Application of an alternative frequency response technique to the durability assessment of engine components

Abstract: Engine components are exposed to vibrations which may lead to fatigue damage. Accurate dynamic simulations are necessary especially during the development process, in order to find a satisfactory component. Currently Scania uses a standard method for dynamic calculations that is based on a frequency response approach. A measured or calculated excitation yields a certain response through the transfer function of the system. The transfer function is obtained through an eigenfrequency calculation and an experience-based estimate of the modal damping. An obvious drawback of this method is that the estimated modal damping strongly affects the calculated response of the system. In this thesis, the method outlined above is compared to an alternative, so-called, inverse method in which the excitation of the system is calculated using a measured response. The advantage is that the modal damping does not affect the result directly since the excitation has been adjusted according to the response. As a demonstration object a charge air pipe and its bracket are used. Acceleration response data is collected from an engine vibration measurement. The calculated safety factor of the demonstration object is reasonable for both the standard method and the inverse method. An estimate of the quality of the model is obtained for the inverse method through statistical measures, which is not the case for the standard method. The excitation for the inverse method is adjusted to the estimated modal damping which is a major advantage since damping is notoriously difficult to quantify in engineering practice. The inverse method has proven to be a useful simulation method for calculations when a prototype of the engine component of interest already exists.