ACMC/SAMPE Conference on Marine Composites Plymouth, September 2003 (ISBN )

Transcription

1 PERFORMANCE OF AUTOMATED ULTRASONIC INSPECTION OF LARGE-SCALE SANDWICH STRUCTURES IN NAVAL SHIPS ABSTRACT Thomas Wulf Plastics and Composites, FORCE Technology, DK-9220 Aalborg East, Denmark On basis of the successful ultrasonic developing work performed within the EUCLID RTP3.21 project [1] and the JP 3.23 THALES project [2], it has now been possible to make clear detection of defects in GRP, CFRP, PVC foam and Balsa wood sandwich structures by means of automated ultrasonic scanner equipment, especially defects located in the core material such as crushed core, core cracks and skin-core debondings. The reasons for these improvements in detection capability on sandwich structures are mainly due to pulser and scanning optimisations such as probe frequency, pulser characteristics, frequency filtering on receiver amplifier, and coupling conditions. Existing portable ultrasonic track scanner (ATS-1) for detailed inspection of large composite structures has been improved. The scanner has been enlarged and is now covering an effective scanning area of 6x0,5 m 2. Within the EUCLID RTP3.21 this scanner has proved to be very flexible in different inspection set-ups, from the inspection of straight and slightly curved panels to the inspection of geometrical complicated joints. Within the JP 3.23 THALES project a new 8 channel Multi Probe Scanner (MPS) has been developed and a prototype is now ready to be tested in the laboratory and on-side naval ships. This scanner is developed for very fast spot-check inspection of large composite structures with an effective scanning time of 2,4 m 2 /min. INTRODUCTION Within the marine industry high-performance materials such as sandwich composites are attractive materials for lightweight constructions. Their increasing applications for especially load-carrying purposes require an extended knowledge of possible hidden flaws incurred after fabrication or in service. Typical and often critical defect types that may effect the strength and durability of sandwich structures are: Fibre fracture, matrix cracking, delamination and dry areas in the skin laminates, debondings between skin and core and defects in the core like shear cracks and core crushing. Almost all these defect are invisible from the outside and therefore the needs for reliable and effective non-destructive testing (NDT) methods exist. As the results of the latest development within the field of NDT on composite materials, new developed automated ultrasonic methods for large-scale inspection of sandwich structures have shown great potential in revealing these typical defects. Mainly these methods have been developed within two European projects called EUCLID RTP 3.21 [1] and JP 3.23 THALES Project [2]. Within the EUCLID RTP 3.21, Survivability, Durability and Performance of Naval Composite Structures, one task is to improve, develop and implement nondestructive inspection techniques for fabrication inspection as well as in-service inspection on fibre composite joints for naval ship super structures. Within this task several different joint designs as T-joints, Metal/Composite joints and Stiffener joints

2 have successfully been tested non-destructively by means of especially Ultrasound. In addition to this testing program, further development of a portable automated ultrasonic track scanner for large-scale inspection of composite structures has been performed. This scanner has proved to be very flexible in different inspection set-ups from the inspection of straight and slightly curved panels to the inspection of geometrical complicated joints. Within the JP 3.23 THALES Project, Inspection and repair of sandwich structures in naval ships abbreviated with sandi, one task is to test and optimise relevant ultrasonic scanning methods in order to find one or more usable inspection method for detection of typical fabrication and in-service defects. A large number of test panels with relevant defects have been manufactured and tested non-destructively. The applied materials covers from GRP and CFRP for the skin laminate to PVC foam and Balsa wood for the sandwich core, in different thickness and densities. This paper summarizes some of the promising results and knowledge gained by FORCE Technology from these projects with special focus on the detection and characterisation capability of relevant defects in sandwich structures by means of automated ultrasonic inspection methods. APPLIED ULTRASONIC TECHNIQUES Through-transmission techniques with separate receiver and transmitter transducers on opposite sides of the composite component, is often used for their testing. The great advances of this technique are that the sound has to travel only once in the thickness direction and the inspection evaluation is simply done by comparing amplitude levels of received signals. However, through-transmission techniques are in general not practicable for in-field inspections, where access is often limited to one side of the structure. A more applicable ultrasonic technique for in-field inspections is the Pulse-Echo technique with a single transducer transmitting ultrasonic waves into the component. The same transducer receives the echo, before the next pulse is sent. This method evaluates the signals based on time spent from pulse initiation to the reflected signal is back in the crystal, and on the amplitude level of the reflected signal. A composite sandwich panel consists of relatively high-density skin laminate materials as GRP and CFRP, and a relatively low-density core material as PVC foam and Balsa core materials. It is often complicated and rather impossible to get useful information about internal defects in composite sandwich structures by means of traditional ultrasonic parameter choice applied on e.g. steel structures. Mainly because of the inhomogeneous composite materials, large density difference between the applied skin and core materials, and also because of the extremely high sound damping within especially the low-density core material. Resolution, sensitivity and propagation of the waves into the material are very frequency dependent, due to the relatively high attenuation in composite components. For the inspection of the skin laminate it is important to use a wavelength (transducer frequency) where the glass or carbon fibres are as close as possible being ignored in order to detect real defects. The best frequency choice for skin laminates varies from MHz on thin (0,5-1 mm) well-consolidated carbon skins to 0,5-2 MHz on thick hand lay-up GRP laminates. New promising results from laboratory Pulse-Echo inspections (inspections only from one side) of heavy sandwich panels shows that it is now possible to make clear detection of skin-core de-bonding, core cracks and impact damage by means of special high damped broadband transducers in the frequency range from 0,1-2 MHz.

