Multi-Method END Study in the Search for 'Defects' in an Aeronautical Radar Element

Non-Destructive Testing (NDT) has gone from being a simple laboratory curiosity to an indispensable tool in the industry to determine the level of quality achieved in its products. The new concepts of Integrated Manufacturing (CIM) bring a more universal concept of quality compared to the past philosophy based on Quality Control as a group specialized in checking whether production works within certain specifications. Even so, NDTs have not lost interest, but have seen their interest increased due to automated inspection techniques. It has become a contribution to the structuring of quality as it allows to move from purely empirical criteria to other more objective and that constitute the link between design and evaluation (Ramirez et al, 1996). The work presented, although it does not propose new methods or techniques of NDT, has the interest of converging into a single object five conventional methods each of which provides partial information about their quality of manufacture and must synthesize the results in order to evaluate it. Furthermore, it shows a situation of the application of NDT in which these must be applied in the absence of reference standards, as they do not exist. This peculiar situation is completely different from the usual situation in the use of NDT in the industry, both in manufacturing processes and in maintenance inspections.


Introduction
In the scientific tradition, observation plays a major role. When the observed phenomenon has been provoked, you have the experiment. Through the experiment, the behavior of Nature is verified under controlled conditions in order to discover regularities that can be described in a logical way and be enunciated as laws. In technology, it is not so much important to discover natural laws as to apply them to questions of immediate utility, and so the experiment shortens its scope and becomes an experiment, in such a way that it can be indicated that an experiment is a technological experiment (Sanglier et al., 2020).
According to the testing of materials, they can be divided into three categories: -Functional tests -Destructive Testing -Non-Destructive Testing This work will refer to the Non-Destructive Tests that should be adapted to the requirements of the sample to avoid its deterioration. This is achieved by resorting to those physical characteristics of the material that are technologically significant, and as these can be very varied (density, thermal conductivity, electromagnetic absorbance, refractive index, crystalline structure, etc.) hence the number of Non-Destructive Tests is potentially large and their basis diverse.
-Electromagnetic waves, including phenomena based on the electrical and/or magnetic properties of the samples -Elastic or acoustic waves -Subatomic particle emission -Other phenomena such as capillarity, watertightness, absorption, etc.
In terms of applications, Non-Destructive Testing methods cover three broad areas: -Defectology: detection of heterogeneities, discontinuities and impurities; evaluation of corrosion and deterioration by environmental agents; determination of stresses; detection of leaks; defects in moving machinery; hot spots.
-Material characteristics: chemical, structural, mechanical and technological characteristics; physical (elastic, electrical and electromagnetic); heat transfer and isothermal trace.
The problem that appears with the application of the different NDT techniques is that it is not very well known since it depends on many factors, which will be the most suitable technique for each type of inspection. For example, in non-destructive testing without contact and in the astronautical field, very high demands are made. The materials used in this very specialized branch of engineering must not only have very high quality levels, but also the tests that require their verification must be such that they do not produce any type of contamination. This severe requirement eliminates many traditional techniques and even some methods. This is the case, for example, with penetrating liquids, magnetic particles or contact ultrasound. In fact, only visual inspection or radiography meets these requirements and, under certain operating conditions, currents induced by mentioning traditional methods. Stimulated by this need, some methods have developed very sophisticated specific techniques that allow physical contact to be replaced by optical contact (surface holography, holographic interferometry, infrared thermography). For example, it is possible to stimulate the generation of ultrasounds in a material by means of laser 'impact' and 'read' by this same means the fine vibrations generated.
The main interest of this work is that it is a rare example of the application in a single object (piece) of a considerable number of non-destructive tests. Nor is´t common that this is done in the absence of specifications, both the procedure for conducting the tests and acceptance-rejection criteria (ASM, 1989;Barbero, 1999). This is unusual in the industrial field where the rule is to follow an operating procedure specification, specially prepared for each specific case, based on solvent standards and containing clear and unequivocal acceptance-rejection criteria.
On the other hand, most often the tests to be performed on each specimen are not numerous for well-defined products. Two or three is normal; sometimes just one. And in any case related, if not in their physical basis, then in their purpose of trying to highlight certain well-determined types of discontinuities and not others (Baker, Jones & Callinan, 1985;Bishop, 1985).
However, when it comes to establishing the degree of confidence that we deserve in the behavior in service of a newly designed object, made perhaps of a rare material of which there are few examples, perhaps only one, and which is also going to serve an application of which there is no direct previous experience, it is clear that the doctrine commonly followed in the industry is not useful (Heru et al., 1997;Scarponi & Briotti, 2000;Miyano et al., 1994).
In these cases, not uncommon in applied research, the only possible guide is a judicious theoretical analysis of the service conditions and the behavior of the material under those conditions and according to a concrete design. The result is a theoretical "model", if you want "virtual", which only has the value of being a reasonable basis for approaching the real behavior of the object (Huang et al., 1998;Kapranos & Priestner, 1987). As this is insufficient, it will be necessary to make representative samples with controlled discontinuities and even better, a prototype to test it under conditions qualitatively similar to those of service and quantitatively more severe, being advisable to prolong the tests until the failure occurs. Only from the experience gathered in such tests and from the previous theoretical analyses is it possible to extract the essential information to draw up quality specifications and to establish reliable acceptance-rejection criteria.

