Radiographic Testing

Radiographic testing

Industrial radiography is used for a variety of applications but is commonly performed using two different sources of radiation, X-Ray and Gamma ray sources. The choice of radiation sources and their strength depends on a variety of factors including size of the component and the material thickness. Within the broad group of X-Ray and Gamma ray sources are a variety of camera choices with varying radiation strengths.

Super Quality Services X-Ray capabilities run the gamut from 10 MeV units utilized to radiograph extremely large and thick castings and forgings, to portable X-Ray cameras used for field weld applications and thin wall material inspection.Gamma sources Iridium (Ir192) used for a variety of weld inspections and Cobalt (Co 60) inspections for thick component testing.

In radiography, the process to produce an image is quite different. The camera is actually a radiation source and it operates quite differently than a photographic camera. The film is not placed inside the camera but instead is placed on the opposite side of the object being imaged.

The radiation is not reflected to the film, but rather passes through the object and then strikes the film. The image on the film is dependent upon how much of the radiation makes it through the object and to the film. Some materials like bone and metal stop more of the radiation from passing through than do materials like flesh and plastic. In NDT, radiography is one of the most important and widely used methods. Radiographic testing (RT) offers a number of advantages over other NDT methods, however, one of its major disadvantages is the health risk associated with the radiation.Both X-rays and gamma rays are electromagnetic waves and on the electromagnetic spectrum they occupy frequency ranges that are higher than ultraviolet radiation. In terms of frequency, gamma rays generally have higher frequencies than X-rays.

Radiographic testing
Radiographic testing

The major difference 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.
Radiographic testing (RT) 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 intensity of the radiation that penetrates and passes through the material is either captured by a radiation sensitive film (Film Radiography) or by a planer array of radiation sensitive sensors (Real-time Radiography). Film radiography is the oldest approach, yet it is still the most widely used in NDT.In radiographic testing, the part to be inspected is placed between the radiation source and a piece of radiation sensitive film.

Types of Radiation Sources

Radiation sources used in Industrial radiography

  • An X- ray machine
  • A radioactive source
    • Ir-192,
    • Co-60,
    • Cs-137

The part will stop some of the radiation where thicker and more dense areas will stop more of the radiation. The radiation that passes through the part will expose the film and forms a shadow graph of the part. The film darkness (density) will vary with the amount of radiation reaching the film through the test object where darker areas indicate more exposure (higher radiation intensity) and liter areas indicate less exposure (higher radiation intensity). This variation in the image darkness can be used to determine thickness or composition of material and would also reveal the presence of any flaws or discontinuities inside the material.

The usual objective in radiography is to produce an image showing the highest amount of detail possible. This requires careful control of a number of different variables that can affect image quality. Radiographic sensitivity is a measure of the quality of an image in terms of the smallest detail or discontinuity that may be detected. Radiographic sensitivity is depends on the contrast and the definition of the image.

Properties of X-rays

  • They have no effect on the human senses
  • They have adverse effect on the body tissues and blood
  • They penetrate matter
  • They move in straight line
  • They are part of electromagnetic spectrum
  • They travel at the speed of light
  • They obey the inverse square law
  • They ionize gases
  • They may be scattered
  • They make certain materials fluoresce
  • They may be refracted, diffracted and polarized

Gamma rays:

Gamma rays are electromagnetic radiation emitted by the disintegration of a radioactive isotope and have energy from about 100 keV to well over 1 MeV. The most useful gamma emitting radioactive isotopes for radiological purposes are found to be

  • Cobalt (Co60)
  • Iridium (Ir192)
  • Cesium (Cs137)
  • Thulium (Tm170).

Properties of Gamma¬rays

  • Gamma rays are emitted from artificial radio active isotope
  • Radioactive isotope is an unstable state of element which has different number of neutrons to the normal state of the same element
  • The radioactive isotope disintegrate continuously releasing electromagnetic energy (gamma rays)
  • Gamma ray sources are usually disc, cylindrical or spherical shape

Radiographic Techniques

  • Singe Wall Single Image
  • Double Wall Single Image
  • Double Wall Double Image
  • Panoramic

Isotope Decay Rate (Decay of the Gamma Source)

Loss of activity of a radioactive nuclease due to Disintegration

Isotope Decay Rate / Decay of the Gamma Source

Darker film zones:

  • Cracks, slag, porosity
  • Incomplete joint penetration

Lighter film zones:

  • Tungsten inclusions
  • Melt through, reinforcement

Radiographic Contrast

Radiographic contrast describes the differences in photographic density in a radiograph.

