& CONCLUSIONSVibration qualification tests are currently indispensable for vehicle manufacturers and their suppliers. Carmakers' specifications are therefore conceived to challenge the mechanical endurance of car components face to numerous inservice detrimental phenomena: fatigue damage, shocks or accidents, and mechanical degradation. The vibration signals used in the automotive industry are of various types (random, harmonic, shock pulses) and are becoming more and more complex (multi-random, swept-sine-on-random etc.), covering different frequency ranges. With automotive business requiring new product development to be faster, a robust methodology that permits to easily compare and quantify the severity of a vibration specification would be advantageous from a design of reliability perspective.The objective of this paper is to illustrate a set of tools that can be applied to the comparison of vibration specification signals used to validate the mechanical endurance of car components undergoing stressing vibration loadings. The paper will describe different criteria and new innovative methodologies adopted to perform the required comparison, from quick and effective specification analysis, to even more complex purposes (for example when building a new generic vibration signal for early product development).Following a brief theoretical introduction, several case studies will show practical applications of the methodology.
As part of the design validation of components mounted on rotating machinery such as a car engine, it is essential to understand the environmental stress in which they operate. In the case of vibration loadings, the reliability must be verified according to accelerated tests that realistically reproduce the in-service stress. Indeed, a representative validation profile permits not only the assessment of the component durability, but also provides a reliable reference for design optimization (downsizing), reducing development cycle costs, and time scale. This paper proposes an innovative methodology to generate tailored vibration signals more representative of the real environment. First, the severity of the test is tailored to the specific engine usage profile, i.e., the time spent at each revolution per minute (RPM) for a given percentile of customer usage. Traditional signals of Power Spectrum Density (PSD) and sine sweep are developed according a fatigue damage approach (test tailoring). Then, a more representative vibration specification is obtained as a multiple-sine-on-random signal. The signal is expressed as a series of multiple sine sweeps (representing the engine harmonics) and a PSD (representing the random leftover signal). A case study applied to an engine mounted heat exchanger (a water-cooled intercooler) is presented. The generated specification is compared to the validation signals developed according to existing procedures. The results show that 1) the tailored approach decreases the risk of unnecessary over-testing and 2) the multi-swept-sine-on-random signal guarantees the best representativeness of the real environment.
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