Ultrasonic Testing Of Carbon Epoxy Laminate Ply Drop

Transcription

1 International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 14 Ultrasonic Testing Of Carbon Epoxy Laminate Ply Drop * R.Sultan, 2 S. Guirguis, 1 M. Younes, $ E. El-Soaly Abstract-- Composite materials are widely used in a number of industrial sectors from aviation, space, to boat building, automotive, and sports goods. Ultrasonic C-scans are used frequently to detect defects in composite materials caused during fabrication and damage resulting from service conditions. Evaluation and characterization of defects and damage require experience and good understanding of the composites. Under low-velocity impact loading delamination was observed. Impacts simulating practical conditions like tool-drop and runway debris induce defects such as delaminations, which are sub-surface in nature and are barely visible on the surface. Hence detection, evaluation and characterization of these delaminations are of major concern in aerospace industries as the life of the component depends on the size and shape of the delamination. In present paper, the results of extensive experimental ultrasonic C- scans using the pulse-echo immersion method on carbon epoxy laminate ply drop specimens impacted by low-velocity impact test were introduced to detect the invisible delamination. The capability of C-scan testing to detect the delamination was investigated. Also an attempt was done to solve the problem of sound wave lack at ply drop zone. Index Term-- Ultrasonic test, C-scan, Carbon epoxy laminate ply drop, Delamination. Nomenclature J Joule MHz Megahertz NDT Non destructive test PC Personal computer 1. INTRODUCTION Carbon fibre-reinforced plastics were widely used in aeronautics and aerospace industry, on account of their high stiffness and strength as well as their low density. These materials presented specific properties such as stiffness/weight and strength/weight ratios higher than those of metallic materials [1, 2]. Examples include the carbon fiber reinforced plastics are tail fin of Airbus A300, A380, and the empennage of the Boeing, they are now considered primary structures. The conventional metallic materials used for the design of structural components like the wing, fin etc. of aircrafts are inhibited by their high densities resulting in a reduction of payload. The evolution of laminated composite materials has opened new vistas in the development of the aircraft industry. The laminated composites offer a weight reduction thereby increasing the payload. A unique feature of laminated composites was the layered construction. The structural components were made by laying plies over one another so that the required thickness of the laminate was achieved. The laminate made in this manner was flat since the thickness of each ply was uniform. In practice, however, tapering of laminates is necessary in many structural components. The tapering in laminates is introduced by terminating plies at different locations. This is known as ply drop-off. The laminate tapers from a thick section to a thin section as a result of ply drop-off. Application of ply drop-off is common in the wing and fin skin structures, helicopter rotor blades, etc. The rotor blades of helicopters or wings of an aircraft have a thick section at the root and a thinner section at their tips. This is achieved by dropping plies internally at different locations. Plies may be inserted, on the other hand, at access holes, lightening holes and at joints or connections to strengthen them. In all these applications, use of ply drop-off results in significant saving in the material and therefore, it is cost effective. Also, the resulting reduction in the weight considerably improves the payload of the aircraft. The termination of internal plies is an effective method for stiffness tailoring which is not possible with constant thickness laminates [3]. While in service, the composite structures may suffer damage from a number of natural and man-made causes, the most significant damages were caused by mechanical impact. Low velocity impacts on composites often lead to non-visual defect on the surface but significant delamination internally. Impact delamination can result from dropped tools in a maintenance hangar or from collision with ground handling equipment. Bird strikes in flight can cause considerable delamination to the random on to the wing leading edge [4]. Complexity of the advanced composite materials manufacturing and composite in service maintenance represents challenges in developing optimized non destructive tools and tests. Composite NDT encompasses a range of modified traditional and new tools including ultrasonic [5]. Ultrasonic technique is one of widely used techniques for non destructive testing of materials. In ultrasonic testing useful information about integrity or geometry of the object under a test is obtained. Measurement configuration often encountered in NDT includes pulse-echo reflection technique. The ultrasonic wave, generated by a piezoelectric transducer propagates through the material and is reflected by defects and back surface of the sample. The signals reflected by defects possess information about defects size and orientation [6]. The Ultrasound C-scan imaging techniques were utilized extensively to detect, evaluate and characterize delamination across the thickness of the laminates [7, 8]. There are many applications of the ultrasonic C-scan technique for the inspection of composite materials, mainly in carbon/epoxy systems, the use of which in advanced structures justifies the cost of such inspections [9]. The main objective of this work was to study experimentally efficiency of the immersion ultrasonic C-scan pulse echo for detection the

