BS 6897 Experimental Determination of Mechanical Mobility in Structures

Mechanical Tests

BS 6897 Experimental Determination of Mechanical Mobility in Structures

EUROLAB laboratory provides testing and compliance services within the scope of BS 6897 standard. The BS 6897 standard, designed by the British Standards Institute (BSI), defines the basis for calibration tests, environmental tests and physical measurements necessary to determine the suitability of impedance heads, force transducers and response transducers for measuring the mechanical mobility of structures.

BS 6897 Experimental Determination of Mechanical Mobility in Structures

It can be applied to measurements of mobility, acceleration or dynamic compatibility, either as a driving point measurement or a transfer measurement. It also applies to the determination of the arithmetic equivalents of these ratios, such as the free effective mass. Although stimulation is applied at a single point, there is no limit to the number of points at which simultaneous measurements of motion response can be made. For example, multiple response measures are required for modal analysis.

The dynamic properties of structures can be determined as a function of frequency from measurements of mobility or from measurements of acceleration and related frequency-response functions known as dynamic cohesion. Each of these frequency response functions is the phasor of the motion response at a point on the structure due to a unit force (or moment) excitation. The magnitude and phase of these functions are frequency dependent.

Acceleration and dynamic adaptation differ from mobility only in that the motion response is expressed as acceleration or displacement, respectively, rather than velocity. To simplify the various parts of ISO 7626, only the term mobility will be used. It is understood that all the test procedures and requirements described also apply to the determination of acceleration and dynamic fitness.

Any excitation waveform whose spectrum spans the frequency range of interest can be used provided the excitation and response signals are properly processed. Early researchers used sinusoidal excitation signals; Under ideal conditions, the steady state response is also a sinusoidal signal. The ratio of the amplitudes of the sinusoidal response and the excitation signals gives the modulus of mobility at that particular frequency, and the phase difference is the argument.

This technique works because the amplitude of a sinusoidal signal is the modulus of the Fourier transform of that signal, so the excitation in itself accomplishes the same end as the Fourier transform of more complex signals. However, it is necessary to stay at each excitation frequency long enough to achieve a steady-state response. This is not necessary if the Fourier transforms of the excitation signal and the response rate are determined. A short duration sine burst can then be used and the ratio of the response and strength spectra gives an accurate value of mobility over a limited frequency range.

Our organization also provides testing services for the Experimental Determination of Mechanical Mobility in Structures, BS 6897, within the framework of laboratory services of materials.

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