X and gamma radiations, because of their unique ability to penetrate material and disclose discontinuities, have been applied to the radiographic (X-ray) inspection of metal fabrications and nonmetallic products.
The penetrating radiation is projected through the part to be inspected and produces an invisible or latent image in the film. When processed, the film becomes a radiograph or shadow picture of the object. This inspection medium, in a portable unit, provides a fast and reliable means for checking the integrity of airframe structures and engines.
Radiographic inspection techniques are used to locate defects or flaws in airframe structures or engines with little or no disassembly. This is in marked contrast to other types of nondestructive testing, which usually require removal, disassembly, and stripping of paint from the suspected part before it can be inspected. Due to the nature of X-ray, extensive training is required to become a qualified radiographer, and only qualified radiographers are allowed to operate the X-ray units.
Three major steps in the X-ray process discussed in subsequent paragraphs are: (1) Exposure to radiation, including preparation, (2) processing of film, and (3) interpretation of the radiograph.
Preparation and Exposure
The factors of radiographic exposure are so interdependent that it is necessary to consider all factors for any particular radiographic exposure. These factors include, but are not limited to, the following:
(a) Material thickness and density.
(b) Shape and size of the object.
(c) Type of defect to be detected.
(d) Characteristics of X-ray machine used.
(e) The exposure distance.
(f) The exposure angle.
(g) Film characteristics.
(h) Types of intensifying screen, if used.
Knowledge of the X-ray unit's capabilities should form a background for the other exposure factors. In addition to the unit rating in kilovoltage, the size, portability, ease of manipulation, and exposure particulars of the available equipment should be thoroughly understood.
Previous experience on similar objects is also very helpful in the determination of the overall exposure techniques. A log or record of previous exposures will provide specific data as a guide for future radiographs.
After exposure to X-rays, the latent image on the film is made permanently visible by processing it successively through a developer chemical solution, an acid bath, and a fixing bath, followed by a clear water wash.
The film consists of a radiation sensitive silver salt suspended in gelatin to form an emulsion. The developer solution converts radiation affected elements in the emulsion to black metallic silver.
These black metallic particles form the image. The longer the film remains in the developer, the more metallic silver is formed, causing the image to become progressively darker. Excessive time in the developer solution results in overdevelopment.
An acid rinse bath, sometimes referred to as a stop bath, instantly neutralizes the action of the developer and stops further development. Due to the soft emulsion and the nonabsorbent quality of the base of most negative materials, only a very weak acid bath is required.
The purpose of the fixing bath is to arrest the image at the desired state of development. When a radiation sensitive material is removed from the developing solution, the emulsion still contains a considerable amount of silver salts which have not been affected by the developing agents. These salts are still sensitive, and if they are allowed to remain in the emulsion, ordinary light will ultimately darken them and obscure the image. Obviously, if this occurs, the film will be useless.
The fixing bath prevents this discoloration by dissolving the salts of silver from the developed free silver image. Therefore, to make an image permanent, it is necessary to fix the radiation sensitive material by removing all of the unaffected silver salt from the emulsion.
After fixing, a thorough water rinse is necessary to remove the fixing agent which, if allowed to remain, will slowly combine with the silver image to produce brownish-yellow stains of silver sulfide, causing the image to fade.
NOTE: All processing is conducted under a subdued light of a color to which the film is not readily sensitive.
From the standpoint of quality assurance, radiographic interpretation is the most important phase of radiography. It is during this phase that an error in judgment can produce disastrous consequences. The efforts of the whole radiographic process are centered in this phase; the part or structure is either accepted or rejected. Conditions of unsoundness or other defects which are overlooked, not understood, or improperly interpreted can destroy the purpose and efforts of radiography and can jeopardize the structural integrity of an entire aircraft. A particular danger is the false sense of security imparted by the acceptance of a part or structure based on improper interpretation.
As a first impression, radiographic interpretation may seem simple, but a closer analysis of the problem soon dispels this impression. The subject of interpretation is so varied and complex that it cannot be covered adequately in this type of document. Instead, this chapter will give only a brief review of basic requirements for radiographic interpretation, including some descriptions of common defects.
Experience has shown that, whenever possible, radiographic interpretation should be conducted close to the radiographic operation. It is helpful, when viewing radiographs, to have access to the material being tested. The radiograph can thus be compared directly with the material being tested, and indications due to such things as surface condition or thickness variations can be immediately determined.
The following paragraphs present several factors which must be considered when analyzing a radiograph.
There are three basic categories of flaws: voids, inclusions, and dimensional irregularities. The last category, dimensional irregularities, is not pertinent to these discussions because its prime factor is one of degree, and radiography is not that exacting. Voids and inclusions may appear on the radiograph in a variety of forms ranging from a two dimensional plane to a three dimensional sphere. A crack, tear, or cold shut will most nearly resemble a two dimensional plane, whereas a cavity will look like a three dimensional sphere. Other types of flaws, such as shrink, oxide inclusions, porosity, etc., will fall somewhere between these two extremes of form.
It is important to analyze the geometry of a flaw, especially for such things as the sharpness of terminal points. For example, in a crack-like flaw the terminal points will appear much sharper than they will for a sphere-like flaw, such as a gas cavity. Also, material strength may be adversely affected by flaw shape. A flaw having sharp points could establish a source of localized stress concentration. Spherical flaws affect material strength to a far lesser degree than do sharp pointed flaws. Specifications and reference standards usually stipulate that sharp pointed flaws, such as cracks, cold shuts, etc., are cause for rejection.
Material strength is also affected by flaw size. A metallic component of a given area is designed to carry a certain load plus a safety factor. Reducing this area by including a large flaw weakens the part and reduces the safety factor. Some flaws are often permitted in components because of these safety factors; in this case, the interpreter must determine the degree of tolerance or imperfection specified by the design engineer. Both flaw size and flaw shape should be considered carefully, since small flaws with sharp points can be just as bad as large flaws with no sharp points.
Another important consideration in flaw analysis is flaw location. Metallic components are subjected to numerous and varied forces during their effective service life. Generally, the distribution of these forces is not equal in the component or part, and certain critical areas may be rather highly stressed. The interpreter must pay special attention to these areas. Another aspect of flaw location is that certain types of discontinuities close to one another may potentially serve as a source of stress concentrations; therefore, this type of situation should be closely scrutinized.
An inclusion is a type of flaw which contains entrapped material. Such flaws may be either of greater or lesser density than the item being radiographed. The foregoing discussions on flaw shape, size, and location apply equally to inclusions and to voids. In addition, a flaw containing foreign material could become a source of corrosion.
Radiation from X-ray units and radioisotope sources is destructive to living tissue. It is universally recognized that in the use of such equipment, adequate protection must be provided. Personnel must keep outside the primary X-ray beam at all times.
Radiation produces changes in all matter through which it passes. This is also true of living tissue. When the radiation strikes the molecules of the body, the effect may be no more than to dislodge a few electrons, but an excess of these changes could cause irreparable harm. When a complex organism is exposed to radiation, the degree of damage, if any, depends on which of its body cells have been changed.
The more vital organs are in the center of the body; therefore, the more penetrating radiation is likely to be the most harmful in these areas. The skin usually absorbs most of the radiation and, therefore, reacts earliest to radiation.
If the whole body is exposed to a very large dose of radiation, it could result in death. In general, the type and severity of the pathological effects of radiation depend on the amount of radiation received at one time and the percentage of the total body exposed. The smaller doses of radiation may cause blood and intestinal disorders in a short period of time. The more delayed effects are leukemia and cancer. Skin damage and loss of hair are also possible results of exposure to radiation.