A mechanical system's modal parameters change when fatigue loading is applied to the system. In order to perform an accelerated vibration-based fatigue test these changes must be taken into account in order to maintain constant-stress loading. This paper presents an improved accelerated fatigue-testing methodology based on the dynamic response of the test specimen to the harmonic excitation in the near-resonant area with simultaneous monitoring of the modal parameters. The measurements of the phase angle and the stress amplitude in the fatigue zone are used for the real-time adjustment of the excitation signal according to the changes in the specimen's modal parameters. The presented methodology ensures a constant load level throughout the fatigue process until the final failure occurs. With the proposed testing methodology it is possible to obtain a S-N point of the Woehler curve relatively quickly and to simultaneously monitor the changes of the specimen's natural frequency and damping loss factor. The presented methodology with real-time control is verified on an aluminium Y-shaped specimen (10E6 load cycles are achieved in 21 minutes) and is applicable to a specimen with arbitrary geometry. Besides the faster completion of the fatigue test the methodology can be adopted for the validation of the vibrational fatigue analysis.
COBISS.SI-ID: 12402971
This study presents an approach to an experimental validation of a complex, large-scale, multi-body mechanism model with diverse body inertia and geometry properties, exposed to an arbitrary kinematic excitation. Such an approach was used to analyze and predict the dynamical behaviour of a complex, 24- DOF, pendulum system with a relatively large dimensions. To obtain the theoretical dynamical response a computational dynamics approach was used in order to establish a constraint-based, simplified, planar, rigid-body numerical model. To validate the model, a special experimental approach was employed that overcame the difficulties related to the analyzed systems size, many rotating components and simultaneous measuring requirements. Consequently, a combination of conventional and wireless signal-streaming measuring setups, which transmitted the signal through the purposely set-up wireless network, were employed. The model is validated by comparing the simulated and experimentally obtained kinematical and dynamical quantities to justify rigidity of the models components and the simplified planar modelling. The validated models ability to predict hard-to-measure systems response is demonstrated.
COBISS.SI-ID: 12156187
A great deal of progress has been made in recent years in the field of global digital image correlation (DIC), where higher-order, element-based approaches were proposed to improve the interpolation performance and to better capture the displacement fields. In this research, another higher-order, element-based DIC procedure is introduced. Instead of the displacements, the elements' global nodal positions and nodal position-vector gradients, defined according to the absolute nodal coordinate formulation, are used as the searched parameters of the Newton-Raphson iterative procedure. For the finite elements, the planar isoparametric plates with 24 nodal degrees of freedom are employed to ensure the gradients' continuity among the elements. As such, the presented procedure imposes no linearization on the strain measure, and therefore indicates a natural consistency with the nonlinear continuum theory. To verify the new procedure and to show its advantages, a real large deformation experiment and several numerical tests on the computer-generated images are studied for the standard, low-order, element-based digital image correlation and the presented procedure. The results show that the proposed procedure proves to be accurate and reliable for describing the rigid-body movement and simple deformations, as well as for determining the continuous finite strain field of a real specimen.
COBISS.SI-ID: 12544795