3 RESULTS FROM LABORATORY PULSE-ECHO INSPECTIONS The experimental work performed within the two mentioned projects has been carried out by means of portable automated scanning equipment from the new P-scan System 4 generation developed by FORCE Technology. In order to be able to gain information on defects in sandwich structures it is necessary to apply special sensor characteristics, special manipulator devices and dedicated post processing software for presentation and evaluation of results. In order to achieve the required sensitivity in the applied contact ultrasonic technique in Pulse-Echo mode, it has been necessary to optimise pulse and scanning parameters such as probe frequency, pulser characteristics, frequency filtering on receiver amplifier, and coupling conditions. Several numbers of sandwich components used for marine applications with different material combinations, qualities and thickness have been inspected within the two mentioned projects. Both virgin samples, samples with inserted artificial defects, and samples with damage from mechanical and blast tests have been inspected. The material used for the skin laminate is either GRP or CFRP in the thickness range from 2-9 mm. The core material is either PVC foam or Balsa wood in the density range from kg/m 3 and in the thickness range from mm. In the following sections the results achieved from the parameter setting optimisation are visualised by presentation of automated scanning results obtained with portable P- scan equipment. Detection of GRP skin/balsa core de-bonding Within EUCLID RTP3.21 it was decided to perform blast tests on large GRP/Balsa sandwich structures in order to test and simulate the survivability of such structures. Before and after the blast test, several ultrasonic inspections were performed in order to reveal possible defects. The responds from the blast in ambient air resulted in some damages in the structure like skin-core de-bonding as illustrated in Figure 1. New developed automated ultrasonic track scanner Visible skin-core debonding defect (Note: not visible when the panel is painted) Figure 1: Automated ultrasonic inspection of skin-core de-bonding defect in sandwich structure exposed for blast in ambient air.

4 Indication of skin-core debonding defect, size: 470x110mm 2 Repetition echoes from skin-core interface reflections Figure 2: P-scan presentation of scanned area with skin-core de-bonding defect. The skin-core de-bonding defects are clearly detected by means of automated ultrasonic scanning in Pulse-Echo mode. The defect is detected by evaluation of the amplitude level of received skin-core interface echoes. The amplitude differences between well bonded areas and de-bonded areas are approx. 6 db (gain factor 2). Detection of core shear crack in PVC foam Within the sandi project several defect types have been simulated in different sandwich test samples. One of the defect types of interest is shear cracks in the core material as indicated in Figure 3. 45º crack GRP skin, thickness = 6 mm H80 PVC core, thickness = 60 mm 50cm 50cm Figure 3: GRP/PVC foam sandwich panel with shear crack

