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New Method and Software for the weldings Non-destructive Testing Eng. Mircea Tîrziu, SC "Symphony" SRL Email: mircea_tirziu@usa.net Contact

Abstract

One of the most appreciate and wide spread method used for the weldings quality testing consists in the radiographic inspection. Any attempt to use the computers to store, to process and to make diagnoses is strong restricted by the very hight price of the scanners, which must work through transmission anf must keep up the errors level at an acceptable level. But, even some companies buy the required scanners, the handled radiographies remains black-and-white, even in other scientific fields are used more techniques to colorize them. In this article the author shows a specialized software that offers to the user the power to make any colorization that he could wish in a very friendly manner. He will define the most relevant colors in a semnificative shorter time as any other program. Due to these, the author was able to discover a new concept in the colorization of the radiographies, that reveals new facts. The obtained images, constructed by using the new technique, named "SuperRainbow", depict a completely new face of the radiographies and the interpretation of these new pictures is only at the beginning. In fact, it could be a promise for a new weldings testing method.
Keywords: welding, radiography, nondistructive testing, color, new method, SuperRainbow

1. Introduction

Wide sprend, the use of the radiographies in the weldings quality testing involves the visual inspection of the black-and-white radiographies. By using the digital techniques the control becomes better: the user can modify the contrast and the brightness of the image, can magnify the image and can apply some kinds of digital filters over the image. By using the computers the radiographies can be stored and it become possible to build a specialized data base and so the user can view and compare a large amount of images, or can make statistical processings. Nevertheless, the use of the digitized radiographies is limited. It could be also observed that the radiographies remains only in a black-and-white shape, even in other research fields, as in medicine, there are ued colorized radiographies. Very interesting, there are no articles concerning any attempt to colorize weldings radiographies.
As I will show, the scanning of a radiography in the nondistructive testing is not an easy thing because the common scanners (that work with reflected light) can hide some relevant aspects (in a radiography, that is semi-transparent). I will show how this facts acts and what it could be make to decrease the errors. The result is that it must be used special scanners for radiographies, that have a high price.
Either there are used common scanners, that hide the relevant aspects, or there are used suitable scanners, that are very expensive, in both cases the effect is a limited spread of the digital processing.
This article describes a technique (named "SuperRainbow") that transforms a black-and-white radiography in a color one in a new way, that depicts in a single image as distinct colorized level of gray as the user needs. With this technique I obtained a new set of images that reveals new facts in a radiography. I supose that the new images can be corellated with the weldings quality and I trace some posible future ways in research. Very important, the images can be obtained with low price scanners and this could help to a spread of the using of the computers in the weldings radiographies testing.
To discover the new techinique I was helped by a specialized software created by me and named "PhotoMir". It contains both the common commands required to make the radiographies analysis and more new commands. One of its new features is a strong set od tools destined to colorize a black-and-white radiography with a minimum of effort and very quickly, with a complete freedom in the colors choosing. The program may be installed with a Setup procedure, has samples and has a rich set of color definitions that allows to the user to make colorizations by a single mouse click.

2. Materials and Methods

The basis of this research consists in the specialized software. At the beginning, the program was created only to colorize the radiographies. In time I understood the problems that occur when someone needs to make an real useful colorization and I modified the programs to help the user. So, I created a powerful set of colorization tools. In the same time, I felt that is better if the user has common commnads in my program than if he should use other software. So, I included in PhotoMir a lot of commands used in the radiographies analysis. I also added more other commands which are used in the image processing (some of its being new), samples, a help file and a Setup program, items that allow the user to be familiar with the program and the new concepts in a short time. I also compressed all these in a zip file that has about 5.5 Mbytes and a low price..

I tested PhotoMir with many kinds of black-and-white images and I modified it to be more and more useful. In the weldings radiographies field I used radiographies made in different conditions and scanned with more kind or scanner. The radiographies presented in this article are made in the nondestructive testing department of a wagons factory, scanned with various kinds of scanners and colorized with my software.

