Industrial Radiography (X-RAY)
The more demanding the requirements…
the more labour-intensive the test will be – and therefore, the higher the costs. However, there is currently no better method for non-destructively locating and identifying imperfections inside a component than industrial radiography; it is used as the standard test method for locating internal defects for all of the materials produced at Brechmann-Guss. With the use of industrial radiography and x-ray technology, practically any material can be inspected for internal imperfections. X-rays are electromagnetic waves with a wavelength in the nanometre range and frequencies from roughly 1017 to 1022 Hz; industrial radiography therefore deals with highly energetic, (very) short-length waves.
High frequency and short wavelength
According to German standard DIN EN 12681 (2003-06, together with the VDG P541 82001 standard for thicker-walled parts) or according to the ASTM E689-10 standard, conventional X-ray (or other radiographic) testing is a non-destructive test for the two-dimensional inspection of components in order to locate internal defects. Due to their high frequency and short wavelength, x-rays can penetrate any material.
The x-rays always follow a straight-line trajectory; they cannot be diffracted or bent (as, for example, light rays). They can, however, be weakened by the atoms of the material as they travel through; parts of the ray are absorbed or diffused. How much of the initial radiation reaches the detector depends on the energy of the radiation, the part measurements (thickness), and the physical properties (density) of the material. Material defects, such as cracks, voids, inhomogeneities, etc., will have a different effect on the test radiation because of the differing density of the material at that point (in comparison to a “perfect” section of material). This difference is then measured by a detector (x-ray sensitive film), which records the intensity of the radiation exiting the test object (i.e., how much of the radiation “made it through”).
The test objects are initially exposed to high-energy electromagnetic radiation, which penetrates and travels through the part; the position and the intensity of the radiation source are specified in industrial standards. Inhomogeneities, material defects, cracks, or deviant material strength – in addition to surface defects (which are, however, not relevant for a test designed to find internal defects) – will differ in how strongly they affect the test radiation. The resulting difference is registered on the x-ray sensitive detector film positioned behind the test object, and produces a two-dimensional image where the differing intensity is represented by varying shades of grey (grayscale); this grayscale mapping is then documented. The evaluation of the film can then be done visually.
Best method safety-critical components
In the past, highly loaded castings were primarily produced from steel. For this reason, almost all of the evaluation guidelines found in standard literature on non-destructive testing for such mappings images are almost exclusively for steel castings. Current standard references are continually being updated and changed; as a rule, the corresponding comparison tables based on DIN EN 12681 should be used for comparison. Until now, the DIN 12681 standard has exclusively referenced the ASTM standards (see Table) for Steel Castings.
Establishing quality levels or a quality classification system for a component (or the critical section of a component) is the responsibility of the designer, and therefore rests on his/her experience with similar components, as well as previous loading analyses – in particular for safety-critical parts. Such specifications always depend on the specific part and application; it is therefore common, even for something as common (and as important) as steering knuckles in vehicles, to differentiate between the critical and uncritical areas of the part, and to specify different quality classes for these areas.
To put it another way: there is a choice. The first choice: Extremely high quality requirements can be universally defined for safety-critical components – which, however, will be paid for later in the form of high testing costs during series production and with a higher number of rejected parts for each one that passes muster, all of which must be taken into account during cost calculations. The second choice is to relax a bit and make use of the years of experience that the design team brings to the table; they’ve learned a few things during their years of practical application. In general, testing results are necessarily subjective, and always depend to some extent on the tester; often, there are multiple evaluations possible and mixed views of the results. At the end of the day, this can only provide one piece of the qualitative picture, as a flat image is being used to describe information about the internal volume of a component.
Independently of the details of the procedure (and the devil hiding in them), however, our goal is always the same: creating durable, safe parts that are fit for service. Whether that means turbocharger housings made of Ni-Resist or pump fittings made from SG cast iron, our guiding principle is that Quality is our benchmark – and the bar is high!
Do you have any questions, don`t hesitate