Tests for sinusoidal vibration impact in compliance with IEC 68-2-6
The purpose of this test is to reveal mechanical defects and/or characteristics deterioration as well as comparison of the results obtained to the requirements specified in the applicable standards and technical documentation in order to reveal resistance of elements, devices and other products (hereinafter referred to as a sample) to the vibration impact of certain severity grade.
In some cases, this test is performed in order to evaluate structural integrity and/or dynamic characteristics of the tested specimen. Besides, based on severity grade, it is possible to classify the elements into different categories. Corresponding standards and technical documentation also specify whether the sample should operate in the course of the tests process, or should it just undergo the tests impact.
Test severity grade is evaluated as a combination of three parameters: frequency range, vibration amplitude, vibration impact duration (represented as swing cycles number or duration).
Recommended frequency ranges are as follows: 1…35, 1…100, 10…55, 10…150, 10…500, 10…2000, 10…5000, 55…500, 55…2000, 55…5000, 100…2000 Hz.
Vibration amplitude (velocity and displacement or both of them) is specified in corresponding standards and technical documentation. Below the set frequency level (known as transition frequency) all the amplitudes are set as constant displacement; and above this frequency level – as a constant velocity. Each displacement amplitude value corresponds to a particular velocity amplitude value, thus, at the transition frequency point, the vibration level is the same. For upper frequency of 10 Hz, it is comfortable to specify the displacement amplitude throughout all frequency range.
Corresponding standards and technical documentation should specify one of the vibration impact durations listed below. In the case, if impact duration is 10 hours or more for each direction or frequency, the impact duration can be separated in time periods provided that the sample mechanical stress (caused by heating or other factors) will not decrease.
In the case of frequency sweep method, the vibration impact duration for each of the axes should be determined by swing cycles number (1, 2, 5, 10, 20, 50, 100) specified by corresponding standards and technical documentation.
There are two types of fixed frequency vibration impact: critical frequencies and pre-set frequencies impact. In the case of critical frequencies vibration impact, the delay time for each of the directions should be chosen from the following list: (10±0,5) min; (30±1) min; (90±1) min; 10 h ±5 min. In the case of pre-set frequency vibration impact the duration, specified in corresponding standards and technical documentation should also describe the full time, during which the sample is exposed to the vibration impact in the course of its use. The upper limit of 107 oscillations should be used for any combination of frequency and axis direction.
In the course of preliminary measurements (if it is required by applicable standards and technical documentation) there should be performed measurements of electrical parameters and mechanical characteristics. The applicable standards and technical documentation should specify the number of axes for vibrational impact as well as their relative positioning. If these parameters are not specified, the sample should be exposed to vibrational impact in three mutually transverse directions that are selected in such a way, so that it would be easy to detect the defects.
The control signal is formed by means of a signal from one or several control points. In the second case, the signals undergo constant arithmetic averaging or other processing specified in applicable standards and technical documentation.
In the course of standards and technical documentation development, the test methods should be based on the stages listed below. The test stages should be performed sequentially for one axis and then for the remaining axes. For the samples that will be used with shock absorbers but are tested without them, special measures should be taken – if it is necessary, the required vibration amplitude control can be implemented with shaker system maximum force limitation.
- evaluation of sample response to vibration impact;
- vibrational impact.
Normally the research of sample response to vibrational impact in the required range should be performed throughout all the swing cycle under the same conditions that during delay time, but the vibration amplitude and sweep speed can be decreased (if compared to the nominal values) if it is necessary to determine the sample response to vibrational impacts more accurately. One should avoid excessive vibration impact time increase. In the applicable standards and technical documentation, there should be specified the necessity of sample operation in the course of tests for vibration impact response. At this stage the sample is tested in order to reveal the critical frequencies that cause:
- sample fault and/or deterioration of its characteristics depending on the vibration level;
- mechanical resonance and accompanied processes like rattling.
The tested specimen response, as well as all the frequency and amplitude values that cause similar processes, should be registered.
There are two methods of sample vibrational impact testing:
- frequency sweep vibration test method;
- fixed frequency level vibrational impact.
Use of frequency sweep vibration impact method is more preferable. Frequency should be variable within certain frequency range while impact amplitude and duration values should correspond to those specified in the applicable standards and technical documentation. If it is necessary, the frequency range can be divided into several sub-ranges, provided that sample mechanical stress will not be decreased.
In the case of vibration impact at fixed frequencies the sample is exposed to vibration at frequencies, revealed in the course of previous sample testing at specified frequencies listed in applicable standards and technical documentation.