NSMS Fundamentals

• Overview
• Externally Mounted Senors
• Various Measurement Capabilities
• Event Time Determination
• Arrival Times to Deflection Conversion
• Deflection to Stress Conversion
• Complete System Characterization
• Flexible Test Scheduling
• Synchronous Realtime Analysis
• Non-Synchronous Realtime Analysis
• High Cycle Fatigue

Blade Crack Detection

The Agilis Measurement Systems Non-intrusive Stress Measurement System (NSMS) a.k.a. Tip Timing, has many capabilities for detection and monitoring of the propagation of blade cracks in turbo machinery. These analysis capabilities include monitoring static blade characteristics such as blade untwist as well as monitoring vibration characteristic such as resonant amplitude, frequency and damping. Trending of these static and vibratory characteristics can be used to detect and monitor propagation of developing cracks in the turbine blades.

Rig Test

The rig test consisted of an electrical powered ducted fan capable of reaching speeds of 5200 RPM.  The fan was a 5 bladed aluminum stage. The excitation was derived from a distortion screen with a 2E pattern which was placed in front of the fan stage. Figure 1 shows the fan and distortion screen.

Figure 1

Probe Configuration

Eight NSMS probes were used in total. Seven of the probes were located above the trailing edge of the blade. These probes were used for measurement of the vibratory characteristics of the blades. The other probe was located above the leading edge and used for the measurement of blade untwist characteristics. Figure 2 shows the probe configuration on the ducted fan case.

Figure 2

Transient Data Set

The test consisted of 70 transients. Each transient consisted of a slow acceleration through the 2E resonance with a 1 minute dwell on the resonant peak. Data was acquired for each transient. The following figure shows a typical transient data set.

Figure 3

Baseline Amplitude & Phase Data

The vibratory characteristics of the mode #1, 2E resonance were extracted for each blade from the acceleration portion of the transient. These included peak amplitudes, resonant RPMs and damping values. The figure 4 displays the baseline amplitude and phase data for each blade across the resonance. This data is used for comparison with the additional transients.

Figure 4

Peak Amplitude, Resonant RPMs, and Damping Values

Peak amplitudes, resonant RPMs and damping values are extracted from these data sets.

Figure 5

Indicators for Crack Initiation and Propagation

The peak amplitudes, resonant RPMs and damping values can be trended across the multiple transients to produce the indicators for crack initiation and propagation. The plots in figure 6 show these trends for the fan crack test.

Figure 6

Drop in Peak Amplitude

The peak amplitudes of the Mode#1 2E resonance dropped as the cracks propagated further into the fan disk. The drop in peak response amplitude was most likely due to the increases in damping from the growing cracks.

Figure 7

Resonant Frequency Shifts

The resonant frequency drops steadily indicating that the cracks began propagating at the beginning of the test. The frequency dropped a total of 9% before the test was terminated. Given this 2E response, resonant frequency shifts of approximately 0.1% represent the minimum detectable shifts.

Figure 8

Video Clip

The damping values increased as the cracks were propagated further into the fan. This is mostly the cause for the drop in resonant frequency amplitude. The following video (flash required) shows all of the resonant characteristics changing across the 70 transients.

Variations in Static Blade Position

The static blade positions can also be monitored for indications of blade crack initiation and propagation. The following plots show the variations in the static blade position from the baseline location at the start of the test.

Static blade positions have shifted approximately 15 mils (0.38 mm) from their baseline locations. These shifts can be monitored real-time at steady- state RPM conditions. The static blade positions are results of changes to untwist and blade lean characteristics from the growing cracks.

Figure 9

Individual Blade Frequencies

Blade buffet response can also be used to determine the individual blade frequencies. This is useful for testing at steady- state RPM conditions where transients through resonant conditions are not possible. The plot below shows mode #1 buffet overlaid with the baseline frequency for transient #1 and transient #70. The drop in Mode #1 frequency is visible as a mismatch with the overlay trace.

Figure 10

Post Test Inspection

Post test inspection clearly showed cracks propagating from the area between the blade inward toward center of the disk. The blades with the largest cracks, blade #4 and blade #5, showed the greatest change in their vibration characteristics. Figure 11 shows one of these cracks after the test was terminated.

Figure 11

Conclusion

The Agilis NSMS system offers several techniques for determining crack initiation and propagation. These include both static and dynamic characteristic trending which can be used together for crack identification. This multiple characteristic approach limits false calls while providing a robust approach for crack identification.