In the past it was thought possible to assess the smallest defect detectable, by fixing a simple type of indicator on the test object during exposure.
This would supposedly guarantee that defects of a certain minimum size, expressed as a percentage of the material thickness, could be detected. In practice, however, this proved not to be achievable.
In particular where small cracks and other two-dimensional defects are concerned, it can never be guaranteed that they are not in fact present when no indication of them can be found in the X-ray image.
However, it is reasonable to expect that at least the quality of the radiographs, and of course the rest of the entire process the film undergoes, meets certain requirements.
The probability is high that defects will be more easily detected when the image quality is high. The exposure technique and required image quality, described in the code, depend on the purpose for which the object involved will be used.
In order to be able to assess and quantify the image quality of a radiograph, it needs to be converted into a numerical value, and to do this “image quality indicators” (IQI) are used, known in the USA as “penetrameters”.
Image quality indicators typically consist of a series of wires of increasing diameters, or a series of small plates of different thicknesses, with holes drilled in them of increasing diameters.
Although codes describe their techniques differently, they agree on the following points:
An image quality indicator shall be placed at the source-side of the object being examined,
If it is not possible to place the indicator on the source-side, it may be located on the film-side. This exceptional situation must be indicated by a lead letter “F” on or directly adjacent to the indicator,
∑ The material of the indicator must be identical to the material being examined.
The image quality of a radiograph is, for example, defined as the number of the thinnest wire still visible, and is generally said to have “image quality number -X-”.
The image quality can also be expressed as a percentage of the object thickness examined.
If, for instance, the diameter of the thinnest wire visible to the naked eye is 0.2 mm and material thickness at the point of exposure is 10 mm, wire discernibility or wire recognizability is quoted as 2 %.
As emphasised above, the use of an IQI does not guarantee detection of defects of comparable size.
It would be incorrect to say that because a wire of 2 % of the object thickness can be seen on the radiograph, a crack of similar size can also be detected.
The orientation, relative to the X-ray beam, of a defect plays an important role in its discernibility (see section 12.1.)
There are various types of IQI, but the four most commonly used are:
the wire type (used in most European countries)
the step-hole type (still occasionally used in France, but the wire type is generally accepted as well.)
small plates with drilled holes, called penetrameters, which are used for ASME-work, although the ASME-code nowadays includes the wire-type IQI.
the duplex IQI.
In some countries (e.g. Japan and France) additional means (such as step-wedges) are used, to verify contrast and check the kV-value used.
At the location of the (step)-wedge, there must be a minimum specified difference in density compared to the density at a location on the film where penetrated material thickness equals nominal wall thickness.
Wire-type IQI according to EN 462-1
EN 462-1 standardises four wire-type IQIs. Each one is made up of seven equidistant parallel wires of various diameters, as shown in figure 1-13.
Table 1-13 shows the wire combinations for the four IQIs according to EN 462-01. The diameters of the wires are given in table 2-13.
EN-type IQIs are manufactured with wires of steel, aluminium, titanium or copper, depending on the type of material to be examined. On each IQI the wire material is indicated. Fe for steel, Al for aluminium, Ti for titanium and Cu for copper.