RADIOGRAPHY

OBJECTIVES

To: • understand the principle of radiographic

testing methods • Know the technique of testing • Become familiar with standards & codes • Learn the applications.

INTRODUCTION

Radiography:

• one of the oldest & the most widely used NDT which uses X-rays or -rays radiation to examine the interior of the materials.

• gives a permanent film record of defects that is easy to interpret.

• applied for assessing the quality of the welded joints also.

• can detect flaws or discontinuities in welds such as cracks, porosity & blow holes, slag, flux or oxide inclusions, lack of fusion between the weld metal & the parent metal, incomplete penetration, tungsten inclusion, etc.

PROPERTIES OF X- RAYS & - RAYS

X- rays are highly penetrating electromagnetic radiations of wave length shorter than UV-rays (X- rays: 5 to 0.0004A, - rays: 0.1 to 0.005A)

These rays have the following properties: • Invisible electromagnetic radiations. • Can penetrate matter. Penetration is less if density of

matter is more and thickness is more • Are differentially absorbed. • Travel in straight lines. • Produce photochemical effects on films. • May be refracted, reflected and diffracted • Damage living tissues • Ionise gases through which it pass.

PRINCIPLE

• Both X-rays & -rays are very short wave length electro magnetic radiation which penetrate opaque materials & obtain a permanent record of the result on sensitized film.

• When these rays pass through a material having defects, the rays passing through the less denser parts of the object are absorbed to a smaller extent than the rays passing through the adjacent sound material (because of difference in density & thickness of the test piece) & are allowed to fall on a light sensitive film placed on the back side of the object to be radiographed.

Contd-

• After exposure for the prescribed time, the film is developed.

• The contrast (difference in density) on the developed film between the image of an area containing a defect & the image of a defect free area of the specimen permits the observer to distinguish the flaw.

• The film is called exograph, if X-rays are used &

gamma ray graph, if -rays are used.

• Both types of film are called radiograph (photographic record of defects)

GENERATION OF X-RAYS

• X-rays are produced by an X-ray tube.

• An X- ray tube is an evacuated tube, usually made of glass, containing an electrically heated filament which liberates electrons & a tungsten anode.

• The electrons liberated from the heated filament are made to impact on the anode with the help of large PD (50 to 2000KV) between the filament & the anode.

• X-rays are emitted due to the impact of high velocity electrons on the anode.

Contd----- • Only about 1% of the total electron energy is converted

into X-rays & the rest of it transforms into heat due to which the anode is heated to a very high temp & a device to cool the anode by water or oil is necessary.

• The X-rays come out of the tube through a window in the form of a beam.

• The intensity of X-rays is directly proportional to the filament current & the wave length is inversely proportional to the voltage between the cathode & anode.

• Both the voltage & the current can be varied due to which the tube may be adjusted over a wide range of operating characteristics depending upon the material to be radiographed.

GENERATION OF X-RAYS

METHOD OF TESTING • The X-ray tube, components to be tested & film are set

up as shown in the fig • The X-rays are allowed to fall upon the test piece • A cassette containing film is placed behind & in contact

with the component lar to the rays. • Since most defects possess lesser density than the

sound parent metal, they transmit X-rays or -rays better than the sound metal does, therefore the film appears to be more dark where defects are in line of the beam.

• Usually a penetrameter is placed on the side of the source adjacent to the weld.

• Thickness of the penetrameter is usually 2% of the thickness of the weld joint.

MARKING OF RADIOGRAPHIC LOCATION, DIRECTION OF WELD BEAD AND PLACING OF PENETROMETER

WELDED COMPONENT

SETTING UP OF CASSETTE

X-RAY MACHINE

RADIOGRAPHY TESTING

RADIOGRAPHY TESTING

GENERATION OF - RAYS

rays, are produced by a radioactive isotope (Radium, Radon, Cobalt, Iridium, Cesium, Thalium, Europium, Cerium, Xenon, Tantalum, etc).

• The gamma ray source (300 mg) is housed in a lead or tungsten alloy container of sufficient thickness to provide necessary protection.

VARIOUS TYPES OF ISOTOPES FOR RADIOGRAPHY ( - RAYS)

Co 60 : This has a half-life of 5.3 years. This is used for radiography of thicker sections of steel (between 2 to 6”).

Ir192 : This has a half-life of 72 days. This is used for thin section. Ir192 will give better radiographic sensitivity than Co 60 .

Cs137 : This half-life of 30 years. Suitable for radiographic examination of steel sections of 1.5 to 2.5 inches.

