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Research on Crack Detection Using Radiographic Testing
Release Date:
2022/10/15
Radiographic Resolution
As shown in Figure 3-3, a beam of radiation perpendicular to the surface of the test specimen is used to radiograph a metallic step block. In theory, the radiographic image on the film should consist of two regions of different blackness: one is a uniformly high-blackness region formed by the AO portion of the specimen, and the other is a uniformly low-blackness region formed by the OB portion. At the interface between these two regions, the blackness should change abruptly and be discontinuous, as illustrated in Figure 3-3a. However, in practice, the blackness variation on the radiographic film is not abrupt.
The “step-edge” image of the test specimen is blurred, and the variation in image density is as shown in Figure 3-3b, with a distinct density transition zone.
Figure c is an enlarged view of b; as can be seen from c, the blackness transition zone is not a simple straight line—it exhibits both a toe and a shoulder.
The variation of blackness in this region is plotted as a curve, referred to as the “blackness distribution curve” or the “unsharpness curve.” Clearly, the wider the region of blackness variation, the blurrier the image outline; therefore, the width of this blackness-variation region is defined as the radiographic unsharpness U.
In practical industrial radiography, numerous factors can lead to image blurring on the radiographic film. If extraneous, random factors—such as movement of the test piece or the radiation source, or poor contact between the intensifying screen and the film—are ruled out, and if the blurring caused by fluorescent scattering from salt-based intensifying screens is disregarded, then the primary contributors to radiographic blur are two main factors: geometric unsharpness Ug, resulting from the finite size of the radiation source, and inherent unsharpness Ui, arising from electron scattering within the photographic emulsion.
The overall unsharpness U on the photographic negative is the combined effect of geometric unsharpness Ug and inherent unsharpness Ui, with geometric unsharpness Ug accounting for the linear portion of the density transition zone, while inherent unsharpness Ui gives rise to the toe and shoulder in the density transition zone, as shown in Figure 3-3c. Currently, the most widely used relationship for describing U, Ug, and Ui is: U = (Ug² + Ui²)¹/².
1. Geometric Unsharpness Ug
Because both the X-ray tube focal spot and the gamma-ray source have a finite size, when radiographing a workpiece, the edges of the workpiece’s surface contours or internal defects will produce a penumbra of a certain width on the radiographic film. This penumbral width is defined as geometric unsharpness, Ug, as shown in Figure 3-4.
The Ug value can be calculated using the following formula: Ug = df × b / (F − b).
In the formula: df — focal spot size; F — distance from the focal point to the film; b — distance from the defect to the film.
The geometric unsharpness that radiographic testing must meet as specified in technical standards generally refers to the maximum geometric unsharpness Ugmax that may occur in the test piece. This is equivalent to the geometric unsharpness produced by a surface defect on the source side or by a wire-type image quality indicator placed on the source side (see Figure 3-5), and its calculation formula is: Ugmax = df × L² / (F − L²) = df × L² / LI.
In the formula: L1 represents the distance from the focal point to the workpiece surface; L2 represents the distance from the work surface to the film.
As evident from the above equation, geometric unsharpness is directly proportional to the focal spot size and the workpiece thickness, and inversely proportional to the distance between the focal point and the workpiece surface.
Given fixed focal spot size and workpiece thickness, achieving a smaller Ug value requires using a larger focal length F during radiography. However, since X-ray intensity is inversely proportional to the square of the distance, maintaining constant film density necessitates either extending the exposure time or increasing the tube voltage when the focal length is increased; therefore, these factors must be carefully balanced and considered in conjunction.
When radiography is performed, the focal spot size varies across the exposure field, with a larger focal spot on the cathode side; consequently, the geometric unsharpness at those locations is also greater.
In fact, because the optical focal point within the irradiation field varies from the cathode side to the anode side, even for longitudinal weld (plate) radiography, the Ug values at different points on the radiograph will also differ.
In contrast, radiographic inspection of circumferential welds (curved surfaces) exhibits even greater and more complex variations in the Ug values at various points on the radiograph due to changes in distance and thickness.
