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the photographsof the fracture surfacesdesignatethe tooth number, the level of magnification,anda length scale. Figure6.4:Low magnification(5.3x) view of tooth#11's fracturesurfaceon concave side. 6.3.2Results The morphologiesof teeth #5 and #11 are very similar. Lines emanating radially from the starternotcharevisible atlow magnifications(4.6x in Figure6.3 and 5.3× in Figure6.4). Theselines areindicativeof fatiguecrackgrowth. On tooth#11, somefatiguestriationscan beseennearthe EDM notch. Region A in Figure 6.5a is the EDM notch. Figure 6.5b is a magnifiedview of the striated region.8As expected,the striationsareroughly parallel to the edgeof the EDM notch. The crack growth direction was perpendicularto the striations and, as mentioned above,from the bottomof the figuretowardsthe top. In general,however,mostof the surfacenearthe notchwasflat with no significantfeaturesor texture. This flat surface leadsto the conclusionthatsignificantrubbingtook place. The rubbing"polished" the surfaceandremovedall featuresthat would haveindicatedthe modeof fracture,e.g. fatigue striations,dimples, etc. The rubbed surfacewas visible over approximately 80%of the surfacewhenmoving awayfrom the notchtowardthe ridge of the fracture. Point B in Figure6.3 is the approximatelocationwhereFigure 6.6 was taken. Figure 6.6 is an example of the typical surface appearancein the rubbed region.9 The 8 Figure B. 1 in Appendix B also contains magnified views of the striated region. 9 Figure B.2 in Appendix B is another picture of the typical surface appearance Point B in Figure 6.4 is the approximate location where Figure B.2 was taken. NAS A/CR--2000-210062 73 in the rubbed region. polishing might operation or rubbing pinion. The hypothesis. Figure with extent 6.5: Fatigue resulted from against a part and 6.3 and of surface polished area) both figures uniformity striations A transition from flattened areas was Figures type have 6.4). was rubbing of the near EDM the flat, observed In Figures found along to the lighter region of the crack of the gearbox flat, notch faces after polished while fracturing surfaces the gear away support was from the in the former on tooth #11 at 30× (a) and at 307× (b). polished further area to one with some texture combined from the notch near the ridge (point C in 6.3 and 6.4, the this combination, transition of the upper is near the toe end of the tooth. line from left corner. Figure or partially the darker Recall 6.7, taken rubbed, region (flat, the light region from region in C in Figure 6.3, shows clearly the features of the partially rubbed surface. The appearance of the raised areas is as if they have been flattened, while the lower lying regions have morphology observed. indicative NAS AJCR--2000-210062 of fatigue. However, no well 74 developed fatigue striations are Figure 6.6: Typical picture of flat, polished taken Figure 6.7: Typical The shows fracture little lighter to no mode, e.g. NAS A/CR--2000-210062 picture region signs area on tooth #5 (410×). near location B in Figure of rubbing. intergranular left The fracture, comers surface ductile 75 Photograph of Figures also was 6.3. of partially rubbed surface (695×). from location C in Figure 6.3. in the upper Photograph shows rupture, 6.3 and no dimpling. was taken 6.4 (point obvious signs Although D) of no striations mode were were this region The lack stages on the surface, observed, lead an applied of rubbing also taken on the load crack type on the load free from location free ended side. 10ad range suggests of crack growth. The combination fact that of surface Therefore, point surface. crack inadequate found Figure Region over by fatigue created in the later 90% rubbed 0.75 mm from striations of surface, 6.9 is the tooth are fatigue in striations. approximately 6.8, of a partially B in Figure there of failure to produce were A is approximately In addition, signs propagated surfaces also side of the tooth. obvious was fracture was 6.9. no other that the which that the A in Figure on the tooth the to the conclusion under the surface was present surface where the evident in Figure 6.8. Because this figure is from the convex side of the tooth, the crack growth direction was from the top of the figure to the bottom. This combination of evidence leads to the conclusion that convex side of the tooth. A light separating that the fatigue shows obvious because band region can the crack be seen continued in Region B and C is assumed C of Figure to be oxidation that the material is capable the areas where this type of fracture occurred SEM. This result also leads to