ANALYSIS OF DRILLING PROCESS OF COMPOSITE STRUCTURES PART II: QUALITY CONTROL OF DRILLED HOLES

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ANALYSIS OF DRILLING PROCESS OF COMPOSITE STRUCTURES

PART II: QUALITY CONTROL OF DRILLED HOLES

Przemysław Sitek

¹

, Andrzej Katunin

^1a

, Anita Kajzer

^2b

1Institute of Fundamentals of Machinery Design, Silesian University of Technology

aandrzej.katunin@polsl.pl

2Department of Biomaterials and Medical Devices Engineering, Silesian University of Technology

banita.kajzer@polsl.pl

Summary

Wide application of polymer composites in various industrial branches and their treatment requires the development and adaptation of suitable methods of quality control after treatment. One of the most often technological operation on composite structures is drilling. The paper focuses on evaluation of drilled holes quality in two composite structures using the most common methods of non-destructive testing (NDT) applied for composite materials. The applied NDT methods cover microscopic imaging, ultrasonic testing, infrared testing and X-ray computed tomography. Moreover, the new delamination factor as well as an automatic detection method of delaminations was proposed. The obtained results of quality control of drilled holes in composites were compared and discussed.

Based on them the recommendations of quality control of drilled holes in composite materials were formulated.

Keywords: drilling of composite structures, quality control, delamination, non-destructive testing

ANALIZA TERMOGRAFICZNA PROCESU WIERCENIA STRUKTUR KOMPOZYTOWYCH – CZĘŚĆ II: KONTROLA JAKOŚCI WIERCONYCH OTWORÓW

Streszczenie

Szerokie zastosowanie kompozytów polimerowych w różnych gałęziach przemysłu i ich obróbka wymagają roz- woju i adaptacji odpowiednich metod kontroli jakości po obróbce. Jedną z najczęściej wykonywanych operacji technologicznych na strukturach kompozytowych jest wiercenie. W artykule skupiono się na ocenie wierconych otworów w dwóch strukturach kompozytowych, stosując metody badań nieniszczących najczęściej wykorzystywa- ne do materiałów kompozytowych. Zastosowane metody badań nieniszczących obejmują obrazowanie mikrosko- powe, badania ultradźwiękowe i termograficzne oraz rentgenowską tomografię komputerową. Ponadto zapropono- wano nowy wskaźnik delaminacji oraz metodę automatycznego wykrywania delaminacji. Uzyskane wyniki kontroli jakości wierconych otworów w kompozytach zostały porównane i omówione. Na ich podstawie sformułowano zale- cenia kontroli jakości wierconych otworów w materiałach kompozytowych.

Słowa kluczowe: wiercenie w strukturach kompozytowych, kontrola jakości, delaminacja, metody nieniszczące

1.

INTRODUCTION

Although the modern manufacturing technologies allow producing composite structures in very complex shapes, additional operation such as drilling is required at the final stage of manufac-

turing or during assembly. Due to the multiplicity of input materials and complexity of internal structure and bonding between particular components of composites several undesirable effects

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usually occur during this operation such as delamination, matrix cracking, fiber breakage, thermal damage, etc. [1]. Therefore, the application of suitable quality control methods became a standard after manufacturing. The most popular type of experiments verifying quality of drilled holes are non-destructive testing (NDT) methods. In order to ensure high quality of products, equipment and constructions, they are used both during manufacturing and during operation stage. The main ob- jective of high quality is ensuring safety of users, which depends from the quality and reliability of the equipment or construction. Generally, objec- tives of such studies are divided into immediate (evaluation and analysis of condition, measurement of geometric parameters and performance, control of production processes) and future (pre- diction by evaluating the risk of rupture, durabil- ity, suitability of operational and technological conditions) [2]. The most popular methods and techniques of NDT in composite elements are visual inspection, optical microscopy, dye pene- trant inspection, ultrasonic testing, laser testing, infrared and thermal testing, and radiographic testing.

