NASA / TM- 1998-206932 Technical Assessment of the National Full Scale Aerodynamic Complex Fan Blades Repair Clarence P. Young, Jr. ViGYAN, Inc., Hampton, Virginia Peter G. Dixon Advanced Technologies, Inc., Newport News, Virginia Terry L. St. Clair Langley Research Center, Hampton, Virginia William E. Johns Washington State University, Pullman, Washington January 1998 The NASA STI Program Office ... in Profile Since its founding, NASA has been dedicated CONFERENCE PUBLICATION. to the advancement of aeronautics and space Collected papers from scientific and science. The NASA Scientific and Technical technical conferences, symposia, Information (STI) Program Office plays a key seminars, or other meetings sponsored or part in helping NASA maintain this co-sponsored by NASA. important role. SPECIAL PUBLICATION. 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Dixon Advanced Technologies, Inc., Newport News, Virginia Terry L. St. Clair Langley Research Center, Hampton, Virginia William E. Johns Washington State University, Pullman, Washington National Aeronautics and Space Administration Langley Research Center Hampton, Virginia 23681-2199 January 1998 Available from the following: NASA Center for AeroSpace Information (CASI) National Technical Information Service (NTIS) 800 Elkridge Landing Road 5285 Port Royal Road Linthicum Heights, MD 21090-2934 Springfield, VA 22161-2171 (301) 621-0390 (703) 487-4650 Foreword This report summarizes technical activities related to problems encountered during fan blade repairs for the National Full Scale Aerodynamic Complex (NFAC) at the NASA Ames Research Center. These activities represent a joint effort between the authors, who were members of an NFAC Blade Repair Technical Review Team, and the NASA Ames Blade Repair Project Team. Disclaimer Use of trademarks or names of manufacturers in this report does not constitute an official endorsement of such products or manufacturers, either expressed or implied, by ViGYAN, Inc., Advanced Technologies Inc., the NASA Langley Research Center, or Washington State University. Table of Contents Page Summary ........................................................................................................................... 4 Introduction ....................................................................................................................... 4 Description of Fan Blade Repak Design and Fabrication ................................................... 5 Fabrication ........................................................................................................................ 5 Problems Encountered During Fabrication ........................................................................ 6 Investigation of Repair Problems ....................................................................................... 7 Action Plan ....................................................................................................................... 8 Resuks and Discussion ...................................................................................................... 9 Prototype Blades Pull Tests .............................................................................................. 11 Lessons Learned ............................................................................................................... 12 Conclusions and Recommendations .................................................................................. 13 References ........................................................................................................................ 13 Acknowledgements .......................................................................................................... 14 Authors Directory ............................................................................................................. 14 Appendix A - Adhesive Film AF-126 Product Specification ........................................... 15 Appendix B - Fatigue Tests ............................................................................................ 18 List of Tables and Figures Tabl.._._e Titl__..._e Table I Pull test results on blade 120 test bed ........................................................ 19 Table II Pull test results for test specimens, three prototype blades ......................... 21 Fik_ure # Titl._...g.e 1. Plan view of NFAC-80x120-ft circuit illustrated. 2. NFAC fan drive configuration - looking downstream. 3. Photo of NFAC fan drive. 4. NFAC blade assembly materials. 5. Physical evidence of cracking in root area of NFAC blade. 6. Estimated Interference (Campbell) diagram for NFAC blades taken from reference 3. o Fan blade assembly repair illustration. 8. Photo of carbon composite inner wrap (patch) installed on blade and metal cuff. 9. Photo of repaired blade upper surface illustrating overwrap of blade root and metal cuff, with chordwise belts. 10. Photo of repaired blade lower surface illustrating overwrap of blade root and metal cuff, with chordwise belts. 11. Finite element model of repaired blade assembly. 12. Finite element model of repaired blade assembly with retention shaft. 13. Thermal expansion properties of Hydulignum and carbon composite materials. 