Synergy between Advanced Composites and New NDI Methods

Transcription

1 Advanced Performance Materials 5, (1998) c 1998 Kluwer Academic Publishers. Manufactured in The Netherlands. Synergy between Advanced Composites and New NDI Methods JERZY P. KOMOROWSKI, RONALD W. GOULD AND DAVID L. SIMPSON Structures, Materials and Propulsion Laboratory, Institute for Aerospace Research, National Research Council Canada, Montreal Rd. M-14, Ottawa, Ontario, K1A OR6, Canada Abstract. A D Sight Aircraft Inspection System (DAIS 500) large area composite impact damage detection capability has been demonstrated using several structures. In order to obtain equivalent impact damage depths, the IM7/ structures had to be subjected to much higher impact energies than conventional AS4/ structures. It is postulated that the application of DAIS with its better than visual inspection sensitivity could lead to both increased design allowables and an alternate approach to certification of composite structures based on impact cumulative probability of occurrence. Further evidence of impact indent depth relaxation (over 30%) was observed. Indent relaxation may have significant implications with respect to the airworthiness of composite structures designed to BVID (barely visible impact damage) limits since this relaxation makes the damage site less apparent. Keywords: aircraft nondestructive inspection, certification, composite structures, D Sight, impact damage Introduction Use of graphite reinforced resins is increasing in airframes of military and civil aircraft. These materials offer high specific strength and stiffness properties and very good fatigue resistance. Unfortunately, the materials are sensitive to low energy impact damage from such common occurrences as hailstones, stones thrown off the runway or tools dropped by maintenance personnel. These impacts may result in significant levels of internal damage while surface damage may be barely or non visible. Operational experience with composite structures indicates that 81% of all damage is due to impact while lightning strikes (10%), overheating (7%) and delamination (2%) constitute the remainder of damage types [1]. Regular in-service inspections of aircraft with scanning devices are not practical due to the cost and time required. Current practices rely on visual detection for impact damage. The United States Air Force (USAF) Damage Tolerance Design Guide for composites requires that composite aircraft structure be capable of carrying ultimate load with visible impact damage defined as a 0.1 in (2.5 mm) deep indentation. Other organizations have established lower thresholds (typically 0.05 in 1.25 mm) while Aerospatiale of France has used 0.3 mm (0.012 in) as the visibility threshold (close visual inspection with 50% probability of detection for the ATR 72 composite wing box). These attempts to lower the threshold are driven by the desire to design lighter structures with higher allowable strain levels. Recent research by Komorowski et al. [2] and at Aerospatiale [3] has shown that significant reductions (up to 45%) in impact dent depths (relaxation) can be caused by viscoelastic

2 138 KOMOROWSKI, GOULD AND SIMPSON effects, cyclic loading, moisture and temperature effects. This implies that if visual inspections are to be used then higher impact energies required to produce visible damage after relaxation (i.e., a residual indent depth equal to the required value) will be needed for certification. As a consequence the allowable design strain levels will have to be lowered even further. A cost effective method for rapid and regular inspection of composite structures with a capability better than the close visual inspection would not only reverse this requirement for higher impact damage and lower strain allowables, but also offer the potential to lower the impact requirement and increase design allowables. This would result in lighter composite structures. Deployment of DAIS will also enhance the safety of operation of current designs where relaxation was not accounted for during certification. Specimen design Two graphite reinforced materials were selected for use in this study, a first generation thermoset resin based and a new material recently selected for wide use in advanced fighter structure design. The first generation prepreg material was unidirectional carbon fibers pre-impregnated with epoxy resin (Hercules ). The carbon fibres were Hercules Corp. Magnamite continuous Type AS4. This material has been widely used in the aerospace industry for over 15 years and is the material used on Canadian CF-18 fighter aircraft. The other composite material system selected for this study was unidirectional carbon fibers pre-impregnated with bismaleimide resin, Cytec s Rigidite The carbon fibers were Hercules Corp. Magnamite continuous Type IM7. For both materials, in-house material characterization was carried out to generate a property database and also to develop processing techniques and to gain handling experience. Two hat stiffened panels in ( mm) with different lay-up configurations were designed. Configuration 1 was designed as a lightly loaded fairing type structure while configuration 2 was a heavily loaded wing skin type structure (Table 1, figure 1). Table 1. Lay-up for two test panel configurations. Laminate thickness (mm) Configuration Plies IM7/ AS4/ Skin 12 (45/0/ 45/90) s Cap 26 (45/0 4 /90/0 3 / 45/0) s Web 12 (45/90/0/ 45) s Flange 5 (45/90/ 45) Skin 48 (45/0/ 45/90) 4s Cap 52 (45/0 4 /90/0 3 / 45/0) 2s Web 24 (45/90/0/ 45) 2s Flange 10 (45/90/ 45)