3. Results

Scanning and errors
The usual scanners introduce a large amount of errors in an image. This surprising fact is caused by the transparent nature of the radiographic film and can be proved by making a scanning of any welding radiography. To explain how apear the errors, it could be necessary to give some descriptions of the scanning process.
Any image can be splitted in very litle areas (as litle as they are named dots, points or pixels) that are arranged in rows and columns as an array. Each element of this "array" has its color. If the image is a black-and-white one each pixel has its level of gray.
To digitalize an image it is used a scanner, that "read" the level of gray of each point. Usually there are used 256 levels of gray and this means that for a black point the scanner will "read" the value 0, for a bright white will "read" the value 255, and for a gray level will read values in the above limits. The image, that are viewed as an array of points, has a correspondent array in the computer memory which holds all the values "readed". So, to scan an image is equal to fill all the elements of the array stored in the computer with the values obtained with the above rule.
One important characteristic of all the scanners is named "resolution" and it describes how many points contains an linear inch. As an example, a resolution of 300 dpi (dots per inch) means that the points have a size of about 1/10 milimeters. To see more details it is required a better resolution and it is a trend to increase this parameter. So, a 9600 dpi resolution is very usually today.
To "read" the images the scanners use two methods. The first uses light rays that are reflected by the scanned picture, a white point reflecting more light that a black one. The reflected light excits an optical device (usualy a photodiode) that transforms the light in electricity and a special electronic device transforms this in a number (usually in the 0-255 range). The second method can be used only for transparent media and uses light rays that passes throught the material. As transparent is a given point, as many light are transmitted to the other side.
Almoust all the scanners thet are on the market works with reflected light.
When I tried to use different kind of scanners to digitalize a welding radiography I obtained the folowing results:

1. Scanner that works with reflected light having a black wall in the back of the radiography. The iamge was almoust black, unusable.
2. Scanner that works with reflected light having a mirror in the back of the radiography. The obtained image had a poor quality, with all the little details (as the pores or inclusions) hidden.
3. Scanner that works with reflected light having an adaptor for transparent media. This means that the above wall has a light source and a disperser that eqalizes the light flow against the image. The image became more clearly and some of the litle details may be observed (with great difficulty). The obtained picture is not suitable to be used for a conventional test.

To explain these results we can observe that the radiographic film has a great thickness. The first two methods use light rays that are reflected at a great angle from the perpendicular to the radiography surface. Due to this fact, the rays colected by a the device that "read" the reflected light (a photodiode) are influenced not only by the readed point characteristics, but also by the characteristics of neighbour points. More, in the radiographyc film there are reflexions, refractions and disperions that allow to rays originates from other points to excit the "reading" device. So, the readed value is more influeced by the neighbour points that the point itself. This phenomenon covers large areas and hides the small details, even the scanner has a high resolution (for opaques images!). It cann't be eliminated because the influence of the neighbour points changes in each point. The difference between the rezults obtained with the first two kind of scanners is due to the mirror: in the first case almoust all the light is absorbed, in the second one there are a great amount of light that are reflected. More, the rays having a given angle are facilitated and so the quality increases.
The third method offers a less influence of the neighbour points and so the quality increase more. Although, the image clarity remains very poor. The explication consists in the direction of the light rays, that are not perpendicular to the film surface because it is used an disperser.
The conclusion is that it is required a transmissive scanner that uses only rays that are perpendicular to the radiographic film. Such a device is very expensive (about 50,000 $).

About the colorization
Even an usual digitized black-and-white radiography has 256 gray levels, a trained man's eye cann't distinguish more that 15-20 distinct levels. Due to this fact, a lot of details remains covered. To help the man to see more levels, in some scientific areas (as in the medicine), the images are colorized. By this technique the image becomes a colored one and the man's eye being much more sensible to the color as to the gray level, the image offers more informations.
Having a black-an-white image with the gray levels in the 0-255 range, to colorize them means to give to each level of gray a color, for example to the 0 gray level we could give the red color, to the 1 gray level we could assign the orange color and so forth. The 256 levels of gray will be transformed in 256 colors. There is not obligatory to use 256 distinct colors. For example, we can set the same color for more neighbour levels of gray (as 3,4 and 5) or for very different level of grays (as 0 and 200).
Any color may be viewed as a combinations of the three basic colors: red (R), green (G) and blue (B). If we use digital colors, we can usually vary each basic color level in the 0-255 range. For example, to generate a light red we will give the maximum value to the red component (255) and the minimum value to the other components (0). We will note this by red=(255,0,0). Others colors are: black=(0,0,0), yellow=(255,255,0), white=(255,255,255) and a mediun gray=(128,128,128),
There are many other methods to generate colors and I will show only one, very useful. It uses the triplet hue (H), saturation (S) and brightness (B). In a few words, the hue is the wavelenght of the color and starts from red and ends in violet, the saturation tells us how many white color is contained in the sampled color (a saturated color has no white, whereas a gray or white color is a completely unsaturated color) and the brightness range from black to the maximum intensity of the color.
We must to establish a link between the gray levels and a parameter of the colorized image. For example, if we transform the black (r) gray (r) white scale in a red (r) yellow (r) green scale, we will know that as green is a given point in the colorized image, as white it is in the original radiography. Making this transform we will build a colorized image in which we will distinguish more levels as in the unprocessed radiography.
But this is only a step because we will find that there are more levels that cann't be separated. We will need to seek other techniquesm more suitable.
One good improvement is to convert the gray scale in a hue scale, when we will be able to see more distinct colors in the image. In many cases this could be enought, but we can observe that remains more colors that cann't be disinguished.