For testing of drilled holes four methods were selected: optical microscopy, thermographic testing, ultrasonic testing and industrial computed tomography. The recent attempts in their application in quality control of drilled holes in composite structures were reviewed here and then applied in experimental studies described below.

The methods based on visual inspection are the most popular in evaluation of quality of drilled holes. To-date, the application of visual inspection methods (mostly using microscopes with low magnification) was reported in numerous studies [3-8].

Microscopic testing is based on the observation of damaged structures in reflected light. Due to the intensive development of computer technology, allowing for graphical presentation of large amounts of data, it is possible to formalize the description of the elements of the microstructure in relation to the analyzed phenomena. This method allows detection of cracks, delamination, surface bubbles and material discontinuity. The big advantage of this method is relatively low cost with respect to the achieved accuracy. However, using this method only surface damage can be detected

which, considering the damage mechanics of composites, is a significant limitation.

There are many ways to measure quality of drilled holes based on microscopic images. One of the most popular is the determination of the de- lamination factor Fd, which is defined as the ratio of the maximum diameter of the observed damage zone (Dmax) to the nominal diameter of a drilled hole (Dnom) [4]:

nom max

D

F_d = D . (1)

The ultrasonic testing methods are also applicable to quality control of drilled holes which is reported in several studies [9-11]. Ultrasonic testing include sending ultrasonic waves into specimen using a piezoelectric ultrasonic flaw detector head.

If the wave does not encounter obstacles, it is passed. However, if the wave encounters a discontinuity or exudate, its part is reflected and returns to head visualizing a discontinuity in the structure.

Due to the applied equipment, the following test methods are used [12]: pulse-echo mode (described above), through-transmission mode (it uses two heads: emitter and receiver) and time of flight diffraction (TOFD). From the available ultrasonic measurement techniques the most suitable and applicable is the C-Scan which allows visualizing a structural condition in planar mode. The main advantages of ultrasonic methods are versatility, short duration of an inspection and direct availa- bility of results. Sensitivity and accuracy at great- er depth is much higher in comparison with the most of available methods. Ultrasonics gives more information about defects around the drilled hole with respect to vision-based methods since it is possible to visualize internal damage sites. Howev- er, the resulting image is two-dimensional which limits information about internal damage to its planar projection. Moreover, it is difficult or impossible to study the very small elements.

Non-destructive experiments of composite structures also includes infrared thermography.

Due to the observed changes of measured temperature at the surface of specimen, this method is particularly used for testing shell structures. The method utilizes disturbances of heat conduction phenomenon or thermomechanical coupling under stress phenomenon [13]. In the case of study thermal response test, emitting-heat structure (passive

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thermography), or heat-excited structure (active thermography) conductivity of damaged areas is different in relation to the rest of the structure resulting in a local change of temperature in the defect.

The newest and the most advanced method of evaluation of damage in composites caused by drilling is the X-ray computed tomography (CT).

This technique is a type of X-ray spectroscopy which allows obtaining a 3D image of the object with a very high resolution. To-date, it is the most accurate method of NDT available. CT uses the specimen images taken from different directions in order to create cross-sectional (two-dimensional) and spatial (three-dimensional) images through beam radiation and registration its intensity on the panel detectors. The method is sensitive enough to detect not only the marco- and mesoscopic damage (such as delamination or debonding), it can identify a damage of micro- and nanoscale (such as single fiber breakage, fiber pull out, etc.). In the evaluation of damage after drilling this method also found wide applicability (see e.g. [10,11]). The main drawbacks are the need of an experienced staff and software and the high cost of research equipment.

The main problem encountered during drilling process of composite materials is a delamination.

The literature distinguishes the loss of cohesion between the various components of the composite (debonding) or between the whole layers of the laminate (delamination). Delamination occurs because the changing of deforming forces and temperature. Another reason of delamination occurrence are technological errors in manufacturing process. Delaminated composite is characterized by significantly reduced stiffness and strength.