3 SYNERGY BETWEEN ADVANCED COMPOSITES AND NEW NDI METHODS 139 Figure 1. Configuration 1: hat-stiffened panel. Damage introduction A total of four stiffened panels were subjected to multiple impact damage (Table 2). Two of each material, one a 12 ply lay-up (No. 856, 860) and the other a 48 ply lay-up (No. 867, 869). Impact damage was introduced in a drop tower using a 0.5 in (12.7 mm) diameter impactor tip. Two separate studies of threshold detectability and relaxation studies were carried out. In the threshold of detectability study, energy levels from 1.36 to 40 J were selected such that damage levels ranged from not detectable (with any of the NDI methods available) to clearly visible. In the impact damage indent depth relaxation study impact energies were selected to demonstrate that the relaxation occurs in the generally accepted BVID (barely visible impact damage) range. Four types of situations were selected for impact damage introduction (figure 2): Table 2. Number of impact sites and energy levels for each panel. Damage by drop tower (12.7 mm dia. tip) Panel no. Material No. of plies Threshold study Impact relaxation study 856 AS4-3501/ sites 1.36 to J 6 sites 6.59 to J 860 IM sites 1.36 to J 5 sites 6.09 to J 867 AS4-3501/ sites 6.78 to J 5 sites to J 869 IM sites to J 7 sites to J Figure 2. Impact damage situation types.

4 140 KOMOROWSKI, GOULD AND SIMPSON Type 1 situations were centered on the skin over the stiffener. Type 2 situations were centered on a web section of the stiffener. Type 3 situations were centered over the flange of the stiffener web. Impacts at Type 3 locations were intended to produce disbond damage between the flange and the skin. Type 4 situations were centered on the skin between stiffeners. The instrumented drop-weight impact test facility was used to introduce impact damage. The system installed at IAR is a Dynatup Model 8200 drop-weight impact system (drop tower) with GRC 730-I data acquisition and analysis instrumentation manufactured by General Research Corporation. The drop tower was modified by increasing the size of the base plate and raising it off the floor to accommodate the positioning of the test specimens. The specimens were fixed in their test position by wedges between the base plate and the specimen. This configuration also allowed for the positioning of the drop tower directly onto very large specimens. D Sight aircraft inspection system (DAIS) and other NDI inspections The panels were inspected with both ultrasonic C-Scan and D Sight techniques, before and after the impact damage introduction. Impact dents were observed visually and depths were measured using a dial gauge wherever possible. The ultrasonic C-Scan inspections were carried out using a reflection pulse-echo method by immersing the specimen in a large water tank. In the reflection pulse echo method, a single transducer is used both as a transmitter and receiver. The transducer is scanned over the test specimen and the reflected signal from a plate located behind the specimen is monitored. Double pass retroreflection (D Sight) is a simple optical set up patented by Diffracto Ltd. The authors suggested that the set up has potential for becoming a rapid large area impact damage, NDI technique in a previous paper [4]. Since then the D Sight Aircraft Inspection System (DAIS) has been developed under IAR-Diffracto collaboration. The DAIS 500 was used for performing the D Sight inspections. The DAIS 500 (figure 3) is currently marketed by Diffracto Ltd., and several units have been delivered to military customers. Most of the D Sight inspections were carried out using Electron TM highlighter liquid used to make the inspected surface reflective for the D Sight inspection. Highlighter is needed for non-reflective surfaces such as aircraft structures coated with typical military matte finish top coats. Recently the authors patented the use of solid films, temporarily placed in contiguous contact with an inspected surface, as a replacement for liquid highlighters. The solid film highlighter protects the surface from contamination and offers consistent reflectivity. Inspection results, threshold study Ultrasonic C-scans of impact damaged panels were inspected and damage size was measured as a diameter of the smallest circle that could be drawn around the damaged (delaminated) area. Figure 4 shows a C-scan of a section of Panel 867.