New colorization methods. The "SuperRainbow" technique
To increase the difference between neighbour colors I varied in the same time not only the hue, but also the brightness and the saturation. I observed that the eye is more sensible to the brightness changes only in a restricted area, around 50% of the bright colors brightness. I alse understood that the man's eye sensibility to the saturation changes is good only for the colors near saturated. For these reasons, this two parameters can be used only for low ranges.
Using these restrictions, I converted the black-and-white radiographies in colored radiographies with colors ranged in the spectrum, with simultaneous changes of the brightness and of the saturation. With this method I obtained useful images in which each level of gray can be viewed as a distinct color.
I used my software, named "PhotoMir", to create this method and, due to the fact that this program allows to change easily the colors, I improved it. The new method consists in:

1. Choosing the gray levels that are interestind in the analysis. For example, 55 to 180.
2. Choosing an interval in which we would like to generate the spectrum colors. For example, 100-129. This means that for the gray levels varying from 100 to 129, the colorized output image will have points with colors in the range red (r) violet
3. The above interval will be repeated over the whole interest interval. This means that the above spectrum will be also generated for the 55-69 (incomplete), 70-99 (complete), 130-159 (complete) and 160-180 (incomplete) intervals.
4. The saturation and the brightness will change along the choosed interval (55-180).

The technique has some variants:
1. It is useful to colorize with the same color more gray level to bring togather more areas that mean the same item.
2. The spectrum can be generated from intervals having different sizes.

Using this technique for the weldings testing I obtained a spectrum that repeats itself, with little color changes in the image plane given by changes in saturation and in brightness. For this reason I named it as "SuperRainbow".

The advantages of this technique are:

1. The image shows very clear all the gray levels.
2. They can be more that 256 intervals that are viewed distinguish

"SuperRainbow" technique and the weldings quality testing - results
I used more weldings radiographies to test the new method. I scanned them and I processed them by using the "PhotoMir" software. In Figure 1 is presented an radiography made in a wagons factory, at its nondistructive testing department. The image was scanned wiith an Hewlett Packard scanner with a transparence adapter. As I shown above, all the little items (as the pores and the inclusions) are near hidden. The image is unusable for any conventional nondistructive test. Figure 2 shows the same image, colorized with the "SuperRainbow" technique. I delimited an interest gray area and all the gray level outside the desired interval were set to black and white. As a first results, the image shows clearly that the welding shape is not circular; it is deformed. The image is also very useful because shows clearly the whole interest interval range at a glance. In the colored image each color corresponds to a range of gray levels. Due to the expected welding tridimensional shape, it could be expected to be seen colored strips, but an inspection over the image shows more interesting items that I named "waves" and "beaks". These details show that the opacity to the X-rays varies not linear and this fact could mean that there are areas where the thicknees of the sample has local changes or there are elements (as pores or inclusions) with changed opacity. These elements could signify local stresses that could make cracks in time and so to be harmful.for the welding. In the same time, they could mean local streches of the material that can modify the elasticity upon a perpendicular direction to the strips. Even if to know if these elements show a better welding or a worse one it must made tests, it is very probable that these elements mean changes of the welding quality. I also supose that the apparition of these items can be used to test the men and machines that make solders.

Fig 1: The scanned radiography of a circular welding

Fig 2: The above radiography colorized with the "SuperRainbow" technique

Fig 3: A radiography that shows a "porosity"

Because the image can show clearly all the interest interval at a glance, it become posible to analize in the same image not only the welding field, but also the welded materials, that could be affected by the welding process.

I would note that I used more variants of the "SuperRainbow" technique and I obtained more different images, showing different aspects of the same structure. I concluded that it is very important to find the right variants and the used software is very suitable to make this in a short time.

In Figure 3 is shown a portion of a radiography of a linear welding in which one can show in the upper-right side a mixture of red, yellow and green large size points. Because the color of the points reflects the opacity of the welding in the mentioned place, I supose that the picture depicts a granulated structure and will be interesting to be made future researches to establish the microscopic state of the weldings that gives such images.

Conclusions

The article presents the results obtained with the author's new software entitled "PhotoMir". One of the main findings is a new technique named "SuperRainbow", that depicts a completely new shape of the weldings radiographies. There are presented some images obtained with this new method, explications and some possible future ways in research. For any problem concerning the new technique or the software the author can be contacted at mircea_tirziu@usa.net

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