It is categorized as peel up (during entry) and push out (at the exit) [14-16]. Peel up arises from peeling of the laminate layers at the junction of the side cutting edges of the drill. When the drill approaches the output side material, uncut layer becomes more sensitive to pressure due to the reduction in a thickness of a material (see Fig. 1).

Fig. 1 Mechanisms of delamination: a) “peel up” at entrance, b) “push-out” at exit [17].

The main goal of the presented study is to compare the most often used methods of quality control of drilled holes and formulate the recommendations of application of considered NDT method for the aforementioned tasks.

2.

TESTING METHODOLOGY

Two types of composites were used for study- ing: glass-epoxy TSE-2 laminate and sandwich laminate. They are described in part I: Evaluation of thermal condition. The holes were drilled using various types and diameters of drill bits. The detailed description of parameters and procedure of drilling was described in [18].

2.1 MICROSCOPIC TESTING

The microscopic testing was performed using Carl Zeiss SteREO Discovery V8 microscope with a camera and a PC. During experiment magnification was changed to obtain similar size of holes.

For the smallest holes (5 mm and 6 mm) magnification 7.5× were selected, for holes 7 mm and 8 mm magnification 6× were selected and for the largest diameters (9.5 mm and 10 mm) magnification 4.8× were selected. Obtained images were saved in jpg format at a resolution of 2560×1920 pixels. Exemplary images are shown in Fig. 2.

Fig. 2 Exemplary images of drilled holes. On the right: a hole in TSE-2 composite drilled using Bosch drill bit, on the left: a hole

in sandwich composite drilled using wood-dedicated drill bit.

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Then, all images were processed using MATLAB algorithm (described in section 3.2).

2.2 THERMOGRAPHIC TESTING

Experimental setup consisted of the following elements: Variocam-HR infrared (IR) camera with IRBIS 3 Plus software installed on PC, vise for mounting specimens and clamps, two halogen lamps OR-3m made by POLAM, 1kW power each.

The halogen lamps were used for thermal excita- tion of tested structures, while the measurements were performed by IR camera.

The experiments were performed using the following parameters: distance between camera and specimen of 30 cm, emissivity of 0.9 and an ambi- ent temperature of 24°C. In order to increase the emissivity of specimens their surfaces were covered by matt black paint. Due to the ability to observe internal defects in the structure, there was chosen a methodology based on transient thermography.

Fig. 3 shows scheme of experimental setup: 1 - computer with software (data acquisition and processing system), 2 - thermographic camera, 3 - halogen headlights, 4 - specimen, 5 - heat, 6 - reflected thermal energy, 7 - lamp switch.

Fig. 3. Scheme of experimental setup

Due to the difficulty in obtaining appropriate focus of the camera it was decided to take picture series for 5 holes for laminate TSE-2, and after 15 holes (3 rows of 5) for the composite sandwich specimen simultaneously. Obtained images are exported to jpg format and then analyzed using MATLAB algorithm.

2.3 ULTRASONIC TESTING

Two techniques of ultrasonic testing were applied in this study: air-coupled technique and pulse-echo technique of ultrasonic testing.

The air-coupled testing was performed using HFUS 2400 AirTech by Ingenieurbüro Dr. Hillger with Hillgus software (Fig.4). Experimental setup is intended for testing by through-transmission mode. The spatial resolution of an ultrasonic scan was set to 125 µm. In accordance with the manu- facturer's instructions the distance between the transmitting head 250 kHz AirTech 4412 and receiving AirTech was set at 10 cm. The gain was experimentally changed in the range from -60 dB to -18 dB. Received sonograms were saved as bitmaps with 16-level color scale. In order to avoid direct contact of the heads and because of small specimens size, their widths were artificially in- creased. The experimental procedure covers the following: specimen preparation (closing holes before scanning), mounting specimens in a clamp, defining of sample size in a software (in X and Y direction) and its resolution, and gain selection.