2. Intrinsic Uncertainty Ui:
Intrinsic unsharpness is caused by the scattering of electrons excited in the emulsion layer by the X-rays that strike the film. As photons pass through the emulsion, they excite electrons within it.
The higher the energy of the X-ray photons, the greater the kinetic energy of the ejected electrons and the longer their range within the emulsion layer.
These electrons scatter in all directions, impacting neighboring silver halide grains; electrons with higher kinetic energy can even penetrate multiple silver halide grains.
Due to the action of electrons, these silver halide grains develop a latent image; consequently, a single photon of radiation does not affect just one silver halide grain but may instead generate a small cluster of latent silver within the emulsion. As a result, not only the point directly struck by the photon is developed, but also the surrounding area is developed as well, leading to diffusion of the image boundaries and blurring of the outlines.
The inherent unsharpness is defined as the average distance over which scattered electrons interact within the film emulsion layer.
The inherent unsharpness is primarily determined by the X-ray energy; within the 100–400 kV range, the empirical formula for expressing inherent unsharpness can be written as: Ui = 0.0013(kV)^0.79.
Table 3-2 shows the values of inherent unsharpness at different X-ray energies.
The curves plotted using the values in Table 3-2 are shown in Figure 3-6. As can be seen, Ui increases continuously with increasing X-ray energy; in the low-energy region, the rate of increase is relatively slow, whereas in the high-energy region, the rate of increase is much faster.
Metal intensifying screens used in radiography absorb X-ray energy, emit electrons, and expose the silver halide in the photographic film, thereby increasing its sensitivity.
The electrons emitted by the intensifying screen also have a certain range of penetration within the emulsion layer, thereby giving rise to inherent blur.
Relevant literature indicates that the material type and thickness of intensifying screens, as well as their usage conditions, all influence inherent unsharpness.
For example, in medium- and low-energy radiography, film equipped with lead intensifying screens exhibits greater inherent unsharpness than film without such screens; moreover, the inherent unsharpness varies with changes in the thickness of the lead intensifying screen.
In gamma-ray and high-energy X-ray radiography, intensifying screens made of copper, tantalum, or tungsten can achieve lower inherent unsharpness than lead intensifying screens; however, if the intensifying screen is not tightly in contact with the film and there is a gap between them, the inherent unsharpness will increase significantly.
The phenomenon of increased inherent unsharpness caused by inadequate contact between the intensifying screen and the film can be explained as follows: When electrons emitted from the intensifying screen leave its surface, if they do not immediately enter the emulsion layer of the film but instead travel a certain distance through the air before doing so, their total effective range will be greater than that of electrons that traverse the emulsion layer entirely. This is because electrons lose less kinetic energy as they move through the air, allowing them to cover a longer distance before being absorbed.
Therefore, the inherent blur increases.
The inherent unsharpness of radiography can be determined using a platinum–tungsten double-wire image quality indicator.
3. The blur curve measured using a microdensitometer
Figure 3-7 shows the experimental records of blur curves measured with a microdensitometer; the variations in blackness of the edge images depicted in these curves are plotted on an enlarged scale.
Figure 3-7 shows the experimental records of blur curves measured with a microdensitometer; the variations in blackness of the edge images depicted in these curves are plotted on an enlarged scale.
Figure a) shows the geometric unsharpness curve (generated by a uniform-focus X-ray tube), which consists broadly of three straight segments, with only very small toe and shoulder regions at the transitions.
b) through d) show the variation of inherent unsharpness as X-ray energy is gradually increased;
e) is the unsharpness curve (hereafter referred to as the U-curve) resulting from the superposition of geometric unsharpness and inherent unsharpness.
As shown in the figure, with increasing X-ray energy, the toe and shoulder regions of curves c) and d) become progressively more pronounced; whereas in curve e), the superposition of inherent unsharpness and geometric unsharpness results in a shortening of the straight-line portion.
Almost all radiographic images exhibit two types of blurring, as illustrated by the curve in Figure 3-7e.
Source: NDT National Standardized Textbook for the Qualification Examination of Nondestructive Testing Personnel for Special Equipment — Radiographic Testing
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