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Analysis of failed aircraft wheel assembly
Engineering Failure Analysis 13 (2006) 65–74 www.elsevier.com/locate/engfailanal
Analysis of failed aircraft wheel assembly Takao Kobayashi, Donald A. Shockey
*
Center for Fracture Physics, Materials Research Laboratory, SRI International, Menlo Park, CA 94025, United States Received 15 November 2004; accepted 5 December 2004 Available online 13 April 2005
Abstract A failed wheel assembly from a Hawker 125-800XP corporate passenger jet was investigated to determine the root cause of the failure. Different levels of damage sustained by the 12 tie bolt/nut couples that had held the flange to the wheel showed that failure was a cascading sequence that began at one couple and spread to adjacent couples. The suspected initiating bolt/nut couple showed stepped threads on one side of the bolt, the characteristics of which suggested that the nut had cracked, disengaged, and induced the failures of the other bolt/nut couples. Since the nut was not available for examination, our conclusion that the root cause of failure was a cracked nut could not be confirmed. Somewhat later, however, a cracked nut was observed in another wheel assembly. Examination of the bolt showed similar thread deformation, supporting the initial conclusion. The nut fracture surfaces showed an intergranular region surrounded by a ductile field – most likely the crack initiation site where the grain boundaries were weak. In our expert opinion, the root cause of the wheel assembly failure was a cracked nut that may have been embrittled by hydrogen during cadmium plating. Ó 2005 Published by Elsevier Ltd. Keywords: Failure analysis; Fracture surfaces; Hydrogen embrittlement; Landing gear; Threaded fasteners
1. Introduction The fleet of Raytheon Hawker 125-800XP corporate passenger jets consists of about 600 aircraft. Each aircraft experiences some 400 takeoff-and-landing cycles annually. The wheel assemblies are periodically inspected and new tires are periodically installed. There exists no criterion to change bolts after some period of service. Consequently, bolts typically remain in service for 4000–5000 cycles. Nuts are changed after about 400 cycles.
*
Corresponding author. Tel.: +1 650 859 2587; fax: +1 650 859 2260. E-mail address: [email protected] (D.A. Shockey).
1350-6307/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.engfailanal.2004.12.046
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Wheel assembly failures are rare, but when they occur, they occur soon after a tire is replaced and the aircraft goes back into service, usually before 20 takeoff-and-landing cycles; some after only a few cycles. The fleet experienced five failures of the main landing gear wheel assembly in the past 12 months. In all cases the wheel flange, which is held to the hub by 12 tie bolts and nuts, separated from the hub. The failures were suspected to result from the nuts loosening on the bolts. In attempting to prevent wheel assembly failures, bolt and nut specifications were checked and numerous internal policy adjustments were made. The wheel manufacturer examined the bolts and nuts carefully and verified conformity to metallurgical and dimensional military specifications (Mil Spec MIL-DTL-25027H). The air station required that (1) two people verify the correct torque of the bolts, (2) two people verify that every maintenance manual step is followed, (3) only new hardware be installed at build-up, and (4) torque seals be checked to verify that the correct torque has been applied. Unfortunately, even with these new procedures the problem continued. The air station asked SRI to examine the bolts and nuts from a failed wheel assembly and find the root cause of the failure. This paper describes the procedures, observations, and conclusions of the analysis.