Definition

Radiographic definition is the abruptness of change in going from one density to another. There are a number of geometric factors of the X-ray equipment and the radiographic setup that have an effect on definition.

Radiographic Contrast
Radiographic Contrast

Sensitivity

The ability of the radiographic technique to detect the smallest possible defect. Sensitivity is measured by using Image Quality Indicators (IQI) also called as Penetrameters.Sensitivity depends on :

  • Radiographic contrast
  • Density

Controlling Radiographic Quality

One of the methods of controlling the quality of a radiograph is through the use of image quality indicators (IQI). IQIs provide a means of visually informing the film interpreter of the contrast sensitivity and definition of the radiograph.

  • Hole-Type IQIs
  • Wire-Type IQIs
Controlling Radiographic Quality
Controlling Radiographic Quality

Film Processing

Processing film is a strict science governed by rigid rules of chemical concentration, temperature, time, and physical movement. Whether processing is done by hand or automatically by machine, excellent radiographs require the highest possible degree of consistency and quality control.

As mentioned previously, radiographic film consists of a transparent, blue-tinted base coated on both sides with an emulsion. The emulsion consists of gelatin containing microscopic, radiation sensitive silver halide crystals, such as silver bromide and silver chloride.

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.

When the film is processed, it is exposed to several different chemical solutions for controlled periods of time.

Film processing basically involves the following five steps:

  • Development
  • Stopping the development (Stop bath)
  • Fixing
  • Washing
  • Drying

After the film processing, radiographs are viewed using a light-box (or they can be digitized and viewed on a high resolution monitor) in order to be interpreted.

Radiation Measures

There are four measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays. These measures are: Exposure, Dose, Dose Equivalent, and Dose Rate.The types of radiation used in industrial radiography, one roentgen equals one rad and since the quality factor for x- and gamma rays is one, radiographers can consider the roentgen, rad, and rem to be equal in value.

Exposure :

Exposure is a measure of the strength of a radiation field at some point in air. This is the measure made by a survey meter. The most commonly used unit of exposure is the roentgen (R).

Dose or Absorbed Dose :

Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter. In other words, the dose is the amount of radiation absorbed by and object. The SI unit for absorbed dose is the gray (Gy), but the “rad” (Radiation Absorbed Dose) is commonly used. 1 rad is equivalent to 0.01 Gy. Different materials that receive the same exposure may not absorb the same amount of radiation. In human tissue, one Roentgen of gamma radiation exposure results in about one rad of absorbed dose.

Dose Equivalent :

The dose equivalent relates the absorbed dose to the biological effect of that dose. The absorbed dose of specific types of radiation is multiplied by a "quality factor" to arrive at the dose equivalent. The SI unit is the sievert (SV), but the rem is commonly used. Rem is an acronym for "roentgen equivalent in man." One rem is equivalent to 0.01 SV. When exposed to X- or Gamma radiation, the quality factor is 1.

Dose Rate:

The dose rate is a measure of how fast a radiation dose is being received. Dose rate is usually presented in terms of R/hour, mR/hour, rem/hour, mrem/hour, etc.

Personnel Monitoring Devices

Film Badges :

Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. The detector is, as the name implies, a piece of radiation sensitive film. The film is packaged in a light proof, vapor proof envelope preventing light, moisture or chemical vapors from affecting the film.

Dosimeter :

Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays. As the name implies, they are commonly worn in the pocket. The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter.

Survey meter :

The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation. A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter.

Alarming Dosimeter :

Audible alarms are devices that emit a short "beep" or "chirp" when a predetermined exposure has been received. It is required that these electronic devices be worn by an individual working with gamma emitters. These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount. Typical alarm rate meters will begin sounding in areas of 450-500 mR/h. It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters.

Advantages of Radiographic Testing

  • It can be used to inspect large areas at one time.
  • It is useful on wide variety of materials.
  • It can be used for checking internal microstructure, misassemble or misalignment.
  • It provides permanent record.
  • No calibration needed on the job site.
  • Devices for checking the quality of radiograph are available.
  • Interpretation of radiographs can be done in comfortable conditions.

