Non Destructive Testing

American National Standards Institute

The American National Standards Institute Web site web.ansi.org/default_js.htm, has been administrator and coordinator of the US private sector voluntary standardization system for 80 years. ANSI promotes the use of US standards internationally, advocates US policy and technical positions in international and regional standards organizations, and encourages the adoption of international standards as national standards where these meet the needs of the user community.

Its primary goal is the enhancement of global competitiveness of US business by promoting voluntary consensus standards. The Institute represents the interests of its nearly 1400 company, organization, government agency, institutional, and international members.

1.US National Standards

ANSI does not itself develop American National Standards (ANSs); rather it facilitates development by establishing consensus among 175 qualified groups. These groups include standards bodies dedicated to NDT (such as the American Society for Nondestructive Testing) and those who have some interest in NDT (such as the American Welding Society). In 1996 alone, the number of American National Standards increased by nearly four percent to a new total of 13 056 approved ANSs.

2 International Standards

ANSI is the sole US representative and dues paying member of the two major nontreaty international standards organizations, the International Organization for Standardization (ISO), and, through the US National Committee (USNC), the International Electrotechnical Commission (IEC). ANSI was a founding member of the ISO and plays an active role in its governance. ANSI is one of five permanent members to the governing ISO Council, and one of four permanent members of ISO's Technical Management Board.

Through ANSI, the US has immediate access to the ISO and IEC standards development. ANSI participates in almost the entire technical program of both the ISO (78 percent of all ISO technical committees) and the IEC (91 percent of all IEC technical committees) and administers many key committees and subgroups (16 percent in the ISO; 17 percent in the IEC). As part of its responsibilities as the US member body to the ISO and the IEC, ANSI accredits US Technical Advisory Groups (US TAGs) or USNC Technical Advisors (TAs). The US TAGs' (or TAs') primary purpose is to develop and transmit, through ANSI, US positions on activities and ballots of the international technical committees.

In many instances, US standards are taken forward, through ANSI or its USNC, to the ISO or IEC, where they are adopted in whole or in part as international standards. Since the work of international technical committees is carried out by volunteers from industry and government, not ANSI staff, the success of these efforts often depends upon the willingness of US industry and the US government to commit the resources required to ensure strong US technical participation in international standards.

European Committee for Standardization
The objectives of the European Committee for Standardization (CEN), Web site www.cenorm.be, are to draw up voluntary European Standards and promote corresponding conformity of products and services in areas other than electrotechnical and telecommunications. In particular, it has an agreement for technical cooperation (the Vienna Agreement) with the International Organization for Standardization (ISO). (There is controversy over whether the CEN standards are voluntary; however, the web page characterizes them as voluntary.)

The CEN members are: Austria (ON), Belgium (IBN/BIN), Czech Republic (COSMT), Denmark (DS), Finland (SFS), France (AFNOR), Germany (DIN), Greece (ELOT), Iceland (STRê), Ireland (NSAI), Italy (UNI), Luxembourg (SEE), Netherlands (NNI), Norway (NSF), Portugal (IPQ), Spain (AENOR), Sweden (SIS), Switzerland (SNV), and the United Kingdom (BSI). The CEN affiliates are: Albania (DSC); Bulgaria (CSM); Croatia (DZNM); Cyprus (CYS); Estonia (EVS); Hungary (MSZH); Latvia (Department of Quality Management and Structure Development); Lithuania (LST); Malta (Malta Standardization Authority); Poland (PKN); Romania (IRS); Slovakia (UNMS); Slovenia (SMIS); and Turkey (TSE). In addition, CEN has the following corresponding organizations: EOS (Egyptian Organization for Standardization and Quality Control); SABS (South African Bureau of Standards); DSTU (State Committee of Ukraine for Standardization, Metrology and Certification); and SZS (Yugoslavian Federal Institution for Standardization).

International Institute of Welding
The International Institute of Welding (IIW), Web site www.aws.org/iiw.htm, was founded in 1948 by the welding institutes or societies in 13 countries to promote international collaboration in welding. With 43 member countries today, the objectives of the organization are:

to promote the development of welding and to provide for the exchange of scientific and technical information on welding research and education
to assist in the formulation of international standards for welding in collaboration with ISO
to promote the organization of national welding associations.