5 a) b) Figure 4: a): Ultrasonic transducer sending waves into the sandwich panel and receiving echoes from backside skin-core interface. b): When the transducer is placed upon the shear crack defect the transmitted waves are scattered in other directions. The backside echoes disappear and the crack is thereby clearly detected by means of large amplitude drop on these backside echoes. The shear crack defect is detected by means of the method indicated in Figure 4. The result of an automated ultrasonic scanning in Pulse-Echo mode of a panel with a shear crack in the core is presented in P-scan mode in Figure 5. 45º core crack Backside skin-core interface Figure 5: P-scan presentation of automated ultrasonic inspection of a panel with shear crack in the core. The shear crack defect is clearly detected by means of large amplitude drop (>10 db) on backside skin-core interface echoes. Detection of impact damage in CFRP/PVC foam sandwich panel Another relevant defect type in sandwich structures is the so-called impact damage, which is characterized by delaminations and cracks in the skin laminate. Depending of the impact energy, the core is often crushed just underneath the damaged skin area in considerable larger areas compared to the damage skin area. Within the sandi project

6 CFRP-PVC foam sandwich panels with impact damage simulated by means of drop tests have been manufactured. One of the test panels has the following specifications: CFRP skin thickness: H80 PVC foam thickness: Impactor: Impact energy level: 2 mm 40 mm 75 mm hemispherical 75 J and 100 J The panel has two impact damages, one with impact energy level of 75 J and one with 100 J. The 75 J impact damage is almost invisible on the skin surface, while the 100 J impact damage appears with skin surface indentation in a relatively small area of 30x50 mm. The panel have been inspected by means of automated ultrasonic scanning equipment in Pulse-Echo mode from the damage size with two different methods. The results are presented in Figure 6 and 7 in P-scan mode. The P-scan presented in Figure 6 is the scanning result from only skin inspection, where no information about possible core damages is obtained. In Figure 7 the result from full panel inspection is presented. Note that this scanning is performed in Pulse-Echo mode as well. 100 J 75 J Figure 6: P-scan presentation of automated ultrasonic inspection of panel with two impact damages in Pulse-Echo mode. Only the damage skin is here inspected. The 100 J damage is visible on left as blue color indications represented by low amplitude levels on echoes from backside skin. Only weak indications are recorded from the 75 J damage.

7 100 J 75 J Backside skin-core interf. Figure 7: P-scan presentation of automated ultrasonic inspection of panel with two impact damages in Pulse-Echo mode. The panel is inspected in full depth by means of special high damped broadband low-frequency probes. As seen on Figure 7 the extent of the core damage is clearly detected by large amplitude drops on backside skin-core interface echoes. The defect size is now much larger (about 100x100 mm 2 ) compared to the indications seen on Figure 6, which indicates large core crushing damage. In order to confirm the indications from these Pulse-Echo inspections, Through- Transmission measurements have been performed as well. This TT inspection was performed with 2 un-damped 1 MHz probes in contact technique with water split coupling. The results from this automated ultrasonic scanning in TT mode are presented in Figure 8.

8 Figure 8: Through-Transmission presentation of automated ultrasonic inspection of panel with two impact damages. The red color indications here represents low amplitude level of transmitted ultrasonic signals. The blue color dots are indications from resin filled transverse holes within the core. As seen on Figure 8 the impact damages are clearly detectable by means of this method. Here the defects are approx. of the same size as indicated on Figure 7. AUT. ULTRASONIC SCANNER EQUIPMENT FOR FIELD INSPECTIONS Within the EUCLID RTP3.21 project some of the efforts have been put into further development of a portable automated ultrasonic track scanner (based on the existing ATS-1 scanner system from FORCE Technology) for large-scale inspection of composite structures. Because the scanner runs on its own track, the scanner is suitable for inspection of large composite structures, non-ferritic structures or structures with complex geometries. The scanner is built up by modules of 2-3m sections, and can, if needed, be enlarged to cover a scanning length of 6m. The scanner unit is controlled by the new P-scan generation called P-scan System 4, developed by FORCE Technology. Figure 9: Left: New developed 6m automated track scanner. Right: PSP-4 and portable PC