Some of the limitations of this Methods are :

  • X rays and gamma rays are hazardous to human health. The IAEA’s Radiation Safety Series are referred for personal safety and radiation protection.
  • It cannot detect planar defects readily.
  • Access to both sides of the specimen is required.
  • Thickness range that can be inspected is limited.
  • Certain areas in many items cannot be radiographed because of the geometric consideration.
  • Sensitivity of inspection decreases with thickness of the test specimen.
  • It is more costly.
  • It cannot be easily automated.
  • It requires considerable skill for the interpretation of the radiographs.
  • Depth of discontinuity not indicated.

Applications

Radiographic Testing is widely used in the :

  • Aerospace industries
  • Military defence
  • Offshore industries
  • Marine industries
  • Power-gen industries
  • Petrochem industries
  • Waste Management
  • Automotive industries
  • Manufacturing industries
  • Transport industries

Dowload Sample Report and Procedure of Radiographic Testing

Radiographic Testing NDT Sample Procedure
Radiographic Testing NDT Sample Test Report Format

This course prepares a candidate to :

  • Setup and calibrate equipment
  • Interpret and Evaluate Results with respect to Applicable Codes, Standards and Specifications.
  • Familiar with the scope and limitations of the Methods
  • Write test reports

Responsibilities of ASNT Level II RADIOGRAPHIC Testing (RT) Personnel

  • A certified ASNT Level II RADIOGRAPHIC Testing (RT) personnel is qualified to select proper Radiographic Testing (RT) technique, film, IQI, equipment as per established procedure.
  • A certified ASNT Level II Radiographic Testing (RT) personnel is qualifiedto setup the proper orientation of the source, object and film distance based on the geometric unsharpness.
  • A certified ASNT Level II Radiographic Testing (RT) personnel is qualified to perform Radiographic Testing (RT) manually.
  • A certified ASNT Level II Radiographic Testing (RT) personnel is qualified to after testing film processing is done manually for high contrast and resolution.
  • A certified ASNT Level II Radiographic Testing (RT) personnel is qualified to interpret the results as per applicable standards.
  • A certified ASNT Level II Radiographic Testing (RT) personnel is qualified to follow the steps in the Radiographic Testing (RT) method.
  • A certified ASNT Level II Radiographic Testing (RT) personnel is qualified to prepare Radiographic Testing (RT) report for acceptance/rejection Criteria.
  • A certified ASNT Level II Radiographic Testing (RT) personnel is qualified to know about the merits and demerits of Radiographic Testing (RT) and other common NDT methods.
  • A certified ASNT Level II Radiographic Testing (RT) personnel is qualified to maintain and handle the Radiographic Testing (RT) equipment carefully.
  • Review of Basic principles of radiation
  • Basic mathematics review
  • Geometrical unsharpness
  • Geometric exposure principle
  • Image quality indicators
  • Darkroom Facilities, Techniques and processing
  • Safe light, processor, viewer lights
  • Film loading
  • Film storage
  • Developer
  • Stop bath
  • Fixer
  • Washing
  • Prevention of water spots
  • Drying
  • Unsatisfactory radiographs
    • High or low film density
    • High or low contrast
    • Poor definition
    • Fog
    • Light leakage
    • Film artifacts
  • Densitometers and step wedge film comparison
  • Multiple film techniques
  • Film latitude
  • Background lighting
  • Penetrameter placements
  • Classification of discontinuities:- inherent, process and service
  • Casting processes and associated discontinuities
  • Welding: Types of welding processes, different types of weld discontinuities

Safety

  • Controlling personnel exposure
  • Time,distance, shielding concepts
  • ALARA concept
  • Radiation detection equipment

Manufacturing Processes and Associated Discontinuities

  • Casting
  • Wrought Processes
  • Welding

Application techniques

Specific Traning

Radiography Testing Procedures

Radiographic Interpretation

  • Illuminator Requirements
  • Pentameter Placement
  • Personnel Dark Adaptation
  • Visual Acuity
  • Film Identification
  • ASME Sec V, Sec VIII
  • API 1104
  • AWS D1.1
  • ASTM E 94