The IIW has over 20 international groups of specialists that meet at least yearly on the invitation of one of the member countries. Besides hosting an international conference on some aspect of welding, three days are spent in parallel sessions for meeting of the various commissions and other working groups. The commissions and other working groups are organized by topic, with Commission V dedicated to quality control and quality assurance of welded products. The yearly meetings (as well as intermediate meetings) are used to stimulate research and to disseminate information on welding processes, their application (including inspection), and other associated subjects. Each year about 400 papers emanate from the IIW working units, of which some are published in the IIW journal Welding in the World, while others become books dealing with recommended practices.

These technical discussions often form the technical basis for standards, and have supplied the basis for the great majority of the welding standards issued by the ISO over the past 30 years. Members of these working units and their employers therefore have a major influence over the content of such standards. Since 1989, the IIW has been authorized by ISO to prepare the final texts of international welding standards as an international standardizing organization. The first ISO standards produced entirely by the IIW were published in 1990.

Thus, participation in the IIW offers a method to gain early access to the development of an inspection standard, before ISO balloting sends the draft standard to the TAGs in the various countries. An example of how the process works is illustrated the current efforts on the design of the IIW ultrasonic inspection block and black light inspection of weldments at elevated temperature.

Commission V became aware of variations in IIW calibration blocks and issued a resolution in 1997:

Commission V advises the other commissions that recent round robins have shown more than 12 dB variation in the sensitivity setting of IIW ultrasonic calibration blocks. Various institutes are now investigating this problem, with the goal of revising the standard (ISO 2400). Any organization interested in participating should contact Hermann Wustenberg at BAM in Berlin, Germany.

Commission V has been investigating procedures for dye penetrant inspection of hot surfaces, under the leadership of the Italian delegation. At the 1998 Assembly, F. Peri offered a draft standard for ISO and the supporting documentation. The Commission passed two resolutions:

Resolution 5: Commission V forwards Document V-1112-98 "Non-Destructive Testing - Characterization of Penetrants for Hot Surfaces in Weld Inspection" to ISO through Route 1.

Resolution 6: Commission V forwards Document V-1113-98 "Penetrants for Hot Surfaces in Welding Inspection: Experimental Work and First Results" for publication in Welding in the World.

Access to the IIW is through the American Council of the IIW at AWS.

The American Society for Nondestructive Testing

The American Society for Nondestructive Testing (ASNT), Web site www.asnt.org, plays a major role in the certification and qualification of NDT personnel by developing and maintaining Recommended Practice No. SNT-TC-1A, known throughout the world as the principal guideline for NDT personnel qualification, and ANSI/ASNT CP-189-1991, the ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel. The Society serves as the major international source for NDT Level III certification by examination; with nearly 3400 NDT professionals in 40 countries holding valid ASNT NDT Level III certificates. ASNT also offers the Industrial Radiography Radiation Safety Personnel (IRRSP) certification program, instituted in cooperation with the US Nuclear Regulatory Commission, which provides a third party national safety program for industrial radiographers. To date, more than 600 radiographers have been ASNT certified by the program. In November 1996, examinations began for ACCP, the ASNT Central Certification Program, a new, independent, portable NDT certification by examination.

In the area of publications, ASNT maintains the world's largest catalog of NDT education and reference materials, providing information on virtually every aspect of NDT. Publications produced by the Society include Materials Evaluation, the Society's archived monthly technical journal, featuring the latest NDT and ASNT news; Research in Nondestructive Evaluation, a quarterly journal publishing original research in all areas of NDT; and the Nondestructive Testing Handbook, recognized worldwide as the definitive NDT reference source, plus Level III Study Guides, the Q&A series, and many key educational materials. The Society also operates the ASNT Information Center, which provides a central archive and retrieval point for NDT related information. Literature searches and document delivery are among the Information Center's key services.

American Society for Testing and Materials
The American Society for Testing and Materials (ASTM), Web site www.astm. org, is a not-for-profit organization that provides a forum for producers, users, ultimate consumers, and others having a general interest (representatives of government and academia) to meet on common ground and write standards for materials, products, systems, and services. Organized in 1898, the American Society for Testing and Materials (ASTM) has grown into one of the largest voluntary standards development systems in the world. From the work of 132 standards writing committees, ASTM publishes standard test methods, specifications, practices, guides, classifications, and terminology. ASTM's standards development activities encompass metals, paints, plastics, textiles, petroleum, construction, energy, the environment, consumer products, medical services and devices, computerized systems, electronics, and many other areas. ASTM Headquarters has no technical research or testing facilities; such work is done voluntarily by 35 000 technically qualified ASTM members located throughout the world.

