A vital question to be asked before performing hammer testing is – how do I support the testing object? What fixing mechanism to use? Or technically, what are the desired boundary conditions? Completely free or completely constrained?
A free-free boundary condition means the object is “suspended” in space without any attachment to the ground at any of its coordinates. In other words, no external load or influence is acting on it. In this condition, all the rigid body modes shall only be determined by the object’s inertia properties, and each mode shall have a natural frequency of 0 Hz. A mode with a natural frequency of 0 Hz will exhibit structural displacement without flexible deformations, hence the term “rigid body”.
However, this perfect condition cannot possibly be achieved in practice. In reality, the testing object has to be supported in some manner. For example, hanging the object using elastic bands, supporting the object on foam or an airbag etc. Due to the external influence acting on the testing object, its rigid modes are affected or coupled to some extent by or with the flexible body modes. That results in all the rigid body modes cannot have zero natural frequencies. A rule of thumb used in practice to judge if the supporting influence can be ignored is if the first flexible mode frequency is ten times higher than the highest rigid body mode frequency.
It is worth noting that a free suspended boundary condition is not always the best choice for investigating the vibration properties of any system. In some cases, a meaningful result only can be obtained by carrying out hammer testing on an object fixed with representative constraints to simulate the real operating condition. For example, we consider a vehicle’s rearview mirror. Its vibration modes in the real operating conditions with the mirror fixed on the vehicle are more interesting than those of a completely freely supported mirror.
Also, in some extreme cases, the size of a testing object is too large to be suspended physically. A constrained fixture will be the only option.
Generally, there is no unique answer to the question of how to support the testing structure. Each test must be considered individually, and it depends on how the results are to be used.
Prosig has many years of experience and knowledge in hammer tests. Please contact us if you have any challenges and would like us to help you get the answers you need.
For further insights on this subject please see
What Is A Hammer Test Or Hammer Impact Test? by Chris Mason
Measuring For Success With A Hammer Impact Test by James Wren
How Do I Calibrate An Impact Hammer? by John Mathey
Dr Cindy (Xin) Wang
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