8.3 Radiographic Testing Principles and Applications

8.3 Radiographic Testing Principles and Applications

Quick Answer: Radiographic testing (RT) forms an image from ionizing radiation transmitted through a part. Thicker, denser, or higher-Z regions absorb more; voids and low-density paths transmit more. X-ray tubes and gamma isotopes differ in generation and operational control. Sharpness, contrast, and density must all be adequate. RT is a premier method for volumetric weld and casting discontinuities when access and safety allow.

Radiography is the other major volumetric pillar next to UT on the Basic exam. Level III candidates must understand how contrast forms, what equipment classes exist, and which discontinuities RT is built to show—without diving into every film-processing recipe reserved for Method exams.

X-Ray vs Gamma Radiation Sources

Both X-rays and gamma rays are electromagnetic ionizing radiation capable of penetrating solids; the practical differences are how they are produced and controlled.

X-ray generators accelerate electrons into a target (typically tungsten) in a vacuum tube. The resulting spectrum is continuous (bremsstrahlung) plus characteristic peaks. Tube voltage (kV) primarily controls penetrating power (energy); tube current (mA) and exposure time control intensity (quantity). When power is removed, useful X-ray production stops—an operational safety advantage.

Gamma radiography uses radioactive isotopes. Common industrial sources include iridium-192 and cobalt-60, selected for energy and half-life suited to material thickness ranges. Output is continuous until the source is shielded; it cannot be “switched off” like a tube—only projected or stored in a shielded projector/camera. Activity decays with half-life, so exposure calculations change over time.

Basic exam items often probe whether you know operational and energy-control differences, not isotope half-life tables by memory alone.

Absorption and Image Formation

As a beam traverses the object, intensity falls roughly exponentially with thickness for a given material and energy, modified by scatter. Local differences create the radiograph:

• Greater thickness → more absorption → lighter film (conventional negative film) or darker/brighter digital presentation depending on display polarity
• Void / gas porosity / cavity → less material in the path → more transmission → indication of higher transmission
• High-density inclusion (e.g., tungsten, some slag chemistries) → more absorption → opposite contrast sense from a void
• Higher atomic number materials absorb more strongly at many energies of interest

Scatter radiation reduces contrast and can fog images; collimation, screens, grids (where used), and technique charts manage scatter and exposure.

Think of RT as a shadowgraph of integrated absorption along each ray path. It does not inherently give a single crisp “depth stamp” the way UT time-of-flight does—depth inference may require multiple shots, IQI placement knowledge, or computed tomography (beyond Basic overview).

Film vs Digital Overview

Industrial film radiography records latent image on silver-halide film, chemically processed to optical density. Film still appears in codes and field work; viewers use high-intensity illuminators and density requirements.

Digital radiography families (overview level):

• Computed radiography (CR): photostimulable phosphor plates scanned to digital images
• Digital detector arrays (DDA / DR): flat panels convert radiation to electronic images more directly

Digital systems enable image processing, archival, and sometimes lower dose or faster cycle times, but they still obey the same absorption physics. Poor geometry or wrong energy produces a sharp-looking but uninformative file. Level III procedure approval must still address energy, geometry, IQI, and acceptance criteria—not only “we went digital.”

Geometric Unsharpness Concepts

Geometric unsharpness (Ug) describes penumbral blur from finite source size. Qualitatively:

• Larger source/focal spot size → more blur
• Larger object-to-detector (film) distance → more blur
• Larger source-to-object distance → less blur (more “point-source-like” projection)

Setup trade-offs: longer SOD improves sharpness but reduces intensity (longer exposures or higher activity/mA). Thick objects and double-wall techniques complicate effective geometry. Codes may limit maximum Ug; the Basic idea is that geometry is a quality parameter, not an afterthought.

Other sharpness factors include motion (part or source movement during exposure) and detector/film inherent unsharpness—mention them as contributors without needing Method-level calculations.

Contrast and Density Basics

Density (film) is the blackness of the developed radiograph; specifications require density within a band so that eyes (or systems) operate in a sensitive range. Too light or too dark, detail vanishes.

Subject contrast arises from real absorption differences in the part. Film/detector contrast and processing or display settings translate that into visible gray-level differences. Energy selection matters: excessively hard (high energy) beams can flatten contrast through thick sections; excessively soft beams may not penetrate enough.

For digital images, window/level tools can enhance display contrast, but they cannot create true subject information that was never recorded. Over-processing can also create misleading artifacts—another Level III caution when approving digital techniques.

Weld and Casting Applications

Welds. RT remains widely specified for new construction piping and pressure-retaining welds where access allows source and film/detector placement. Typical detectable volumetric indications include porosity, slag inclusions, and some incomplete penetration or root defects that present enough thickness difference. Interpreters also evaluate undercut, burn-through, and certain alignment issues when geometry shows on the image. Tight cracks and incomplete fusion parallel to the beam may be faint or invisible—pair with UT/MT as required.

Castings. RT is classic for shrinkage cavities, gas porosity, inclusions, and core-related voids. Section thickness variations in castings demand careful energy and exposure planning; multiple shots cover complex shapes. Acceptance standards (ASTM and product specs) classify severity levels for many casting indications.

Interpreting Volumetric Flaw Images

Interpretation maps gray-level patterns to discontinuity types using shape, location, and process knowledge:

• Rounded porosity: dark (high transmission) spots, isolated or clustered
• Elongated slag: often irregular linear/worm-like darker indications along weld layers
• Wormhole / piping porosity: elongated gas paths
• Shrinkage: dendritic or cavity patterns in castings, often at last-to-freeze regions
• Burn-through / excess penetration: localized thickness anomalies at roots

Always separate true discontinuities from film artifacts, screens marks, light leaks, and geometric shadows of normal part features. Process knowledge from earlier chapters (welding, casting) is what turns a gray blob into a defendable call.

Level III Selection Note

Specify RT when the discontinuity population of concern is volumetric, when a permanent image record is required, and when two-sided access (or approved single-wall/double-wall technique) and radiation safety controls are achievable. Do not specify RT as a substitute for surface crack detection or as a guaranteed crack finder. Section 8.4 covers IQI, orientation limits, and safety principles that complete the RT decision package.