2 100 mm 3 mm International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 15 invisible delamination in carbon epoxy laminate ply drop specimens exposed to low velocity impact test and to become familiar with general procedure of immersion ultrasonic C- scan system. The attempt was done to overcome the limitation of the present testing due to the lack of sound wave in ply drop zone because of inclination of this zone to detect the delamination. 67 mm mm 67 mm 5 mm 2.2 Description of equipment for C-scan ultrasonic testing The system comprised of the following components as shown in Figure 2: An ultrasonic generator USD 30 and echo acquisition instrument. 3.5MHz normal focused immersed probe for both emission and reception normal longitudinal waves. PC with dedicated software, that allows a several operations such as scan map plotting in real time and image treatment. Display screen to show the several echo conditions. Impacted zone Flat surface Ply drop zone surface Flat surface Water 2. EXPERIMENT Test specimen Fig. 1. Geometry of specimen It is carbon epoxy laminates ply drop T700/M21, consist of 20 plies at thick part and 12 plies at thin part (Test specimen drop from 20 to 12 plies) with plies orientation (90, 45, 0 A, 0,-45 A, 0, 45 A, 0 A,-45, 0) S. The specimen impacted at ply drop zone on the flat surface by 15J, as shown in Figure 1, to present a damage that may happen to the aircraft during life cycle, like a bird impact, hail stone impact, or technical mistake by the operator. This impact induces defect such as delamination. Hence detection, evaluation and characterization of this type of damage are of major concern in aerospace industries and maintenance organization to keep the high level of safety. The delamination detection in composite materials is often difficult visually and is limited by the testing method. Immersion focused probe Fig. 2. Schematic setup for ultrasonic immersed C-scan imaging system Specimen Special stand 2.3 Procedure of ultrasonic testing by normal angle probe The test specimen cleaned very well and fixed on special stand by making the flat surface as upper one. The special stand with specimen positioned well inside the water tank of the testing machine and the movement of probe lever automatically by motor control.

3 International Journal of Engineering & Technology IJET-IJENS Vol:13 No: Results and discussion The machine starts to scan the whole area of specimen with right angle probe to the specimen, the image start to appear during the scanning process. At the end of C-scanning, the full image with delamination was observed as shown in Figure 3. specimens was not very much clear or may not sufficient to give the exact award about this type of specimens by using normal focused probe at ply drop zone due to high reflection of sound wave away from normal probe. In present experiment the movement of inclined probe automatically concentrated on ply drop zone only, because our problem concerned with this zone due to the lack of sound wave at this zone. 0 Inclined probe 7 Normal probe Inclined prope 7 0 Ply drop zone Delamination Fig. 4. Positioning of probe Inclined probe 7 Fig. 3. C-scan image The delamination at the ply drop zone due to the impact concentrated on this zone and the delamination that extended in thick and thin part could reflect some of sound waves to the normal focused probe as shown in C-scan image with different colors. The C-scan imaging system can secure all the parameters (size, shape, location) concerned with these delaminations, like the delamination of green color which the nearest delamination to the flat surface and the delamination of brown color was located away from flat surface. Appropriate tuning of the device software resulted in effective C-scan images, allowing the determination of the position and the size of the delamination, as well as the determination of the specimen thickness. The delamination very obvious by using present method at the impacted zone and there is no sound wave at ply drop zone (white zone in Figure 3) due to high reflection of sound waves away from the normal focused probe when the waves impacted on the slope of ply drop zone. 3. EXPERIMENT 2 In present experiment the angle of probe changed from normal position to inclined position as shown in Figure 4 to get normal sound waves to the ply drop zone as shown in Figure 5, because there was lack of sound wave at this inclined zone due to the high reflection of sound wave away from normal probe when sound wave impacted on ply drop zone by using normal probe technique. The previous situation (normal probe technique) made the test of carbon epoxy laminates ply drop 20 plies Reflected wave Refracted wave Ply drop zone Inclined Longitudinal wave 12 plies Fig. 5. Reflection of sound waves by using inclined probe Results and discussion The inclined probe produced sound waves perpendicular to ply drop zone as shown before in Figure 5, so that the movement of this inclined probe over the flat surfaces (thin and thick part) of specimen will not give any result due to the high reflection of sound wave away from the inclined probe. During the C-scan by using the inclined probe, the front echo was clear in A-scan screen because the inclined probe produces normal sound waves (front echo) to the inclination of ply drop zone and the back echo was weak when comparing with front echo as shown in Figure 6 because the transmission of the sound waves from the inclined probe through the specimen was impacted the lower straight back surface of specimen by inclined angle not normal angle, so that the most of these waves (back echo) were not reflected back to the inclined probe as shown before in Figure 5.

