Crystal Instruments is excited to announce the release of our new VCS software module, Blade Fatigue Testing (BFT). Fatigue testing of blades is a critical step in mechanical design, whether it be turbine, compressor, or even propeller blades. Repeated cyclic loading can deteriorate the structural integrity of a blade over time inducing a reduction in overall strength, this is known as fatigue damage. The BFT software provides engineers with important data that can be analyzed to determine how a certain blade behaves under fatigue.

The BFT software is a unique and well-suited solution for aerospace, automotive, energy and civil engineering industries. BFT provides the ability to search and acquire resonant frequencies of a desired blade using a sine sweep. These frequencies along with the corresponding amplitudes are displayed as well as the phase and Q factor of the blade. Users have multiple choices for dwell (fatigue) testing: basic dwell, tracked dwell and phase tracked dwell. BFT also includes the ability to control dwell frequency, amplitude and phase manually.

Blade Fatigue Testing Setup

A typical test setup for blade fatigue testing is shown in the figures above. A blade is selected and held using a specially designed fixture which is then fastened to either the head expander or armature of the shaker. It is important to understand the Q-factor and material properties of the blade under test. Materials with a low Q-factor tend to have a high coefficient of damping which requires more drive from the shaker to excite the specimen at a desired deflection amplitude. Material properties such as Q-factor, mass, and stiffness are always important to consider when selecting the correct shaker to use for blade fatigue testing. Sensors used generally include a laser displacement sensor (focused on the blade tip) and an accelerometer (placed on head expander or armature). For the Sine Sweep portion of the test, the laser displacement sensor is set as the monitor and the accelerometer is set as the control. When performing the Dwell test, the displacement sensor is set as the control and the accelerometer is set as the monitor. Controlling the displacement amplitude directly provides better data and more accurate strain values.

The laser displacement sensor shown in Figure 2 is a Micro-Epsilon OptoNCDT ILD1900, a common sensor for blade fatigue applications. This sensor has a frequency range of 10 kHz, a measurement distance of 25 mm and a sensitivity of 400 mV/mm. Laser displacement sensors are generally mounted in place using an adjustable tripod stand. However, custom sensor mounts may be used as well, depending on the size of the shaker and the DUT. Other sensors used in BFT applications include accelerometers and strain gauges. For control of the armature, IEPE accelerometers are a popular choice due to their unrequired need for a charge converter. Foil strain gauges are commonly used for measuring blade deflection and strain; Different configurations may be used such as linear, half-bridge, full-bridge, or rosette.

BFT - Major Software Features

Specialized Input channel table for sweep and dwell entries
The input channel table includes two new columns, these being the sweep channel and the dwell channel.

Many standards require the blade amplitude to be controlled using a displacement sensor for dwell entries, because of this individual channel types can be pre-set to eliminate the need to swap monitor and control after sine sweeps.

Sine Sweep Resonance Search
After performing a sine sweep, users may use the Resonance search feature to search and acquire the natural frequencies, amplitudes, phase and Q factor from the FRF. Fixed dwell, tracked dwell or phase tracked dwell entries may then be quickly configured and added to the run schedule.

Phase Tracked Dwell
Phase tracked dwell allows for the amplitude, frequency, and phase to be tracked automatically during the dwell. As fatigue damage starts to develop in the material of the blade, the natural frequency will shift. The phase tracked dwell algorithm tracks this change and adjusts the dwell accordingly.

Initial dwell profile can be configured and includes parameters such as natural frequency, phase, phase slope, dwell time/cycle#, amplitude level and sweep level.

Manual Dwell control
Manual Dwell control allows for incremental changes in the frequency, amplitude and or phase. Additional parameters such as dwell cycles, dwell time and tracking speed may also be adjusted manually. Manually controlling the dwell can provide a more accurate resonant frequency and gives the user more flexibility on how the dwell is performed.

Digital Readout Chart
BFT includes digital readout values for amplitude, frequency, cycle count, time and many more parameters.

*Figure 7. Digital Readout Chart and dwell table*

Strain measurement integration
BFT includes the option to incorporate strain measurements for monitoring and control.

A variety of strain gauges are compatible with our spider hardware and allow for easy setup/configuration.

*Figure 8. Blade and fixture attached strain gauges.*

BFT vs. RSTD Software
Blade fatigue testing and resonance search and dwell are two similar software modules offered by crystal instruments; However, there are several important advantages to using BFT over RSTD. Blade fatigue testing allows for easier test configuration and sensor setup. Unlike RSTD, BFT provides users with custom pre-set input channels for Sine sweep and dwell tests; this eliminates the need to switch monitor and control when performing different tests. Another advantage of BFT is the ability to configure a sweep profile and a dwell profile at the same time; whereas in RSTD there is only one profile breakpoint table which needs to be updated based on the test type being performed. Additional benefits of BFT include Digital readouts and dwell tables for live measurements. Lastly, BFT provides improved DSP algorithms for tracking amplitude and phase of blades with high Q-factors.

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