2. Investigation of a failed wheel assembly The particular wheel assembly sent to us for examination had failed after 15 flight cycles. In the condition it was received, only two of the bolts still had nuts attached, and those bolts were bent. The other 10 bolts had been removed from the flange holes and the nuts were lost. We examined the 12 bolts visually and with a light microscope and photographed each at four 90° angles. Four bolts were bent, including the two that still had nuts attached; four bolts exhibited stripped threads near the bolt ends; and four bolts had no damage obvious to the unaided eye. Fig. 1 shows a bolt from each damage category. The type and extent of damage indicated the sequence of bolt failure and the positions of the bolts relative to each other in the wheel flange. The two bent bolts with washers and nuts intact were the last two bolts connecting the flange and the hub. Since the other 10 bolt/nut couples had failed, the separating flange exerted large lateral stresses on the bolts, causing the bolts to bend. We designated these as Bolts #1 and #2 and arbitrarily placed them in the bolt ring as shown in Fig. 2. The two bent bolts without a nut (#3 and #4) were bent to a lesser degree by the separating flange, but the threads at the bolt ends were stripped when the nuts sheared off. Bolts #3 and #4 were probably the next pair to fail and were probably located on either side of Bolts #1 and #2. Bolt #4 was probably adjacent to Bolt #2 and Bolt #1 was probably adjacent to Bolt #3, because the bend angles of these pairs were similar. The stripped threads were near the ends of the bolts, indicating that the nuts had already backed off several threads when the stripping occurred. Furthermore, the threads were stripped all around the bolts (360°). Bolts #5–#8 were not bent, but their threads were stripped near the bolt ends. The locations of the stripped threads showed that the nuts had already moved to near the bolt ends when the load exerted by the partially separated flange was exerted. Thus, these four bolts were probably located two each on either side of the four bent bolts as shown in Fig. 2. We used the number of stripped threads to position these bolts. The final four bolts (#9–#12), which showed no apparent thread damage, were probably the first to lose their nuts, and did so before the flange separated appreciably from the hub. Therefore, one of these bolt/nut couples probably started the sequence of events that led to wheel assembly failure. Presumably the nut on one of these bolts loosened, caused the next-neighbor bolt/nut couples to loosen, which in turn caused their next-neighbor couples to loosen, and so on until only two bolt/nut couples
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Fig. 1. Photographs of bolts from each damage category: (a) bent; (b) stripped (note last four threads of Bolt #5); and (c) no apparent damage.
(those with bent bolts) remained. Thus, wheel failure was a cascading event beginning with one bolt/nut couple. To identify the first-to-fail bolt and the probable root cause of the wheel failure, we examined the four suspect bolts (Bolts #9–#12) carefully, quantitatively characterizing the thread profiles, interrogating the thread surfaces and metallographic cross sections, and measuring hardness. 2.1. Examination of bolt thread profiles We imaged the threads of the four suspect bolts with an optical comparator, producing a profile every 15 degrees around the circumference. No profile anomalies were observed on suspect Bolts #9, #11, and #12. Bolt #10, however, exhibited steps on the load-bearing faces of certain threads, indicated by the arrows in Fig. 3. The steps occurred on only 7 threads; threads on either side of this group of 7 showed no steps. The stepped threads lay under the nut when the nut was in its tightened position. The location of the step on the thread face increased from thread to thread almost linearly with distance from the bolt end (shown more clearly in Fig. 7). Fig. 4 shows profile images at several angular positions. The two lines on the center thread show the magnitude of the step at several angular positions. A plot of step magnitude as a function of the angle (Fig. 5) shows that the steps existed on only one side of the bolt. Bolts #1–#8 were examined in a similar manner. Bolts #3–#8 had stripped threads, but the profile of threads adjacent to the stripped threads did not have steps similar to those found in Bolt #10.
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Fig. 2. Wheel flange and relative bolt positions based on bolt damage observations.
Fig. 3. Optical comparator image of threads of Bolt #10, showing the steps on thread faces.
2.2. Metallographic cross-section of Bolt #10 We investigated these steps further by cross-sectioning Bolt #10, then polishing and etching the surface of the section. Consistent with the comparator results in Fig. 3, steps were evident on certain threads, namely those that engaged the nut while the nut was in the tightened position (Fig. 6). Threads on either side of the tightened nut position exhibited no steps. And, as also observed with the comparator, the steps existed on only one side of the bolt. The position of the step on the thread face shifted linearly from one thread to the other toward the bolt end (Fig. 7), providing a clue about how the nut interacted with the bolt.
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Fig. 4. Optical comparator images of steps found on the threads of Bolt #10 as a function of position around the axis of the bolt.
The optical micrograph of the three threads in Fig. 8 shows that the top portion of the threads was pushed to the left (toward the bolt end), producing a step on the load-bearing thread face and a protrusion and fold on the unloaded face, indicating that these threads experienced overloading from the wheel flange. 2.3. Summary of Bolt #10 observations Steps on the faces of certain threads of Bolt #10 were evident, as shown in Figs. 3, 6, 7 and 8. Step characteristics are as follows.
