Understanding Impact Testing

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Balanset-4

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Magnetic Stand Insize-60-kgf

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Reflective tape

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Dynamic balancer “Balanset-1A” OEM

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Definition: What is Impact Testing?

Impact testing (also called impulse testing or impact modal analysis) is a modal testing technique using an instrumented impact hammer to apply broadband force impulses to structures while measuring the resulting vibration response with accelerometers. The technique calculates frequency response functions (FRFs) showing how structures respond at each frequency, revealing natural frequencies, mode shapes, and damping ratios essential for understanding dynamic behavior and diagnosing resonance problems.

Impact testing is the practical field alternative to shaker modal testing, providing similar information without requiring heavy, expensive electromagnetic shakers and complex mounting fixtures. It’s widely used for resonance troubleshooting, structural modification validation, and finite element model correlation in machinery and structural dynamics applications.

Equipment

Instrumented Impact Hammer

• Force Transducer: Piezoelectric sensor in hammer head measures impact force
• Hammer Mass: 0.1-5 kg depending on structure size and frequency range
• Interchangeable Tips: Hard (steel), medium (plastic), soft (rubber)
• Output: Force signal synchronized with response measurement
• Typical Cost: $500-3000

Response Sensors

• Accelerometers at points of interest
• Single roving accelerometer or multiple fixed sensors
• Good frequency range matching test requirements

Data Acquisition

• Minimum two channels (force and response)
• Simultaneous sampling essential
• FFT analyzer or modal analysis software
• Transfer function and coherence computation

Test Procedure

Single-Point FRF

1. Mount Accelerometer: At response location
2. Select Hammer Tip: Match to structure and frequency range
3. Strike Structure: Firm, quick impact at excitation point
4. Record Data: Force and response signals
5. Compute FRF: H(f) = Response(f) / Force(f)
6. Average: Repeat 3-10 times, average FRFs
7. Check Coherence: Verify data quality (coherence > 0.9)

Multiple-Point Testing

• Roving Hammer: Impact multiple points, fixed accelerometer
• Roving Accelerometer: Impact fixed point, move accelerometer
• Result: FRFs from multiple locations reveal mode shapes
• Grid Testing: Systematic grid of points for complete structural survey

Hammer Tip Selection

Effect on Frequency Content

• Hard Tip (Steel): Short impact duration, high-frequency content, good for stiff structures and high frequencies (to 10+ kHz)
• Medium Tip (Nylon/Delrin): Moderate duration, balanced spectrum, general purpose (to 2-5 kHz)
• Soft Tip (Rubber): Long duration, low-frequency emphasis, large/flexible structures (to 500-1000 Hz)

Matching Structure

• Light Structures: Small hammer, soft tip (avoid damage, ringing)
• Heavy Structures: Large hammer, harder tip (adequate excitation)
• Rule of Thumb: Structure should respond but not excessively (peak acceleration 1-10g typical)

Data Quality

Good Impact Technique

• Quick, clean impact (no double hits)
• Hammer pulled away immediately (doesn’t stay in contact)
• Strike perpendicular to surface
• Consistent strike location
• Appropriate force level

Coherence Validation

• Coherence function shows measurement quality
• Coherence near 1.0 (> 0.9) = good data
• Low coherence = poor impact, noise, nonlinearity
• Reject poor impacts, repeat test

Results and Interpretation

Frequency Response Function

• Magnitude plot shows amplification vs. frequency
• Peaks = natural frequencies/resonances
• Peak height = amplification factor (inverse of damping)
• Phase plot shows 180° shifts through resonances

Natural Frequency Identification

• List all peaks from FRF
• First mode typically lowest frequency peak
• Higher modes at higher frequencies
• Compare to operating frequencies for interference check

Mode Shape Determination

• From multiple-point testing
• Relative response amplitudes at resonance define deflection pattern
• Animation possible with software
• Identifies nodes and antinodes

Applications in Machinery Troubleshooting

Frame Resonance Investigation

• Impact motor or fan frame
• Identify frame natural frequencies
• Compare to blade passing, motor electromagnetic frequencies
• If match found → resonance is problem

Foundation Testing

• Impact baseplate or foundation
• Determine foundation natural frequencies
• Verify adequate stiffness and frequency separation

Before/After Comparisons

• Test before structural modification
• Test after (stiffening, damping, mass changes)
• Verify modification achieved desired effect
• Quantify improvement

Impact testing is a practical, cost-effective modal analysis technique accessible to field vibration specialists. Using only an instrumented hammer and vibration analyzer, impact testing identifies structural resonances, validates modifications, and provides the dynamic characterization needed to solve resonance problems and optimize structural designs in machinery and structural applications.

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