Radiographic Testing Techniques A focus on X-Ray and Gamma ray testing principles

Introduction

• In general, Radiographic Testing is method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials.

• The major distinction between X-rays and gamma rays is the origin where X-rays are usually artificially produced using an X-ray generator and gamma radiation is the product of radioactive materials.

Basic principle of radiation testing

Properties of X-Rays and Gamma Rays • They are not detected by human senses (cannot be seen,

heard, felt, etc.).

• They travel in straight lines at the speed of light.

• Their paths cannot be changed by electrical or magnetic fields.

• They can be diffracted, refracted to a small degree at interfaces between two different materials, and in some cases be reflected.

• They pass through matter until they have a chance to encounter with an atomic particle.

• Their degree of penetration depends on their energy and the matter they are travelling through.

• They have enough energy to ionize matter and can damage or destroy living cells.

Advantages of Radiation testing techniques • Both surface and internal discontinuities can be detected.

• Significant variations in composition can be detected.

• It has a very few material limitations.

• Can be used for inspecting hidden areas (direct access to surface is not required)

• Very minimal or no part preparation is required.

• Permanent test record is obtained.

• Good portability especially for gamma-ray sources.

Disadvantages of exposure to radiation • Hazardous to operators and other nearby personnel.

• High degree of skill and experience is required for exposure and interpretation.

• The equipment is relatively expensive (especially for x-ray sources).

• The process is generally slow.

• Highly directional (sensitive to flaw orientation).

• Depth of discontinuity is not indicated.

• It requires a two-sided access to the component.

X- Ray Non Destructive testing

• Radon mathematically showed that it‍ is possible to calculate the density‍ distribution of an object from its x-ray‍ projection.

• Based on this principle, 2D-tomography‍ was developed and currently used for NDT testing of equipments.

• 2D-tomography‍ reconstructs the attenuation values of‍ single planes inside the object.

PFD of a typical X-Ray RT setup

2-D Tomography

• A large electrical potential is created between the cathode and the anode by the high-voltage generator. Electrons that break free of the cathode are strongly attracted to the anode target. The stream of electrons between the cathode and the anode is the tube current.

• A focusing cup is used to concentrate the stream of electrons to a small area of the target called the “focal spot”

• In addition, X-ray generators usually have a filter along the beam path (placed at or near the x-ray port). Filters consist of a thin sheet of material (often high atomic number materials such as lead, copper, or brass) placed in the useful beam to modify the spatial distribution of the beam

Tomographic films construction • X-ray films for general radiography basically consist of an

emulsion-gelatin containing radiation-sensitive silver halide crystals.

• The emulsion is usually coated on both sides of a flexible, transparent, blue-tinted base in layers about 0.012 mm thick.

• Additionally, the film is placed between two sheets of intensifying material, which enhance the radiation emitted.

• This material in sheet form is called an intensifying screen or simply a screen.

• By intensifying the radiation, exposure time can be reduced.

Extracting image from the film

• When X-rays, gamma rays or light rays strike the crystals or grains, some of the Br- ions are liberated leaving the Ag+ ions.

• In this condition, the radiograph is said to contain a latent (hidden) image because the change in the grains is virtually undetectable, but the exposed grains are now more sensitive to reaction with the developer.

• The image is developed from the film in a similar way to how a photographic image is developed from a negative.

A Sample of 2-D Computed Tomography(CT)

Tomogram of a plate of carbon fibre reinforced‍ plastic after impact damage

3-D Tomography

• In 3D x-ray-computed tomography (3D-CT), not only one plane but also the volume of an object‍ is reconstructed using the 2-dimensional (2D) radiographic projections of the object.

• For 3D-CT,‍ typically cone beam geometry is used.

• The object is rotated in a x-ray cone beam‍ produced by a x-ray tube to generate the 3D tomogram.

A sample of 3-D CT

A 3-D Image of a battery cell constructed by 3-D CT.

Gamma Ray Testing

• Manmade radioactive sources are produced by introducing an extra neutron to atoms of the source material.

• Two of the most common industrial gamma-ray sources for industrial radiography are Iridium-192 and Cobalt-60.

• Because the radiation emitted by a gamma-ray source cannot be varied in quality there is a limited range of material thickness for which each source can produce satisfactory radiographs

Gamma Ray Testing Thickness ranges in steel for which gamma-ray sources will produce

satisfactory results

Source Thickness(approx.), mm

Iridium 192 10-75

Cobalt-60 50-150

Caesium-137 25-100

Caesium-134 35-100

Thulium-170 5-10

Ytterbium-169 2-10

Selenium-75 4-35

Process of Testing

• 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

• 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%.

