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RADIOGRAPHIC INTERPRETATION.
-The
usefulness of the information obtained
from the radiographic process
depends upon the intelligent interpretation
of the derived image. To successfully interpret
the radiograph, the radiographic interpreter must
have a working knowledge of the component or material
and be able to relate the images to the conditions
likely to occur. Specifications are used to spell
out the discontinuities that maybe considered detrimental
to the function of the part and the acceptable magnitudes
of the discontinuities. It is the duty of the film
interpreter to recognize the various discontinuities, their
magnitudes, and be capable of relating them to the particular
specification required. The responsibility and capability
of the radiographic interpreter cannot be overemphasized.
Often, many human lives and investments
of millions of dollars are depending on the judgement
of the radiographic interpreter. RADIATION HAZARD. -Radiation from X-ray units is destructive to living tissue. It is universally recognized that in the use of such equipment, adequate protection must be provided to personnel. Personnel must keep outside the primary X-ray beam at all times. Radiation produces changes in all matter that it passes through. 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 parts are in the center of the body; therefore, the more penetrating radiation is likely to be the more harmful in these areas. The skin usually absorbs most of the radiation; therefore, it 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.
Figure 15-11.-Coupting of search unit to test part for transmission of ultrasonic energy.Ultrasonic Inspection The term ultrasonic means vibrations or sound waves whose frequencies are greater than those that affect the human ear (greater than about 20,000 cycles per second).Ultrasonic inspection is a method of inspection that uses these sound waves. The ultrasonic vibrations are generated by applying high-frequency electrical pulses to a transducer element contained within a search unit. The transducer element transforms the electrical energy into ultrasonic energy. The transducer element can also receive ultrasonic energy and transform it into electrical energy. Ultrasonic energy is transmitted between the search unit and the test part through a coupling medium, such as oil, as shown in figure 15-11, for the purpose of excluding the air interface between the transducer and the test part. The ultrasonic vibrations are transmitted into and through the part. When the beam strikes the far surface of the part or strikes the boundary of a defect, the beam reflects back towards the transducer, travels through the couplant, and enters the transducer, where it is converted back into electrical energy. Then the information is displayed on a cathode-ray tube (CRT,) screen.Ultrasonic inspections can be separated into two basic categories-contact inspection and immersion inspection. In the contact method, the search unit is placed directly on the test part surface by using a thin film of couplant, such as oil, to transmit sound into the test part. In the immersion method, the test part is immersed in a fluid, usually water, and the sound is transmitted through the water to the test part (fig. 15- 12). The immersion-type method is used to inspect materials while they are immersed in a suitable liquid, such as water or oil. This method proves more satisfactory than contact testing for irregular-shaped surfaces. Immersion inspection also permits use of a wider range of testing frequencies. The three general
Figure 15-12.-Immersion method. methods of contact inspections are straight-beam, angle-beam, and the surface-wave method.STRAIGHT BEAM. -The straight-beam method is used to detect discontinuities parallel to the test surface, and is generally used on material 1/2 inch thick or greater. Most straight-beam methods are applied by using the pulse-echo technique (transmitting and receiving search unit or units placed on the same surface). Certain applications use the through-transmission method (transmitting search unit placed on one surface, and receiving search unit placed on the opposite surface). In the through-transmission method, discontinuities block the passage of sound. This results in a reduction of the received signal (fig. 15-13). With this method, echoes from the discontinuities are not shown on the CRT. Therefore, depth information on the discontinuities is not determined. Typical discontinuity examples are laminations, corrosion, and cracks. |
 
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