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1995 | Buch | 2. Auflage

Introduction: capabilities and limitations of radiographic inspection Kapitel lesen Erstes Kapitel lesen

Industrial Radiology

Theory and practice

verfasst von: R. Halmshaw, MBE, Ph.D., ARCS, C. Phys., F. Inst. P., Hon. F. Brit. Inst. NDT, Hon. F. Indian Soc. NDT

Verlag: Springer Netherlands

Buchreihe : Non-Destructive Evaluation Series

Enthalten in: Professional Book Archive

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Über dieses Buch

Industrial radiography is a well-established non-destructive testing (NDT) method in which the basic principles were established many years ago. However, during 1993-95 the European Standards Organisa­ tion (CEN) commenced drafting many new standards on NDT including radiographic methods, and when completed these will replace national standards in all the EC member countries. In some cases these standards vary significantly from those in use in the UK at present. These CEN standards are accepted by majority, not unanimous voting, so they will become mandatory even in countries which vote against them. As most are likely to be legal by the time this second edition is published, they are described in the appropriate places in the text. The most important new technical development is the greater use of computers in radiology. In the first edition, computerized tomography was only briefly mentioned at the end of Chapter 11, as it was then largely a medical method with only a few equipments having found a place in industrial use. The method depends on a complex computer program and a large data store. Industrial equipments are now being built, although their spread into industry has been slow. Computer data storage is also being used for radiographic data. Small computers can now store all the data produced by scanning a radiographic film with a small light-spot, and various programs can be applied to these data.

Inhaltsverzeichnis

Frontmatter
Introduction: capabilities and limitations of radiographic inspection
Abstract
Before starting on the details of the principles and techniques of industrial radiology, it will be useful to offer some general comments on the inherent capabilities and limitations of NDT, particularly in relation to flaw detection. Although most NDT methods are used for many applications other than flaw detection, for example, checking the correctness of construction of assemblies, measuring components and spacings, measuring thickness, coatings etc., flaw detection in both metals and non-metals, particularly in welds, is a major application.
R. Halmshaw
1. Principles of radiography
Abstract
Before discussing the properties of ionizing radiation in detail it is desirable to establish the basic principles of radiography. X-rays are generated in an X-ray tube when a beam of electrons is accelerated on to a target by a high voltage and stopped suddenly on striking the target. The X-rays produced have different wavelengths and different penetrating powers according to the accelerating voltage. Gamma-rays have the same physical nature as X-rays and are emitted by certain radioactive substances.
R. Halmshaw
2. Basic properties of ionizing radiations
Abstract
X-rays and gamma-rays are forms of electromagnetic radiation having wavelengths roughly in the range 102212;9–10−13 m. X-rays are produced by allowing a stream of high-energy electrons to strike a metal target, and they originate in the extra-nuclear structure of the target atoms. Gamma-rays are emitted from the nucleus of radioactive elements. Both X- and gamma-rays travel at the speed of light (c = 3 × 108 m s−1). They travel in straight lines and are invisible. The essential difference between light, ultra-violet, infra-red, radio-waves and X-rays is one of wavelength and frequency. X-rays and gamma-rays have the property of being able to penetrate matter which is opaque to light and they have a photographic action very similar to that of light. They pass through material of low density more readily than through high density material, and their penetrating ability depends on their wavelength.
R. Halmshaw
3. X-ray sources
Abstract
X-rays are produced whenever electrons are suddenly brought to rest by colliding with matter. It is necessary therefore, for the following conditions to prevail:
1.
a means of producing and sustaining a stream of electrons, i.e. a good vacuum, with a source of electrons which in most modern X-ray tubes is a heated filament;
 
2.
a means of accelerating the electrons to a high velocity, i.e. a means of producing and applying a high potential difference between the source of electrons (the filament) and the target;
 
3.
a ‘target’ for the electrons to strike, the face of which must also be in the vacuum, aligned with the electron beam, and is therefore an intrinsic part of the X-ray tube.
 
