Opening the Vault

As ASNT marks its 85th anniversary this year, Materials Evaluation is opening the ME Vault to revisit notable articles from across the profession’s history. New this year and free to ASNT members, the ME Vault provides access to archived journal content dating back to 1942.

The following reprint, originally published in 1943, captures an early and important shift in industrial radiography: the move from individual judgment toward standardized, repeatable procedure. In “A Method for Compiling an Accurate X-Ray Technique Chart,” published in Industrial Radiography (April issue), J.J. Allen Jr. describes a system for controlling variables and building practical technique charts for production work—an approach aimed at making radiographic inspection more accurate, efficient, and dependable. The article also reflects the wartime urgency of its moment, when laboratories were expanding rapidly under what Allen called “the present emergency.”

Throughout ASNT’s 85th anniversary year, this series will highlight selected articles that reflect key moments in the evolution of nondestructive testing and evaluation. Chosen for both historical significance and continuing relevance, these reprints trace how the profession advanced through new methods, stronger standards, and changing industry demands. Visit asnt.org/me/vault for more information.

Jill Ross, CAE, is ASNT Director of Publications. She can be reached at jross@asnt.org.

A Method for Compiling an Accurate X-Ray Technique Chart

By J.J. Allen, Jr.

X-Ray Technician, Fairchild Aircraft, Hagerstown, Maryland

Accurate technique charts are an important source of information in any X-ray laboratory. It is believed that up to this time there has been no graph or chart which was more than a starting point from which to establish a final technique. At the Fairchild Aircraft Laboratory a system has been developed for making accurate charts, which are readily applicable in the radiography of small parts such as airplane castings.

These charts, such as our chart for aluminum alloy (see Figure 1) are made for each material to be radiographed; and are in the form of a graph with kilovoltage plotted against part-thickness, all other factors being held constant. With such a chart, it is possible immediately, and accurately, to obtain the best kilovolt setting for any given part thickness by reading the middle line. It is also possible to determine the maximum and minimum part-thickness range in which two per cent defects are discernible on the films in which each kilovolt setting is used, by reading the upper and lower lines respectively.

Fortunately, we may discard as a variable the composition of an alloy. Modern foundry methods assure us of reasonable uniformity of structure in parts of the same alloy.

The first step, therefore, in the elimination of variables, is the standardization of darkroom procedure. Large film manufacturers have determined the best darkroom standards and these are well known. Best results are obtained in routine work when solutions and washes are maintained at a temperature of 68 degrees F. This is a constant in the Fairchild Laboratories and is maintained with automatic temperature regulating devices.

To obtain good definition on the film, and yet stay within reasonable bounds, it is generally agreed that a ratio of twenty to one should be maintained between target-part and part-film distance. In the case of most small parts handled in industrial laboratories, a target-film distance of thirty-six inches serves to reasonably maintain this ratio.

Either a cardboard lead-backed film holder, or a cassette employing lead or calcium tungstate screens will be used depending upon the density of the alloy radiographed. In the Fairchild laboratories, cardboard film holders are used as standard, for the work consists chiefly of radiography of aluminum alloy castings.

The factors of kilovoltage, milliamperage, and time remain for consideration. From experience it was found that the lowest practical kilovoltage was always best. A standard milliampere-second factor was therefore determined which gave adequate film density for the range of thickness ordinarily covered in the radiography of small parts, that is, from one-eighth to two inches, and yet one which did not call for an unreasonably long exposure.

To determine what kilovoltage setting was proper for each part thickness, a standard penetrameter scale (see Figure 2) was radiographed. Penetrameters were made in accordance with Army Air Corps Specifications, AN-QQ-M-188. These were formed from rolled aluminum alloy (24S-T). Penetrameters were made by dividing it into strips one-half by one and a half inches in dimension. The thicknesses of these strips were then reduced to thicknesses equivalent to two percent of one-eighth, two-eighths, and so on by eighths up to two inches.

The penetrameter scale block was placed on a 14 by 17 inch loaded cardboard film holder in such a manner that it was farthest from the central ray (see Figure 3). The factors previously discussed were maintained as constants, and exposures were taken, starting at the lowest useful kilovolts.

The films obtained when processed and read contained the data needed to compile the technique chart.

It is logically assumed that, where the penetrameter detail can be clearly seen on the negative, since they simulate two percent defects, then actual defects of this minimum magnitude can be discerned on radiographs of an actual part by following the same technique.

The same method that we have used in making our chart for aluminum castings can be used to compile a chart for any alloy. All that is necessary is an accurate penetrameter scale of material similar to the one to be radiographed.

This method is used in the Fairchild laboratories to standardize procedure and to facilitate operation of the laboratory. It certainly seems worthy of investigation especially in those laboratories where personnel lacking a high degree of skill are being employed as a result of the present emergency. It reduces an often difficult and lengthy problem to a simple matter of measurement of the part and consulting a chart for the proper kilovolt setting.