14. Finite element model illustration of blade repair overwrap with slots. 15. Photo of pull test rig installed on blade 120 test bed. 16. Photo illustrating pull test cylinder plugs from blade 120 test bed. 17. Fatigue test fixture. 18. Finite element model of NFAC blade installed in fatigue test fixture. Summary This report describes the principal activities of a technical review team formed to address National Full Scale Aerodynamic Complex (NFAC) blade repair problems. In particular, the problem of lack of good adhesive bonding of the composite overwrap to the Hyduliginum wood blade material was studied extensively. Description of action plans and technical elements of the plans axe provided. Resuks of experiments designed to optimize the bonding process and bonding strengths obtained on a full scale blade using a two-step cure process with adhesive primers axe presented. Consensus recommendations developed by the review team in conjunction with the NASA Ames Fan Blade Repair Project Team are provided along with lessons learned on this program. Implementation of recommendations resulted in achieving good adhesive bonds between the composite materials and wooden blades, thereby providing assurance that the repaired fan blades will meet or exceed operational life requirements. Introduction The National Full Scale Aerodynamic Complex (NFAC) is located at the NASA Ames Research Center, Moffett Field, California. The wind tunnel operates in both a closed circuit (40x80) or open (80x120) circuit configuration as shown in figure 1. The wind tunnel has six fans with 15 wooden blades per fan as illustrated in figures 2 and 3. Recent cracking of the National FuU Scale Aerodynamic Complex (NFAC) wooden fan blades has been attributed to higher mean loads and higher cyclic loads due to complex fan inflow disturbances. The blade configuration and material composition is shown in fig. 4. Hydulignum is a compressed material that is manufactured from birch veneers approximately 1/16 in. thick (ref. 1). Hyduliginum is a very dense material with tensile strengths up to 30 ksi in the grain direction. Sitka Spruce is a lighter wood with lower strength, but has long been the standard material used for wooden fan blades construction for NASA Wind Tunnels, see ref. 2. For the NFAC blades, the hyduliginum laminates (pressed boards) indicated in fig. 5, are approximately 3Ainch. thick. Physical evidence of cracking is illustrated in fig. 5. Based on diagnostic test data obtained in 1996, and utilizing full scale fatigue test data (ref. 1), all evidence and analyses indicate that the blade(s) cracked in the manner expected, and at about the operational hours expected. Because of fan inflow disturbances, the high. 1/Rev. blade dynamic loading is the main contributor to fatigue damage (i.e. cracking). Also, a 4/Rev. blade dynamic loading was found to be significant, and higher than expected. There is ample evidence that the close proximity of the first fundamental mode frequency to a strong 4/Rev. disturbance (see fig. 6 taken from ref. 3) has contributed to the higher than expected 4/Rev. cyclic loading. As a resuk of the fan blade cracking problem, repairs are being implemented for all of the fan blades in order to keep NFAC operating safely until a new replacement set can be designed and manufactured. The goal was to achieve the best repair possible. The present fan blade repair concept was formulated in 1996 and utilizes a carbon composite overwrap (glove) that is designed to transmit 30% of the mean (static) plus dynamic loads to the threaded metal cuff. The repair concept is illustrated in figure 7. The carbon fiber inner wrap (patch) illustrated in figure 7 is shown installed on the blade in the photo of figure 8. The metal cuffs are flange connected to blade retention shafts which are attached to the fan hub. Based on detailed structural analysis, 4 operationalloadsmeasurementsa,ndfull scalefatiguedatadevelopedwhentheinitial setof bladeswerebuilt(ref.1),transferof30%oftheloadawayfromthehighlystressedt,hreaded portionofthewoodenbladeattachroot(wherefatiguecracksinitiated)wouldassure30months ormoreofoperationallife,whilereplacemenbtladesarebeingprocured.Inaddition,theblade repairconceptwouldprovidesomebladecontainmenctapabilityi.e. would reduce the risk of catastropic failure such as blade shear out at the blade attachment. The repair design would also prevent the loss of large sections of the blade due to crack propagation in the spanwise direction. After extensive lab and prototype testing by NASA Ames Project personnel, the initial set of 5 repaired blades were cured in an autoclave. However, for the second set of 5 blades, the composite overwrap delaminated (disbonded) from the fan blade Hydulignum material after removal from the autoclave. As a result of problems encountered with the blade repair fabrication, a technical review team was formed to work with NASA Ames project personnel to solve the problem. The purpose of this report is to document the technical investigations and report on the results achieved to solve the repair fabrication problem in order to assure a blade repair that meets or exceeds operational life requirements. Description of Blade Repair Design and Fabrication An illustration of the repair design is given in figure 7. Photos of a