14.1 Radiographic Testing (RT) & Film Quality
Key Takeaways
- Radiographic testing uses X-rays or gamma rays to image through-thickness density differences and is strong for volumetric discontinuities such as porosity and slag when technique is correct
- Planar defects (cracks, lack of fusion) can be missed when the beam is not aligned near edge-on to the defect plane — geometry and orientation matter as much as source strength
- Film (or digital image) quality is judged by density, contrast, and IQI/penetrameter sensitivity — not by whether the inspector likes the weld appearance
- IWI-S competence under IAB-041 §4.2 is to verify radiographic film quality adequacy; it is not to act as a Level 2 radiographer interpreting defects for acceptance
- Geometric unsharpness, source-to-film distance, object-to-film distance, and identification markers are control points the welding inspector checks against procedure and standard
14.1 Radiographic Testing (RT) & Film Quality
Quick Answer: RT images a weld by transmitting X-rays or gamma rays through the joint onto film (or a digital detector). It is excellent for many volumetric discontinuities (pores, slag, some incomplete penetration views) but can miss planar defects that are not favourably oriented to the beam. IWI-S competence (IAB-041 §4.2) is to verify film quality adequacy — density, sensitivity/IQI, identification, geometry — not to interpret and accept/reject defects as a certified radiographer.
Chapter 13 covered surface methods (VT, PT, MT). This section opens volumetric non-destructive testing under WI2.6 and locks one of the most examined IWI-S boundaries: film quality verification versus defect interpretation.
Principles — How RT Works
Ionising radiation passes through the welded joint. Material thickness, density, and voids change how much radiation reaches the detector:
- Thinner paths or gas-filled voids → more transmission → darker regions on conventional film (negative image)
- Thicker metal or dense inclusions → less transmission → lighter regions on film
X-ray sources are electrically generated tubes (voltage and current settable; beam can be switched off). Gamma sources (commonly Iridium-192 or Cobalt-60 depending on thickness range) are radioisotopes with fixed energy spectra, useful for field work and thicker sections but always “on” for dose-control purposes and governed by strict radiation safety rules.
For the inspector, the physics takeaway is simple: RT maps radiation path length / attenuation, not “crack colour.” Interpretation of what a dark line means belongs to qualified NDT personnel working to a procedure and acceptance standard. Your job as IWI-S is to confirm the image is good enough to support that interpretation and that the report package is complete.
Strengths and Weaknesses (Volumetric vs Planar)
Strengths
- Detection of volumetric discontinuities: porosity, wormholes, many slag inclusions, incomplete root penetration when open to the beam path in a way that changes thickness
- Permanent record (film or digital file) for traceability and third-party review
- Suitable for many butt welds in plate and pipe when access and geometry allow double-wall or single-wall techniques as specified
Weaknesses and traps
- Planar defects (cracks, lack of side-wall fusion, tight LOF) may produce little or no image if the defect plane is nearly parallel to the beam (not edge-on). RT does not magically find every crack
- Complex geometry, heavy reinforcement, backing bars, and multi-pass layering can mask or mimic indications
- Sensitivity limits: small flaws below the IQI/sensitivity capability of the technique may not be revealed even when present
- Safety and logistics: radiation controlled areas, access for source and film, production interruption
Exam scenario pattern: a tight lack-of-fusion plane almost parallel to the radiation beam is not a guarantee of a clear RT indication. Pair RT with process knowledge and, where required, complementary methods (often UT for planar flaws).
Film / Image Quality — What IWI-S Verifies
IAB-041 frames Standard-level competence as verifying radiographic film quality adequacy with the explicit caveat of no interpretation of defects as the radiographer. Treat that as a hard exam line.
Density
Optical density of film in the weld area must fall within the specified band of the applicable RT standard/procedure (commonly a range such as roughly 1.8–4.0 or as stated by the governing code — always use the project standard, not a memorised single number from a textbook). Under-dense film is pale and low-contrast; over-dense film is too dark to read reliably. Check densitometer readings recorded on the report against requirements.
Contrast and definition
Adequate subject contrast and definition allow small thickness changes to appear. Poor processing, wrong energy for thickness, fog, or scatter reduce useful contrast. Digital systems use different metrics (grey levels, SNR, basic spatial resolution) but the inspector still verifies that procedure-required quality indicators are met and documented.