5 SYNERGY BETWEEN ADVANCED COMPOSITES AND NEW NDI METHODS 141 Figure 3. DAIS 500, shown are, optical sensor, computer and remote pendant with touch sensitive screen for controlling image acquisition. Figure 4. Post-damage C-scan of a section of Panel 856. Damage map on the right was obtained by subtracting the pre-damage C-scan from post-damage C-scan. Based on the data in Table 3, the largest impact energy required to produce D Sight detectable damage is associated with situation type 2 (over the web of the stiffener). C-Scan indications (i.e., figure 4) for this situation type were also small and generally D Sight indications were observed before C-Scan damage was reported (figure 5). Situation type 1 (at the center of the stiffener) required the least energy to produce detectable damage.

6 142 KOMOROWSKI, GOULD AND SIMPSON Table 3. Test matrix and inspection results, section of Panel No (D detected, N not detected). Impact Impact energy Site Situation Plan Test Load no. type (J) (J) (kn) Indent depth (mm) C-Scan (mm) D/N D Sight DAIS 500 D/N D D /46 D /51 D D D D D D D not av. not av. not meas. N N Situation type 4 (mid-bay between stiffeners) required less energy than situation type 2 but more than situation type 3 (over the stiffener flange). This impact energy versus situation scale was the same for both panel materials and while the energy required for thicker panels was obviously larger, the scale was the same (figure 6). Since the highest reductions in panel compressive strength were observed for panels impacted at mid-bay [5], the fact that at these situations smaller dents could be expected should be considered in future studies of stiffened panels and in certification testing. Table 4 summarizes the data for each panel. When conducting the analysis it should be kept in mind that the impact energies used in the stiffened panel tests were intentionally small. The aim was to generate damage which would allow an evaluation of the DAIS 500 s threshold of detection capability. Thus out of 178 impacts only 136 produced detectable damage (either through C-Scan (113 calls), dent (128 calls) or DAIS 500 (111 calls)). It should be noted that some dents were extremely small and could be located only with DAIS. The DAIS 500 inspection took place several months after the impact events and dent measurements. Because of this time lag and the dent relaxation phenomena (discussed in the next section), there were 21 (out of 128) sites where dents were not detected with DAIS 500, but were observed just after impact visually with full knowledge of the exact impact location. Impact indentation produces a very distinct D Sight signature which is rather easily distinguished from D Sight signals originating from other surface perturbations (see figure 5). This observation is supported by the fact that only 4 false calls (out of 115) were produced during stiffened panel DAIS 500 inspections (a false call rate of 3.5%). In some of the more significant impacts, apart from the dent D Sight signature, the delamination signature could also be observed. This further facilitates impact damage identification. In Table 4 it can be seen that in Panels 856, 860 and 867 situation 2 impacts resulted in 15 impacts detectable with DAIS 500 but not identified as damaged on C-Scans. This is partly related to the rigid support provided in this situation by the web of the stiffener and the

7 SYNERGY BETWEEN ADVANCED COMPOSITES AND NEW NDI METHODS 143 Table 4. Number of impact events per situation type and numbers of sites detected using various NDI methods. Total damage DAIS 500 DAIS 500 DAIS 500 not C-Scannot C-S or DAIS Situation Total Total Total C-Scan+ dent DAIS 500 confirmed confirmed detected detected not detected Panel type impacts C-Scan dent + DAIS 500 detected by C-Scan by dent by C-Scan by D Sight by dent Total DAIS 500 inspection produced 4 false calls. Not detected by dent dent not visually detectable immediately after impact.

