Tests for random vibration impact
in compliance with IEC 68-2-34
Test for random vibration impact is a complex test procedure.
In this test type, such term as ASD (acceleration spectral density) is often used. This term should be interpreted as “random vibration acceleration spectral density measured as “squared acceleration divided by frequency (g2/Hz)”. ASD spectrum determines ASD measurement method within the specified frequency range.
Tests purpose is to determine devices, elements and products resistance to random vibration impact having certain severity grade as well as to detect mechanical defects and/or specified product characteristics deterioration in order to estimate sample validity. In the course of test performance, the sample is exposed to random vibration impact having a certain value within a broad frequency band. Due to the complex mechanical response of the sample and its mounting fixture, the tests process requires special care in its preparation and implementation.
For simplification of test method implementation, the test program is divided into 4 sections:
- tests Fd. IEC 68-2-34;
- tests Fda. IEC 68-2-35;
- tests Fdb. IEC 68-2-36;
- tests Fdc. IEC 68-2-37.
The last three clauses are represented by comprehensive tests methods with recommended verification procedures specified in the annexes. All the information necessary for the developers of corresponding standards and technical documentation is available in the description of Fd test. The information necessary for test engineer is available in Fda, Fdb, Fdc test description (depending on the particular information required).
Tests performance theory
All test methods require a certain degree of repeatability – especially for qualification and acceptance tests used for one and the same sample type by different organizations (such as manufacturer and consumer of electronics).
The word “repeatability” does not stand for the similarity of the test results obtained in the laboratory environment and real conditions – this term means obtaining similar tests results in different test laboratories. Great variance in requirements for tolerance values at certain severity grade as well as the necessity to provide results validity leads to introduction of three repeatability classes: high (control points tolerance: ±3 dB; measurement points tolerance: ±5 dB), medium (control points tolerance± 6 dB), low (no tolerance values requirements). For each of the repeatability classes, it is possible to choose the verification method taking into consideration tested sample dynamic characteristics and particular testing equipment used.
Repeatability requirements also include vibration level control within the narrow frequency band. Despite of the fact that frequency alignment in narrowband spectrum provides better repeatability if compared to the broadband range, frequency alignment in the narrowband spectrum does not allow to fully control environmental impact on the tested object. However, broadband frequency alignment often leads to resonance change inside of the tested specimen resulting in peak and valley values of the initial test level parameters. Real environment conditions often lead to peak and valley values inside of the tested object. Besides, it is unlikely that these peak and valley values may coincide with those occurring in the course of laboratory tests.
Vibration severity rate for tests performance is determined by correlation of the following parameters:
- frequency range;
- ASD level;
- exposure time.
In order to simplify the process, even spectrum is used in this test. Under special circumstances, the use of a different spectrum shape may be possible (specified as frequency function). Most often used frequency ranges are as follows: 20…150, 20…500, 20…2 000 Hz.
Nominal ASD level in the set frequency range should be selected from the following values: 0,0005; 0,001; 0,002; 0,01; 0,02; 0,05; 0,1; 0,2; 0,5; 1; 2; 5; 10 g2/Hz.
Exposure time should be selected from the values specified below. If the required exposure time for each of the directions is 10 hours or more, this time can be divided into 5 hour periods provided that sample mechanical stress (caused by heating or other factors) will not decrease. Any set time is a resulting exposure time *(30 s, 90 s, 3 m, 9 m, 30 m, 90 m, 3 h, 9 h, 30 h) that should be equally distributed between each set direction.
Prior to tests performance, it is necessary to expose the sample to sinusoidal vibration impact in order to obtain FR value and to perform resonance detection tests.
For FR value detection the sinusoidal vibrations tests are performed throughout the frequency range in both directions (sinusoidal excitation amplitude corresponds to random vibration test severity degree).
In order to find resonance frequencies during tests performance, frequencies, causing mechanical resonance and similar processes (for instance, fault of normal operating mode) are compared. This also allows to obtain information of late effects caused by random vibration impact. Applicable standards and technical documentation should specify necessary actions in the case of any changes in resonance frequency.
In the case, if the applicable standards and technical documentation require implementation of preliminary measurements of electrical parameters and mechanical characteristics, such measurements are performed in the course of primary measurements prior to the exposure. During exposure, the sample is influenced by random vibration at the specified level. The samples are sequentially exposed to vibration in three mutually transverse axes unless otherwise provided by applicable standards and technical documentation. Vibration impact directions are selected in such a way, so that to easily detect all possible defects. Unless otherwise provided by applicable standards and technical documentation, the tested specimen should also be in operating condition so that it would be possible to detect both fault state and mechanical defects. Standards and documentation should also specify whether it is necessary to perform measurement of electrical and mechanical parameters during the exposure (and at a certain exposure stage).
There are three random vibration testing methods:
- broadband random vibration impact;
- narrowband random vibration at fixed frequencies;
- frequency sweeping random vibration.
These three vibration testing methods are not equivalent and shall be considered to be independent testing methods. Broadband vibration test is of special importance, besides, from the technical point of view, this test method is the most comprehensive one.
Test methods for random vibration impact are used in addition to already existing IEC 68-2-6 sinusoidal vibration tests which are a great advance in real environment vibration impact reproducing (similar to that occurring in the course of the device use). Random vibration impact test should be used in all cases when it is financially possible since there are some defects that can not be revealed by sinusoidal vibration impact, while they are easily detected by random vibration impact.