The frequency‐domain approach to fatigue life estimation in random loading has been largely investigated due to its computational advantages, and several methods for the frequency translation of the most common time‐domain methods have been proposed. Between the most known frequency methods, there are Bendat's method, valid for narrow‐band signals, and Dirlik's formula, which ifis considered the best result for wide‐band signals. However, a great part of the frequency methods takes the rainflow count as a reference time‐domain method and uses the rainflow damage computation as the exact value to emulate. Therefore, very few experimental data for the fatigue life of mechanical components subject to random loads are available in the literature. This work presents the setup for a series of experimental tests for specimens subjected to random loads, aiming at achieving experimental data to compare with the results provided by frequency methods. After a brief description of the materials used for the setup, the two‐step test concept is described: first, the specimen will be subjected to random loads obtained by a certain power spectral density for an amount of time which should nominally cause a 30% of damage; then, the fatigue test will be ended on a resonance testing machine to compute the actual residual fatigue life of the specimen; this two‐step testing also allows to reduce the time requested for the tests. The test bench developed for the experimental investigation is described in the paper, together with the results of some preliminary tests, aimed at verifying the feasibility of the conceived procedure.
The frequency-domain approach to fatigue life estimation in random loading has been largely investigated due to its computational advantages, and several methods for the frequency translation of most common time-domain methods have been proposed. However, the largest part of frequency methods only focuses on the estimation of the marginal amplitude probability density function of cycles and do not consider the damage increment caused by the positive mean value of the counted cycles. This paper aims to deepen the effect of the random mean value of the cycles on fatigue damage estimation by simulating a large number of time histories with different power spectral densities and computing the resulting damage for a steel material using time-domain instruments like rainflow count, Goodman correction for mean value and Palmgren-Miner linear rule for damage cumulation. The influence of the presence of a non-zero global mean value of the process on the damage computation due to the random cycle mean value has been investigated. The study of the effect of signal PSD resulted in a non-monotonic influence on the ratio between the total damage and an approximated damage that does not consider random mean values. Finally, some further simulations have been done to study the effect of variations in the S-N curve of the material.
This work deals with the development of a simplified and time-efficient phenomenological model for chain transmissions efficiency. Firstly, a complete multibody model for a final transmission of a motorcycle is developed in MSC Adams View environment, and is properly validated in terms of efficiency with experimental tests on a real chain system. Results from multibody simulations are used to develop a new analytical model, that relates all the main operating and geometry parameters of the transmission system to the transmission efficiency with simple relations. In particular, the influence of the polygonal effect on system efficiency is investigated, finding a relationship with the angle of the sprockets pitch polygon. A new parameter, called chain tension efficiency, is introduced to model the distribution of losses within the system. A linear relationship between this parameter and the number of sprockets teeth and with the system efficiency was assumed and validated, with good results. In addition, the dependence of slack span tension from several parameters, like speed, torque and number of teeth of sprockets, is investigated. In particular, it is highly dependent on the chain peripheral speed, both for a centrifugal tension and for a further linear component of tension, which might be originated by friction between links. The presented simple model can describe the chain system dynamics with low computational effort, allowing the designer to use a smart tool to select the proper transmission parameters. Due to its computational efficiency, the model is also useful for real-time and hardware-in-the-loop simulations.
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