Fig. 4. Experimental setup for ultrasonic testing in air-coupled technique: 1 - ultrasonic heads, 2 - control unit, 3 - manipula-

tion system SUSE, 4 - grips

The pulse-echo ultrasonic tests were performed using Sonoscan c-sam series D-9000 scanner with a waterproof SK 15/0752 head (frequency 15 MHz) and dedicated software. The spatial resolution was set to 125 µm. The experimental procedure covers the following: filling reservoir with demineralized water, fixing the samples in water, setting height of the head, setting size and resolution of a scanning area. Received sonograms were saved in jpg format and then processed using the MATLAB algorithm.

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Fig. 5.Scanning head of Sonoscan c-sam series d-9000

2.4 X-RAY TOMOGRAPHIC TESTING

The experiment was conducted using v|tome|x L450 manufactured by General Electric Sensing &

Inspection Technologies GmbH. The collected images were represented as three-dimensional matrix of voxels and reconstructed in a dedicated software VGStudio.

In order to perform tomographic experiment specimens need to be properly prepared – cleaned and placed with a distance one upon the other, to scan all specimens in one scan. The experimental procedure covers the following: mounting specimens in three-jaw chuck, setting appropriate oper- ating parameters tomography (power, distance from the specimen, etc.), taking images with 0.25º rotation, reconstruction and data processing on workstation with VGstudio software. The experimental setup is presented in Fig.6.

Voltage lamps were set to 150 kV and a current to 140 µA which gave total power of 21 W. The resolution of a single image was 2048×2048 pixels.

Fig.6 . Tomograph view: 1 – X-ray tubes, 2 – detector, 3 – rotary three-jaw chuck, 4 – inverters, 5 – specimens

3. EVALUATION OF

EXPERIMENTAL RESULTS

3.1 DEVELOPMENT OF A TOOL FOR

SUPPORTING QUALITY CONTROL OF DRILLED HOLES

The proposed algorithm is based on the observ- er methodology and, as the name suggests, it is under the control of the user, and the quality of the results depends mainly on his experience. The advantage of the algorithm is that it can be used in any available advanced graphic processing software such as Adobe Photoshop or free ImageJ.

Since the algorithm can be automatized, tools available in the Matlab Image Processing Toolbox software were used.

General diagram of the quality control algorithm consists of four stages. The first stage includes all the operations related to the preparation of the input image, such as convert the image to the adopted format, cropping the image to the ROI and brightness alignment. In this case, the JPEG format is adopted due to the fact that the great part of images already been acquired in this format.

The second step is a segmentation. It consists of operations aimed at separating areas of the image due to the criterion of measuring the quality of holes. In an exemplary algorithm it was assumed that quality control is based on the delami- nation factor Fd (1) except that circles need not be concentric. This allows for a more accurate result coefficient, when the damage is configured asym- metrically. Therefore, it was necessary to obtain the image data of two circles diameters – the maximum diameter of damage and the nominal diameter of the drilled hole. Resulting image was converted to a grayscale and then binarized using the simplest and the computationally fastest thresholding methods. We also tested various types of colors, and their isolation, but will ulti- mately remain the embodiment simple conversion to gray scale based on the weighted sum:

B G

R

I=0.2989 +0.587 +0.114 , (2) where I is the output image in scale of gray, and R, G, B, are intensities of red, green and blue, respectively.

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The thresholding level was determined based on a histogram (Fig.8A) and various algorithms of manual (Fig.7) and global thresholding using the Otsu method (Fig.8B), entropy-based Kapur method (Fig.8C) and iterative Ridler-Calvard method (Fig.8D) were analyzed. The local thresholding methods such as the Bernsen method were rejected because of the long time algorithm.

