing distance required between these two discontinuities for acceptance?