What is Radiographic Testing ?
Radiography setup using a X-Ray tube
PRINCIPLE
Radiography is a NDT method based on the high energy electromagnetic waves that penetrate through the test material and are attenuated to different degrees depending on the amount of material loss. Radiography is a thru transmission method where the object is placed in between the radiation source and detector. A defect is a loss of material and therefore lesser attenuation and higher transmission. This difference in transmission is detected by the radiation sensitive detector that produces grey images where changes in density (darkness) are used to detect discontinuities. Radiation energy used is in the range 50 KV to 1 MeV.
EQUIPMENT
Basic equipment for radiography includes a radiation source – X-Ray tube or Gamma ray source, detector, density meter, IQI. Radiation sources can be either a X-Ray tube or a gamma ray source such as Selinium-75 or Iridium-192. Radopgraphs produced by X-Rays are better quality than with the gamma ray sources. Detectors can be film or digital computed radiography (CR) or digital detector arrays (DDA). Safety equipment required includes a GM counter, a dosimeter and TLD badges.
RADIOGRAPHIC QUALITY
The following define radiographic quality
- Sensitivity. This is verified by using image quality indicators (IQI). IQIs can be either wires or shims with small holes
- Unsharpness/Image Spatial Resolution. Placing source and film relative to object so that the penumbra of the image does not exceed 0.5 mm. For digital detectors, resolution is measured with duplex wire pair IQI to meet ISO 17636-2. For weld inspection image spatial reslution should meet Class B of ISO 17636-2
- Density/SNRn. Density is used for films. The film should be gray and neither too white (low density) nor too black (high density). Normalized signal to noise ratio (SNRn) is used for digital methods as per ISO 17636-2
Several variables go into making a good radiograph and these include the type of radiation source (X-rays or gamma ray), X-ray energy level, film grain size, exposure time, etc. Images produced using X-rays produce images with better contrast as compared to gamma rays.
APPLICATIONS
Radiography is a volumetric NDT technique and has a variety of applications primarily being weld inspections in piping and pressure vessels. Inspection is done either with the source placed inside the pipe/pressure vessel and the film outside, called single wall exposure technique, or by placing source and detector on the outside the pipe, called double wall exposure technique.
Radiography is extensively used in the aerospace industry for inspection of airframe structures. Digital RT is more common in aerospace industry.
INDUSTRY CODES
ASME V, Article 2
ISO 17636-1 Film Radiography
ISO 17636-2 Digital Radiography
TRAINING
RT Level I, Course no – NDT-112
Duration – 40 hours
RT Level II, Course no – NDT-211
Duration – 40 hours after completion of Level I
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