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Notching is a vibration testing technique used to reduce test input levels at specific resonant frequencies where excessive structural loads may occur. During launch vehicle qualification or acceptance testing, dynamic amplification can produce interface loads that exceed those expected during flight. Notching helps control these loads while still demonstrating compliance with environmental verification requirements.
Within an ECSS verification programme, notching is typically applied during sine vibration and random vibration testing after modal analysis and structural assessment have identified critical resonances. The objective is to avoid overtesting flight hardware while maintaining confidence that the equipment can survive its operational environment.
Engineers use notching to balance realistic environmental testing with protection of sensitive flight hardware. When correctly implemented, it supports qualification, reduces technical risk and provides evidence that interface loads remain within acceptable limits.
Notching is the deliberate reduction of the vibration input level or spectrum during a vibration test to limit structural responses at resonant frequencies and prevent over-testing of the test item.
To prevent unrealistic load amplification while maintaining representative environmental verification.
Design and structural analysis
Manufacturing verification
AIT activities
Environmental testing
Flight readiness assessment
Protects hardware from excessive test-induced loads.
Supports ECSS environmental verification requirements while avoiding unnecessary overtesting.
Provides evidence that qualification loads remain representative of flight conditions.
Helps ensure flight hardware survives launch environments without introducing unnecessary risk.
ECSS-E-ST-10
ECSS-E-ST-10-03
ECSS-E-ST-10-02
ECSS-Q-ST-20
Notching criteria, bandwidths and allowable load limits are project specific and should be justified through analysis and testing.
Customer verification requirements may impose specific notch limits, approval processes and reporting obligations.
Identify resonances through analysis and modal assessment.
Define instrumentation (including response accelerometer locations), damping assumptions and interface load limits.
Apply controlled notches during sine or random vibration testing.
Assess measured responses and confirm interface load compliance.
Document final notch levels, results and verification evidence.
Modal analysis
Damping assessment
Interface load calculation
Random vibration notching
Sine vibration notching
Measurement planning
Test correlation activities
When a structure is excited at one of its natural frequencies, the vibration response can become significantly larger than the applied input. This phenomenon is known as dynamic amplification.
A simple example is a child on a swing. Small pushes applied at the correct frequency create large motions. Space hardware behaves similarly during vibration testing.
The amplification factor is influenced by:
Natural frequency
Structural damping
Modal mass
Excitation level
Boundary conditions
Near resonance, a relatively small shaker input can generate very large interface forces.
A vibration test specification defines the input applied by the shaker.
The hardware responds according to its dynamic characteristics.
Input acceleration ≠ structural load
In many cases the highest structural loads occur because the hardware amplifies the shaker input through resonance.
This is why engineers monitor:
Input control accelerometers
Response accelerometers
Interface loads
Strain measurements
Notching is often driven by response rather than input.
Launch vehicle environments are normally specified as acceleration levels.
Hardware failure, however, occurs because of force and stress.
A resonant structure may experience interface forces significantly greater than those predicted from input acceleration alone.
Notching is therefore used to control force rather than simply control acceleration.
At frequencies where excessive interface loads are predicted or measured, the vibration input is reduced.
The resulting notch:
Reduces force transmission
Limits dynamic amplification
Protects flight hardware
Maintains realistic verification conditions
The objective is not to make the test easier.
The objective is to prevent unrealistic overtesting.
The challenge with notching is balancing two competing objectives:
Test hard enough to verify the design.
Avoid creating loads that would never occur in flight.
Too little notching can damage hardware.
Too much notching can invalidate the test.
Good notching is therefore a structural dynamics exercise rather than a test laboratory adjustment.
Many engineers use the terms response limiting and notching interchangeably. Although closely related, they are not the same process.
Notching is a pre-defined reduction in vibration input at specific frequencies before the test begins. The notch profile is typically derived from structural analysis, modal testing, interface load predictions or previous test data.
The objective is to prevent excessive loads from occurring during the test.
Example:
A resonance is identified at 180 Hz.
Analysis predicts interface loads exceeding qualification limits.
The input spectrum is reduced around 180 Hz.
Testing proceeds using the modified spectrum.
Response limiting is an active control process performed during testing. Instead of relying solely on the shaker control accelerometer, additional control response channels monitor structural response or interface loads in real time.
Example:
A response accelerometer is installed on a sensitive avionics enclosure.
During the test the response reaches a predefined limit.
The vibration controller automatically reduces shaker input.
The response remains below the allowable threshold.
| Method | Characteristic |
|---|---|
| Notching | Predictive |
| Response limiting | Reactive |
Notching uses analysis to anticipate problems.
Response limiting uses measurements to prevent problems.
Many modern spacecraft vibration tests employ both techniques simultaneously.
Response limiting can provide greater confidence because it is based on measured hardware behaviour rather than analytical prediction alone.