the conclusion Figure in fatigue 6.9. mode along the The darkened region of the fracture surface. Recall striations in Figure 6.8 are from location A. The surface in Region C signs of duct_e rupture, Figure 6,i0, This observation is encouraging it demonstrates on the concave to Wow and convex 6'8: Picture (825×). NASA/CR--2000-210062 sides Photograph fracture, and was fatigue. surface was taken by ductile should be obvious and visible under the that the primary mode of crack growth of the tooth of paVia]lynChed of failing with fatigue at location 76 striations A in Figure on load free side 6.9. Figure6.9: Low magnification(31.4×)view of tooth#5's fracturesurfaceon convex side. A third tooth (#9) was also observedwith the SEM. All of the features observedon teeth #5 and #11, with the exception of the ductile fracture area, were observed observations on tooth #9. made on all of the fractured Figure No additional of teeth could are good view of ductile rupture be seen. It is concluded representations of the crack that the patterns teeth. 6.10: Magnified NAS A/CR--2000-210062 features #5 and #11 77 at location C in Figure 6.9 (1670x). Figure 6.11 summarizesthe surfaceappearanceon the loadedand load free sides of the fractured pinion teeth. A scenario of crack growth progression is developedbasedon theseobservations. The fatigue crack growth initiates from the EDM notch. The growth continuesinto the rim and at a larger rate towardsthe toe thanthe heel sinceit is assumedthatthe rubbedareasarethe older surfaces.Oncethe crackreachesthe ridge,the crack continuesto grow towardthetoe end. Figure6.12is a sketchof this scenarioon the loadedside. The numbersin the sketchcorrespondto the progressionof the Crack front' When the Crack re_es the tooth surface at the toe end, the extent remaining convex side, becomes of crack ligament. growth and progresses sufficiently close ligament• After remaining has dramatically Consequently, the by fatigue changed crack along front the convex to the root of the convex this, any the stress turns additional side. side, ductile load on distribution toward the When approximately the middle of the tooth the crack rupture the tooth occurs causes tearing of the ligament on the heel end. Figure 6.2 sketches the crack the tooth width. This sketch is applicable to cross-sections from in the fillet on the front in the the torsional growth through the toe end to length. No rubbing Toe Heel Tooth root concave side Ridge \ Heel Partially tile rupture root convex side Tooth Figure 6.11" Sketch appearance NASA/CR--2000-210062 of loaded along and load free sides the length. Toe rubbed Orientation 78 of a pinion is consistent tooth's fracture with SEM pictures. surface Ridge Toe Heel Tooth root concave Figure 6.12: Sketch 6.4 Chapter of crack This chapter was devoted The test was conducted root of nine of the Limited observations the fracture surfaces pinion teeth rupture successfully. removed to serve of the crack were observed the microscopy signs to presenting of on loaded from EDM as starter growth during with a SEM. identified rubbing, any discernable data by NASA/GRC. Overall, significant propagation scenario fracture surfaces. side devised from Summary test. ductile side fatigue cracks crack regions addition, the had "polished" features for fatigue growth crack face the pinion into the crack and, growth. as a result, and regions morphology surface. on the majority bevel introduced made, In surface spiral were the test were which fracture an OH-58 notches This of showed polishing of the surfaces. The signs of fatigue on the loaded and load free sides of the fracture surface indicated the majority of the crack growth was attributed to fatigue. At the ridge near the toe end, the surface surface jagged showed was little created and tom appearance the last remaining side. ligament Due to the dearth no observations were of rubbing removed Nevertheless, compares NASA/CR--2000-210062 test results latter of rubbing. stages of crack of the fracture of the tooth surface after rupture of well-developed made all a scenario these to no signs in the fatigue This observation growth. near suggested It was the heel inferred that the from the that this region was occurred in the root of the convex striations on the fracture surfaces, on the crack growth rates. In addition, the large amounts indications of crack front shape during propagation. of crack propagation to the simulations results 79 was from devised. Chapter The 5. next chapter CHAPTER DISCUSSION AND SEVEN: SENSITIVITY STUDIES 7.1 Introduction In Chapter moving, 5, fatigue non-proportional fatigue crack compares