Structural Dynamics

Structural Dynamics

Structural dynamics deals with the characterization of structural properties. All structures are subject to physical forces that affect their performance. From a wind turbine blade vibrating in an offshore gale, an aircraft experiencing turbulence during a flight to machinery exposed to self-generated vibration, these forces test the integrity of structures.

Yet, while structures must be resilient and rigid, over-engineering them can be both unnecessary and costly – especially when weight is an issue. And some structures, such as engine mounts, must not be too rigid. They must absorb vibration to maximize comfort.

Introduction to Structural Dynamics

Understanding how structures behave in service enables engineers to optimize their designs, monitor structural integrity, and maximize performance. These articles are intended to:

  • Describe what structural dynamics measurements and analysis is, why it is important to perform it and how it is typically done
  • Explain the difference between testing and simulation and how the combined use can be beneficial
  • Explain the difference between signal analysis and system analysis
  • Give an overview of the most frequently used applications
  • Highlight important trends in structural dynamics.

Structural Characterization

Structural dynamics is about the characterization of structural properties and the behavior of structures. Structural properties are expressed in a set of modal parameters, each consisting of mode shape with an associated natural (resonance) frequency and damping value. The modal parameters are derived from a mathematical model describing the relationship between the inputs and outputs and can be obtained using classical modal analysis or operational modal analysis (OMA).

In the classical modal analysis, the structure is excited using impact hammers or modal exciters (modal shakers), whereas, in operational modal analysis, natural excitation is used. In both cases, the response is typically measured using accelerometers.

Determining how shocks affect a structure is a special type of structural characterization. For this purpose, the shock response spectrum (SRS) calculated from transients in the time domain is used.

Structural behavior is observed using techniques such as operating deflection shapes (ODS) analysis for determining the vibration patterns of structures under various operating conditions or using permanent structural health monitoring (SHM) to follow the structural state continuously and determine the required health management of the structure.

Integrating Test and Simulation

Structures are often designed using finite element (FE) models, and their geometry models and results predictions are very useful for optimizing the tests.

Importing detailed FE models not only allows you to create simpler test models that are highly accurate. FE Models also helps you to define optimal excitation and response DOFs to get the best possible test results. FE predictions can be correlated with the test results, and the test data can be imported back into the simulation tools for updating the FE models.

  • Operating Deflection Shapes (ODS)

    Operating Deflection Shapes (ODS) analysis is a very versatile application for determining the vibration patterns of machinery and structures under various operating conditions.

  • Classical Modal Analysis

    Classical modal analysis is used for modal parameter identification of structures under controlled boundary and environmental conditions using hammer or shaker excitation.

  • Ground Vibration Test (GVT)

    In aircraft ground vibration testing (GVT), large modal tests for different aircraft configurations on the ground are performed to update the flutter boundary predictions before the first test flight is made.

  • Operational Modal Analysis (OMA)

    Operational Modal Analysis (OMA) is used instead of classical modal analysis for accurate modal parameter identification of structures under actual operating conditions, and in situations where it is difficult or impossible to artificially excite the structure.

  • Structural Health Monitoring (SHM)

    By performing long-term continuous Structural Health Monitoring (SHM) it is possible to monitor and track a structure’s state and carry out condition-based maintenance to ensure structural integrity.

  • Test-FEA Integration

    Integrating testing and Finite Element Analysis (FEA) helps to cut development costs, reduces the number of physical prototypes, and shortens the time from concept to production.

  • Shock Response Spectrum (SRS)

    The Shock Response Spectrum (SRS) is used to determine the damage potential of components and systems from transient events, such as pyroshocks, in order to ensure their survival in known environments.

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STRUCTURAL DYNAMICS 

NIELS JØRGEN JACOBSEN Niels-Jørgen Jacobsen

With degrees from the Technical University of Denmark and Copenhagen Business School, Niels-Jørgen has published numerous technical papers and articles on structural dynamics.