As one can notice in the histogram (Fig.7A)), it is possible to apply the Otsu method and it gives satisfactory results due to the fact that the distri- bution is bimodal. Also very similar results were obtained using iterative method. The next step was image binarization with a predetermined threshold by Otsu method, and detection a circle using the Hough transform. It special attention should be paid to the argument of the function referred as a sensitivity, since it is depends on the effectiveness of detection of a circle. As the sensitivity factor value increases it detects more circu- lar objects, including weak and partially obscured circles, but exaggerated values increase a risk of false detection. The function returns coordinate values and diameters of detected circles.

The next step is detection of diameter of an area of delamination. For this purpose a previously created image in grayscale was used and denoised using the Wiener filter.

Fig. 7. Manual thresholding of an image – A) input image, B) converted image to grayscale, C) threshold = 120, D) threshold = 140, E) threshold = 160, F) threshold = 180.

The next step was to highlight the outer by using the operator of the first kind – the Roberts cross operator. The advantage of application of such a filter is a shortening of processing duration comparing to the other edge detection algorithms such as Sobel, Canny or Prewitt. Then convex hull was generated and the center of gravity was determined in order to determine the center of the circle around delamination. Next, a double loop was used to find the greatest distance between the center of gravity and the far part of the environment which was adopted as the radius of delamination. It should be mentioned that it is not possible to define a circle on each convex polygon.

The third step is a calculation the delamination ratio, and the last is displaying and analysis of the accuracy of detection of the circle (Fig.9). If the detected circles seems correct, it is possible to automate operations for all similar images and determine delamination factors.

3.2 RESULTS OF VISION-BASED INSPECTION

Based on micro-examination it is possible to determine external damage. The table below pre- sents the average delamination factors that were detected by the delamination factor algorithm (Tab.1). It turned out that for the TSE-2 composite the best (from among the tested ones) are the drills Bosh HSS-R dedicated for metal, whereas for sandwich structures the least damage sites were observed using wood-dedicated drill bit. The worst results were obtained for 7 mm unbranded HSS drill bit, most probably because of the poor quality of the drill. Except of the obvious factors, remains of inaccurately cut edges had an influence on the final delamination factors. These results addition- ally confirms the results of thermal analysis of drilling process presented in [18].

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Fig. 9. Specimen view after finding both circles and evaluation delamination factor

In the tested sandwich structure all the holes are characterized by large delamination factors, the drills are completely unsuitable for processing of such a composite. One can clearly see not only delaminations and cracks, but also inaccurately cut parts of a core and face sheets (Fig. 10).

Tab. 1. Delamination factors form microscopic tests.

material drill bit type Ø, mm

AVG Fdel

TSE-2

Bosch HSS-R 5 1.26

Bosch HSS-R 7 1.292

Bosch HSS-R 9.5 1.294 unbranded HSS 5 1.277 unbranded HSS 7 1.888 unbranded HSS 9.5 1.273 wood-dedicated 6 1.316 wood-dedicated 8 1.288 wood-dedicated 10 1.304

sandwich

Bosch HSS-R 5 1.543

Bosch HSS-R 7 1.751

Bosch HSS-R 9.5 1.5

unbranded HSS 5 1.606 unbranded HSS 7 1.532 unbranded HSS 9.5 1.611 wood-dedicated 6 1.415 wood-dedicated 8 1.538 wood-dedicated 10 1.323

Fig. 10. Specimen honeycomb view

Probably, it would be more suitable to use pol- ycrystalline diamond bit.

In Tab. 2 the greatest detected hole diameters used for estimating delamination factors together with the nominal diameter holes are presented. In order to evaluate real diameters of the detected circles based on the diameters expressed in pixels the information about the zoom of the image was received. The greatest diameter error is 13%.

Taking into account that because of the perspective view of the circles it was the inner circle that was subjected to detection, the error is quite small.

It should be noted that the drill has a less diameter than its mark. This follows also from the exe- cution tolerance usually from h8 to h12. If in the hole remained uncut material, then the algorithm reveals a less diameter. Thus, only maximal diameters were taken into consideration.

