Volume 5, Number 1		January 2006

 

 

This article provides an overview of radiographic film and an investigation of two different radiographic applications. Radiography is the use of X-rays or gamma rays to expose film. Since the rays utilized possess great penetrating capabilities, information about the condition of a part or component placed between the source and film can be obtained. The greatest attribute of film radiography is its flexibility of application.

Radiographic film starts out as a blue-tinted sheet of thin plastic polyethylene. An adhesive base is applied to aid in the adhesion of a film emulsion that is composed of a gelatin binder and grains of silver bromide. When X-rays, gamma rays, or visible light interact with the silver bromide, ionization of the compound results. When developed, unexposed crystals are removed and the remaining exposed silver bromide crystals appear dark. The different densities obtained on the film determine the image of the part being viewed. Some basic characteristics of film follow.

Film Speed.  Film speed is determined by the size of the silver bromide grain clusters used in a particular film. The larger the grain clusters, the faster the film reacts to ionization (Fig. 1).

 

Film Sensitivity.  Radiographic film sensitivity refers to the smallest detail that can be seen on the radiograph. The smaller or finer the grain size, the greater the film sensitivity. However, increased sensitivity requires a corresponding increase in exposure time. Use of high speed films results in the loss of sensitivity. Other factors affecting sensitivity are exposure geometry (Ug) and the quality (kilovoltage) of the radiating energy selected. Sensitivity is usually measured with an image quality indicator (IQI). A hole in the indicator with a code specified diameter must be visible to the film interpreter in order to meet the code requirements. A wire-type indicator uses a series of fine wires instead of holes to indicate sensitivity (Fig. 2a and b). 

Radiographic Film Contrast.  Based upon the exposure of individual grains, radiographic film contrast is the measurement of the difference in intensities or densities between the lightest and darkest area of the captured image (Fig. 3a and b). Film density is measured with a densitometer. For an application such as viewing  corrosion, a film possessing high contrast will increase the accuracy of thickness correlations.

Radiography of Welds

The interpretation of radiographs for weldments is not only dependent upon parameters such as exposure geometry and film speed, but also on factors such as type of base material, joint configuration, and the welding process used. If any one of these factors is not taken into consideration, a false or irrelevant indication could be confused with a true discontinuity. Even worse, a true discontinuity could be disregarded as an irrelevant or false indication.

Material Characteristics.  Certain materials produce unique or characteristic results when subjected to the various welding processes. Knowledge of these characteristics is valuable for accurate interpretation of radiographic images. Without knowledge of the material being examined, a correlation between characteristic indications induced by the material and the discontinuities introduced by the welding process cannot be made.

Joint Configuration.  To accurately assess an indication’s position within the volume of a weld, the joint configuration must be known. For example, lack of penetration at the root of a weld can only occur in the center of a butt-joint. A specific type of discontinuity might appear in one place when using a certain welding sequence and joint preparation, and in another place when using a different welding sequence and joint preparation. Non-fusion between fill passes and base material could be confused with internal undercut or slag inclusions, as an example. To facilitate interpretation, a weld should be subject to visual examination by quality control inspectors or the radiographic technician prior to being radiographed. The inspector should determine that the surface of the weld and surrounding base material are free of any anomalies that may mask or be confused with any subsequent indications within the image of the weld on the final radiograph.

Process Discontinuities.  Different welding processes may have discontinuities specific to the process. For instance, slag inclusions can only be induced in a weld that uses a flux shielding. A radiographer could expect slag inclusions in a weld produced utilizing the shielded metal arc welding process (SMAW) or flux-cored arc welding process (FCAW), but would not expect the same discontinuity in a gas metal arc welding (GMAW) weld. Tungsten inclusions can only be present when using the gas tungsten arc welding (GTAW).

Discontinuities have different densities from the parent material. Discontinuities such as slag inclusions or porosity allow more radiation to pass through because they are less dense than surrounding metal. The process of recording the varying degree of absorption of penetrating radiation through an object is the entire premise on which the radiographic test method is based. More ionizing radiation interacting with the film produces a darker area in the final radiographic image of the weld. A highly dense material such as tungsten attenuates a greater amount of energy than a discontinuity such as a gas pore. Base material appears as one density while the pore is darker than base metal. A tungsten inclusion produces a very distinct, bright, star-like indication on the radiograph.

Interpretations.  Typically, in producing a radiograph of a weldment, there are minimum requirements stipulated by governing codes or specifications. These may include sensitivity within the image, proper identification of the weld, location markers, and a specified maximum and minimum density of the image in the area of interest. The first step in interpreting a radiograph is to make an overall observation of the film to ensure that minimum requirements are met.

During interpretation, thought must be given to the origin of the various indications within the radiographic image. Are the indications false indications, artifacts, or a true discontinuity? If a true discontinuity, what type of discontinuity is it?

Process Discontinuities.  Radiographers must possess a working knowledge of basic welding processes and the potential discontinuities associated with each. Knowledge of where within a weld or particular joint configuration weld defects can occur improves the ability to interpret film. Defects resulting from the welding process can include: (1) lack of fusion, (2) lack of penetration, (3) porosity, (4) excessive or lack of fill, (5) slag, (6) transverse or longitudinal cracking and (7) crater or star cracking.