Steps appear only on 7 threads. Threads to either side have no steps. The stepped threads all lie under the nut in the tightened position. Step size (lateral dimension) is greatest on the center threads and decreases to either side. Step location on thread faces is highest near the flange, lowest near the bolt end. Steps exist only halfway around the bolt circumference; threads on the other half have no steps.
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Fig. 5. Step magnitude as a function of angular position for Bolt #10.
Fig. 6. Macro SEM image of Bolt #10 cross-section.
2.4. Implications of the observations The steps were produced by an overload transmitted by the nut. The nut was in its tightened position when the steps were produced. Since the steps exist only around half (approximately) of the circumference,
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Fig. 7. Increasing position of step on the face of adjacent threads.
Fig. 8. Metallographic image of three deformed threads.
the overload was applied only on one side. This indicates that the nut split, relieved the load on the bolt threads on the split side, and overloaded the opposite side, causing plastic deformation of the threads. The progressive change in step height on successive threads and the absence of steps on the opposite side of the bolt suggest that the nut was cocked when it deformed the bolt threads. The load exerted on the bolt thread was reduced on the broken side of the nut and concentrated on the other side, and the difference in the load lines generated a moment, as shown in Fig. 9. A further indication that the nut split is that the thread-stripping process was interrupted. We hypothesize that this interruption occurred because the nut deformed, the split widened, and the nut disengaged and fell away. We conclude that a cracked nut was the root cause of the wheel assembly failure. The nut could have contained a crack when it was fabricated, or could have acquired a crack when it was torqued during installation or when the aircraft was in service (an unusually heavy landing load, for example). Moreover, the high-strength steel nut could have been weakened by hydrogen effects during the cadmium coating process.
Fig. 9. Moment caused by the difference in the load lines.
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Unfortunately, the nut on Bolt #10 was lost while the aircraft was in service and thus was not available for examination, and so the cracked-nut hypothesis could not be confirmed.
3. Analysis of a cracked nut/bolt couple from a second wheel assembly Soon after reporting our findings and our opinion that the root cause of the wheel assembly failure was a cracked nut, we were told that a maintenance worker had newly observed a wheel assembly that had a cracked nut still on the bolt. When we were provided with the bolt and nut, we measured the bolt thread profile with the optical comparator and examined a cross-section, as we did for Bolt #10, to check for steps and determine their characteristics. We compared the results with those from Bolt #10 to check our cracked-nut hypothesis. We also examined the nut fracture surfaces to seek the root cause of nut failure. 3.1. Examination of second bolt threads and comparison with Bolt #10 Optical comparator images of thread profiles at 15° intervals were produced just as for Bolt #10. Steps on certain thread faces were evident. The two bolts were similar in that all stepped threads lay in the tightened nut zone (and nowhere else), and the steps were larger on one side of the bolt. However, the second bolt had only five stepped threads and the steps were less severe and less localized. We sectioned the bolt axially on the 90°/270° plane and polished and etched the cross-section in a 2% nital solution to observe the subsurface deformation and microstructure. As with Bolt #10, the threads with steps were in the tightened nut position zone and the height of the steps above the thread groove increased from thread to thread. Higher magnification metallographic images showed that the threads at the 90° position had steps about three times larger than threads at the 270° position. However, the steps were smaller and less distinct than those in Bolt #10, and were not associated with clear slip lines. In summary, the deformation observed in the bolt with the split nut had similar characteristics as those found in Bolt #10. The extent of deformation in Bolt #10 was greater, perhaps because the cracked nut on the second bolt had acted on the bolt for a shorter time. 3.2. Examination of fracture surface of cracked nut The fracture surfaces of the cracked nut were exposed by cutting through the wall opposite the crack. Examination with light microscopy showed no obvious signs of corrosion, fatigue, or overload failure. The macro view in Fig. 10 shows a flat center section surrounded by slanted shear surfaces. Linear markings radiate back to an area near the base of the nut. Examination of this area at higher magnification (Fig. 10, inset) showed exposed intergranular facets, suggestive of hydrogen embrittlement. 3.3. Hydrogen embrittlement The nut specification calls for cadmium coating for protection against corrosion. To this end, the nut is placed in an acid pickling bath to clean it before electrodeposition plating. Both the pickling and plating operations subject the nut surface to atomic hydrogen, some of which diffuses into the steel. Hydrogen is attracted to the grain boundaries and weakens them. Upon application of load, a crack may initiate and propagate over time through the section. The fracture surfaces of hydrogen-affected material typically exhibit exposed grains and grain boundary cracks extending into the fracture surface.