Tab. 2. Comparison of detected diameters of holes to drill sizes for TSE-2 composite.

drill bit type Ø,

mm max size error

Bosch HSS-R 5 4,79 5%

Bosch HSS-R 7 6,1 13%

Bosch HSS-R 9.5 8,46 11%

unbranded HSS 5 4,71 6%

unbranded HSS 7 6,6 6%

unbranded HSS 9.5 8,47 11%

wood-dedicated 6 5,24 13%

The main disadvantage of the presented algorithm is the necessity of continuous monitoring which consists in verifying the accuracy of detec-

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tion circles on the resulting images and make adjustments sensitivity and filter parameters. If algorithm works continuously with images of one type (e.g. only with TSE-2 composite having a predetermined diameter and similar lighting conditions) a precise selection algorithm parameters is possible.

If the bit is improperly matched to the material, the damage zone can reach values exceeding twice nominal diameter of the hole. Both tested laminates require specific cutting tools.

3.3 RESULTS OF THERMOGRAPHIC ANALYSIS

While conducting research it was made an at- tempt to register the sequences of individual holes.

However, as the distance from the camera was very small, the proper adjustment of the focus was impossible. Initially, it was assumed that the presented above algorithm would suit also the analysis the thermographic images. Yet, due to very low precision, and thus, resolution of the images, only the visual evaluation was performed.

What is interesting, the better effect was achieved while the heating, not the cooling down.

In this case brighter areas with the higher temperature are visible. Their faster heating up was caused by smaller thermal capacity.

The fact that the choice of thermographic analysis is appropriate was confirmed after analysis of samples with holes drilled using 35 mm Forstner drill bit. One can see extreme examples of damage.

Standard drill bit of such type is not suitable for processing the examined composites, especially for the great diameters, the core drill bit would be fit for use. On the areas, where, as a result of high temperature, the resin around the holes had evap- orated one can see extensive delamination (Fig. 13). The quality of the holes is very low.

Fig. 11. View of specimens: on left: TSE-2, Forstner, on right:

sandwich. 1 – delaminations, 2 – cracks, 3 - inaccurately cut edges

3.4 RESULTS OF AIR-COUPLED ULTRASONIC ANALYSIS

The obtained images were very blurred (see Fig.12). Hence, the analysis of these images did not have sense. There was conducted a trial of scanning after removing the filler. However, the results obtained this way were still unclear. It is interesting that the wave resistance of the plasti- cine used for filling the holes is very similar to that of laminate 'TSE-2'. This caused another difficulty in interpretation of the results. One can conclude that this NDT technique is not suitable for damage assessment after drilling of composites.

Fig. 12. “Honeycomb” specimen sonogram

3.5 RESULTS OF PULSE-ECHO ULTRASONIC ANALYSIS

In Tab. 3 the delamination factors together with their averages for TSE-2 composite were presented. The obtained delamination factors are considerably less. This is a consequence of the lack of perspective and fact that the most of the surface damage sites that were visible when using the microscopic method are not noticeable in this case.

Thanks to the ultrasonic analysis it is possible to obtain information about inner defects. For the TSE-2 composite the least damage was detected for holes drilled using wood-dedicated drill bits, the largest – using unbranded HSS drill bits. Due to the fact that the surface damage sites are almost invisible, it is impossible to compare reason- ably the obtained results with those obtained by

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microscopic method. The results are rather a supplement to the microscopic ones. In the case of sandwich composite the damage sites on the edges of holes could not be detected because of the speci- ficity of a tested structure. However, detecting cracks is immediate. Moreover, the obtained image is susceptible for disturbances like water drops.

Summing up, C-scans give the information only about the damage laying in a 'flat' manner that leads to significant loss of information about damage in the direction that is normal to the examined surface. Therefore, the method is usually of an approximate character.

