WHEN an engineer makes measurements of a structure, it is not only to determine its physical dimensions but also to establish its mechanical characteristics.
Mechanical characteristics that particularly interest the engineer are those features that determine how a structure moves or flexes when it is shaken, bumped, hit or pounded: in other words, excited.
Any physical system will vibrate if excited. The frequency at which vibration naturally occurs is called resonance or natural frequency. The shapes which the resonating (vibrating) system assumes are physical properties of the structure and can be measured experimentally by a vibration analyser and displayed by using modal analysis.
Impulse hammers are used by many engineers to “excite” a structure in order to measure the distribution and dissipation of energy (transfer function) across the structure. The size of the hammer used must be sufficient to create a sufficient impulse i.e. it must induce sufficient energy into the structure to cause it to vibrate.
One such hammer kit, supplied by Kingdom from Dytran Instruments, is the 5803A. This 5.4 kilogram sledge hammer was recently purchased by the University of Wollongong for studies into the dynamic characteristics of railway rail tracks. The researchers from UoW recently used this system to conduct an in-field investigation of the dynamic properties of heavy-haul coal lines in Queensland with the support from Queensland Rail (QR).
The Dytran 5803A incorporates a force sensor in the head of the hammer and is teamed with an accelerometer placed on the rail to measure the vibration response resulting from an impact. Dytran also supplies accelerometers, pressure, force, and impact sensors and electronics.
Modal analysis determines the fundamental vibration mode shapes and corresponding frequencies. The use of specialised vibration analysers such as SignalCalc Ace-Quattro 4 channel analyser can make either a simple mechanical system or an extremely complicated structure easy to qualify. Then the accurate determination of the natural frequencies and mode shapes can proceed using advanced modal analysis software such as ME’scope Visual Engineering Series, as recently purchased by the Defence Science and Technical Organisation at Fishermans Bend in Victoria. The results of the experimental work can then be used to confirm or modify an analytical design, prepared perhaps by Finite Element Analysis.
So, if I had a hammer, there would be more to discover about the built environment and structures in general.