Structural Dynamics

What is structural dynamics?

Structural dynamics is concerned with the dynamic and transient behaviour of structures due to external forces.

For instance,

  • how does a structure behave once and after a shock load is applied the that structure?

  • How do the main elements of structure move, bend, at what frequency, how does the strain and stress vary in the structure elements?

Why is structural dynamics important?

The way in which the different elements of a structure behave under vibration determines:

  • the level of noise radiated from the machine, equipment and object, which is the transmission medium from vibration to air-borne noise.

  • The location of anti vibration mounts for the most effective isolation of the structure to input forces and or isolation from transmitting vibration

  • The action or design improvements required to get a more favourable behaviour under transient and impulse conditions.

Structural dynamics is fundamental in vibration control and noise reduction of equipment and machinery.

How do we improve the structural dynamics of machines, equipment and structures?

To improve the dynamic behaviour of any structure, we must understand the behaviour, in order to model it mathematically. (This can be done with software like MATLAB and Finite Element packages.)

We model the structure as per the design drawings and ascertain the dynamic behaviour by determining:

  • The natural frequencies (eigenvalues)

  • The mode shapes (eigenvectors)

  • And the damping coefficient of the structure at every mode shape

The results of the finite element model, is dependent on the constraints and damping applied to the model.

But are these results correct?

To verify the results of the model, ideally experimental measurements on the structure should be performed and compared to those from the finite element analysis.

We instrument the structure with transducers the measure the physical properties that are of interest to us.

By using accelerometers, strain gauges etc. to understand the structure under transient conditions, typical to that of the environment it will operate.

But to verify whether the natural frequencies and mode shapes are correct in the model, as constraint assumptions were made during modelling, we performed modal analysis using an impulse force (shock impulse) or a dynamic force (sinusoidal wave or random input)

The results and analysis thereof, provide the mode shapes and their corresponding natural frequencies.

These results are compared with those from the modal testing and analysis. If they differ, model updating of the finite element model is performed to ensure that the model behaves like the physical structure.

What if this is not possible?

Sometimes there are valid reason why this approach cannot be followed, like:-

  • Too costly

  • And modelling of a sub-component is not effective or realistic, i.e. too small and complex.

Too expensive

The approach must be implemented via a closed feedback loop process, and over a long period of time. Building these capabilities takes time, but becomes a true investment for the future, as the constraints and assumptions taken for modelling are refined by the experimental results, and can be used for future modelling.

Too complex

A good example of this, is a electronic circuit on a printed circuit board (PCB).

In our experience, during flight certification using RTCA/DO-160 and Mil-Std-810, there are components that fail during vibration testing on a vibration shaker.

These are usually

  • Fine wired components

  • Internals of components

  • Relatively heavy components detaching

  • Capacitor legs fatiguing due to vibration

  • Ceramic components cracking

These are typical failures that we have seen over the last 25 years on PCBs…

We have generated a lessons learnt database that assists us in solving these failures cost effectively, and quickly.

There are also other aspects that contribute to failures on PCBs. And they are:

  • Designers not adequately informed of the complete nature of the tests that the component will undergo

  • A lack of structural dynamics understanding

  • The incorrect fixings and the locations of the mounts or stand-offs

This is where the true understanding of structural dynamics is an asset to any design and manufacturing company.

So if you need help with anything afore-mentioned, please get in touch.

We do not charge for phone calls and advice. We are just passionate about this field of engineering, and happy that we can help you.

What have we done for companies like yours?

Vibration testing of various assemblies on a vibration shaker table that includes:

  • Electric motors under high vibration levels, as experienced on a nacelle

  • Experimental modal analysis of commercial vehicle structures

  • Updating of fabricated structures to pass vibration testing

  • Modification of fabricated structures to manipulate the natural frequencies and mode shapes for a healthier behaviour, i.e.

  • reduce high strain areas where fatigue occurred.

  • Isolating machinery from transmitting vibration

These are just a few examples. If you have any queries, please contact us on 01908 643 433.

Structural dynamics

We perform structural dynamic assessments on existing and problematic equipment. This assessment will include modal testing to identify the natural frequencies and mode shapes. (eigenvalues and eigenvectors). Using the results of the experimental analysis, we'll update the analytical structural model.

We use established methods to analyse the structural model. Once determining the actual dynamic behaviour, we will update the analytical model. Then verify its behaviour to establish similarities.

Our experience includes measuring, assessing and analysing structural dynamics on:

  • buildings

  • construction sites

  • offshore structures

  • large off-highway vehicles

  • mine shaft lifts/skips

  • articulated truck trailers

  • aeroplane engine cradles

  • jet engine mode shapes

  • vehicle dynamics

  • design for vibrational environments