Image Quality Indicators (IQI)
It is important that the end user of industrial radiography has some means to determine if the quality of the radiograph is satisfactory. Just looking at a radiograph can provide part of the answer, but it is neither a sufficient nor reliable means to truly assess quality or to establish a measure of the sensitivity of the technique used.
Sensitivity is measured using a device known as an Image Quality Indicator or IQI The purpose of an IQI is to indicate the overall sensitivity of the technique and as a measure of how well the radiograph will reveal discontinuities. As the name implies, an IQI is an indicator of the quality of the radiographic image.
An IQI is a device made from the same material as the test specimen. It is placed on the test specimen in a position where its image will be recorded on the radiograph.
There are three types of IQI in common use which are specified in Australian standard AS2177:
- Wire type IQI
- Step hole IQI
- Plaque hole IQI
Wire type IQI
This type of IQI consists of a series of metal wires mounted in a flexible plastic holder. The wires are mounted parallel to each other about 5 mm apart and are each about 60 mm long. The standard models consist of three series, each containing seven wires, numbered 1 to 7, 6 to 12 and 10 to 16.
Actual wire diameters are:
Wire No.-Diameter (mm)
This type of IQI is normally available in steel, copper and aluminium.
When radiographing a weld, the IQI should be placed with the wires across the weld area being examined.
Step hole IQI
This type of IQI consists of a series of steps, each one having one or more circular holes of a diameter equal to the thickness of the step and drilled at right angles to the surface of the step. The standard models consist of 3 step hole IQI’s each containing six steps, numbered 1 to 6, 7 to 12 and 13 to 18.
Actual step thicknesses (and hole diameters) are:
Step No.-Thickness (mm)
This type of IQI is normally available in steel.
When radiographing a weld, this type of IQI should be placed adjacent to, and parallel to the weld. The thickness of metal beneath the IQI should the same as the throat thickness of the weld being examined. If necessary, a shim of the same metal should be placed under the IQI to ensure this.
Plaque hole IQI
This type of IQI consists of a number of uniform thickness strips, or “plaques”, each containing three holes drilled through the thickness at right angles to the surface. Hole diameters are one, two and four times the plaque thickness. Plaques are identified by a number, ranging from 5 to 200, that is the plaque thickness in thousandths of an inch. For example plaque 5 is 0.005 inches thick. These are used extensively in America where they are known as ASME Penetrameters, the American term for an IQI.
When radiographing a weld, this type of IQI should be placed adjacent to, and parallel to the weld. A shim of the same metal should be placed under the IQI so that the thickness of metal beneath the IQI is the same as the throat thickness of the weld being examined.
The smallest element readily visible in the area under inspection is used to determine IQI sensitivity
For the wire type IQI: %Sensitivity = Diameter of the smallest vissible wire / Thickness of metal * 100
For the step-hole IQI:
%Sensitivity = Diameter of the smallest vissible hole / Thickness of metal * 100
For the plaque hole IQI there are two different approaches as follows:
The first approach is to use the Australian Standard formula for sensitivity defined in AS2177:
%Sensitivity = sqrt(AxB/2)
- A is the thickness of the smallest plaque image visible, expressed as a percentage of metal thickness
- B is the diameter of the smallest hole visible, expressed as a percentage of metal thickness
The second approach is to use the American (ASME) system for IQIs. American standards set a required IQI sensitivity in terms of the plaque thickness and the smallest hole visible in the radiograph. A code may require a 2-2T sensitivity or a 1-2T sensitivity. No reference is made to percentage sensitivity.
The first number refers to the size of the plaque to be used, a plaque whose thickness is 2% in the first case, or 1% in the second case, of the thickness of the metal being radiographed. The second number (2T) states that the 2T (twice the thickness) hole shall be visible in the radiographic image. Hence it is a go/no-go test, with no determination of actual percentage sensitivity. This is a far more sensible use of the plaque hole IQI. The Australian approach requires affixing several plaques to the test object to find the thinnest visible plaque and the smallest visible hole to arrive at a meaningless number.
There are two important factors relating to IQI sensitivity
- The sensitivity determined is a function of the type of IQI used. A 2% sensitivity with a wire type IQI is not the same as a 2% sensitivity with a step hole or plaque hole IQI.
- The IQI sensitivity is not a measure of the smallest detectable discontinuity in the test object. IQI sensitivity is a measure of the quality of the image, and is a means of controlling and assessing image quality.
IQI’s should be placed in a particular location on the test object
The IQI should, wherever possible, be placed on the source side of the test object and towards one end of the area being radiographed with its smallest element furthest from the centre line of the beam of radiation.
There are cases where it is not possible to place the IQI on the source side of the test object, such as when radiographing the far wall of a pipe by projecting the radiation through both walls. In such cases, the IQI can be placed on the film side, but with a letter ‘F’ beside the IQI to indicate that it is a film side IQI. Also in these cases, a reference shot should be made using the same technique on a section of pipe where IQI’s can be placed on both sides of the surface.
Selecting the correct IQI is important
Typical radiographic sensitivities range from around 1% to 4%, depending on:
- the thickness of the item being radiographed
- the type (energy) of the radiation
- the type of film
- other factors discussed in the previous task.
In some cases, a code, specification or customer order will state the required sensitivity.
Selecting the correct IQI involves:
- Selecting the correct metal (steel, aluminium, copper).
- Selecting the correct type, again normally specified in the code or specification.
- Selecting the correct model, i.e. the model with an element that will correspond with the required sensitivity. Hence, if radiographing 20 mm steel for a 2% sensitivity, with the wire type IQI, the model with a wire closest to 0.4 mm (2% of 20 mm) is required. This is wire number 10, so the 6 to 12 wire type IQI is required. If the step hole type is specified, then the model with a 0.4 mm step is required. This is the 1 to 6 model.
- Inspection Methods: