11.3 Vibration testing methods

Vibration testing methods are crucial for understanding how structures respond to dynamic forces. From impact and shaker tests to operational modal analysis, these techniques help engineers measure and analyze vibrations in various scenarios.

Advanced methods like random vibration and multi-axis testing simulate complex real-world conditions. By mastering these techniques, engineers can design safer, more reliable structures and products that withstand the rigors of their intended environments.

Vibration Testing Methods

Impact, Shaker, and Operational Modal Analysis

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• Impact testing excites a structure with a calibrated hammer and measures its response
  • Used for modal analysis and frequency response function measurements
  • Allows quick setup with minimal equipment
• Shaker testing applies controlled excitation forces to a structure using electrodynamic or hydraulic shakers
  • Enables precise input control and frequency sweeps
  • Useful for product qualification and fatigue testing
• Operational modal analysis (OMA) uses only output responses of a structure under normal operating conditions
  • Eliminates need for artificial excitation
  • Requires advanced signal processing techniques

Advanced Vibration Testing Techniques

• Random vibration testing applies broadband random excitation to simulate real-world environments
  • Provides realistic representation of complex vibration scenarios
  • Commonly used in product qualification and fatigue testing
• Sine sweep testing applies sinusoidal excitation force varying in frequency over time
  • Identifies resonances and damping characteristics
  • Measures transmissibility of structures
• Multi-axis vibration testing excites a structure in multiple directions simultaneously
  • Uses multiple shakers for more realistic representation of complex environments
  • Requires careful coordination of multiple exciters
• Acoustic vibration testing uses high-intensity sound waves to excite structures
  • Particularly useful for large or sensitive structures
  • Avoids direct mechanical contact with test specimen

Vibration Experiment Design

Sensor Selection and Data Acquisition

• Choose and place sensors to capture relevant vibration modes
  • Accelerometers most common sensor type (piezoelectric, capacitive)
  • Consider sensitivity, frequency range, and mounting method
• Configure signal conditioning and data acquisition systems for accurate measurements
  • Set appropriate sampling rate (typically 2.5 times highest frequency of interest)
  • Implement anti-aliasing filters to prevent signal distortion
  • Ensure sufficient dynamic range to capture both low and high amplitude vibrations

Test Setup and Excitation Methods

• Design proper fixtures to represent structure's boundary conditions
  • Avoid introducing unwanted dynamics (stiff mounting, minimal mass loading)
  • Simulate real-world constraints (free-free, fixed-fixed, simply supported)
• Select excitation methods based on structure characteristics and test objectives
  • Consider force level, frequency range, and spatial distribution
  • Match excitation type to desired test outcomes (modal analysis, fatigue testing)
• Plan tests with clear objectives and parameters
  • Define pass/fail criteria based on relevant standards (ISO, MIL-STD)
  • Establish test duration, frequency range, and amplitude levels

Data Analysis and Validation

• Perform experimental modal analysis to extract modal parameters
  • Measure frequency response functions (FRFs)
  • Calculate natural frequencies, mode shapes, and damping ratios
• Conduct data quality checks to ensure measurement reliability
  • Evaluate coherence functions (should be close to 1 for good data)
  • Perform reciprocity checks (FRFs should be symmetric)
• Apply advanced signal processing techniques for complex scenarios
  • Use operational modal analysis methods for OMA data
  • Implement time-frequency analysis for non-stationary signals (wavelet transforms)

Vibration Testing Methods: Advantages vs Limitations

Comparison of Excitation Techniques

• Impact testing offers quick setup and minimal equipment
  • Advantage: Versatile and cost-effective for many applications
  • Limitation: May struggle to excite higher frequency modes consistently
• Shaker testing allows precise control of input forces and frequencies
  • Advantage: Excellent for swept-sine and controlled random testing
  • Limitation: Can introduce mass loading effects on lightweight structures
• Operational modal analysis eliminates need for artificial excitation
  • Advantage: Tests structure under real operating conditions
  • Limitation: Challenges in identifying closely spaced modes

Specialized Testing Methods

• Random vibration testing simulates many real-world environments
  • Advantage: Provides realistic excitation for product qualification
  • Limitation: May not adequately excite specific resonances of interest
• Sine sweep testing effectively identifies resonances and measures transmissibility
  • Advantage: Clear identification of system response at discrete frequencies
  • Limitation: May not represent actual operating conditions for many structures
• Multi-axis vibration testing offers comprehensive evaluation of structural dynamics
  • Advantage: Simulates complex, multi-directional vibration environments
  • Limitation: Requires sophisticated test setups and control systems
• Acoustic vibration testing excites large structures without direct contact
  • Advantage: Suitable for sensitive or difficult-to-mount specimens
  • Limitation: May have limitations in low-frequency excitation and precise force control

Vibration Test Results Interpretation

Frequency Domain Analysis

• Analyze frequency response functions (FRFs) to understand system dynamics
  • Identify resonant frequencies from amplitude peaks
  • Estimate damping from peak width or phase change rate
• Use coherence functions to assess data quality
  • Values close to 1 indicate good correlation between input and output
  • Low coherence may indicate noise, nonlinearities, or insufficient excitation
• Apply modal parameter estimation techniques to extract system properties
  • Curve fitting methods (circle fit, line fit)
  • Polyreference techniques for closely spaced modes

Time Domain and Statistical Analysis

• Perform operating deflection shapes (ODS) analysis to visualize structural behavior
  • Animate mode shapes at specific frequencies
  • Identify areas of high displacement or stress
• Utilize time-frequency analysis for non-stationary vibration signals
  • Short-time Fourier transforms reveal frequency content over time
  • Wavelet analysis provides better time-frequency resolution
• Conduct statistical analysis of vibration data for random testing
  • Calculate power spectral density (PSD) to describe frequency content
  • Determine probability distribution functions (normal, Rayleigh)

Model Validation and Refinement

• Compare experimental modal parameters with finite element model predictions
  • Natural frequencies should match within 5-10% for good correlation
  • Mode shapes compared using modal assurance criterion (MAC)
• Use stabilization diagrams to identify genuine structural modes
  • Consistent poles across model orders indicate true modes
  • Spurious poles often appear inconsistently
• Perform model updating to improve analytical predictions
  • Adjust model parameters (material properties, boundary conditions)
  • Minimize differences between experimental and analytical results