Products operating in aerospace, defence, automotive and industrial environments rarely experience a single, clean vibration profile. In actual service, a product may simultaneously experience broadband random vibration, engine orders, gear-mesh frequencies, structural resonances and other dynamic excitations. Mixed-Mode Vibration Testing is used to reproduce these complex environments in a controlled laboratory setup and evaluate whether the product can withstand them.
At Envitest Lab, we are technically equipped to perform mixed-mode vibration testing in accordance with the applicable requirements of IEC 60068-2-80, supporting evaluation of product robustness under combined vibration environments.
What is Mixed-Mode Vibration?
Conventional vibration testing generally uses either sine or random excitation. Mixed-mode testing combines different vibration components to create a more representative operational environment.
Two important approaches are:
Sine-on-Random (SoR): A deterministic sinusoidal component is superimposed on a broadband random vibration background. The sine component can represent known excitation sources such as engine orders, rotating machinery, gear meshing or structural resonance, while the random component represents the general vibration environment.
Random-on-Random (RoR): Additional narrow-band random vibration components are superimposed on a broadband random background. This can represent environments where multiple vibration sources exist and the narrow-band energy varies randomly with time, frequency and phase.
How Is the Test Performed?
A successful mixed-mode vibration test starts with understanding the actual operating environment of the Equipment Under Test (EUT).
The test engineer first identifies the vibration sources, operating conditions, mounting configuration and critical frequencies. This information is then converted into an appropriate test profile.
For a SoR test, the broadband random background is normally defined using a Power Spectral Density (PSD) profile, while the sinusoidal component is specified by parameters such as frequency, acceleration and sweep or fixed-frequency conditions.
For RoR testing, both the broadband and narrow-band random components are represented through appropriate PSD specifications.
The next step is to configure the vibration controller. The controller must simultaneously generate and control the required vibration components while maintaining the specified overall profile within the defined tolerances.
Test Setup
The EUT is mounted on the vibration fixture in the same orientation and configuration specified by the applicable test requirement or representative of its actual installation.
Before starting the test, the laboratory verifies:
- EUT mounting and fixture integrity
- Accelerometer installation and orientation
- Control and response measurement locations
- Fixture resonance and suitability
- Cable routing and strain relief
- EUT operating condition
- Functional monitoring requirements
- Test profile and acceptance criteria
Instrumentation is critical because mixed-mode testing involves multiple excitation components that must be controlled simultaneously.
Functional Monitoring During Testing
One of the major advantages of mixed-mode testing is the ability to observe the EUT while it is being subjected to a realistic vibration environment.
Depending on the product, monitoring may include:
Electrical continuity, output signals, communication, temperature, pressure, switching operation, mechanical movement or other functional parameters.
The objective is not simply to determine whether the structure survives the vibration. It is also important to identify whether the product experiences intermittent or functional failures during vibration.
Such failures can be extremely important in real-world applications.
What Can Mixed-Mode Testing Reveal?
Mixed-mode vibration can expose failure mechanisms that may remain hidden during conventional single-mode testing.
These can include:
- Structural resonance
- Fatigue accumulation
- Component loosening
- Fastener failure
- Mounting weaknesses
- Connector intermittency
- Solder or PCB failures
- Cable damage
- Bracket or enclosure fatigue
- Functional degradation
- Frequency-specific response problems
The combination of broadband and discrete or narrow-band excitation can create a much more representative stress environment than testing with a single vibration component.
Why It Matters for Aerospace and Defence
Consider a helicopter transmission system. The product may experience broadband vibration from the overall airframe while simultaneously experiencing strong tonal components associated with rotating shafts, gears and engine orders.
Similarly, equipment installed on a tracked vehicle may experience broadband terrain-induced vibration together with specific structural or drivetrain excitation.
Aircraft and jet-engine environments can also involve multiple simultaneous vibration sources.
A simple random test may reproduce the general vibration energy but fail to reproduce the specific frequency-related excitation that drives a particular component into resonance.
From Test to Engineering Improvement
At Envitest Lab, the objective is not simply to complete the test and issue a report.
The test results can be used to understand how the product responds to its operational vibration environment and identify areas requiring improvement.
If a resonance is detected, the engineering team can investigate changes to stiffness, damping, mounting, material selection or structural design. If intermittent electrical behaviour occurs, connectors, solder joints, cables or PCB assemblies can be investigated.
This makes mixed-mode vibration testing valuable not only for qualification but also for design verification, failure investigation and product improvement.
The Envitest Lab Approach
Mixed-mode vibration testing requires more than a vibration shaker. It requires appropriate control, instrumentation, fixtures, test profiles, response monitoring and engineering interpretation.
At Envitest Lab, we focus on replicating the relevant operational environment as closely as practical, while maintaining controlled and repeatable laboratory conditions.
The objective is simple:
Test. Validate. Identify. Improve.
Envitest Lab – Empowering Products.