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Fig. 10. SEM photograph of the fracture surface of the cracked nut.
Stress is required for hydrogen cracking. Below a certain stress level, cracking will not occur. This critical level is known as the static-fatigue limit. The cracked nut suggests the nut was tightened past its static fatigue limit or subjected to unusually large service loads (a hard landing, for example). Hydrogen cracking can sometimes be avoided without appreciable change in hardness by baking after plating, usually at temperatures around 375–425 °F in air for a suitable time, depending on the hardness, size, and finish of the nut. ASTM standards B 850-94 and B 242 describe bakeout procedures. For the nuts used on the Hawker wheel assemblies, the specified Rockwell C hardness is 46–50, for which a bakeout time of 20–24 h is recommended. We measured the hardness of Bolt #10 and the second bolt and obtained Rockwell C values of 51.9 and 50.8, respectively. These hardness values differ insignificantly from the specification, but are in the range of hydrogen embrittlement susceptibility and support the observation of intergranular fracture shown in Fig. 10. Thus, it appears likely that the failure of the nut was due to time-dependent hydrogen embrittlement under stresses imposed by installation or service.
4. Conclusions We conclude that the root cause of the wheel assembly failure was a cracked nut. When the nut cracked, the load on the tie bolt was released and was taken up by the adjacent bolt/nut couples. Under the increased load, the nuts on the adjacent bolts backed off, transferring their load to the next adjacent bolt/nut couples,
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which then failed, and so on around the bolt circle. Later bolt/nut couples failed more violently as the flange separated from the wheel casing, as manifested by thread stripping and bolt bending. The likely root cause of the nut cracking was hydrogen embrittlement. Hydrogen in the steel from pickling and cadmium coating operations may not have been sufficiently baked out, and could have resulted in crack initiation and propagation under tightening or service loads.
Analysis of failed aircraft wheel assembly Takao Kobayashi, Donald A. Shockey
*
Center for Fracture Physics, Materials Research Laboratory, SRI International, Menlo Park, CA 94025, United States Received 15 November 2004; accepted 5 December 2004 Available online 13 April 2005
Abstract A failed wheel assembly from a Hawker 125-800XP corporate passenger jet was investigated to determine the root cause of the failure. Different levels of damage sustained by the 12 tie bolt/nut couples that had held the flange to the wheel showed that failure was a cascading sequence that began at one couple and spread to adjacent couples. The suspected initiating bolt/nut couple showed stepped threads on one side of the bolt, the characteristics of which suggested that the nut had cracked, disengaged, and induced the failures of the other bolt/nut couples. Since the nut was not available for examination, our conclusion that the root cause of failure was a cracked nut could not be confirmed. Somewhat later, however, a cracked nut was observed in another wheel assembly. Examination of the bolt showed similar thread deformation, supporting the initial conclusion. The nut fracture surfaces showed an intergranular region surrounded by a ductile field – most likely the crack initiation site where the grain boundaries were weak. In our expert opinion, the root cause of the wheel assembly failure was a cracked nut that may have been embrittled by hydrogen during cadmium plating. Ó 2005 Published by Elsevier Ltd. Keywords: Failure analysis; Fracture surfaces; Hydrogen embrittlement; Landing gear; Threaded fasteners
1. Introduction The fleet of Raytheon Hawker 125-800XP corporate passenger jets consists of about 600 aircraft. Each aircraft experiences some 400 takeoff-and-landing cycles annually. The wheel assemblies are periodically inspected and new tires are periodically installed. There exists no criterion to change bolts after some period of service. Consequently, bolts typically remain in service for 4000–5000 cycles. Nuts are changed after about 400 cycles.
*
Corresponding author. Tel.: +1 650 859 2587; fax: +1 650 859 2260. E-mail address: [email protected] (D.A. Shockey).
1350-6307/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.engfailanal.2004.12.046
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T. Kobayashi, D.A. Shockey / Engineering Failure Analysis 13 (2006) 65–74
Wheel assembly failures are rare, but when they occur, they occur soon after a tire is replaced and the aircraft goes back into service, usually before 20 takeoff-and-landing cycles; some after only a few cycles. The fleet experienced five failures of the main landing gear wheel assembly in the past 12 months. In all cases the wheel flange, which is held to the hub by 12 tie bolts and nuts, separated from the hub. The failures were suspected to result from the nuts loosening on the bolts. In attempting to prevent wheel assembly failures, bolt and nut specifications were checked and numerous internal policy adjustments were made. The wheel manufacturer examined the bolts and nuts carefully and verified conformity to metallurgical and dimensional military specifications (Mil Spec MIL-DTL-25027H). The air station required that (1) two people verify the correct torque of the bolts, (2) two people verify that every maintenance manual step is followed, (3) only new hardware be installed at build-up, and (4) torque seals be checked to verify that the correct torque has been applied. Unfortunately, even with these new procedures the problem continued. The air station asked SRI to examine the bolts and nuts from a failed wheel assembly and find the root cause of the failure. This paper describes the procedures, observations, and conclusions of the analysis.
2. Investigation of a failed wheel assembly The particular wheel assembly sent to us for examination had failed after 15 flight cycles. In the condition it was received, only two of the bolts still had nuts attached, and those bolts were bent. The other 10 bolts had been removed from the flange holes and the nuts were lost. We examined the 12 bolts visually and with a light microscope and photographed each at four 90° angles. Four bolts were bent, including the two that still had nuts attached; four bolts exhibited stripped threads near the bolt ends; and four bolts had no damage obvious to the unaided eye. Fig. 1 shows a bolt from each damage category. The type and extent of damage indicated the sequence of bolt failure and the positions of the bolts relative to each other in the wheel flange. The two bent bolts with washers and nuts intact were the last two bolts connecting the flange and the hub. Since the other 10 bolt/nut couples had failed, the separating flange exerted large lateral stresses on the bolts, causing the bolts to bend. We designated these as Bolts #1 and #2 and arbitrarily placed them in the bolt ring as shown in Fig. 2. The two bent bolts without a nut (#3 and #4) were bent to a lesser degree by the separating flange, but the threads at the bolt ends were stripped when the nuts sheared off. Bolts #3 and #4 were probably the next pair to fail and were probably located on either side of Bolts #1 and #2. Bolt #4 was probably adjacent to Bolt #2 and Bolt #1 was probably adjacent to Bolt #3, because the bend angles of these pairs were similar. The stripped threads were near the ends of the bolts, indicating that the nuts had already backed off several threads when the stripping occurred. Furthermore, the threads were stripped all around the bolts (360°). Bolts #5–#8 were not bent, but their threads were stripped near the bolt ends. The locations of the stripped threads showed that the nuts had already moved to near the bolt ends when the load exerted by the partially separated flange was exerted. Thus, these four bolts were probably located two each on either side of the four bent bolts as shown in Fig. 2. We used the number of stripped threads to position these bolts. The final four bolts (#9–#12), which showed no apparent thread damage, were probably the first to lose their nuts, and did so before the flange separated appreciably from the hub. Therefore, one of these bolt/nut couples probably started the sequence of events that led to wheel assembly failure. Presumably the nut on one of these bolts loosened, caused the next-neighbor bolt/nut couples to loosen, which in turn caused their next-neighbor couples to loosen, and so on until only two bolt/nut couples
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Fig. 1. Photographs of bolts from each damage category: (a) bent; (b) stripped (note last four threads of Bolt #5); and (c) no apparent damage.
(those with bent bolts) remained. Thus, wheel failure was a cascading event beginning with one bolt/nut couple. To identify the first-to-fail bolt and the probable root cause of the wheel failure, we examined the four suspect bolts (Bolts #9–#12) carefully, quantitatively characterizing the thread profiles, interrogating the thread surfaces and metallographic cross sections, and measuring hardness. 2.1. Examination of bolt thread profiles We imaged the threads of the four suspect bolts with an optical comparator, producing a profile every 15 degrees around the circumference. No profile anomalies were observed on suspect Bolts #9, #11, and #12. Bolt #10, however, exhibited steps on the load-bearing faces of certain threads, indicated by the arrows in Fig. 3. The steps occurred on only 7 threads; threads on either side of this group of 7 showed no steps. The stepped threads lay under the nut when the nut was in its tightened position. The location of the step on the thread face increased from thread to thread almost linearly with distance from the bolt end (shown more clearly in Fig. 7). Fig. 4 shows profile images at several angular positions. The two lines on the center thread show the magnitude of the step at several angular positions. A plot of step magnitude as a function of the angle (Fig. 5) shows that the steps existed on only one side of the bolt. Bolts #1–#8 were examined in a similar manner. Bolts #3–#8 had stripped threads, but the profile of threads adjacent to the stripped threads did not have steps similar to those found in Bolt #10.
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Fig. 2. Wheel flange and relative bolt positions based on bolt damage observations.
Fig. 3. Optical comparator image of threads of Bolt #10, showing the steps on thread faces.
2.2. Metallographic cross-section of Bolt #10 We investigated these steps further by cross-sectioning Bolt #10, then polishing and etching the surface of the section. Consistent with the comparator results in Fig. 3, steps were evident on certain threads, namely those that engaged the nut while the nut was in the tightened position (Fig. 6). Threads on either side of the tightened nut position exhibited no steps. And, as also observed with the comparator, the steps existed on only one side of the bolt. The position of the step on the thread face shifted linearly from one thread to the other toward the bolt end (Fig. 7), providing a clue about how the nut interacted with the bolt.
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Fig. 4. Optical comparator images of steps found on the threads of Bolt #10 as a function of position around the axis of the bolt.
The optical micrograph of the three threads in Fig. 8 shows that the top portion of the threads was pushed to the left (toward the bolt end), producing a step on the load-bearing thread face and a protrusion and fold on the unloaded face, indicating that these threads experienced overloading from the wheel flange. 2.3. Summary of Bolt #10 observations Steps on the faces of certain threads of Bolt #10 were evident, as shown in Figs. 3, 6, 7 and 8. Step characteristics are as follows.
Steps appear only on 7 threads. Threads to either side have no steps. The stepped threads all lie under the nut in the tightened position. Step size (lateral dimension) is greatest on the center threads and decreases to either side. Step location on thread faces is highest near the flange, lowest near the bolt end. Steps exist only halfway around the bolt circumference; threads on the other half have no steps.
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Fig. 5. Step magnitude as a function of angular position for Bolt #10.
Fig. 6. Macro SEM image of Bolt #10 cross-section.
2.4. Implications of the observations The steps were produced by an overload transmitted by the nut. The nut was in its tightened position when the steps were produced. Since the steps exist only around half (approximately) of the circumference,
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Fig. 7. Increasing position of step on the face of adjacent threads.
Fig. 8. Metallographic image of three deformed threads.
the overload was applied only on one side. This indicates that the nut split, relieved the load on the bolt threads on the split side, and overloaded the opposite side, causing plastic deformation of the threads. The progressive change in step height on successive threads and the absence of steps on the opposite side of the bolt suggest that the nut was cocked when it deformed the bolt threads. The load exerted on the bolt thread was reduced on the broken side of the nut and concentrated on the other side, and the difference in the load lines generated a moment, as shown in Fig. 9. A further indication that the nut split is that the thread-stripping process was interrupted. We hypothesize that this interruption occurred because the nut deformed, the split widened, and the nut disengaged and fell away. We conclude that a cracked nut was the root cause of the wheel assembly failure. The nut could have contained a crack when it was fabricated, or could have acquired a crack when it was torqued during installation or when the aircraft was in service (an unusually heavy landing load, for example). Moreover, the high-strength steel nut could have been weakened by hydrogen effects during the cadmium coating process.
Fig. 9. Moment caused by the difference in the load lines.
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Unfortunately, the nut on Bolt #10 was lost while the aircraft was in service and thus was not available for examination, and so the cracked-nut hypothesis could not be confirmed.