However, response limiting requires additional instrumentation, careful setup and clear acceptance criteria.
One of the most important concepts in vibration testing is Dynamic Amplification Factor, or DAF.
DAF describes how much larger a structure’s response becomes compared with the applied input.
Every structure possesses natural frequencies.
When excitation occurs near one of these frequencies, energy accumulates within the structure.
The resulting vibration level can become significantly larger than the original input.
This phenomenon is resonance.
| Parameter | Value |
|---|---|
| Input acceleration | 1 g |
| Measured structural response | 12 g |
| Calculation | DAF = Response ÷ Input |
| Result | DAF = 12 |
The structure is amplifying the input by a factor of twelve.
Structural damping
Natural frequency
Modal mass
Joint stiffness
Boundary conditions
Mounting configuration
Highly damped structures dissipate energy efficiently.
Lightly damped structures store energy and exhibit much larger resonant responses.
This explains why lightweight aerospace structures often require careful notching and response limiting strategies.
Large DAF values can generate interface loads much higher than flight predictions.
A qualification test that ignores amplification effects may unintentionally become a structural proof test.
Understanding DAF helps engineers:
Predict interface loads
Develop notch strategies
Set response limits
Protect expensive hardware
Improve correlation between analysis and test
One of the most common misconceptions in vibration testing is that shaker acceleration determines structural severity.
In reality, hardware fails because of loads and stresses.
Input acceleration is the vibration level commanded by the shaker.
Examples include:
20 g sine vibration
14.1 Grms random vibration
Specified launch vehicle PSD environments
Input acceleration describes the environment applied to the test item.
It does not directly indicate structural stress.
Interface loads are the forces and moments transferred through the mounting points connecting the hardware to the spacecraft, launch vehicle or test fixture.
These loads drive structural stress.
Engineers therefore pay close attention to:
Axial force
Shear force
Bending moment
Torsional moment
Newton’s Second Law states:
Force = Mass × Acceleration
When acceleration increases because of resonance, force increases accordingly.
A modest shaker input may therefore generate very large interface loads.
| Parameter | Value |
|---|---|
| Equipment mass | 20 kg |
| Input acceleration | 1 g |
| Response acceleration | 15 g |
| Increase in resulting force | Factor of fifteen |
This is why interface loads often become the primary driver for notching decisions.
The objective of vibration testing is not to maximise acceleration.
The objective is to reproduce realistic flight loads while maintaining verification confidence.
Well-executed notching allows engineers to:
Control interface loads
Prevent unrealistic overtesting
Protect flight hardware
Maintain representative verification conditions
Incorrect notch levels may result in overtesting or undertesting.
Additional analysis and review cycles can affect programme schedules.
Extra instrumentation and modelling effort may increase project costs.
Insufficient evidence may compromise verification acceptance.
Poor justification or traceability can delay approval.
Maintain configuration control
Ensure requirement traceability
Use calibrated test equipment
Perform independent technical reviews
Correlate analysis and test data
Document all notch assumptions and approvals
Notching should never be used to compensate for inadequate structural design. If excessive notching is required, the root cause may be insufficient stiffness, poor load paths or inadequate structural margins.
Every notch should be linked to an identified structural mode, measured response or validated analysis. Applying arbitrary notches introduces verification risk and reduces confidence in test results.
Excessive notch depth or bandwidth can remove important flight-like energy from the test environment. This may result in undertesting and reduced verification confidence.
Incorrect damping values can significantly affect calculated amplification factors and interface loads. Damping assumptions should be supported by analysis or test evidence.
Where practical, analytical predictions should be supported by modal surveys, low-level sine testing or measured response data.
Changes to mounting arrangements, mass properties or hardware configuration can invalidate previously approved notch profiles.
Low-level signature sweeps before and after testing help confirm that no structural changes or hidden damage occurred during the campaign.
For smaller equipment, local dynamic responses at higher frequencies may become more critical than global modes and should not be overlooked.
Verification Plan
AIT Plan
Test Specification
Test Procedure
Test Report
Verification Control Document
NCRs and RFWs
Requirement compliance achieved
Interface loads remain within limits
No hardware damage following testing
Verification evidence accepted
Flight readiness supported
Verification planning support
Structural dynamics guidance
Vibration test campaign development
Documentation and reporting support
Independent technical review
Qualification and acceptance test assistance
A method of reducing vibration input levels at specific resonances to control structural loads.
To prevent unrealistic overtesting while maintaining verification objectives.
Yes, when technically justified and properly documented.
Using structural analysis, modal testing and interface load assessments.
Yes, provided the methodology is technically justified.
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Need support with vibration testing, structural dynamics or ECSS verification? Contact Resonate Testing for independent guidance on notching strategies, qualification testing and flight hardware verification.
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