Vibration testing is a simulation of the various vibration environmental impacts that a product encounters in transportation, installation, and use environments. This test simulates the various vibration environmental impacts that a product encounters in transportation, installation, and use environments to determine its ability to withstand various environmental vibrations. Vibration testing is the evaluation of the resistance of components, components, and the entire machine in the expected transportation and usage environment.
The Bede test believes that the most commonly used vibration methods can be divided into two types: sinusoidal vibration and random vibration. Sinusoidal vibration is a commonly used testing method in the laboratory, mainly used to simulate the vibration generated by rotation, pulsation, oscillation (which occurs on ships, aircraft, vehicles, and space vehicles), as well as to analyze the resonance frequency of product structures and verify the resonance point residence. It is further divided into sweep frequency vibration and fixed frequency vibration, and its severity depends on the frequency range, amplitude value, and test duration. Random vibration is used to simulate the overall structural seismic strength evaluation of the product and the transportation environment in the packaging state, and its severity depends on the frequency range, GRMS, test duration, and axial direction.
The reciprocating motion of an object or particle relative to its equilibrium position is called vibration. Vibration can be further divided into sinusoidal vibration, random vibration, composite vibration, scanning vibration, and fixed frequency vibration. The main parameters describing vibration include amplitude, velocity, and acceleration.
Vibration testing standards:
GJB 150.25-86
GB-T 4857.23-2003
GBT4857.10-2005
WJ231-77
Testing of physical or model vibration systems on site or in the laboratory. A vibration system is a mass elastic system that is excited by a vibration source, such as a machine, structure or its components, organism, etc. Vibration testing has developed from the aerospace sector and has now been promoted to various industrial sectors such as power machinery, transportation, construction, and environmental and labor protection. Its application is becoming increasingly widespread. Vibration testing includes response measurement, dynamic characteristic parameter measurement, load identification, and vibration environment testing.
Response measurement
Mainly for measuring vibration levels. In order to verify the operational quality, safety and reliability of machines, structures, or their components, as well as determine environmental vibration conditions, it is necessary to measure the vibration level of each selected point and direction of the vibration system under various actual working conditions, and record the relationship between the vibration value and time variation (referred to as time history). For periodic vibration, the main measurement is the vibration level (amplitude or effective value of displacement, velocity, acceleration or strain) and vibration period; For transient vibration and impact, the maximum peak value and response duration of displacement or acceleration are mainly measured; For stationary random vibrations, the main measurement is the mean and variance of the time history of force and response; For non-stationary random vibrations, time can be divided into many small segments, and the mean and variance of the time history within each segment can be measured to find their relationship with time and use this as a measure of vibration level.
The vibration speed of many machines is almost constant over a wide frequency range, so the maximum effective value of the vibration speed measured at a selected point on the machine can be used as an indicator of the intensity of machine vibration (known as vibration intensity).
Parameter measurement
In order to design and trial produce new machines or solve vibration reduction problems when renovating old machines, as well as to improve the efficiency of vibration machinery, it is necessary to understand the dynamic characteristic parameters of the system. There are many dynamic characteristic parameters, and for linear systems, the most commonly used are modal parameters, including natural frequencies, vibration modes, modal mass or stiffness, and modal damping ratio. Modal parameters can be converted into mechanical parameters in physical coordinates (i.e. geometric coordinates), including concentrated mass, stiffness, and damping matrices.
Determination method
In engineering design, sometimes only low order (such as first and second order) natural frequencies, vibration modes, and damping coefficients need to be known, and these parameters can be measured using simple methods:
① Natural frequency measurement involves tapping or sudden unloading to generate free vibration in the system, recording its attenuation waveform and comparing it with the time scale signal in the instrument, or inputting the fixed frequency sine wave and attenuation waveform generated by the signal generator into a ray oscilloscope, and obtaining the first and second order natural frequencies from the Lissajous diagram displayed on the oscilloscope. If there is an exciter or vibration table, the system can be subjected to step frequency excitation or low-speed sweep frequency excitation to find the resonance frequency. When the damping is small, the resonance frequency is approximately equal to the natural frequency.
② Measurement of vibration mode: Handheld wooden or aluminum probes are used to contact various points of the tested system, and the positions of all non vibrating points, namely the nodal line positions, are determined by the impact sound (or by hand feeling). For a horizontally placed flat plate system, sand particles can be sprinkled on the flat plate. During vibration, the sand particles will gather on the nodal line, and the vibration mode can be roughly determined based on the distribution of the nodal line.
③ Damping measurement can be carried out using attenuation vibration method, resonance method, and phase method. The attenuation vibration method uses a recorder to record the attenuation waveform of free vibration, and calculates the damping value from the attenuation rate of two or several adjacent amplitudes in the same direction; The resonance method calculates the damping value based on the amplitude during resonance and the frequency bandwidth of the resonance region; The phase method calculates the damping value based on the relationship between the phase of the resonant region and frequency.