8 144 KOMOROWSKI, GOULD AND SIMPSON Figure 5. Section of Panel 856 (the same as in figure 4). DAIS 500 image (top) and a drawing identifying impact indents located in the DAIS image (see Table 3). Impact at site 29 was not detected in the C-scan or in the DAIS-500 image. The lines running at 45 to the stiffener direction are traces of machining marks from the lay-up tool. difficulty of performing C-Scan inspections in this area. When situation 2 is excluded from the consideration, then the remaining 82 DAIS 500 calls were all confirmed by C-Scan. The largest C-Scan observed damage without D Sight indication in 12 ply panels was 20 mm diameter (situation 3), in the 48 ply panels this damage was 24 mm. The indent depths could not be measured reliably using a dial gauge for some D Sight detected impact sites (below mm) in hat stiffened panels. These small indents were measured in the earlier coupon test study and compared to DAIS detection rate. Based on the two

9 SYNERGY BETWEEN ADVANCED COMPOSITES AND NEW NDI METHODS 145 Figure 6. Situation type sensitivity to impact damage as measured by impact energy required to generate D Sight detectable damage. Figure 7. Data from mm coupons, simply supported and impacted with 12.7 mm diameter steel ball. C-scans measurements are diameters of equivalent area circle. Curves are shown only to indicate trends. T800/3900 is a toughened graphite/thermoset matrix system. studies it can be concluded that dents 0.1 mm deep will be reliably detected. The impact of incorporation of DAIS on current and future certification approaches should be considered briefly. At lower impact energies (13.6 to 20.4 J) impact dents in IM7/ panels are about 30% smaller than those observed in AS4/ while C-Scan damage sizes are comparable. At higher energies (over 68 J) the dent depths in IM7/ are less than 50% of the dent depths in AS4/ panels of the same thickness and construction. This observation is also supported by data in figure 7 obtained in a previous study [6]. Current certification requirements for composites are driven by impact visibility or barely visible impact damage which is measured by impact dent depth. Thus, for certification

10 146 KOMOROWSKI, GOULD AND SIMPSON Table 5. Impact indent depth reduction measurements. Test no. Situation type Energy (J) Damage Indent depth (mm) C-Scan (mm) Initial Final % change Note BTP BTP / BTP / / / BTP / BTP / BTP / / / / BTP BTP Initial measurements taken immediately after impact. Final measurements taken at least 24 hours later. C-Scan damage diameter or rectangular dimensions shown. BTP broken top plies. purposes, structures manufactured from tougher systems (i.e., IM7/ or T800/3900) have to be subjected to much higher impact energies resulting in higher damage levels than AS4/ structures. As shown by the above data, DAIS offers an alternate means of compliance to the BVID inspection requirement. By applying DAIS inspections, the indent depth requirement can be lowered significantly. This has implications with respect to design allowables and to the use of toughened resin systems. There is limited benefit from a lowered indent requirement for thin structures (up to 20 plies). A 0.36 mm deep dent required a 4.3 J impact in AS4/ versus 6.3 J in IM7/ In these thin panels the current BVID limits typically results in penetration