4 Plydrop zone Delamination International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 17 Front echo Back echo Fig. 6. A-scan at ply drop zone The delamination appeared clearly for the first time in ply drop zone as shown in Figure 7 by green color. Previously the detection of this delamination in ply drop zone by using normal probe technique was not possible, due to high reflection of sound wave away from the normal probe at ply drop zone, but this time by using inclined probe the delamination (green color) was detected in ply drop zone and this appearance of delamination for the first time solved our problem about ply drop zone. Spots of colors distributed on ply drop zone due to the weakness of back echo. Fig. 7. C-scan by inclined probe 7 on ply drop zone The configuration of delamination in Figure 3 with respect to the present configuration of delamination in Figure 7 even though they were the same specimen, but they have different configuration of delamination, because of using different angles of probe (normal and inclined position).in Figure 3 the delamination that extended in thin and thick part of specimen could observed by using normal probe technique but the delamination in ply drop zone couldn t detected by this technique as appeared by green color in Figure 7 due to high reflection of sound wave away from normal probe. 4. CONCLUSIONS During the first experiment (normal angle probe), the delamination was due to the impact at ply drop zone and the C-scan image shows the delamination extended in thin and thick part of specimen that could reflected the sound waves to the normal probe, but the remaining area of ply drop zone there was no sound wave due to inclination of ply drop zone that reflected the sound waves away from the normal probe. The C-scan images can easily secure all the general parameters concerned with this delamination. There was a lack of sound wave at ply drop zone due to the high reflection of sound waves away from the normal probe by using normal probe technique. In second experiment the delamination in ply drop zone very clear by using inclined probe technique which could not detected by using normal probe technique due to high reflection of sound wave away from normal probe at ply drop zone when the waves impacted on ply drop zone, this situation prevented the appearance of this delamination in ply drop zone by using normal probe technique. To give the full description about this type of specimens (carbon epoxy laminates ply drop) by using only the normal angle probe was not enough, because of high reflection of sound wave away from the normal probe at ply drop zone, which affect the quality of the produced image and thereby affecting the quality of results. Therefore the C-scan by inclined probe technique combined with normal probe technique can give general configuration with precise information about delamination in carbon epoxy laminate ply drop specimens because they are complementary to each other. Phased array ultrasonic testing can be proposed as future work for detection delamination in complex geometry especially at ply drop zone, also the C-scan ultrasonic testing is helpful tool for porosity detection in composite laminate. REFERENCES [1] David K. Hsu Non destructive Inspection of Composite Structures methods and practice 17th World Conference on Non destructive Testing, Shanghai, China Oct [2] Sun-Kyu Kim, David Hsu, Young-Hwan Seo, Kwang-Hee Im Detection of stacking orientation error of CFRP composite laminates using ultrasound waves Transactions of non ferrous metals society of china,2009. [3] A. Mukherjee, B. Varughese Development of a specialised finite element for the analysis of composite structures with ply drop-off Composite structures,1999. [4] Cheng-Shung Wang, Yi Zhao and Tuan-Khoi D. Nguyen Prediction of After-Impact Compressive Strength for Laminated Composites ASEE Gulf-Southwest Annual Conference, Southern University and A & M College,2006. [5] B. Boro Djordjevic Non destructive test technology for composite The 10 th International Conference of the Slovenian Society for Non- Destructive Testing 2009.

5 International Journal of Engineering & Technology IJET-IJENS Vol:13 No:01 18 [6] D. Pagodinas Ultrasonic signal processing methods for detection of defects in composite Materials ISSN ULTRAGARSAS, Nr.4 (45) [7] N.K.Ravikiran, A.Venkataramanaiah, M.R.Bhat and C.R.L.Murthy Detection and Evaluation of Impact damage in CFRP Laminates Using Ultrasound C-Scan and IR Thermography National Seminar on Non- Destructive Evaluation NDE, Hyderabad [8] Hasiotis T., Badogiannis E. and Tsouvalis N.G. Application of Ultrasonic C-Scan Techniques for Tracing Defects in Laminated Composite Materials 4 th international conference on NDT [9] Sheng-Tsong Chiu, Yie-Yih Liou, Yuan-Chang Chang, Ching-long Ong Low velocity impact behavior of prestressed composite laminates Materials chemistry and physics,1997.