INTRODUCTION

Radiography is a sensitive method used for detecting flaws found in castings, welds, and various metal structures.

Picture 1: Radiographic testing 

It is the most reliable form of nondestructive inspection, used extensively in the welding industry for critical situations, such as pipe welding and pressure vessels. This inspection method is similar to x-rays used by hospitals to detect a broken bone.

The x-ray machine sends invisible radiation through the body to a sheet of photographic film similar to photographic film used in cameras. Since bones absorb more x-rays than the surrounding flesh, they appear lighter on the developed film. Breaks show up darker because x-rays pass through them more easily.

Picture 2: The body X-ray film 

RADIOGRAPHIC TESTING

Picture 3: Principle of Radiographic Testing

Radiographic inspection of welds is done with a similar x-ray generating machine or with a radioactive material called isotopes. These generate higher energy gamma rays.

It provides data for a more exacting evaluation and a permanent record in the form of a photographic image called a radiograph.

When radiation is passed through the item being tested, flaws appear lighter or darker than the surrounding metal.

A radiograph of a piece of metal with varying thicknesses will show up on the processed film as different shades of darkness. The thinner materials show up darker because it allows more energy to reach the film.

Picture 4: A metal radiographic film show a crack in joint

The amount of radiographic energy absorbed is based upon the material’s mass or density. For example, when steel and aluminum blocks of the same size are exposed to the same intensity of radiographic radiation, the aluminum will cause a darker image because it isn’t as dense as the steel and absorbs less energy.

An internal flaw, such as scale, trapped slag, or porosity will show up as a darker area on the film. When evaluating these indications, care must be exercised. False images can be caused by dirt, static electricity, or poor film processing, leading to the wrong conclusion.

Picture 5: RT equipment of Aitech JSC.

Because of many variables such as exposure, intensity of radiation and film-processing techniques, a device called an image quality indicator (IQI), or penetrameter, is often used. It is placed on the object being radiographed, usually off to one side of the desired weld section.

There are two basic types of IQIs: hole type and wire type. Hole type penetrameters are usually rectangular pieces of material with the same radiograpic absorption as the material being tested. They vary in thickness and have a series of holes with different diameters. Wire type penetrameters use wires of varying diameters. With both types of IQIs, the sensitivity of a radiographic image is determined by which of the holes or wire diameters is visible.

A densitometer measures the film density. This indicates if the correct lightness or darkness of the film has been achieved in order to better detect flaws during interpretation.

The success of radiographic inspection depends greatly upon the training and experience of the operator.

SAFETY

Picture 6: Radiographic safety in work

Radiographic inspection must be performed by a qualified operator because of the potential dangers from radiation which can cause death or serious injury if safe practices are not followed.

Do not enter areas where x-ray or gamma ray sources are used without permission or supervision of an experienced radiographer.

For gamma ray methods, the radioactive isotopes are always on or emitting radiation. The must be stored and transported in special containers referred to as pigs. Isotopes should be used only by a qualified radiographer. Radiation detecting devices must be worn in order to detect radiation. Examples of such devices are survey meters and special film badges.

CONCLUSION

All aspects of radiography must be handled only by an experienced operator who knows the importance of safety and has the skill to interpret results and maintain proper records

Picture 7: RT work of Aitech in aviation