11 SYNERGY BETWEEN ADVANCED COMPOSITES AND NEW NDI METHODS 147 of the laminate by the impactor. Thus durability becomes a problem, while inspectability is straightforward. In moderately thick panels (up to 40 plies), the impact energy versus indent curve is relatively flat for both material types, therefore there is not a real advantage in lowering the BVID threshold. Since the impact energy is not reduced significantly, the impact damage reduction will be small and the design allowables will be unaffected. This figure shows explicitly that the 2.54 mm (0.1 in) BVID indent size requirement results in test energies for tougher laminates as much as two times higher than those required for first generation thermosets (figure 7). The residual compressive strength of structures manufactured from both materials with the same BVID indent depth would be comparable thus rendering the new material less attractive due to its higher costs. For these moderately thick composites, lowering the BVID limit by taking advantage of DAIS capability as an alternate means of compliance to the visual inspection, would not be very effective (figure 8) for the same reason that for the design allowables. For thicker composites (>40 plies), DAIS becomes very attractive as an alternate means of compliance (figure 9). The energy versus impact depth curve has a steeper slope, therefore moving the detectability limit to a smaller value implies a significant reduction in impact energy. The smaller delamination sizes associated with the lower impact energies and lower damage levels which imply significant increases in design allowables. Additionally, using DAIS to reduce the indent size detected has the potential to increase the attractiveness of the toughened systems. Significantly lower impact energies could be used which would result in lower damage levels and an associated rise in design allowables. The advantage they hold over more brittle systems would be the higher durability of tougher matrix system structures. Figure 8. Data for 24 ply coupons (figure 7) plotted against typical damage tolerance assumptions for composite structures (energy cut-off and BVID).

12 148 KOMOROWSKI, GOULD AND SIMPSON Figure 9. Data trends for 48 ply coupons (figure 7) plotted against typical damage tolerance assumptions for composite structures (energy cut-off and BVID). Figure 10. Data trends for 48 ply coupons (figure 7) plotted against proposed damage tolerance approach assuming combined DAIS and cumulative energy probability approach. The increased detectability of indents also offers the potential of a different certification approach to BVID and would allow full advantage to be taken of the increased capability of the toughened systems. In particular, a once in a life-time (cut-off energy) assumption is proposed. While the certification for composites based on impact cumulative probability

13 SYNERGY BETWEEN ADVANCED COMPOSITES AND NEW NDI METHODS 149 of occurrence [7] for some structures could make inspection for impact redundant, a more likely scenario is shown in figure 10. Based on a consideration of cumulative impact energy probability, material design allowables and minimum durability requirements, a test energy will be selected. While this impact energy would result in impacts visually non detectable (especially in tougher materials), the energy level could be selected to ensure that DAIS would detect indentations in both new and older systems as shown in the figure 10. For the example shown (based on the data from this study), this test energy would be approximately 60 J. This approach would result in a significantly lower damage level in the tougher system than in the older one (figure 7). Thus the new inspection (DAIS), combined with a cumulative impact energy probability approach, would allow designers to take full advantage of new material (combined higher design allowables and higher durability). Although technically feasible, a trade-off study is necessary to establish the cost and operational benefits of operating lighter aircraft structure requiring DAIS deployment against those of operating heavier, less efficient structure. Such a study would be of immediate use in light of the weight problems of some widely publicized programs (i.e., V-22). The authors also suggest that studies be undertaken to assess the possibility of two adjacent impact events, both below the BVID threshold, reducing the strength of a composite primary structure to below the required residual strength (limit load). No such studies were published in the open literature. Impact damage relaxation study Several impacts were performed on stiffened panels with the aim of monitoring indent depth evolution with time. Because most impacts in the panels were performed at very low energy levels in order to establish the threshold of detection for the DAIS 500, these often have resulted in indent depths at the limit of practical depth measurement with a dial gauge. Other methods of measurement (i.e., Shadow Moiré) are too cumbersome for use on a large number of indents. Thus, in the relaxation study, somewhat larger impact energies were used. This was also done in order to demonstrate that the relaxation occurs in the generally accepted BVID zone of indent depths (1.2 to 2.5 mm). Impacts were limited to situation types 1 and 4. The test results are presented in Table 5. Indent reductions of over 30% of the original depth were measured. This is similar to the reductions reported by the authors earlier [2] using simple coupon specimens. However, in this study only the factor of time is included. Larger reductions could be expected if the panels were exposed to cyclic loading, temperature and humidity. The data in Table 5 show that impacts which initially are above BVID can relax below that level. Since most aircraft structures in service today have been certified without taking account of this phenomenon, it is possible that critical damage may remain undetected for an extensive period of time seriously degrading the residual strength of these structures. The magnitude of the observed reductions indicates that indent relaxation must be accounted for in the certification process for composite structures. The impact energies used in this part of the project were generally larger than in the threshold study. However, they provided further evidence that IM7/ requires higher