More than 10 000 ASTM standards are published each year in the 72 volumes of the Annual Book of ASTM Standards. These standards and related information are sold throughout the world.

ASTM technical committees are the specific arenas in which ASTM standards are developed. There are 132 ASTM main technical committees and each is divided into subcommittees. The subcommittee is the primary unit in ASTM's standards development system, as it comprises the highest degree of expertise in a given area. Subcommittees are further subdivided into task groups. Task group members do not have to be ASTM members; many task groups seek non-ASTM members to provide special expertise in a given area.

Most of the inspection topics are handled by Committee E-7, Nondestructive Testing. Its work is divided by topic among the following subcommittees

E07.01 Radiology (X and Gamma) Method
E07.02 Reference Radiological Images
E07.03 Liquid Penetrant and Magnetic Particle Methods
E07.04 Acoustic Emission Method
E07.05 Radiology (Neutron) Method
E07.06 Ultrasonic Method
E07.07 Electromagnetic Method
E07.08 Leak Testing Method
E07.09 Nondestructive Testing Agencies
E07.10 Emerging NDT Methods
E07.91 USA Participation in ISO TC193/
and some administrative subcommittees.

The standards produced and maintained by this committee are included in Volume 3.03, Nondestructive Testing, of the Annual Book of ASTM Standards. This volume presently contains 129 standards.

American Welding Society

One area of interest of the American Welding Society (AWS), Web site www.aws. org, is how nondestructive testing is applied in welding. Two aspects of this are through its interactions with the NDT interests in the IIW (particularly Commission V) and in the area of certification of personnel.

AWS is designated by the IIW as the main member society for the US (which includes duties on the transmission of IIW documents to the IIW Delegates and Experts in the US), and serves as the secretariat for the American Council of the IIW. The American Council of the IIW has served to bring together the various welding interests in the US, including those within both AWS and the Welding Research Council, to select the official representatives for the various IIW working parties, and to promote the transfer of information between US experts and those in other countries. R. French is the secretary of the American Council and can be reached at the e-mail address listed in the IIW Web site.

The other AWS interest is in the certification of weld inspection personnel. While much weld inspection is visual, various other technologies are also applied. AWS offers certification for the following different levels: Certified Associate Welding Inspector, Certified Welding Inspector, Senior Certified Welding Inspector, and NDT Inspector-Radiographic Interpretation.

American Society for Quality

The ISO 9000 standards apply to quality management systems. The ISO 14000 standards apply to environmental management systems. Note that these standards do not apply to products but to management systems. The QS-9000 requirements developed by the Big Three automakers are based on the ISO 9000 standards.

As the secretariat for the American National Standards Institute's (ANSI) ASC Z-1 Committee on Quality Assurance, the American Society for Quality (ASQ), Web site www.asqc.org/standcert.html, provides direction on and builds consensus for national and international standards. ASQ volunteers play key roles in developing the ISO 9000 series standards, originally adopted nationally as the Q90 series standards and recently revised and redesignated as the Q9000 series standards. They do so through their involvement in the US Technical Advisory Group for ISO Technical Committee 176, administered by ASQ on behalf of ANSI.

Following are some major companies operating in the Middle East who provide Non Destructive Testing services. Their services mostly include

X and Gamma radiography
Phased Array Ultrasonic Testing.
Time of Flight Diffraction Ultrasonic Testing.
Conventional Ultrasonic Testing.
Magnetic Particle and Dye Penetrant Testing.
Eddy Current Inspection
Tank Floor Mapping.
Positive Alloy Material Identification.
Hardness Testing.
Coating Inspection.
Vacuum Box Testing.
Pre and Post Weld Heat Treatment.
Refractory Dry Outs.
Portable and fixed furnaces.
Internal Gas Firing.

INTERTEK GLOBAL INTERNATIONAL
BUILDING NO 242 CRING ROAD
DOHA QATAR

PIH Services ME Ltd
Al Qusais, P O Box 62574,
Dubai, U.A.E

Tel : +971 4 267 99 89
Fax : +971 4 267 95 85
Email:sales@pihme.com
Web:www.pihme.com

Branch offices at Qatar, Bahrain, Oman, Abudhabi

Abudhabi Branch
P O Box 42766
Tel: +9712 555 1933
Fax: +9712 555 1966
email: sales@pihme.com

Qatar Branch
P O Box 14975
Tel: +974 4501921
Fax: +974 4501922

Oman Branch
P O Box 289, Code No 115
Madina Qaboos
Tel: +968 24595 766
Fax: +968 24595 073