Methods
For some time now, INTA has been developing a radar system for stereometric surveying of the relief of large areas of land. As the resolution is given by the opening of the system and this depends on the size of the antenna, to achieve high resolutions would be necessary very large antennas. As, on the other hand, the surveys are  (Potel et al., 1998;Clauser, 1952;Rosethal & Trolinger, 1995;Globe, 1979;Bales & Bishop, 1994;David, Hsu, Barnard & Daniel, 2004).
The applied essays (the basis of which is given as known) are the following: -Visual. -Radiographic. -Ultrasound. -Sonic. -Thermography. -Ultraviolet.
Penetrating liquids and in general any technique that involves contaminating the material with a coupling fluid were discarded (Wan et al., 2016;Silk, 1982).
The results achieved are described below, with emphasis on a comparison of those provided by one or another method.

Operating Conditions
Although depending on the method applied and the area of the parts where the specific operating conditions are applied, as a guideline we summarize below those that can be considered typical: -Visual: Light levels between 500 and 1000 lux. Observation with the naked eye or with low power magnifiers (< 10 d).
-Radiographic: X-rays from 40 to 45 KV. Exposures of 7 to 8 mA x min. without filters. Type I film without ASTM. Without reinforcement sheets. Focus-film distance irrelevant (900 mm was used). Focus of 0,8 x 0,8 mm.
-Sonic: Scanning with continuous Lamb wave between 2.5 and 70 KHz.

Discontinuities
Given the nature of the material, the usual designation of discontinuities may have a different meaning than the common one in NDT, which normally comes from those found in metallic materials. Therefore, an attempt will be made to introduce descriptive names for most discontinuities (Mouritz, Townsend & Shah Khan, 2000;Mitrevski, Marshall & Thomson, 2006;Hasiotis, Badoggiannis & Tsouvalis, 2007).
On the other hand, the visual definition of certain discontinuities is affected by the translucent nature of the material which can make its appearance diffuse. Finally, it´s necessary to note the difficulty of graphic reproduction of a good part of the images obtained, so that those included in this work give a very poor idea of the originals (Lawson & Sabey, 1970).

Test Pieces
In order to test the effectiveness of different methods in detecting flat discontinuities, a master specimen was used in which different artificial discontinuities were introduced, such as peel-offs between "skin" and core on both sides of the specimen and core breaks in the central plane of the specimen.
First, the specimen was inspected with ultrasound transmission in total immersion. The image shown in figure 2 was obtained, where it can be seen that all the discontinuities (dark areas) existing in the upper and lower bands of the test tube have been detected, as well as other discontinuities, located in the central band of the test tube, which were not suspected. In the image you can see four holes (light circles) located in the corners, around which there are also areas with detachments.          e. Figure 9b s while figures spectively.
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Limitations
In the case of visual inspection, the method provides immediate indications that often do not require elaborate interpretation. However, one should not be tempted to believe that 'what you see' is 'how you look'. In many cases, what is seen must be elaborated according to the materialographic aspects of the problem and even of the observation technique itself, such as the color of the light used, whether or not it is polarized, incidence of illumination, etc.
Radiographic inspection of this type of material does not present any difficulty. These materials are not very absorbent and require lower energies for inspection. If the thickness to be examined is small, the problem may be its excessive transparency that will force to have X-ray tubes with very low inherent filtration and even to dispense with the film cover, which will force to work under safety light. However, although it does not pose serious radiographic problems, the defectology of the composite material is usually not very conducive to radiographic inspection, since it consists of laminar-type discontinuities (delaminations) arranged parallel to the image and whose radiographic detectability is extremely poor. Another issue is the cracks or pores that could be present in the inspected piece, which do not present a problem to be evidenced, however this type of discontinuities are less frequent than the delaminations.
Manual testing by ultrasound requires highly qualified personnel. The large number of critical decisions to be made, together with the poor documentation of results, reduces the reliability of the tests, pushing this problem to the development of automated procedures. Small, irregular, rough or thin material samples are difficult to inspect. Heterogeneities very close to the surface are difficult to detect. Calibration of the test system and determination of certain characteristics of the defects to be inspected requires the use of standard or reference samples.
Defect detection techniques using Lamb waves (sonic techniques) allow continuous monitoring of the state of the structure and allow the integration of detection systems in critical areas of the part that require constant maintenance. These waves are dispersive, varying their speed of propagation according to the frequency. The efficiency of the transducer can be affected when the elements that compose it are close to the edge of the plate, and the latter can act in a reactive manner by interfering with the frequency band of the nodes. For the study of the edges, finite element analysis techniques are proposed.
The inspection using ultraviolet light does not detect too deep defects, and requires an electric power source. The roughness of the surface under study can affect the sensitivity of the test.
The methods of infrared thermography pulsed by reflection (infrared camera in front of the part as well as the heat source) and by transmission (infrared camera behind the part) present good results for small thicknesses, however, it is necessary a good adjustment on a test specimen with defects of different sizes and at different depths to adjust well this technique. It is very important the adjustments of the heat power provided by the focus and the times of incidence of the heat in the part since the processes of heat transfer could result negative in the visualization of the possible defects in the part.