The last step in the interpretation process is to compare the weld discontinuities to a specified code or standard. A value is assigned to a particular discontinuity based upon characteristics such as size, shape, number of pores and location according to the specified requirements. To do this, the limits of the indication’s length and width must be determined and evaluated in accordance with the code. If a discontinuity does not meet the requirements of the code, it is classified as a defect or reject. It should be noted that a true depth measurement of a discontinuity can only be determined by using radiographic parallax techniques or by employing other nondestructive test methods.

Radiographic Examination of Aircraft

Radiographic inspection of aircraft structures is similar to radiography of steel welded structures, forgings and castings. The basic physics in each case are the same. Both X-ray and gamma ray techniques may be used and both film and digital imaging techniques may be utilized.

Radiographs produced for aircraft inspection are usually performed with a directional X-ray tube head. The use of 360 degree emitter tube heads (panoramic) can be used for circular sections such as fuselage structures, engine nacelles, and combustion chambers. Items inspected can include many different structure types including, skins, spars, ribs, cast components such as landing gear components, machined components such as wing attachment fittings, and forgings such as landing gear torque links.

Reasons for performing inspections on aircraft vary and may include detection of corrosion, fatigue cracks, stress corrosion cracking, erosion, misplaced items or foreign object debris (FOD).

Special Considerations.  For most aircraft applications, the radiation energy level used is considerably less than that used for steel structure inspection. A 160 kV tube head has sufficient penetrating ability for most aircraft applications. This is due to the relatively thin structure and the lower atomic number of the materials used in aircraft construction. Aluminum, magnesium, titanium and non-metallic composite materials are frequently used for most aircraft construction due to their high strength-to-weight ratio. To achieve the necessary subject contrast with these materials, the energy used is often less than 100 kV. A low kV tube also offers the advantage of small physical size and reduced weight. These are valuable attributes in the inspection of aircraft where, in many instances, accessibility to locations for both tube and film is not ideal or when placement of a larger tube head in the correct position to meet the exposure requirements of alignment and distance is physically impossible. The weight of large tube heads makes maneuvering the tube to the required position difficult and may risk damage to fragile aircraft structures when placing the heavy tube head inside.

Film Choice.  The choice of film type can vary considerably. Critical inspections of fatigue or stress corrosion cracking require a slow speed, fine-grained film while the detection of FOD can usually be done with a high speed coarse grain film. This is based on the high degree of subject contrast produced by a loose fastener (such as a rivet) when seen against a lightweight aluminum skin for example.

Film Packaging.  Film packaging is another important consideration especially if the film cassette has to be placed in a fuel tank in a wing cavity. This use requires vacuum sealed film packs made from a thin vinyl or similar polymer material to prevent the film from becoming wetted by the fuel. Most techniques that require film to be placed within a fuel tank state that the tank should be drained and purged. However, even the best cleaning techniques can leave pockets of fuel in areas where the film is to be placed.

Film placement.  Film placement can also prove to be tricky due to the presence of wiring looms, insulation, interior trim, and hydraulic pipes within the aircraft structure. A solution often employed for placing film behind wiring looms without crimping is to attach the film cassette to a long sheet of thin aluminum with a width comparable to the film. The aluminum is used to slide the film cassette behind the wiring looms into the area of inspection.

When inspecting fuselage frames, complete coverage of the frame’s circumference is typically required. The tube head is commonly placed  inside the fuselage and the film is placed on the outside of the fuselage skin. Each time the tube head is repositioned, the film is changed. In this application, when coverage of the entire frame circumference is required, a panoramic tube head is valuable. Use of a panoramic tube head allows the entire circumference to be covered in one shot. This efficiency results in significant savings in time and labor, factors that offset the initial added expense of the equipment. The tube head is centrally positioned within the fuselage. A roll of film is wrapped around the full circumference with a slight overlap. In some instances, a longer exposure time may be required because the focal spot to film distance (FFD) may need to be increased to position the tube head centrally.

Conclusion

Radiography’s greatest benefit is its flexibility of application. Knowledge of the principles of film radiography allows the radiographer to use it to obtain information on a wide variety of components and in many applications. The use of radiography in the examination of welds and in aircraft structure are but two of an increasingly wide variety of applications in today’s industry. TNT

Jeff Arveson is Radiation Safety Officer and Quality Control Manager for Kakivik Asset Management LLC and an ASNT certified Level III in MT, PT, RT, UT, LT and VT. (651) 470-8830, e-mail <jarveson@kakivik.com>.

Richard A. Harrison is President of T.E.S.T. NDT, Inc. and ASNT Level III in UT, MT, PT, RT, ET and VT.  He is also a member of the Technicians Advisory Committee. (714) 255-1500, e-mail <ndtguru@aol.com>

Ray Shepard is Quality Assurance and Training Manager for Kakivik Asset Management LLC and a former professor of welding and NDT at the University of Alaska, Anchorage. (907) 770-9418, e-mail <rshepard@kakivik.com>.

 

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