- RAYS

- RAYS TESTING

PRECAUTION

• Every radiation worker and area require some radiation measurment method in order to have safty aspect

• There are two type of radiation monitoring instrument

– Personnel monitoring

– Lab /area monitoring

• Portable unit measuring range upto 20 mr /hr

• TLD – Thermo Luminenscec Dosimeter

• Film Badge Reader.

RADIOGRAPHIC FILM

• Basically X-ray film is made of:

– Base: a transparent blue tinted cellulose derivated

– Emulsion: consists of gelatin containing microscopic, radiation sensitive silver halide crystals such as silver bromide & iodide

• Usually, the emulsion is coated on both sides of the base in layers about 0.0005 inch thick. Putting emulsion on both sides of the base doubles the amount of radiation-sensitive silver halide & thus increases the film speed.

RADIOGRAPHIC FILM

• The emulsion layers are thin enough so developing, fixing & drying can be accomplished in a reasonable time.

• The emulsion is very sensitive & when x-ray, -ray strikes it, a change takes place in its physical structure.

• The change is of a nature that it cannot be detected by ordinary physical method. However, when the exposed film is processed

FILM PROCESSING • When the film is processed, it is exposed to several

different chemicals solutions for controlled periods of time. Processing film basically involves the following four steps. – Development - The developing agent gives up

electrons to convert the silver halide grains to metallic silver. Grains that have been exposed to the radiation develop more rapidly, but given enough time the developer will convert all the silver ions into silver metal. Wash the developer away with water

– Fixing - Unexposed silver halide crystals are removed by the fixing bath. The fixer dissolves only silver halide crystals, leaving the silver metal behind.

Contd-----FILM PROCESSING

– Washing - The film is washed with water to remove all the processing chemicals.

– Drying - The film is dried for viewing.

• Processing film is a strict science governed by rigid rules of chemical concentration, temperature, time, and physical movement.

FILM HANDLING X-ray film should always be handled carefully to avoid

physical strains, such as pressure, creasing, buckling, friction, etc. Whenever films are loaded in semi-flexible holders and external clamping devices are used, care should be taken to be sure pressure is uniform.

• Marks resulting from contact with fingers that are moist or contaminated with processing chemicals, as well as crimp marks, are avoided if large films are always grasped by the edges and allowed to hang free.

• Clean towels should be kept close at hand to dry the hands.

• Another important precaution is to avoid drawing film rapidly from cartons, exposure holders, or cassettes.

VIEWING RADIOGRAPHS

• Radiographs (developed film exposed to x-ray or gamma radiation) are generally viewed on a light-box.

• Because the energy of electromagnetic radiation is directly related to their frequency, X rays are much more energetic and penetrating than light waves as well.

INTERPRETATION OF RADIOGRAPH

For successful interpretation of a radiograph, one should have as wide a knowledge of the form of images characteristics or various defects. A radiograph contains light & dark areas. The latter representing parts of the material having a lower density. Various defects appear on the negatives of a radiographs as follows:-

• Gas cavities & blow holes appear as well defined circular dark areas

• Shrinkage cavity appears as a fibrous irregular dark region having an indistinct outline

INTERPRETATION OF RADIOGRAPH

• Cracks are indicated by darkened areas of variable width

• Sand inclusions appear as grey or black spots of an uneven or granular texture with indistinct boundaries.

• Inclusions in castings appear as dark spots of definite outline. In light alloys, the inclusions may be of high density & thus cause light spots.

CONDITION FOR GOOD RADIOGRAPHY

• The distance between the source and the material under test should be always as great as in practical.

• The film should be as close as possible to the object being radiographed.

• The central rays should be as nearly perpendicular to the film as possible to minimize distortion.

ADVANTAGE & DISADVANTAGE OF X-RAY -RAY

• Advantages:

• Disadvantages:

X-ray - ray •Exposure times much shorter •More sensitive & is better on extremely thin sections.

•suitable for out door •Portable because the source is small. •Very suitable for out door work & in confined spaces, since it requires neither electric power nor water supply for operation.

X-ray - ray •Requires trained operator •Involves radiation hazards

•Requires trained operator •Requires more exposure time due to lower sensitivity which involves more radiation hazards •involves radiation hazards •source loses strength continuously Trained operator is required

RADIOGRAPHY TESTING

Neutron radiography:- It is used to solve complex problem that can not be

solved by X-rays or Y-rays. It is used for detecting internal cracks of very light materials like plastic, rubber components, etc.