X Ray NDT
X-ray, Radiography Industrial Inspections and Training
Radiation is dangerous and radiography is the application of radiation as an investigative tool. For Nondestructive Testing RT Radiography Testing, SRT Source Radiography, DT Digital Radiography, CR Computed Radiography. They all achieve the same similar results with the variance in sensitivity and amount of material which can be penetrated by the process.
Because of the penetration and absorption capabilities of x and gamma radiation, radiography is used to test a variety of products such as welds, castings, forgings, and fabrications. Radiography is generally a good process for the inspection of metals of less than 4 inch. For deeper penetrations ultrasound NDT testing usually becomes a more appropriate inspection method.
NDT Training and Services When performing x-ray to a specific code, a form of sensitivity verification is required to ensure the appropriate energy is used for the material. This is usually achieved with a penetrameter. These devices come in plaque or wire type and essentially achieve the same goal of verifying the integrity of the x-ray energy to material.
Advantages of radiography (RT)
- Can be used with most materials -This includes cell phones, money, food, airbags anything an engineer would like to take a closer look at.
- Provides a permanent visual image – This may be accomplished by digitally media or on conventional radiographic film.
- Reveals the internal nature of material – Is the part in a good sound condition or is there corrosion or flaws affecting the material.
- Discloses fabrication errors – Was the airbag of a vehicle assembled correctly?
- Reveals structural discontinuities – Cracks, and other detrimental defects on a structure
Limitations of (RT)
- Impracticable to use on specimens of complex geometry Provides – Radiography is a single dimension answer to a 3D world.
- The specimen must lend itself to two-side accessibility – The part must be accessible to placing the source on the opposite side of the film.
- Laminar type discontinuities are often undetected by RT – Given the nature of radiography if a defect does not posses a large density variance it may not be detected.
- Safety considerations imposed by x and gamma rays must be Considered – Radiation is lethal, if not used correctly loss of limb or life may occur.
- It is a relatively expensive means of nondestructive testing – a basic radiography set up costs in excess of $100,000.
In order to provide nondestructive testing you usually need to be qualified and certified in accordance with a standard, procedure or practice. You will often here people say you need to meet ASTM, which guides you to SNT-TC-1A a recommended practice or NAS-410 a standard. The important part is to know which you meet so you will know which procedures you can follow or which parts you are authorized to inspect.
A properly qualified and certified Level II technician should be able to make the determination if an indication is a defect or not per an applicable code. Note that qualified is in reference to the person’s documented training, examinations and experience. Certification is in reference to a level III attesting in writing to the person’s qualifications for work to be accomplished per a company’s procedure. Qualification stays with you through the years, certifications usually expire after 5 years.
In order to achieve certification a technician needs to go through an examination process which is also part of qualification. This usually includes a General knowledge (40 question closed book examination), Specific (30 question open or closed book examination specific to a procedure) and practical examinations to demonstrate the capability of the inspector to perform an inspection. If the inspector is going to be developing techniques the standard may also require the Level II inspector to develop a technique in order to be certified to the appropriate level. For radiography technique development should always be a part of the Level II certification because of the complexity of the process. By requiring a technician to develop a technique on a complex part an examiner can genuinely see the level twos capabilities in the method. In order to be certified the inspectors average score from all examinations should be 80% with all the tests being greater than 70%.
X and gamma rays possess the capability of penetrating materials. In passing through matter, the amount of absorption at any point is dependent upon the thickness and density at that point. Density is the amount of radiation absorbed by the emulsion of film. When this variation is detected and recorded (usually on film), a Means of imaging the internal structure of the material is available. The latent image which is present but not visible produced in the film emulsion becomes a shadow picture of the specimen when the film is processed.
The formation of an image on the film depends upon the amount of Radiation received by different sections of the film. A discontinuity such as a void represents a Thickness difference in the specimen and will appear as a dark spot On the developed film and dark or light on a digital image.
Several special types of x-ray film have been designed for the radiography of materials. Some types work best with lead screens, or without screens. Other types are intended primarily for use with fluorescent intensifying screens. X-ray films are commonly coated with emulsion on both sides of the support–the superposition of the radiographic images of the two emulsion layers doubles the density and hence greatly increases the speed. X-ray films coated on one side only (single-coated films) are available for use when the superposed images in two emulsions might cause confusion. This becomes evident in micro radiography where geometric unsharpness of fuzziness is critical.
How to become a Level II NDT Explained