Admittance method
Mechanical admittance is a characteristic parameter in the frequency domain of a system (see mechanical impedance). The natural frequencies of large and complex structures are numerous and dense, and the vibration modes are very complex, which cannot be measured using simple methods. However, the response of the system to the excitation force can be tested first to obtain mechanical admittance, and then graphical identification (i.e. amplitude frequency, phase frequency, real frequency, imaginary frequency, or vector plot recognition of mechanical admittance) or computer identification can be used to determine modal or physical parameters.
Time domain recognition
Directly use the time history of vibration to determine the modal parameters of the system. For free vibration, the modal parameters can be directly calculated by the relationship between free vibration and pulse response function (one of the time-domain characteristic parameters of the system, its Fourier transform is mechanical admittance). For forced vibrations, numerical time series analysis methods or other methods (such as random decrement method, filtering method, etc.) can be used to calculate modal parameters. The advantage of time-domain identification method is that it can utilize the vibration signals of machines in operation, and is suitable for large structures that cannot be tested in the laboratory; The disadvantage is that the excitation force of natural vibration sources is often unable to be measured and controlled, and can only be identified by response values, resulting in low accuracy.
Load identification
It refers to the analysis and determination of the properties of the vibration source (whether it is regular or random? Is it a fixed force or a fixed motion?...), the propagation path, and the load spectrum (i.e. the time history of the load) applied by the vibration source to the system. Load identification, also known as environmental prediction, can provide data for analyzing the dynamic response and vibration causes of a system. The loads borne by large structures are very complex and difficult to directly measure, but the loads borne by the system can be inferred from the response signals of the structure and the known mathematical models of the system. Then, statistical and comprehensive analysis can be conducted based on the data obtained under various working conditions to obtain the load spectrum. The properties and propagation paths of the vibration source can be obtained using power spectrum analysis or correlation analysis methods.
Environmental testing
In order to understand the stability of the product's vibration resistance life and performance indicators, identify weak links that may cause damage or failure, and conduct assessment tests on the system under simulated vibration and impact conditions in actual environments. The testing specifications for finalized products are usually standardized, and suitable testing methods need to be developed for new products. The testing methods are divided into two categories: ① standard testing, including resistance to predetermined frequencies, resistance to resonance, sine scan testing, broadband random vibration testing, impact testing, acoustic vibration testing, and transportation testing; ② Non standard testing, including transient waveform vibration testing, narrowband random vibration testing, random wave reproduction testing, sine wave and random wave hybrid testing, etc. (See vibration environment testing)
The large amount of raw data obtained from vibration testing data processing and analysis testing must undergo various processing before it can be used as the basis for engineering design calculations. The original recorded data for testing is the time history of parameters (the relationship between displacement, velocity, or acceleration values and time). Through intuitive analysis, the data can be divided into three types: transient, periodic, random, or non random continuous non periodic. Then, statistical analysis, correlation analysis, and spectral analysis can be conducted in the time domain (including the time difference domain, i.e. the time difference between two signals with independent variables), frequency domain, and amplitude domain, To obtain various functions that characterize the characteristics of time history. The processing methods can be divided into analog processing method and digital processing method. The former has simple equipment, but poor accuracy and long processing time; The latter requires converting the original recorded analog quantity into digital quantity and then using a digital computer for processing. Due to its high accuracy and extremely fast speed, with the emergence of various fully functional specialized data processors (such as fast Fourier analyzers), digital quantity processing has gradually replaced analog quantity processing.
Data processing and analysis
The large amount of raw data obtained from testing must undergo various processing before it can be used as the basis for engineering design calculations. The raw recorded data of testing is the time history of parameters (displacement, velocity, or acceleration values related to time). Through intuitive analysis, the data can be divided into three types: transient, periodic, random, or non random sustained non periodic, Furthermore, statistical analysis, correlation analysis, and spectral analysis are conducted in the three major domains of time domain (including time difference domain, where the independent variable is the time difference between two signals), frequency domain, and amplitude domain, in order to obtain various functions that characterize the characteristics of time history. The processing methods can be divided into analog processing method and digital processing method. The former has simple equipment, but poor accuracy and long processing time; The latter requires converting the original recorded analog quantity into digital quantity and then using a digital computer for processing. Due to its high accuracy and extremely fast speed, with the emergence of various fully functional specialized data processors (such as fast Fourier analyzers), digital quantity processing has gradually replaced analog quantity processing.
Testing equipment
It can be roughly divided into excitation equipment, vibration measurement equipment, and analysis equipment, which correspond to the three parts I, II, and III in Figure 2, respectively. The single line arrow in the figure represents the transmission path of electrical signals, while the double line arrow represents the transmission path of mechanical quantities (force, velocity, acceleration, etc.). Some equipment or devices in the figure are explained as follows: ① Excitation equipment: can be divided into two types: exciter and vibration table. Currently, excitation equipment with vibration control instruments has been gradually adopted, which can excite according to the required waveform or spectral shape. ② Sensors: can be divided into three types: force measurement, motion measurement, and impedance measurement (simultaneously measuring force and motion at a point) Filter: It can perform functions such as anti-interference, denoising, and extracting useful signals. Before using a digital computer for processing, it is necessary to pass the signal through a low-pass filter (called an anti aliasing filter) to avoid the possible aliasing phenomenon that may occur after the signal is discretized into digital quantities.