R. Halmshaw
4. Gamma-ray sources and equipment
Abstract
In 1896 Becquerel discovered that certain of the heavier elements emitted penetrating radiation and were unstable; the earlier concept that elements represented the most stable form of matter had therefore to be abandoned. There are three radioactive series known in nature, the parent elements of which are uranium-238, uranium-235 and thorium-232. Each of these decays through a series of daughter elements, which are also radioactive, to a final stable element, which in each series is one of the several isotopes of lead. Radium is one of the daughter elements in the uranium-238 series. The disintegration of the nucleus of a naturallyoccurring radioactive substance is accompanied by the emission of one or more forms of radiation which were named alpha-, beta- and gammarays. Gamma-rays were shown to be penetrating electromagnetic radiation of the same physical nature as X-rays, and it is the radioactive substances emitting gamma-rays which are used in radiography. Alpharays consist of the nuclei of helium and although they may have a considerable kinetic energy they will penetrate only very small thicknesses of material such as, for example, very thin foil. Beta-rays are electrons and also have only a low penetrating power.
R. Halmshaw
5. Recording of radiation
Abstract
When X-rays pass through matter they are absorbed to an extent which depends on the thickness and density of the material and on the wavelength of the radiation. The properties of differential attenuation and the essentially linear propagation of X-rays provide the fundamental bases of radiography. The presence of internal cavities and inhomogeneities in an object will produce local variations in the spatial intensity of the emergent beam of an initially homogeneous beam of X-rays, in other words, the emergent beam will contain an ‘image’ of the internal structure of the object. However, X-rays are not directly perceptible to the human senses and the information contained in the X-ray beam must be converted to some form which can be appreciated by eye. A detecting medium which will reveal this information by means of a secondary interaction of X-rays with matter is therefore necessary. Such a detecting medium may give an indication of the spatial variations in intensity over an area, i.e. a two-dimensional detector, or it may be necessary or desirable to obtain a point-by-point measure of radiation intensity by a scanning procedure.
R. Halmshaw
6. Radiographic techniques: principles
Abstract
Ideally, the choice of a technique ought to be made in terms of a required defect sensitivity, but it is seldom possible for users to state categorically the size of the smallest defect which they wish to detect. Consequently, the choice of technique is frequently made in terms of some other factor such as convenience or availability of equipment, time involved etc. Not all radiographs are required to have high defect sensitivity, but it is seldom that sensitivity is not an important factor. It is necessary, therefore, to understand the interplay of the various parameters in each technique from the viewpoint of their effect on sensitivity.
R. Halmshaw
7. Radiographic techniques: sensitivity
Abstract
A distinction must be made between radiographic quality and radiographic sensitivity. In many types of radiographic work the two are taken to be synonymous and this is particularly so in the field of flaw detection, where the ability to show a smaller flaw, i.e. an increase in flaw sensitivity, is regarded by the radiographer as an improvement in radiographic quality. However, in the inspection of irregular-thickness castings for example, ability to cover a range of thickness on a single radiograph with a reasonable sensitivity everywhere may be regarded as the criterion of quality and may be preferable to having maximum sensitivity in one particular thickness of the specimen. Thus, in this case, quality requires consideration of thickness latitude as well as sensitivity. Mechanisms and assemblies may be radiographed in order to measure some dimension such as a narrow gap, or the orientation of a component part, and then the most important criterion may be the definition of detail, sharpness of images of edges, or even correct angulation of the component relative to the radiation beam. In these cases radiographic flaw sensitivity may be a relatively minor factor in defining high-quality radiography.
R. Halmshaw
8. Sensitivity performance
Abstract
Chapter 6 and 7 discussed in detail how radiographic techniques are developed, the effects of the various technique parameters and the image quality indicator (IQI) patterns which are available. It is now necessary to establish attainable IQI sensitivity values.
R. Halmshaw
9. Interpretation of radiographs
Abstract
The basic purpose of radiographic inspection is to obtain information about abnormalities in the specimen. These abnormalities may be flaws in metal (e.g. cracks, inclusions), errors in assembly, wrongly-positioned components or dimensional errors, and may or may not be significant. An understanding of the image provided assumes therefore that an image of a ‘correct’ specimen is available for comparison.
R. Halmshaw
10. Safety problems in radiography: units of radiation
Abstract
The energy absorbed from ionizing radiation when it is incident on living tissue can result in damage or destruction of the cells. The principal reactions are burns, dermatitis, cancer induction and blood changes, and there may be direct damage to chromosomes in individual cells which are responsible for reproduction. It is necessary, therefore, to consider both radiation-induced disease in the individual and genetic damage to part of the total population, which may affect future generations. Both types of damage are considered in fixing internationallyagreed dose limitations. At present it is assumed that there is no wholly safe dose of ionizing radiation, so the risk assumed by an individual or a population group must be balanced against the benefits derived from the uses of X-rays etc. An attempt is also made to ensure that the genetic consequence of the agreed dose limit, in the foreseeable future, has an acceptable limit. The International Committee for Radiological Protection (IRCP) is the international body from which recommendations on protection originate, and these are endorsed by national committees and made law in individual countries or by bodies such as the EC, sometimes with minor changes in the detail of recommended procedures. The international regulations are periodically updated [1‐3].
R. Halmshaw
11. Fluoroscopy, image intensifiers, television systems and tomography
Abstract
The direct production of a visible image on a fluorescent screen, X-ray fluoroscopy, has been known and used since the early days of X-rays but now has few applications in industrial radiography. In fluoroscopy the X-radiation that is transmitted through the specimen falls on to a screen which fluoresces, that is, emits light within the visible part of the spectrum; a visible image is produced on the fluorescent screen due to differential absorption in the different thicknesses of the specimen. The thinner, less absorbent parts of the specimen are seen as brighter areas on the screen, so that the tonal range is reversed compared with a film radiograph seen on an illuminated film-viewing screen. Cavities are brighter, not darker. The potentialities of ‘real-time’ imaging, particularly in medical diagnostic radiology, and the ability to see moving images, have led to major developments, e.g. image intensifiers and CCTV-fluoroscopic systems.
R. Halmshaw
12. Special methods
Abstract
There are a few special methods or applications of radiology which have not yet been covered. The most important is the use of atomic particles, such as electrons, neutrons and protons, to form ‘radiographic’ images. Probably the most important of these is the use of neutrons.
R. Halmshaw
Backmatter
Metadaten
Titel
Industrial Radiology
verfasst von
R. Halmshaw, MBE, Ph.D., ARCS, C. Phys., F. Inst. P., Hon. F. Brit. Inst. NDT, Hon. F. Indian Soc. NDT
Copyright-Jahr
1995
Verlag
Springer Netherlands
Electronic ISBN
978-94-011-0551-4
Print ISBN
978-94-010-4244-4
DOI
https://doi.org/10.1007/978-94-011-0551-4