·Table of Contents
·Methods and Instrumentation
Radioscopy - The Prevalent Inspection Technique of the Future!?M.Purschke
RICH.SEIFERT & CO., Ahrensburg, Germany
|Fig 1: Schematic array of a radioscopic system|
Roughly speaking, an X-ray image is created in a sequence of two steps:
The X-radiation which is produced by the radiating unit passes through the object to generate an attenuation image of said object on the incoming screen of the detector. The geometric resolution or rather the resulting unsharpness in the radioscopic image is set by the Object-to-Detector distance. The relation between the Focus-to-Detector distance (FDD) and the Focus-to-Object distance (FOD) determines the geometric magnification of the image. An image sensor, e.g. an X-ray image intensifier transforms the attenuation image into a visible X-ray radioscopic image.
The transfer chain which is made up of a lens system, a camera and a monitor must then render the emergent image of the X-ray image intensifier visible on the screen so that the operator may view it. Digitization enables the transfer of the image to an image processor for processing or evaluation of the radioscopic image.
|Fig 2: System for manual inspection of aluminium parts (about 1948)|
In the mid-fifties, X-ray image intensifiers with a binocular were introduced to radioscopic inspection. Next to electrically powered object manipulators, the early sixties launched the image intensifier / TV camera chain which is common to this day, with the difference, however, that the formerly known analogue TV cameras have given way to CCD cameras.
|Fig 3: Inspection systems with image intensifier / TV camera chains for the examination of castings and welds (about 1970)|
In the early seventies, continuing advancement of the imaging components and the inspection techniques led to a marked increase in the demand for radioscopic systems, especially for those that are intended for the examination of castings and welds. (Figure 3).
In the early eighties, digital image processing techniques were increasingly applied in radioscopy. Digital image processing for the purpose of image enhancement and automatic image assessment must nowadays be regarded as state of the art.
Drawing up of a Standard that governs radioscopy, Schooling and training of the NDT personnel and high-performance Systems designed for radioscopic inspection laid the foundation for the success of this examination technique and for a strong increase in applications. These "Three Important S" of radioscopy will continue to determine the future success of this test method. They will be discussed in the following chapters.
4.1 Schooling and Training
In the early nineties, the rising pressure exerted on users of radioscopic test methods to employ certified NDT personnel triggered an increased demand for the training of such personnel. Since 1994, a two-level training course, called RS 1 / RS 2, has been offered by DGZfP in cooperation with renowned manufacturers.
RS 1, a four-day training program, introduces the participants to the basics of radioscopic inspection and the characteristics of different types of imaging systems. Each day comprises four hours of theory and four hours of practice. Up to March 2000, 168 participants had enrolled in the RS 1 course. Figure 4 shows the percentage distribution of participants from different fields of application.
|Fig 4: Participant distribution for different application fields|
68% of the participants cover the light alloy metal casting industry. This is the largest group. 10% and 20% respectively represent weld inspection and various other applications fields. This participant distribution certainly reflects the application of radioscopy in the various industrial sectors. The large number of participants from the casting industry proves the validity of this training concept that was initiated by DGZfP together with leading manufacturers. The convincing success of the RS 1 course would have been impossible had it not been for the manufacturers' direct approach of potential users with a keen interest in NDT training, the reason being that it is exactly the users in the casting industry who have no idea whatsoever about the responsibilities that DGZfP has assumed and about the opportunities that DGZfP offers.
Starting in 1996, the RS 2 course picked up momentum as well. It operates within the same time frame as the RS 1 course, but places its main emphasis on digital image processing. The 54 participants (until March 2000) who have enrolled so far demonstrate a noticeable shift toward NDT personnel from the field of seam weld inspection. This is a logical consequence of the fact that radioscopic weld inspection can only be performed successfully with the aid of digital image enhancement techniques. And it is exactly these techniques that are stressed in the RS 2 training course. But the course also features another topic of major importance which is the standardization of radioscopic inspection techniques.
During the past few years, great efforts have been made to draw up a radioscopy standard . It was in particular the International Institute of Welding (IIW) that made extensive contributions. The sub-commission V A working party in charge of "Radioscopic systems for weld inspection" agreed to prepare a three-part standard resulting in prEN 13068 draft European Standard "Radioscopic Testing Part 1-3".
Part 1 of the standard defines the basic requirements that testing equipment has to meet as well as the approach on how to determine relevant parameters for the imaging system. These specific parameters serve to compare different radioscopic systems in terms of their component properties. Part 3 assigns these specific parameters to different classes of requirements so that each application is associated with certain minimum requirements that the imaging system in use must fulfill.