Bahrain Branch
P O Box 26152
Manama
Tel: +973 1783 1120
Fax: +973 1783 0034



SIEVERT INDIA PVT LTD

Telephone No.+91 22 27727060, 27726693, 27727118
Tele fax No.+91 22 27727076
e- mail ID:sievert@mtnl.net.in, sievert@vsnl.com

Contacts
Mr. Sudhakar R Rao
Managing Director.
Mob: +91 98203 81766

Mr. Prabhakar Raghavendra
Director - Finance
Mob: +91 98200 55717

S.Venkataraman,
Director-Operations
Mob: +91 98201 30661

Qatar Branch
Post Box 24544, Doha , State of Qatar.
Telephone No.+ 974 4602421
Tele fax No.+974 4602425
e- mail ID:sievertndt@qatar.net.qa

Contacts
Mr. S.N.Moorthy
Director - Overseas Operations
GSM : +974 5843639

Mr.K.G.K.Chowdary
Country Manager
GSM: +974 5838930

Oman Branch
Post Box 851, P.Code 130, Azaiba, Sultanate of Oman.
Telephone No.(968) - 24485770, 24485710, 24480724
Tele fax No.(968) - 24485772
e- mail ID:sievert@omantel.net.om

Contacts
Mr. Sudhakar Rao
Managing Director.
GSM : 00+968 99384654

Mr. W.V.Seshagiri Rao
Country Manager
GSM : +00968 99440938


Amsyco International (NOW GIS)

Address: PO Box 22410 Doha State of Qatar, Qatar
Tel: +974 460 4550
Fax: +974 460 4054
Website: www.amsyco.com


Inspection Corrosion Engineering Services

Doha, Qatar
+974-4369953
+974-4369976
+974-4505983

Salwa
+974-4603971
+974-4505982

Fax(Qatar)
+974-4505984
+974-4311589

E-Mail: info@icesqa.com
http://www.icesqatar.com

Geometric unsharpness refers to the loss of definition that is the result of geometric factors of the radiographic equipment and setup. It occurs because the radiation does not originate from a single point but rather over an area. Consider the images above which show two sources of different sizes, the paths of the radiation from each edge of the source to each edge of the feature of the sample, the locations where this radiation will expose the film and the density profile across the film. In the first image, the radiation originates at a very small source.

Since all of the radiation originates from basically the same point, very little geometric unsharpness is produced in the image. In the second image, the source size is larger and the different paths that the rays of radiation can take from their point of origin in the source causes the edges of the notch to be less defined.

The three factors controlling unsharpness are source size, source to object distance, and object to detector distance. The source size is obtained by referencing manufacturers specifications for a given X-ray or gamma ray source. Industrial x-ray tubes often have focal spot sizes of 1.5 mm squared but microfocus systems have spot sizes in the 30 micron range. As the source size decreases, the geometric unsharpness also decreases. For a given size source, the unsharpness can also be decreased by increasing the source to object distance, but this comes with a reduction in radiation intensity.

The object to detector distance is usually kept as small as possible to help minimize unsharpness. However, there are situations, such as when using geometric enlargement, when the object is separated from the detector, which will reduce the definition.

Codes and standards used in industrial radiography require that geometric unsharpness be limited. In general, the allowable amount is 1/100 of the material thickness up to a maximum of 0.040 inch. These values refer to the degree of penumbra shadow in a radiographic image. Since the penumbra is not nearly as well defined as shown in the image to the right, it is difficult to measure it in a radiograph. Therefore it is typically calculated. The source size must be obtained from the equipment manufacturer or measured. Then the unsharpness can be calculated using measurements made of the setup.

Formula for calculating the geometric unsharpness

Ug = Ft/d

Ug = geometric unsharpness
F = size of source
t = Specimen thickness at point of contact with film.
(Object - film distance)
d = Source to object distance.

The purpose of an IQI is to indicate the overall sensitivity of the technique and as a measure of how well the radiograph will reveal discontinuities. As the name implies, an IQI is an indicator of the quality of the radiographic image. An IQI is a device made from the same material as the test specimen. It is placed on the test specimen in a position where its image will be recorded on the radiograph.