3. Analysis of a cracked nut/bolt couple from a second wheel assembly Soon after reporting our findings and our opinion that the root cause of the wheel assembly failure was a cracked nut, we were told that a maintenance worker had newly observed a wheel assembly that had a cracked nut still on the bolt. When we were provided with the bolt and nut, we measured the bolt thread profile with the optical comparator and examined a cross-section, as we did for Bolt #10, to check for steps and determine their characteristics. We compared the results with those from Bolt #10 to check our cracked-nut hypothesis. We also examined the nut fracture surfaces to seek the root cause of nut failure. 3.1. Examination of second bolt threads and comparison with Bolt #10 Optical comparator images of thread profiles at 15° intervals were produced just as for Bolt #10. Steps on certain thread faces were evident. The two bolts were similar in that all stepped threads lay in the tightened nut zone (and nowhere else), and the steps were larger on one side of the bolt. However, the second bolt had only five stepped threads and the steps were less severe and less localized. We sectioned the bolt axially on the 90°/270° plane and polished and etched the cross-section in a 2% nital solution to observe the subsurface deformation and microstructure. As with Bolt #10, the threads with steps were in the tightened nut position zone and the height of the steps above the thread groove increased from thread to thread. Higher magnification metallographic images showed that the threads at the 90° position had steps about three times larger than threads at the 270° position. However, the steps were smaller and less distinct than those in Bolt #10, and were not associated with clear slip lines. In summary, the deformation observed in the bolt with the split nut had similar characteristics as those found in Bolt #10. The extent of deformation in Bolt #10 was greater, perhaps because the cracked nut on the second bolt had acted on the bolt for a shorter time. 3.2. Examination of fracture surface of cracked nut The fracture surfaces of the cracked nut were exposed by cutting through the wall opposite the crack. Examination with light microscopy showed no obvious signs of corrosion, fatigue, or overload failure. The macro view in Fig. 10 shows a flat center section surrounded by slanted shear surfaces. Linear markings radiate back to an area near the base of the nut. Examination of this area at higher magnification (Fig. 10, inset) showed exposed intergranular facets, suggestive of hydrogen embrittlement. 3.3. Hydrogen embrittlement The nut specification calls for cadmium coating for protection against corrosion. To this end, the nut is placed in an acid pickling bath to clean it before electrodeposition plating. Both the pickling and plating operations subject the nut surface to atomic hydrogen, some of which diffuses into the steel. Hydrogen is attracted to the grain boundaries and weakens them. Upon application of load, a crack may initiate and propagate over time through the section. The fracture surfaces of hydrogen-affected material typically exhibit exposed grains and grain boundary cracks extending into the fracture surface.
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Fig. 10. SEM photograph of the fracture surface of the cracked nut.
Stress is required for hydrogen cracking. Below a certain stress level, cracking will not occur. This critical level is known as the static-fatigue limit. The cracked nut suggests the nut was tightened past its static fatigue limit or subjected to unusually large service loads (a hard landing, for example). Hydrogen cracking can sometimes be avoided without appreciable change in hardness by baking after plating, usually at temperatures around 375–425 °F in air for a suitable time, depending on the hardness, size, and finish of the nut. ASTM standards B 850-94 and B 242 describe bakeout procedures. For the nuts used on the Hawker wheel assemblies, the specified Rockwell C hardness is 46–50, for which a bakeout time of 20–24 h is recommended. We measured the hardness of Bolt #10 and the second bolt and obtained Rockwell C values of 51.9 and 50.8, respectively. These hardness values differ insignificantly from the specification, but are in the range of hydrogen embrittlement susceptibility and support the observation of intergranular fracture shown in Fig. 10. Thus, it appears likely that the failure of the nut was due to time-dependent hydrogen embrittlement under stresses imposed by installation or service.
4. Conclusions We conclude that the root cause of the wheel assembly failure was a cracked nut. When the nut cracked, the load on the tie bolt was released and was taken up by the adjacent bolt/nut couples. Under the increased load, the nuts on the adjacent bolts backed off, transferring their load to the next adjacent bolt/nut couples,
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which then failed, and so on around the bolt circle. Later bolt/nut couples failed more violently as the flange separated from the wheel casing, as manifested by thread stripping and bolt bending. The likely root cause of the nut cracking was hydrogen embrittlement. Hydrogen in the steel from pickling and cadmium coating operations may not have been sufficiently baked out, and could have resulted in crack initiation and propagation under tightening or service loads.