Radiographic Testing (RT)

Overview

RT is based on the ability of high-energy radiation to penetrate solid materials. Variations in thickness, density, or internal discontinuities affect the absorption of radiation, creating an image on film or a digital detector. This image reveals defects such as porosity, cracks, lack of fusion, and inclusions. RT is extensively applied in industries like oil & gas, aerospace, power generation, and manufacturing.

Apparatus and Working

Apparatus

• Radiation Source: X-ray tube or gamma-ray isotopes (Ir-192, Co-60).
• Detector/Film: Radiographic film, computed radiography plates, or digital flat-panel detectors.
• Image Quality Indicators (IQI): Wire or hole-type gauges to verify sensitivity.
• Markers: Lead letters/numbers for identification and orientation.
• Protective Equipment: Lead shielding, collimators, and dosimeters for safety.

Working

1. The test object (e.g., a weld) is placed between the radiation source and the detector/film.
2. Radiation passes through the object; areas with flaws allow more radiation to pass, creating darker spots on the image.
3. The detector/film captures the transmitted radiation, forming a radiograph.
4. The radiograph is developed (film) or processed digitally, then interpreted by a qualified inspector.

Principle

RT works on the principle of differential absorption of ionizing radiation. When radiation passes through a material, its intensity decreases depending on the thickness and density of the material. Internal flaws such as voids or cracks reduce the effective thickness, allowing more radiation to reach the detector, which appears as darker regions on the image. Conversely, denser areas appear lighter.

Advantages and Disadvantages

Advantages

• Detects internal and surface-breaking defects.
• Provides a permanent record (film or digital image).
• Applicable to a wide range of materials and thicknesses.
• High accuracy in detecting volumetric defects like porosity and slag inclusions.

Disadvantages

• Radiation hazards require strict safety measures.
• Less effective for planar defects (e.g., tight cracks parallel to the beam).
• Equipment is expensive and requires skilled operators.
• Time-consuming compared to some other NDT methods.

Applications

• Weld Inspection: Pipelines, pressure vessels, boilers, and structural welds.
• Castings: Detection of shrinkage cavities, porosity, and inclusions.
• Aerospace: Inspection of turbine blades, airframes, and composite structures.
• Petrochemical Industry: Examination of critical joints in refineries and offshore platforms.
• Power Plants: Inspection of nuclear reactor components and steam lines.

Weld defects detectable by RT

• Porosity (gas pores): Spherical or elongated cavities; isolated or clustered (stringers).
• Slag inclusions: Entrapped non-metallics (slag/flux/oxides), irregular or elongated dark areas.
• Tungsten inclusions: Dense foreign particles from GTAW; appear as bright/light spots on film.
• Lack of fusion: Incomplete bonding between weld and base metal or between passes; linear dark indications.
• Incomplete penetration (root/LOP): Weld does not extend through joint thickness; dark line at root.
• Undercut (when severe): Edge groove reducing thickness; may be visible depending on orientation.
• Burn-through: Excessive penetration causing holes; clear localized dark spots.
• Cracks (limited sensitivity): May be seen if oriented favorably and open; RT is less reliable for tight planar cracks.
• Overlap (limited): Surface-laid metal without fusion; generally poorly revealed unless geometry favors detection.
• High/low reinforcement (profile effects): Variations in weld crown/root that affect image density.