The quality of a radiographic image can be assessed in terms of three factors:

1. Image sharpness. Usually, in radiography, the inverse of sharpness - unsharpness or blurring - is used.
2. Image contrast. The density change on a film for a given thickness change in the specimen. If a small image detail shows only faintly, this is a low contrast image; if the detail is easily seen, this is a higher contrast image.
3. Image Noise. For radiography-on-film, this is effectively graininess. In radioscopy, there are additional features affecting image noise.

A good design of IQI should be able to show changes in all these three factors.

There are four broad patterns of IQI in use today - the wire type, the step/hole type, the hole-in-plaque type and the duplex wire type.

Probably the most widely used IQI in radiography-on-film is the wire type. Various patterns of wire type IQIs are described in German, Scandinavian, British, Japanese, Chinese, American CEN and ISO Standards. These consist of a series of straight wires of the same or similar material to the specimen, the wires being of different diameters taken from the series 0.10, 0.125, 0.16, 0.20, 0.25, 0.32, 0.40, 0.50, 0.63, 0.80, 1.00. The wires may be 10, 20, 30, 50mm according to the various Standards and are held parallel to one another in a low density plastic mount, with appropriate identification symbols.

Placement of IQIs
Standard practice is to place the IQI on the side of the specimen facing the radiation source, on the grounds that this is the region of the image where geometric unsharpness and therefore image blurring, will be the largest. Most tables of IQI values assume that the IQI is in this position.

In the case of double-wall/single-image pipe weld radiography, where there is no access to the inside of the pipe, the only practical place for the IQI is on the film side of the weld, that is, effectively on the film cassette. If the IQI is used in this position, a lead identification letter (F or FS) should be placed on the IQI. In this position, the IQI image is completely insensitive to geometric unsharpness factors such as source-to-film distance or source diameter and is an indication of contrast only. Normally, the IQI readings are 1 - 2 wires or holes better than with the IQI on the source side of the same specimen. Some Standards ask for a trial exposure on a sample piece of pipe, with IQIs on both source and film sides of the weld, for comparison. Some experts regard the IQI reading from the film side position as misleading and of little practical value, but tables of IQI sensitivities for the IQI on the film side are included in EN 1453:1997 and EN 462-3:1997.

The smallest element readily visible in the area under inspection is used to determine IQI sensitivity

For the wire type IQI:

sensitivity % equals diamter of smallest wire divided by thickness of metal times 100

For the step-hole IQI:

sensitivity % equals diamter of smallest hole divided by thickness of metal times 100

Selecting the correct IQI is important

Typical radiographic sensitivities range from around 1% to 4%, depending on:

the thickness of the item being radiographed
the type (energy) of the radiation
the type of film
other factors discussed in the previous task.

In some cases, a code, specification or customer order will state the required sensitivity.

Selecting the correct IQI involves:

Selecting the correct metal (steel, aluminium, copper).
Selecting the correct type, again normally specified in the code or specification.
Selecting the correct model, i.e. the model with an element that will correspond with the required sensitivity. Hence, if radiographing 20 mm steel for a 2% sensitivity, with the wire type IQI, the model with a wire closest to 0.4 mm (2% of 20 mm) is required. This is wire number 10, so the 6 to 12 wire type IQI is required. If the step hole type is specified, then the model with a 0.4 mm step is required. This is the 1 to 6 model.


An example of IQI selection as per client requirement in radiography (Qatar Petroleum requirement)

Image Quality Indicators

Image quality indicators conforming to ASTM E747 shall be employed to verify the radiographic sensitivity achieved. Subject to clients approval prior to commencement of work. Selection of indicator shall be by the sensitivity requirement for the material thickness under examination and shall be compatible with material involved e.g. Steel for steel etc.

The image quality indicator shall be selected for single wall thickness plus maximum permissible reinforcement next to the film without regard to backing strips or rings. The image quality indicator shall be located on the source side of weld under examination however where not practical then indicator shall be placed on film side with identifying lead marker "F" placed adjacent to the indicator. Dimensions of marker shall be similar to those of identification markers. Image quality indicator shall be placed at 90 deg. to the weld axis at the furthest point of radiographic interest on radiograph except where panoramic exposure technique is utilized; a minimum of three shall be placed equidistant around the circumference of the weld.


Some Excercises on IQI

1. What model wire type IQI would you select in the following cases?
25 mm thick plate where the weld has 3 mm reinforcement and a 2% sensitivity is specified.
40 mm thick casting where a 2.5% sensitivity is specified.

