Non-Gaussian PDF modeling of turbulent boundary layer fluctuating pressure excitation

The purpose of the study is to investigate properties of the probability density function (PDF) of turbulent boundary layer fluctuating pressures measured on the exterior of a supersonic transport aircraft. It is shown that fluctuating pressure PDFs differ from the Gaussian distribution even for surface conditions having no significant discontinuities. The PDF tails are wider and longer than those of the Gaussian model. For pressure fluctuations upstream of forward-facing step discontinuities and downstream of aft-facing step discontinuities, deviations from the Gaussian model are more significant and the PDFs become asymmetrical. Various analytical PDF distributions are used and further developed to model this behavior

Shaker testing simulation of non-gaussian random excitations with the fatigue damage spectrum as a criterion of mission signal synthesis.

Experimental simulations of random excitations are nowadays performed digitally by applying the Inverse Fast Fourier Transform (IFFT) to the desired Power Spectral Density (PSD) profile, in combination with randomized IFFT phases. However, the excitations generated in this way will always have a Gaussian probability distribution, whereas real-life random excitations are typically non-Gaussian. For example, in the case of land transportation some distinctive peaks will occur which exceed the average level of vehicle vibration. The sta-tistical parameter known as kurtosis can characterize this feature and could be controlled in experimental simulations in addition to the PSD. The so-called \u201ckurtosis control\u201d can be achieved by special phase manipulation instead of selecting the phases randomly. By increasing the kurtosis, it is furthermore also possible to obtain an accelerated qualification test, whereby the time-to-failure (TTF) is decreased in a controlled manner. It is known that the response of a lightly-damped linear system is closer to Gaussian than the applied excitation. Therefore, in order to increase the response kurtosis in an accelerated test, the kurtosis control method must be able to effectively generate extra kurtosis. In this work a method was used which indeed achieves a high excitation kurtosis, which moreover passes into the re-sponse of the structure. According to the Fatigue Damage Spectrum (FDS) model, a single-degree-of-freedom system was hereby considered in order to calculate the structural response. Furthermore, the rainflow counting procedure and the Miner damage accumulation rule were employed to predict relative TTFs for operational excitation and accelerated test mission. Finally, the considered method of non-Gaussian shaker testing simulation was also advanced from kurtosis control to direct application of the FDS as a criterion for mission signal synthesis. An extensive experimental campaign was carried out, where an example of a real-life vibration excitation measured on the cabin floor of a car was considered. Shaker testing was performed for a cantilevered test specimen subjected to various simulated Gaussian, non-Gaussian, accelerated non-Gaussian, and real road excitations

Synthesis of Sine-on-Random vibration profiles for accelerated life tests based on fatigue damage spectrum equivalence

In many real life environments, mechanical and electronic systems are subjected to vibrations that may induce dynamic loads and potentially lead to an early failure due to fatigue damage. Thus, qualification tests by means of shakers are advisable for the most critical components in order to verify their durability throughout the entire life cycle. Nowadays the trend is to tailor the qualification tests according to the specific application of the tested component, considering the measured field data as reference to set up the experimental campaign, for example through the so called “Mission Synthesis” methodology. One of the main issues is to define the excitation profiles for the tests, that must have, besides the (potentially scaled) frequency content, also the same damage potential of the field data despite being applied for a limited duration. With this target, the current procedures generally provide the test profile as a stationary random vibration specified by a Power Spectral Density (PSD). In certain applications this output may prove inadequate to represent the nature of the reference signal, and the procedure could result in an unrealistic qualification test. For instance when a rotating part is present in the system the component under analysis may be subjected to Sine-on-Random (SoR) vibrations, namely excitations composed of sinusoidal contributions superimposed to random vibrations. In this case, the synthesized test profile should preserve not only the induced fatigue damage but also the deterministic components of the environmental vibration. In this work, the potential advantages of a novel procedure to synthesize SoR profiles instead of PSDs for qualification tests are presented and supported by the results of an experimental campaign