data these investigates assumptions. from two the crack loads growth was a tested OH-58 sets of results sensitivity of the determine the accuracy Section prediction variations spiral method. bevel commonly contact adopted load. two loading (HPSTC) in past When be implemented pinion. The success under experimental present of the chapter predictions to variations in the and methods are compared to the experimental crack trajectories are evaluated to Sensitivity contact crack studies position growth and are conducted magnitude rate models in and the to variations in since methods' 7.2 Comparisons from the moving load analyses (Section from analyses that consider only highest loading research using HPSTC there in Section because loading, is a single results proportional load method; which produces reasonable designer. 7.4. HPSTC it is a more simplified existing load fatigue location are compared loading growth and proportional to evaluate the need the least computationally intensive crack growth results is the most same as those reported from measurements 5.5) are point of has approach crack been than theories the can loading. The for the moving, non- model practical and method for a gear of Crack Growth Results Figure 7.1 shows the predicted and experimental trajectories and through the cross section of a pinion tooth. The predicted surface pinion 6 presented parameters. tooth moving the results fatigue Crack growth predictions compared to crack growth results single bevel simulations lives and in tooth of the crack-closure-based the model spiral of the prediction 7.3 to explore sensitivity OH-58 Chapter to evaluate In Section 7.2, the crack growth of a tested pinion; the fatigue results in the predicted. in Section and 5.5. photographs The experimental of the failure trajectories surfaces on the tooth results are the are approximated of tooth #11. As discussed in Chapter 6, the failure associated with tooth #11 is representative of all the failures in the tested pinion. In addition, the size of the initial flaw in the predictions was taken from the dimensions of the EDM notch in tooth #11. A ridge because because As a result, it was concluded and further propagation explanation for changes scenario in the predicted crack path through the simulations were stopped prematurely. The the trajectory along the tooth surface on the toe end the experiment. accurate is not observed for this discrepancy in the load shape will be investigated NAS A/CR--2000-210062 cross section simulations were varied significantly that the simulations of the crack would the were lead to no additional halted from not completely insights. An might be that the simulation did not properly account and location as the crack grew. This changing load further in Section 81 7.3.3. Predicted'\ a) Tooth _, ""',\ _ Predicte "" d _ , surface / / / \ "_ / / / / / / i / [ J t I / J / j, / ,/ \\, Experimental / / ,, ', Predicted \\ ,/ ". / \ /" X b) Figure Qualitative be made. experiment. crack The the middle observed section 7.1" Predicted comparisons numerical analyses in both more rapidly at location predict toward tooth the crack results toe pinion results is concordant and the crack trajectories. which study than of initial to the tested failure, the fractography numerical heel. with analyses, In both the can the the test and a portion of the tooth at the heel end remains intact. Additionally, the predict the crack propagating along a steeper trajectory into the gear rim in of the tooth length than on the toe end of the length. in the tested pinion. The final predicted trajectory through well with the initial path in the experiment. the formation of a ridge if the simulations trajectory, however, does not completely predict the toe end of the crack turning behavior is not seen in the tested pinion. the loading of midpoint and experimental of the analyses' In addition, propagates simulation, simulations Cross 7 / conditions for the simulations the thickness It is assumed were continued. This behavior of the tooth is also a_ees very that this path could lead to The entire predicted crack match the tested pinion. The simulations up the tooth height at a steep angle. This One reason for the discrepancy could be that were not identical to the test. The gear was tested at increasing torque levels over the 4.9 million cycles. The simulations, however, were performed under a constant torque level. The increase in torque should affect the tooth contact, which, in turn, will influence the crack trajectory. The influences of the torque NASA]CR--2000-210062 level and contact location 82 on crack trajectories are explored in