Vibration testing is a lab method that checks whether products, components, and packaging can survive mechanical oscillation during transport, storage, and service. It applies controlled vibration to a mounted specimen, then compares the results against defined acceptance criteria. This guide explains the working principle, specimen preparation, test methods, key parameters, instrumentation, and standards.
Principle & mechanism
A vibration test reproduces dynamic excitation by driving a shaker table with a controlled oscillatory motion. Mechanical energy moves into the mounted specimen through a fixture. Electrodynamic shakers are the most common excitation source. A power amplifier drives a current through a coil suspended in a magnetic field. The resulting Lorentz force moves the armature and table along the excitation axis. Hydraulic and mechanical shakers serve lower-frequency or high-thrust applications. The control loop compares the response signal measured at the control point with a demanded reference profile. The controller adjusts the drive signal until the achieved motion matches the reference within a specified tolerance.
Damage mechanisms of interest include resonance amplification. In this case, structural natural frequencies amplify input displacement and cause overstress. Fatigue accumulation from repeated cyclic stress is another mechanism. Relative motion between parts is a third. It produces wear, fretting, and loosening of fasteners. Understanding these mechanisms allows the engineer to select excitation types, locate resonances, and interpret functional failures observed during or after exposure.
Test specimen preparation & mounting
Preparation begins with a documented baseline. The specimen is inspected visually, photographed, and functionally verified. This ensures that any post-test change can be attributed to the vibration exposure. Samples should represent the production configuration, including actual mass distribution, connectors, and internal mounting hardware. Dummy fillers or ballast may be used where specified by the relevant test plan.
The specimen is attached to the shaker table or slip table through a fixture. The fixture must be stiff enough to avoid adding resonances within the test frequency range. Fixture response is normally verified by a resonance survey before formal testing. Mounting orientation follows the intended field installation. Multiple axes are tested by reorienting the specimen or using orthogonal fixtures.
Accelerometers for control and monitoring are mounted at defined points with studs, adhesive, or wax. Placement follows the standard's requirement for control-point locations. Threads are torqued to specification. Cables are strain-relieved to prevent noise and whip. Clearances around the moving assembly are checked so that the armature travel limits are not reached during low-frequency excitation.
Sinusoidal, random & shock methods
Three excitation methods address different field environments. Sinusoidal testing applies a single-frequency tone. The frequency is swept up and down across a defined band, either at constant acceleration, constant displacement, or constant velocity, with cross-over frequencies between control modes. It is used for resonance search, resonance dwell, and endurance sweeps, because response peaks are easy to identify at each frequency.
Random testing excites all frequencies in the band at the same time, with a statistically defined energy distribution. It matches the broadband vibration of transport vehicles, aircraft, and rocket payloads. Shock testing applies a short-duration transient pulse. Common waveforms are half-sine, sawtooth, or trapezoidal. It reproduces handling drops, pyrotechnic separation, or impact events. Shock can be produced on a shaker with waveform synthesis or on a dedicated drop-table rig.
A vibration program frequently combines these methods in sequence. A sinusoidal resonance survey comes first, followed by random endurance, and finished with shock pulses. Each method needs its own reference profile, control strategy, and notching rules. The sequence should reflect the service environment documented for the product.
Key parameters: frequency, acceleration, PSD
Frequency defines the spectral band of excitation, typically from a few hertz to two thousand hertz or more. The range depends on the specimen mass and the standard applied. Acceleration is expressed in units of g or meters per second squared. It quantifies the severity of sinusoidal excitation and shock pulses. Displacement limits govern the low-frequency end, where large excursions are needed.
For random testing, the governing quantity is power spectral density, expressed in g² per hertz. It describes how vibration energy is spread across frequency. Integrating the PSD over the band yields the root-mean-square acceleration. This single figure is most often quoted for overall test severity. Test levels are usually given as a breakpoint table of frequency versus PSD, connected by straight lines on log-log axes.
More parameters include sweep rate for sinusoidal sweeps, dwell time at resonances, cycle or pulse counts for shocks, axis sequence, and total duration. The engineer should verify that the demanded profile is achievable within shaker force, displacement, and load limits befOre testing begins. Any deviations agreed with the client must be documented.
Sensors & measurement instrumentation
Measurement quality depends on transducer selection and placement. Piezoelectric accelerometers are the standard sensors. They are chosen for their wide frequency response and robustness. IEPE types with built-in electronics dominate general laboratory use. Charge-output types suit high-temperature zones. Sensitivity, mass loading on lightweight specimens, and mounted resonance frequency are the main selection criteria.
Control accelerometers are fixed at the specimen-table interface as the standard requires. Monitoring accelerometers may be placed at critical internal locations to observe response amplification. The vibration controller generates the drive signal and closes the loop in real time. It averages the control channels and equalizes the spectrum toward the reference. A charge amplifier or signal conditioner feeds the controller. A data-acquisition system records additional channels for strain, displacement, or functional signals from the operating specimen.
Calibration of the measurement chain against a traceable reference is mandatory at defined intervals. Cable routing, grounding, and anti-aliasing filtering must be checked. Electromagnetic noise and loose sensor mounts are the most frequent causes of distorted control data.
Acceptance criteria & test standards
Acceptance is judged against criteria fixed in the test plan before exposure begins. Typical requirements are the absence of visible damage such as cracks, loose fasteners, or deformed supports. The specimen must survive without change in functional performance, verified immediately after each axis and at the end of testing. Resonance frequencies must remain within agreed shift limits. No structural failure at mounting points is allowed. Measured data must stay within the tolerance bands around the reference profile. Any notching or level reduction must be documented and approved.
Widely used framework standards include the environmental testing series IEC 60068-2, which covers sinusoidal, random, and shock methods. MIL-STD-810 is the general specification for environmental engineering of defense hardware. ASTM D999 covers vibration of shipping containers. Packaging and transport simulation standards add random road spectra. Sector specifications may invoke these base methods with defined levels and durations.
The test report should state the standard, profile, levels, axes, durations, sensor locations, observed responses, and the conformity conclusion. The report is the deliverable used for design sign-off and customer acceptance.
Frequently Asked Questions
What
products and material samples are suitable for vibration testing?
Applicable items cover electronic assemblies, automotive and aerospace components, medical devices, industrial equipment, and transport packaging of various types of samples. Any product that will face oscillation or shock during transport, installation, or service can be evaluated. The specimen must be representative and rigidly mounted to the shaker or slip table within the force and displacement limits of the facility.
What
does a vibration test report contain and how is it used?
A typical report records the applied standard and method. It includes the sinusoidal, random, or shock profile with frequency, acceleration, and PSD levels, plus axes and durations. Control and monitoring sensor locations, resonance survey results, and any deviations or notching are also recorded. Post-test inspection and functional verification findings are stated with a conformity conclusion. Clients use the report for design verification, transport validation, and customer acceptance submissions.
What
should be communicated with the laboratory before submitting samples for vibration testing?
Key points include the applicable standard or a client-supplied test plan, the required excitation methods and levels, axes and mounting orientation, and specimen mass and dimensions. Also include functional operation during test if required, and the acceptance criteria agreed in advance. Notify the laboratory of fragile features, operating power needs, and special fixture requirements. This allows the fixture and control strategy to be prepared correctly before scheduling the test.