AGMA 93FTM9-1993 pdf free pdf download.Gear Tooth Bending Fatigue Crack Detection by Acoustic Emissions and Tooth Compliance Measurements.
A typical acoustic emissions plot for an 8620 gear is ilustrated in Figure 3.. The graph consists of strip chart recorder output in which the y-axis represents acoustic emissions counts, and the x-axis represents time, which has been calibrated to show the corresponding number of cycles as determined usmg the strip chart recorder speed and the test frequency.
In this case, crack nutiation appears to occur at approximately 61000 cycles. It should be noted, however, that because a physical definition of crack initiation has yet to be defined, and because the acoustic emissions detection system has a finite sensitivity, the term aack initiation simply refers to the time at which the fast detectable emissions are recorded. It may be possible to further refine the acoustic emissions detection system in order to increase sensitivity. Such refinement may indude adjustments to the natural frequency of the tramdncer, the threshold value, the amplifier gain, cc the number and placement of transducers.
Notice that prior to the initiation point, the number of recorded acoustic emissions counts is non-zero. This is the result of background noise which is generated by the testing macthe Because it is difficult to separate the noise signal from the signal generated by the growing crack, the sensitivity of the detection system must be maintained at levels which mitixnize the amount of background interference.
As seen in the figure, the emissions slowly increase until, at approximately 87000 cycles, the rate of emissions increases dramatically, leading to final fracture at 89000 cycles. This acoustic emissions faotprint is lical of the 4118 and 4320 gears as well,
In both the peened and unpeened 9310 gears, however, the crack initiation mode was very different, as illustrated in Figure 4. As the gear reached the critical point, it instantaneously cracked along the thidmess When this happened, the gear let out an audible pop that sounded like the snapping of one’s fingers. As the same time this pop was beard, the acoustic eimsstcins rose very rapidly. After this abrupt jump in emissions the curve flattens out during propagation, then once again increases very rapldly unul ure.
It is proposed that in the 9310 gears, a small discontinuity is growing below the surface near the case-core transition, After this crack reaches a critical length. it quickly propagates to the surface resulting in the sudden burst of acoustic emissions. This type of failure is consistent with failures that occur below the surface in carburized steel in which the endurance llnát of the core is much lower that the endurance limit of the case. The endurance limit in directly proportional to the hardness of the material in steels.AGMA 93FTM9 pdf download.