repaired blade are provided as figures 8 and 9. The repair consists of a carbon composite patch that is tailored to transmit 30% of the peak loads (static + dynamic) to the exterior of the metal cuff. The patch is then overwrapped with carbon and fiberglass composite lay-ups around the blade root section and metal cuff. The loads are transmitted by shear through the bond between the blade and composite overwrap into the steel cuff. This unloads the blade by 30 percent in the highly stressed threaded root inside the metal cuff (fig. 4). Chordwise beks around the blade at the root and outboard edge of the lay up (see figures 9 and 10) were added to retard crack growth, provide transition and containment. A principal decision was the selection of an adhesive that would provide a strong bond between the composite material, the wooden blade and steel cuff. A Minnesota Mining and Manufacturing (3M) AF-126 film adhesive was selected for bonding the composite material to both the wood and steel See the adhesive specification provided as Appendix A. Extensive NASTRAN finite element analyses were performed for the fan blade assembly with the repair installed (see figures 11 and 12). Note that in figure 12, that the blade retention shaft is modeled along with the blade assembly. The most recent finite element analysis indicates that the overwrap is transfering about 40% of the load instead of 30%, which would give the repaired blades even longer operational life. Fabrication The first step in the repair process was to inject resin into the existing cracks. This was done successfully, and verified by ultrasonic examination. The second step was to do a wet lay upof the composite materials on the blade and steel cuff. However, due to problems encountered during prototype testing and adhesive selection, it was determined that the repair would have to be cured at high temperature and pressure in an autoclave. A one-step cure cycle of 12 hrs. at a maximum temperature of 185°F and 90 psi pressure was selected. The autoclave cure temperature was limited to 185°F to prevent degradation of the Hydulignum strength properties (ref. 1). With the selected one step cure cycle it was expected that the composite overwrap and film adhesive could be co-cured to achieve the desired repair. Problems Encountering During Fabrication The first set of five blades that came out of the autoclave appeared to have cured properly. However, after the second set of 5 blades was removed from the autoclave, 4 out of the 5 blades delaminated during cool down, i.e. the carbon composite overwrap disbonded from the blade over the portion of the blade made from Hydulignum. This was determined to be an adhesive failure to the wood. The fifth blade was put into a refrigerator at around 15°F and it too delaminated. Evaluation of the problem suggested that the delamination occurred as a result of residual thermal strains induced as a result of large variations in coefficient-of-thermal-expansion (C.T.E) between the dissimilar materials. Typical C.T.E. values for the various materials are as follows: (1) Hydulignum, 25x10 "6in./in./°F in the transverse direction, and 15x10 "6in the longitudinal direction (fig. 13). (2) Carbon composite wrap is 3.3 to 5.5x10 _sin./in./°F; (fig. 13) (3) Sitka Spruce, 10xl0 -6 in./in./°F and (4) steel cuff, 6x10 _ in./in./°F. The very large (-800 lbs) and thick wooden blade assembly is a large heat sink with poor heat transfer properties. In an effort to relieve the build-up of thermal strain, and allow more heat penetration into the wood at the edges of the composite overwrap bond line, slots were cut into the overwrap, 2 each on the upper and lower surface of the blade and in the sharp transition root area. The slots are shown in the finite element model illustration in figure 14 and in the final repair photos of figures 9 and 10. The slots are not desirable from a structural design point of view since the slots introduce structural discontinuities and stress risers, in both the overwrap and blade. Finite element analysis of the overwrapped blade with slots was used to examine the change in load transfer into the root section and metal cuff, and associated stress distributions to assure that the composite material and wooden blade stresses were acceptable. The third set of 5 blades was fabricated with slots in the overwrap and went through the autoclave cure cycle. These blades appeared to cure properly, but delaminations at the Hydulignum interface again occurred when exposed to cold temperatures (~ 20°F). It now became apparent that the bond between the composite wrap and blade had little or no strength even with the slotted wraps. This was verified by examining large pieces of the unbonded composite overwrap which had virtually no wood present at the failure surface (bond line). In particular, it was observed that the delaminations were occuring only over the Hydulignum surface. The adhesive was bonding well to the Sitka Spruce blade material. Also, in areas thought to be good (i.e. tap tests did not indicate a problem) for the first set of blades cured, composite material samples were removed, which visually showed lack of composite material bond to the Hydulignurn surfaces on the blade, ie. no failure in the wood at the bond line.