Abstract
In the field of military land vehicles, random vibration processes generated by all-terrain wheeled vehicles in motion are not classical stochastic processes with a stationary and Gaussian nature. Non-stationarity of processes induced by the variability of the vehicle speed does not form a major difficulty because the designer can have good control over the vehicle speed by characterising the histogram of instantaneous speed of the vehicle during an operational situation. Beyond this non- stationarity problem, the hard point clearly lies in the fact that the random processes are not Gaussian and are generated mainly by the non-linear behaviour of the undercarriage and the strong occurrence of shocks generated by roughness of the terrain. This non-Gaussian nature is expressed particularly by very high flattening levels that can affect the design of structures under extreme stresses conventionally acquired by spectral approaches, inherent to Gaussian processes and based essentially on spectral moments of stress processes. Due to these technical considerations, techniques for characterisation of random excitation processes generated by this type of carrier need to be changed, by proposing innovative characterisation methods based on time domain approaches as described in the body of the text rather than spectral domain approaches.
Highlights
In the context of design of structures under extreme stresses, it is usually accepted that stationary random excitation processes for these extreme stresses can be characterised by their acceleration PSD over a relevant frequency range, usually covering the LF [1 to 50 Hz] and MF [50 Hz to 500 Hz] ranges for the case of wheeled vehicles
– α = 0, 1%: risk associated with the development of “secure” mechanical equipment and/or structures forming part of a functional system for which performance is important for the final client, but for which the client requirement level is high. This is the case for devices associated with the principal functions of the System, for which high functional performance levels specified by the final client cannot be achieved without risk unless the loads assumed in the development are well controlled. From these technical considerations, during the period from 2005 to 2010 [4, 7] it became clear that major changes needed to be made to Response Spectrum (RS) techniques to get a better understanding of these extreme value exceedance risks, in the case of excitation processes generated by Land military vehicles
Extreme Response Spectrum (ERS) time domain and XRS time domain techniques have been developed for this purpose and tested for Land Armament programs (VBCI and ARAVIS), highlighting their robustness and calculation speed characteristics
Summary
In the context of design of structures under extreme stresses, it is usually accepted that stationary random excitation processes for these extreme stresses can be characterised by their acceleration PSD over a relevant frequency range, usually covering the LF [1 to 50 Hz] and MF [50 Hz to 500 Hz] ranges for the case of wheeled vehicles. Methods of characterising extreme values generated by these random acceleration processes, based on conventional Gaussian signal assumptions, can be used This is the case for characterisation techniques using the Response Spectrum (RS) method widely used by manufacturers in the Civilian and the Military fields for writing vibration design specifications. This effective masses criterion consists of using all modes for which the sum of effective masses is 90% of the total mass of the structure to be designed [1] At this stage, this is a generalisation of the Biot model historically used in calculation of the Shock Response Spectrum (SRS) of transient excitations and that is adapted to the case of random vibrations [2] referred to as an Extreme Response Spectrum (ERS).
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