Part 2 focuses on quality control and long-term stability of the imaging system. Generally speaking, the performance of a radioscopic system should always be checked with the aid of testing devices having natural flaws. For added comparability, standardised image quality indicators as in EN 462-1 and 5 must be used to monitor the principal image quality parameters such as spatial resolution or unsharpness and contrast sensitivity.
Part 3 of the radioscopy standard defines minimum requirements that all radioscopic systems must meet regardless of their specific technical application. As could be expected, this issue triggered off vehement discussions since such a common denominator in terms of application criteria for radioscopic inspection systems is difficult to establish when, in fact, these systems cover a diversity of industrial applications.
The performance of the entire system (imaging system and geometric imaging conditions) and its suitability for a certain inspection job are checked by means of specified image quality indicators (IQIs), namely the DIN wire IQI (EN 462-1) and the platinum duplex wire IQI (EN 462-5). The use of these two indicators enables a nearly independent definition of geometric as well as contrast resolution. In analogy to radiography, two test classes are defined for each application: a standard class SA and a more stringent class SB.
Parts 1 and 2 of the standard have successfully passed all enquiries of CEN. The final vote for Part 3 is to be initiated still this year. Depending on the final vote of the CEN members, the availability of the long awaited radioscopy standard is now within reach.
The performance capability of a radioscopic inspection system (Figure 5) is judged first and foremost by the parts throughput, i.e. by the inspection rate.
|Fig 5: Fully automated inspection system|
Schematic Inspection Sequence:
Inspection of the object starts when the system, i.e. the mechanical manipulator is being charged. Immediately after having approached the first test position, fully automated systems will start with the exposure. The second position is being approached after image acquisition has been completed. While the object is being positioned, calculation of the test decision takes place. These steps will be repeated until all test positions have been executed and the object has been discharged.
A marked increase in the inspection rate and thus in the parts throughput depends very much on the concept of the inspection system. The times needed for the loading, unloading and changing of the object must be kept to a minimum. So-called twin-manipulators are ideally suited to handle these tasks (Figure 6).
|Fig 6: Twin-manipulator concept|
Schematic Inspection Sequence with Twin-Manipulator:
The twin-mechanism of a parts manipulator enables parts transport into and out of the radiation protection cabinet with only minimum down-times. The loading and unloading of the parts manipulator takes place outside the radiation protection cabinet, parallel with the inspection.
Fully automated evaluation , implying radioscopic image diagnostics with the aid of digitized image evaluation, constitutes by now an integral part of radioscopic inspection systems that are intended for the continuous testing of high-quality safety-relevant aluminium castings in the automotive industry. For economic reasons, such fully automated inspection systems are the only answer to ensuring both a high parts throughput and reliable test results.
Furthermore, late in the 90s, different X-ray sensitive panel-like digital sensors (flat panel detectors) found the way out of the development. An advantage compared to X-ray image intensifier / CCD camera systems is the low weight of such a flat panel detector. This offers the possibility to develop faster mechanical components for a further increase of throughput of fully-automated radioscopic inspection systems. But there is also a serious disadvantage of this flat panel detectors: most of them are not able to fulfill the video real-time requirements. This means, at least in the moment, a significant restriction for the applicability of flat panel detectors in the visual radioscopic inspection. Nevertheless flat panel detectors are highly suitable for the realization of three dimensional computer tomography.
As computer hardware has undergone rapid advancement in recent years, R&D centres are by now in a position to run highly complex, yet cost-effective computational processes when using their radioscopic equipment for three-dimensional computed tomography [3,4]. This method lets the object rotate in equidistant angular increments to mathematically reconstruct the X-rayed volume (Figure 7).
|Fig 7: Schematic array of three-dimensional computed tomography (3D CT)|
The 3D-CT technique allows an exact defect localization in the casting . Thus it will be of great future importance in the design process of new castings and their moulds and dies. Consequently, the central issue already at the developmental stage must be an optimization of both the casting design and the casting process so that a later emergence of defects can be prevented.
Figure 8 shows a test object consisting of aluminum and plastic blocks with a thickness of approx. 1cm. In the blocks are bore holes drilled through and milling grooves of approx. 0,5 cm. The section image shows the plastic blocks darker than the aluminium blocks due to their lower density. Figure 9 shows the reconstruction of the test object volume.
|Fig 8: Test object of Al and plastic (approx. 15 cm x 15 cm x 1 cm) left: test object right: 2D CT||Fig 9: Reconstruction result and volume visualization of the test object|
Furthermore high accuracy three dimensional measurements of structures within the specimen are important task especially for very complex castings. This requires the generation of volume data in CAD-data quality. Figure 10 shows an example of a 3 D-CT of a casting visualized with a usual CAD system (resolution approx. 200µm).
|Fig 10: CAD visualization of 3D-CT of a casting (left: casting, right: detail)|
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