14 150 KOMOROWSKI, GOULD AND SIMPSON impact energies to create indents with a depth equal to those observed in the AS4/ panels (Table 5). Conclusions The ability of the DAIS 500 to detect impact damage at very low impact energy levels is comparable to ultrasonic C-Scan (113 versus 111 calls). A very low false call rate in impact damage detection was observed with the DAIS 500 (3.5%). The extent of impact damage (C-Scan signature and indentation) in stiffened panels depended largely on the impact location versus stiffener position. Significant differences were observed in the response to impact between the AS4/ and IM7/ Impacts of the same energy produced indents 30% to 50% less deep in the IM7/ panels. In order to generate an indent of the same depth in both material systems nearly 70% higher energy was required in IM7/ DAIS offers significant potential as an alternate means of compliance to visual detection of BVID. The ability of DAIS to detect smaller indents can be used to reduce the influence of the impact damage requirement on design allowables. DAIS, in conjunction with a cumulative probability of occurrence approach to certification, offers the potential for alternate approaches to certification that would alleviate the penalty on toughened materials due to increased energy levels for BVID. Significant structural weight savings should be possible through synergy between advanced materials and new inspection methods. Feasibility and trade-off studies should be undertaken. Impacts which initially are above the BVID limit can relax to below that level. Since most aircraft structures in service today have been certified without taking into account of this phenomenon, it is possible that critical damage may remain undetected for an extensive period of time with the potential of seriously degrading the residual strength of these structures. The magnitude of the observed reductions (over 30%) indicates that indent relaxation must be accounted for in damage tolerance tests of composite structures. Acknowledgments This paper reports part of work which was carried out under a DND/USAF Contract No. F , DSS No. 017SV.070C3-93-2, Large Area Composite Inspection System LACIS-D. The authors would like to acknowledge the direction provided by Mr. C. Buynak of the USAF and Mr. R. Hastings of DND. The staff of Diffracto Limited have again proven to be excellent partners. The authors also thank Mr. C.E. Chapman and Mr. A. Marincak of IAR for providing NDI support. References 1. F. Schur, Inspection of carbonfibre repairs, Air Transport Association Non-Destructive Testing Forum, Long Beach, CA (1991).

15 SYNERGY BETWEEN ADVANCED COMPOSITES AND NEW NDI METHODS J.P. Komorowski, R.W. Gould, and A. Marincak, Study of the effect of time and load on impact damage visibility, Proceedings of the Second Canadian International Composites Conference and Exhibition (CANCOM), edited by W. Wallace, R. Gauvin, and S.V. Hoa (Ottawa, Ont., 1993), pp M. Thomas, Study of the evolution of the dent depth due to an impact on carbon/epoxy laminates. Consequences on impact damage visibility and on in service inspection requirements for civil aircraft composite structures, presented at MIL-HDBK 17 meeting March 1994, Monterey, CA. 4. J.P. Komorowski, D.L. Simpson, and R.W. Gould, A technique for rapid impact damage detection with implication for composite aircraft structures, Composites, (1990). 5. R.S. Whitehead, Certification of primary composite aircraft structures, Proceedings of the 14th Symposium ICAF 1987, pp J.P. Komorowski, R.W. Gould, and A. Marincak, Location of impact damage sites in composite aircraft structures, NRC/IAR, LTR-ST-1904 (1992). 7. H.P. Kan, R.S. Whitehead, and E. Kautz, Damage tolerance certification methodology for composite structures, NASA CP3087, pp Final Manuscript March 13, 1997