Modal testing is an experimental method used to determine the natural vibration characteristics and behavior of a structure. This method plays an important role in the design, construction, and operation of ships. Modal testing can be used to identify and solve problems that could affect the ship's noise and vibration levels, performance, and safety. If the sound and vibration levels on a ship are at a disturbing level for passengers and crew, the ship's vibration problems can be identified and solved using the results of modal testing.
The most common method of modal testing is hammer impact testing. In this test, the ship's vibration is measured by striking the ship at different points with a hammer impact. The measured vibration data is analyzed to determine the ship's modal parameters. Modal parameters are the ship's natural frequencies, modal masses, modal damping ratios, and mode shapes.
The main purpose of hammer impact testing is to understand how structures respond to external forces. During the test, a special hammer with a load cell at the end is used to measure the hammer force on the structure being tested and to obtain the FRF (Frequency Response Function). When structural resonances occur, there will be an increase in the response, which can be clearly seen by examining the FRF.
An ideal impulse to a structure is an impulse of infinitely short duration that causes a constant amplitude in the frequency domain; this causes all vibration modes to be excited with equal energy. Hammer impact testing is designed to mimic this; however, in reality, a hammer impulse cannot last for an infinitely short period of time, but has a known contact time. The duration of the contact time directly affects the frequency content of the force, with a longer contact time leading to a smaller bandwidth. A load cell is attached to the end of the hammer to record the force.
For a linear system, the larger the force applied to the structure, the stronger the response, therefore the transfer function is not affected by the level of the excitation force applied to the structure each time. Therefore, if the linearity assumption is correct, the individual FRFs calculated each time can be averaged to obtain a smoother FRF.
Excessive hammer force can excite the nonlinear modes of the structure and contaminate the transfer function matrix. Additionally, transfer functions calculated when hammer testing is performed on a highly damped complex coupled structure typically exhibit poor linearity.
The reciprocity assumption is fundamental when trying to reduce the number of hammer measurements made on a structure. Reciprocity states that the transfer function composed of a response measured at point B on a structure and an excitation applied at point A is the same as the transfer function obtained when the response is measured at A and the same excitation is applied at B. Reciprocity can improve the efficiency of hammer testing. Because, if reciprocity is met, many responses can be measured at the same time for a single hammer.
Modal testing is a non-destructive technique used to identify and solve problems that could affect a ship's noise and vibration levels, performance, and safety. It is a valuable tool for ensuring the safety and performance of ships.