Resonance Frequency for System Cushioning
What is Electrolysis and galvanic corrosion?
Galvanic Corrosion is the corrosion that occurs when two dissimilar metals are used together in a structure and exposed to an electrolyte (salt water, chemical, petrol) and the less noble of the 2 metals will corrode. For example; aluminum sheet with steel fasteners on a boat. Some pairings of metals are more at risk of galvanic corrosion. Check a galvanic series or chart.
Electrolysis is the acceleration of the galvanic corrosion when electricity is introduced to the metals in question. Connected by an external electrical source, the less noble metal experiences accelerated galvanic corrosion.
while both processes involve metals and electrolytes, their dependence on an external voltage source distinguishes them.
The resonance frequency for a package should be designed so that it does not match, nor be near to, dominant forcing frequencies. High and almost continuous working accelerations can possibly cause a cushion to lose thickness due to fatigue, resulting in a loss of performance with respect to shock and vibration protection.
Natural Frequency of Components
The acceptable acceleration level an object or its components will withstand during vibration conditions without damage ranges far below the level for a single impact or shock. This is because the fatigue stress of materials is much lower than the allowable stress for a single deformation.
Ideally, the acceptable maximum continuous working acceleration level of the item should be determined as a function of frequency/vibration fragility and compared with the acceleration imposed on the item during actual shipment. This method, however, involves complexities which normally cannot be researched economically. The lowest natural frequencies of the components of the object, below 200 Hz, however, should be identified.
A cushion should be designed so that it isolates harmful vibrations for the item packed. If vibration fragility of the item is determined, a straightforward test of the proper working of the cushion can be carried out. If information only on the lower natural frequencies of the object is available, the resonance frequency for the cushion system must be located far below the first natural frequency of the object contained. The cushion will work as a filter, attenuating the forcing frequencies close to the natural frequency, having insufficient power to generate vibrations violent enough to cause damage.
If it is not possible, or economically feasible, to lower the resonance frequency far enough, a solution must be sought in increasing the natural frequency of key components by changing the system's construction or by reducing the deformation under vibration by independently blocking the sensitive components at risk.
The resonance frequency can be changed by manipulating the cushion with respect to material, static stress, or thickness, within the limits given by the calculation for shock protection.
It is always advisable to perform vibration tests to simulate the effects of real shipment conditions on the packed object. This is especially important when it is expected that dominant forcing frequencies match or are near the resonance frequency of the package and/or natural frequencies of components of the packed item.