2. Determine the IQI sensitivity in the following cases:

in a weld in a 20 mm thick plate, number 10 is visible in the parent metal, and wire number 8 is visible in the weld. The total weld thickness is 24 mm.

in a 40 mm thick casting, wire number 5 is visible in the area being examined.

Gamma radiation sources, most commonly Iridium-192 and Cobalt-60, are used to inspect a variety of materials. The vast majority of radiography concerns the testing and grading of welds on pressurized piping, pressure vessels, high-capacity storage containers, pipelines, and some structural welds. Other tested materials include concrete (locating rebar or conduit), welder's test coupons, machined parts, plate metal, or pipewall (locating anomalies due to corrosion or mechanical damage). Theoretically, industrial radiographers could radiograph any solid, flat material (walls, ceilings, floors, square or rectangular containers) or any hollow cylindrical or spherical object.

For purposes of inspection, including weld inspection, there exist several exposure arrangements.

First, there is the panoramic, one of the four single wall exposure/single image (SWE/SI) arrangements. This exposure is created when the radiographer places the source of radiation at the center of a sphere, cone, or cylinder (including tanks, vessels, and piping). Depending upon client requirements, the radiographer would then place film cassettes on the outside of the surface to be examined. This exposure arrangement is ideal - when properly arranged and exposed, all portions of all exposed film will be of the same approximate density. It also has the advantage of taking less time than other arrangements since the source must only penetrate the total wall thickness (WT) once and must only travel the radius of the inspection item, not its full diameter. The major disadvantage of the panoramic is that it may be impractical to reach the center of the item (enclosed pipe) or the source may be too weak to perform in this arrangement (large vessels or tanks).

The second SWE/SI arrangement is an interior placement of the source in an enclosed inspection item without having the source centered up. The source does not come in direct contact with the item, but is placed a distance away, depending on client requirements. The third is an exterior placement with similar characteristics. The fifth is reserved for flat objects, such as plate metal, and is also radiographed without the source coming in direct contact with the item. In each case, the radiographic film is located on the opposite side of the inspection item from the source. In all four cases, only one wall is exposed, and only one wall is viewed on the radiograph.






Of the other exposure arrangements, only the contact shot has the source located on the inspection item. This type of radiograph will expose both walls, but will only resolve the image of that wall which is nearest the film. This exposure arrangement takes more time than a panoramic, as the source must now penetrate the WT twice and travel the entire outside diameter of the pipe or vessel to reach the film on the opposite side. This is a double wall exposure/single image DWE/SI arrangement.



Another is the superimposure (wherein the source is placed on one side of the item, not in direct contact with it, with the film on the opposite side). This arrangement is usually reserved for very small diameter piping or parts. The last DWE/SI exposure arrangement is the elliptical, in which the source is offset from the plane of the inspection item (usually a weld in pipe) and the elliptical image of the weld furthest from the source is cast onto the film.


























The beam of radiation must be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam. The length of weld under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.

The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of radiation reaching each area. It is known as a radiograph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a negative, without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served.

Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface irregularities (which will be visible on the radiograph) may make detecting internal defects difficult.

After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, which is important both for the setting up of the equipment and for the choice of the most appropriate technique.

Non-destructive testing (NDT) is an analysis technique used in scientific fields to determine the state or function of a system without the use of invasive approaches like disassembly or failure testing. Because NDT does not require the disabling or sacrifice of the system of interest, it is a highly-valuable technique that saves both money and time in product evaluation, troubleshooting, and research. Common NDT methods include acoustic testing, liquid penetrant testing, and radiographic testing. NDT can be used with any isolated input / output system, and is a commonly-used tool in electrical engineering, civil engineering, systems engineering, and medicine.

The need for NDT

It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.

During their service lives, many industrial components need regular non-destructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:

Aircraft skins need regular checking to detect cracks;
Underground pipelines are subject to corrosion and stress corrosion cracking;
Pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
Reinforced concrete structures may be weakened if the inner reinforcing steel is corroded;
Pressure vessels may develop cracks in welds;
The wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.

Finished machined parts, such as bearings, that have newly been assembled can be tested for missing pieces, such as a ball or roller bearing, or grease within the housing non-destructively with a checkweigher. A roller motor for a conveyor can be tested for the proper level of oil, without disassembling the finished product. Thousand of manufactured products can benefit from this form of testing.

Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique.

Use of X-rays for NDT is a common way of examining the interior of products for voids and defects, although some skill is needed in using radiography to examine samples and interpret the results. Soft X-rays are needed for examining low density material like polymers, composites and ceramics.