9 The automated track scanner consists of several mechanical parts: Surface attachment, structural parts, X-Y scanner, and probe fixture. In order to get a rigid attachment to the surface of the structure, and at the same time to get an easy portable unit, the scanner is provided with 5 vacuum suction pads (only 4, when the scanner length<3 m), 2 suction pads in both ends and one in the middle to maintain a proper stiffness and stability. The suction pads are supplied with pressurized air (min. 4 bar) from a compressor (min. 2 HP) or by means of a proper vacuum pump (min. 0,4-0,6 bar vacuum). In order to establish a good and stable surface attachment, the surface has to be dry and cleaned for dust and dirt. The structural part of the scanner consists of a 6m straight alu-tooth bar reinforced by a rigid alu-profile, which is connected to the suction pads through two 2D pivot joints, in order to make the scanner flexible. This scanner has proved to be very flexible in different inspection set-ups from the inspection of straight and slightly curved panels to the inspection of geometrical complicated joints. In the following figures the applicability and flexibility of this scanner are illustrated. Figure 10: Left: Ultrasonic scanning of composite joint in large-scale sandwich structure. Right: Ultrasonic scanning of composite ship hull. Scanner for very fast spot check inspection of large composite structures The speed/cost parameter is often one of the crucial parameters that determines what kind of NDT method is the most effective. Therefore some of the work in the sandi project is focused on developing faster inspection and scanning techniques. The critical defect size in large sandwich structures are of course an important parameter and varies from relatively small areas near highly stressed structural parts to larger areas in low-stressed areas. However, compared to the aerospace industry, where small defects in the mm-size level are not acceptable, the critical defect size in the marine industry is probably closer to the m-size level. An effective method for fast scanning is by applying several transducers scanning the surface in lines (B-scan inspections) instead of the more time consuming area scanning technique (C-scan inspection) with only one or two transducers. This new scanning concept has been implemented in the development of a new 8 channel portable Multi Probe Scanner (MPS).

10 A prototype version of this MPS with 8 transducers placed in a fixed array, with mutual distance of 100 mm, has now been developed. The new scanner is illustrated in Figure 11. Figure 11: Prototype of a new portable 8 channel Multi Probe Scanner The scanner only moves the transducers in one direction; hence only 8 B-scans are recorded and therefore this technique is dedicated to spot check inspection. The scanning speed is 50 mm/sec which results in an effective scanning time of 2,4 m 2 /min. CONCLUSION On basis of the successfully ultrasonic developing work performed within the EUCLID RTP3.21 project [1] and the JP 3.23 THALES project [2], it has now been possible to make clear detection of defects in GRP, CFRP, PVC foam and Balsa wood sandwich structures by means of automated ultrasonic scanner equipment. The reasons for these improvements in detection capability on sandwich structures are mainly due to pulser and scanning optimisations such as probe frequency, pulser characteristics, frequency filtering on receiver amplifier, and coupling conditions. For only skin inspection the best frequency choice varies from MHz on thin (0,5-1 mm) wellconsolidated carbon skins to 0,5-2 MHz on thick hand lay-up GRP laminates. New promising results from laboratory Pulse-Echo inspections (inspections only from one side) of heavy sandwich panels show that now it is possible to make clear detection of skin-core de-bonding, core cracks and impact damage by means of special high damped broadband transducers in the frequency range from 0,1-2 MHz. Existing portable ultrasonic track scanner (ATS-1) for detailed inspection of large composite structures has been improved. The scanner has been enlarged now covering an effective scanning area of 6x0,5 m 2. Within the EUCLID RTP3.21 this scanner has proved to be very flexible in different inspection set-ups, from the inspection of straight and slightly curved panels to the inspection of geometrical complicated joints. Within the JP 3.23 THALES project a new 8 channel Multi Probe Scanner (MPS) has been developed and a prototype is now ready to be tested in the laboratory and on-side naval ships. This scanner is developed for very fast spot-check inspection of large composite structures with an effective scanning time of 2,4 m 2 /min.

11 ACKNOWLEDGMENT ACMC/SAMPE Conference on Marine Composites The author acknowledge the EUCLID financial support for the development work and approval of this publication given by the Ministry of Defences of Denmark, Norway, United Kingdom, The Netherlands, France and Italy. This work was part of the SaNDI (THALES JP3.23) project with participants from Norway, Denmark, Sweden, Finland and the United Kingdom. The support of the Ministries of Defence of the five participating nations is gratefully acknowledged. All the partners including the shipyards within the international industrial consortium in both project [1][2] are acknowledged for the design and manufacturing of test specimens. REFERENCES