Modal testing: Difference between revisions

Revision as of 22:31, 27 May 2022

Revision as of 19:40, 22 April 2023

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== Impact Hammer Modal Testing ==

[[File:Modal hammer.jpg|thumb|Model hammer with temporal and frequency response from various interchangeable tips.]]

An ideal impact to a structure is a perfect impulse, which has an infinitely small duration, causing a constant amplitude in the frequency ___domain; this would result in all [[modes of vibration]] being excited with equal energy. The impact hammer test is designed to replicate this; however, in reality a hammer strike cannot last for an infinitely small duration, but has a known contact time. The duration of the contact time directly influences the frequency content of the [[force]], with a larger contact time causing a smaller range of bandwidth. A [[load cell]] is attached to the end of the hammer to record the force. Impact hammer testing is ideal for small light weight structures; however as the size of the structure increases issues can occur due to a poor [[signal-to-noise ratio]]. This is common on large [[civil engineering]] structures.

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Small objects or structures can be attached directly to the [[Simulation table|shaker table]]. With some types of shakers, an armature is often attached to the body to be tested by way of piano wire (pulling force) or stinger (Pushing force). When the signal is transmitted through the piano wire or the stinger, the object responds the same way as impact testing, by attenuating some and amplifying certain frequencies. These frequencies are measured as modal frequencies. Usually a load cell is placed between the shaker and the structure to obtain the excitation force.

For large civil engineering structures much larger shakers are used, which can weigh 100[[kg]] and above and apply a force of many hundreds of [[newtons]]. Several types of shakers are common: rotating mass shakers, electro-dynamic shakers, and electrohydraulic shakers. For rotating mass shakers the force can be calculated fromby knowing the mass and the speed of rotation; for the electro-dynamic shaker, the force can be obtained through a load cell, or an accelerometer placed on the moving mass of the shaker. Shakers can have an advantage over the impact hammer as they can supply more energy to a structure over a longer period of time. However, problems can also be introduced; shakers can influence the dynamic properties of the structure and can also increase the complexity of analysis due to [[window function|windowing]] errors.

==See also==