As the name implies, the sinusoidal vibration test is to test the vibration response characteristics of the product itself and its ability to maintain functional and performance indicators under the condition of simulating the engineering product to be subjected to "sinusoidal" vibration excitation in the laboratory. Since any complex time-domain vibration waveform can be transformed into a collection of countless continuous sinusoidal vibrations, when assessing the fatigue life of engineering products, sinusoidal fixed-frequency vibration or sinusoidal swept-frequency vibration is also used as the environmental condition of external excitation. Therefore, the purpose of the sinusoidal vibration test is:
a) Identify mechanical weaknesses (structural integrity) and performance degradation (changes in dynamic characteristics) to help stakeholders decide whether the product should pass;
b) Assess the fatigue life of engineering products to confirm the service life of the products.
In the existing general environmental test standards (specifications), sinusoidal vibration tests are commonly used in the following environmental test situations.
1) Environmental adaptability test
Early vibration test standards were limited by environmental test equipment, environmental adaptability tests, environmental stress screening, and
Sine fixed frequency vibration test or sine swept frequency vibration test is often used for testing of vibration characteristics. The sine swept frequency vibration test procedure is clearly defined as follows:
(1) Put the test sample in working state, perform up-sweep and down-sweep sinusoidal scanning cycles in the entire test frequency range from the lower limit working frequency to the upper limit working frequency, record the acceleration response curve of a certain position on the selected test sample and determine the critical frequency.
The principles for confirming critical frequencies (or dangerous frequencies) are:
a) The mechanical vibration resonance frequency at which the ratio of the peak acceleration amplitude to the input acceleration amplitude in the response acceleration data on the test sample is greater than 2 ;
b) The frequency with which the performance indicators or main functions of the product change significantly.
(2) Keep the test sample in working condition and perform sinusoidal vibration sweep cycles for a minimum duration of 1 hour. Pay close attention to any changes in critical frequencies that occur during the test and record them.
If the enhanced test of fixed-wing aircraft is implemented, the four most severe frequencies (Note: the ratio of peak acceleration amplitude to input acceleration amplitude is the largest) are selected from the key frequencies of the test sample's acceleration response curve, and the sinusoidal dwell vibration test is implemented at each selected frequency point. Due to slight changes in the structure and installation status of the test equipment or product, the actual test frequency may drift slightly. Therefore, during the resonance dwell test, attention should be paid to timely adjust the test frequency value to ensure that the resonance peak of the sinusoidal dwell vibration test can always be maintained at the maximum acceleration response state. After the sinusoidal dwell test is completed, the sinusoidal sweep frequency cycle is continued. It is also necessary to record the acceleration response curve of a certain position on the selected test sample and determine the key frequency. Any changes in the key frequency should be recorded to provide data support and evidence for the analysis of the test results.
2) Propeller aircraft (fixed-wing aircraft) Test spectrum
For equipment installed on propeller aircraft, the vibration environment is mainly induced by the rotation of the propeller. The vibration spectrum is composed of a broadband random vibration background superimposed on some narrowband peak vibration signals. The background spectrum of vibration is generated by various random vibration sources. The narrowband peak vibration signal is often caused by the low-order periodic component caused by rotating machinery (such as engines, gearboxes, rotating shafts, etc.), and its peak is generated by the pressure field of the propeller blade rotation. When implementing this type of broadband random vibration superimposed on multiple narrowband random (or sine swept) vibration tests, the "vibration source dwell" (note: not a single sine dwell) vibration method is usually used to achieve laboratory vibration environment simulation.
3) Helicopter (rotorcraft) vibration test spectrum
The vibration environment of a helicopter is composed of a broadband random vibration background superimposed with a single-frequency peak vibration signal. The peak is composed of sinusoidal vibration and its harmonics induced by the movement of the main rotating parts or machinery (such as the helicopter main rotor, tail rotor, engine, gearbox, rotating shaft, etc.). The normal operating speed of these rotating parts is generally fixed. Even if there is an error in control accuracy, the rate of change does not exceed 5%. Therefore, the vibration caused by these rotating parts or machinery can be identified as a single-frequency sinusoidal vibration signal. In other transport machinery with rotary engines, steam turbines, gas turbines and other power equipment, their vibration environment can generally be classified into this state, but the number of single- frequency peak signals should be determined according to the specific situation. This test spectrum of periodic vibration induced by the imbalance of rotating machinery plus broadband random vibration background is also referred to as broadband random plus sinusoidal fixed frequency vibration test spectrum.
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