There has been some concern that the blades in the real engine operating environment may not always behave in a linear manner. The non-linearity can arise from friction contact surfaces (i.e. blade dovetails and discs), non-linear material properties (i.e. Young’s modulus, non-linear temperature dependence of modulus), component manufacturing variability, and component design geometry. The vibration forcing itself can also cause multi-modal responses when applied as multi-mode excitation. The present study aims at investigating the effects of static contact friction loads on the blade vibration responses. Moreover, some natural frequencies of the blade investigated here are commensurable and thus leading to internal resonance in the system and nonlinear interactions between involved modes.
This investigation shows that some blade vibration modes are more sensitive to the blade root friction loads than others. This sensitivity is associated with modeshape localisations. The other source of non-linear behavior is related to internal resonances. This particular blade geometry affects the blade stiffness in such a way that some natural frequencies are commensurable. For instance, there is an internal resonance between the first and second torsion modes. The modal frequency of second torsion is twice the first torsion frequency. Both the non-linearity effects associated with contact friction loads and internal resonances seem to result from the interactions of two or more natural vibration modes. There is a dominant modal response among the interacting modes. Fast Fourier Transforms (FFT) of response histories also reveals the contribution of individual modes in the multi-modal response. This paper attempts to address this non-linear blade behaviour by conducting both experimental tests and numerical simulations using an in-house forced response code.