3.6 RESULTS OF X-RAY COMPUTED TOMOGRAPHY

The information about the damage in this method can be considered as a sum of information of all other methods, i.e. we get the information about the external defects as well as the inner ones.

Fig. 13. Exemplary sonogram of a sandwich structure Tab. 3. Averages of detected diameters for TSE-2 composite

drill bit type Ø, mm average

Bosch HSS-R 5 1.149

Bosch HSS-R 7 1.165

Bosch HSS-R 9.5 1.157

unbranded HSS 5 1.263

unbranded HSS 7 1.438

unbranded HSS 9.5 1.094

wood-dedicated 6 1.098

Since the obtained 3D images have very high quality, the tomography enables verification and analysis of each case deeply and separately. The images are very detailed and accurate. This shows high potential of X-ray CT. A disadvantage that follows from the accuracy of the model is a huge amount of data that requires very efficient experimental set-ups. Because of the small amount of results (only three holes of the greatest diameters in material of each type) it is only possible to compare the results based on one drill bit diameter. One can observe the damage around the drilled holes in TSE-2 composite (Fig.14). The core and face sheets damage is observed in Fig.15.

Their inappropriate processing is noticeable. Based on the obtained models it was noticed that in the case of TSE-2 composite the least damage was caused by Bosch drill bit, whereas in the case of sandwich composite the wood-dedicated drill bits are a little bit better than the others. In general, the quality of holes drilled in the sandwich structure is very low. This proves the claim that the used drill bits are not suitable for such composites.

Fig.14 . Reconstructed model of TSE-2 specimen

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Fig.15 . Reconstructed model of honeycomb specimen Based on observations of delamination factor performed on successive layers of the image using developed algorithm (Fig. 16) at most samples had the highest ratios delamination at the exit of the material.

Fig.16 Graph showing the relationship between the thickness and delamination factor of the material of laminate "TSE-2"

This is due to push out the output layer while drilling without the support and exceeding the critical pressure (see Fig.17 for instance).

Fig.17 Cross-section in the location of a drilled hole with visible output delamination

4. CONCLUSIONS AND DISCUSSION Quality control of holes in parts made of a composite material still causes many technical difficulties. These complications are the result of delamination of the material and internal damage, often invisible from the outside. Quality control is an essential part of the manufacturing process and should be performed fast and accurate. The conducted experiments show that not every method gives a satisfactory result for each type of composite, because each case is slightly different and it is impossible to develop a method and algorithm ideal using any measurement equipment. Current- ly, the most popular and giving the most reliable results of the holes inspection is computed X-ray tomography. Most prototypes and random elements are controlled by this method. As only they allow for an almost ideal verification by comparing obtained CT scan with CAD model. Another widely used method for quality control of composites is thermography, but it is impossible to detect small defects using the standard equipment.

Microscopic testing method can give equally satisfactory results. Based on study and analysis of the literature it can be concluded that a properly prepared environment for automated video testing (adequate lighting, high contrast) is a fast and effective tool for controlling the quality of drilled holes. Due to the relatively low price of implemen- tation and adaptability one can conclude that under certain specific assumptions and applications is a good tool for a comprehensive assessment of the characteristics of holes in laminates. The proposed algorithm, despite its simplicity is fast enough. It is possible to increase its accuracy, by changing the quality assessment. An important advantage of discussed testing methods is the fact that they are of a non-destructive ones. It is obvious that destructive testing, may also provide valuable information about a structure, method or drilling parameters, but prevent further use of the item, causing cost increase.

The research presented in this paper was partially realized by the first author during the research visit at the Institute of Lightweight Engineering and Polymer Technology, Technical University of Dresden, with- in the framework of EU project no. UDA-POKL.04.01.01-00-078/13-00 “Boundless engineer – new forms of education based on international cooperation”. The authors acknowledge Marek Dańczak and Dr Paweł Kostka for their assistance and scientific support during ultrasonic and tomographic tests.

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