IQI / penetrameter sensitivity
An Image Quality Indicator (IQI) — wire type or step/hole penetrameter depending on standard — is placed on the specimen to prove that a defined sensitivity was achieved. You verify:
- Correct type and designation of IQI for material and thickness
- Correct placement (source side or film side as required; location relative to weld)
- That the required wire or hole is visible on the radiograph
- That the report records IQI identification and achieved sensitivity
If the required IQI feature is not visible, the film is not adequate for acceptance decisions — regardless of whether someone claims “I can still see the weld fine.”
Identification and coverage
Every film/image set needs unambiguous identification: job, weld ID, date, location markers, shot number, welder/procedure references as required by the RT procedure and ITP. Location markers (lead letters, belts) must demonstrate full coverage of the weld length or circumference claimed. Missing ID or incomplete coverage is a quality failure even if density looks perfect.
Geometric Unsharpness Concepts
Geometric unsharpness (Ug) is the penumbra blur caused by finite source size and geometry:
- Larger source (focal spot / gamma effective size) → more blur
- Larger object-to-film distance → more blur
- Larger source-to-object distance (source-to-film distance minus object-to-film) → less blur
Inspectors do not usually calculate Ug on the shop floor, but they understand that tight film contact, correct SFD, and procedure limits on maximum unsharpness protect definition. Excessive stand-off of film from the object, wrong collimation, or wrong source choice can produce a “soft” image that fails definition criteria even when density is in band.
Related technique checks (procedure-dependent):
- Single-wall vs double-wall exposure
- Source inside pipe vs outside
- Number of shots for full circumferential coverage
- Screens, filters, and scatter control where specified
IWI-S Role vs Radiographer Role
| Task | IWI-S (typical) | NDT radiographer (ISO 9712 Level 2, etc.) |
|---|---|---|
| Select RT as method in ITP / technique review | Identify/verify relevance with welding knowledge | Develop detailed RT technique card |
| Verify density, IQI, ID, coverage, geometry records | Yes — film quality adequacy | Produce and self-check images |
| Interpret indications as cracks, slag, porosity | No — not as radiographer | Yes — within certification and procedure |
| Accept/reject weld per RT acceptance criteria | Decide using quality documents from qualified NDT, not by personal film reading outside competence | Report findings; acceptance often by welding inspector / engineer using report |
Trap language on the exam: “IWI-S interprets radiographs for crack length acceptance under ISO 5817.” False under the IAB-041 task wording for film quality. Correct: IWI-S verifies film quality adequacy and then uses qualified NDT reports (and applicable acceptance criteria) when taking decisions on quality documents.
Practical Witness and Review Checklist
When RT is on the ITP hold/witness point:
- Confirm procedure number, revision, and applicability (material, thickness, technique class)
- Confirm personnel certification (ISO 9712 or national equivalent as contract requires) is current for RT
- On film/digital package: ID, location markers, densitometry, IQI visibility, coverage
- Confirm report lists technique essentials (source type/energy, SFD, screens, geometry class if required)
- Route interpretation results from the certified interpreter; do not invent your own defect map from a quick look
- Tie results to weld map / joint ID and any repair loop
Link Forward
Section 14.2 covers ultrasonic testing, which complements RT for many planar flaws. Section 14.3 consolidates method selection, IQI concepts across methods, and ISO 9712 personnel levels. Remember: excellent RT film quality does not mean “no defects” — it means the image is fit for qualified interpretation.
Exam Focus
- Volumetric strength vs planar orientation weakness
- Density + IQI + identification = quality triad
- Geometric unsharpness drivers (source size, distances)
- IWI-S verifies film quality; does not replace radiographer interpretation (IAB-041 §4.2 critical point)
Under IAB-041 §4.2 framing for IWI-S, what is the inspector’s primary radiography-related competence regarding film?
Why can a tight lack-of-fusion plane be difficult to detect with RT even when present?
Which set best describes checks used to judge radiographic film (or image) quality adequacy?
Which change tends to increase geometric unsharpness (blur) on a radiograph, all else equal?