Gamma Ray Testing

• The specimen to be inspected is placed between the source of radiation and the detecting device and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.

• The radiograph is examined as a negative because, in printing, some of the detail is always lost and no useful purpose is served.

Applications of Radiographic testing in Equipment Design • 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.

Radiographs

Following is required to understand a radiograph:

• A sound basic knowledge of all aspects of the specimen being examined and the types of defects likely to arise in it.

• A knowledge of the basic principles of the radiographic technique used interpretation cannot be left to non-radiographic personnel.

• it is necessary to have information in advance concerning the types of defects or other abnormalities likely to be encountered.

Viewing a radiograph

• Films should be handled with care and viewed correctly so as to determine precisely the various defects in equipments in various stages.

• A radio graph is examined by placing it on an illuminated viewing screen of appropriate luminance.

• For the examination of small areas of film of high density, a Photoflood bulb in a box with a restricted aperture on the front can be used.

• The radiograph may be a few centimetres from the glass of the bulb and the film is moved slowly across the aperture

Weld Verification

• Welds are commonly used to join two or more metal parts.

• There is a chance that welding may fail if not created to proper specification. For example, the base metal must reach a certain temperature during the welding process, must cool at a specific rate, and must be welded with compatible materials or the joint may not be strong enough to hold the parts together, or cracks may form in the weld causing it to fail.

• Typical welding defects are lack of fusion of the weld to the base metal, cracks or porosity inside the weld, and variations in weld density.

Weld Verification(cont.)

• Welds may be tested using NDT techniques such as industrial radiography or industrial CT scanning using X-rays or gamma rays, ultrasonic testing, liquid penetrant testing, magnetic particle inspection or via eddy current.

• 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 or pipe wall (locating anomalies due to corrosion or mechanical damage).

Weld Verification(cont.)

• The most widely used welding process to which radiographic inspection is applied is the metallic-arc fusion butt weld.

• Welds form the cores of equipment design with each weld contributing to hold up the equipment.

Weld Verification(cont.)

• Using of radiography to determine defects in welds becomes crucial for proper maintenance of the equipment and to compute the falling efficiency of the yield with time.

• This gives us insights to the supports/girders that the equipment might require at some point in the future.

Surface defects

•Undercutting

• Incompletely-filled groove

• Dressing, chipping, grinding marks

• Spatter

• Excessive underbead (excessive penetration)

• Shrinkage groove

• Overlap: an overflow of weld metal on to the surface of the parent

• Metal

• Misalignment

• Crater pipe: due to shrinkage at the end of a weld run.

Surface defects

Undercutting

Excessive Underbead

Internal defects

a) Blowhole: a large cavity due to entrapped gas. This term is conventionally applied to cavities exceeding 1 mm diameter, and such cavities are easily detected by radiography.

b) Gas pore: a small cavity due to entrapped gas. Similarly, this term is conventionally applied to cavities less than 1 mm diameter.

c) Porosity: a group of gas pores, often lying in strings, i.e. linear porosity.

d) Pipe or worm hole: this is an elongated cavity, and is frequently end-on to the X-ray beam, so that the image is circular.

Internal defects

e) Inclusion: slag or other foreign matter trapped during welding. These occur as isolated inclusions, clusters of inclusions, or lines of inclusions.

f) Incomplete root penetration

g) Lack of sidewall fusion

h) Cracks

i) Undercutting

j) Shrinkage grooves

k) Misalignment

Applications of Radiographic testing in Equipment Design

Casting

• X-raying of cast products has been used primarily as a Quality Assurance tool to inspect castings for various anomalies like shrink and porosity.

Defects in Cast components

• Microporosity : very small cavities occur, usually around the grain boundaries. This defect is common in aluminium alloys.

• Porosity : individual pores less than about 1 mm diameter are present.

• Sponginess : there is a system of interconnected small cavities.

• Airlock : sometimes air is trapped in the casting mould by the inflowing molten metal.

• Shrinkage : cavities caused by contraction of metal. They are usually extensive cavities and their form depends on the metal thickness and the rate of cooling

Defects in Cast components

• Cracks: These occur as hot tears or stress cracks.

• Cold shut : formed when two streams of molten metal fail to unite.

• Unfused chaplets: Chaplets are devices for maintaining the position of mould cores. Sometimes does not fuse properly and sometimes produces gas and inclusions.

• Segregations : This is a condition resulting from local concentrations of any of the constituents of an alloy. It may be general, localized or banded

Applications of Radiographic testing in Equipment Design

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