EXPERIMENTAL-STUDY OF A MECHANICAL SYSTEM CONTAINING A LOCAL CONTINUOUS STIFFNESS NONLINEARITY UNDER PERIODIC EXCITATION AND A STATIC LOAD

Local stiffness non-linearities under dynamic (periodic) and static (t ime-invariant) loads exist in many complex mechanical systems, oftenti mes at the junctions of assembled components. Unlike in linear systems , a static load may significantly alter the nature of the non-linearit y and dynamic response, in terms of amplitude, frequency content and s tability. To examine such phenomena, a controlled laboratory experimen t with a local continuous stiffness non-linearity has been designed, f abricated, instrumented and analyzed. It consists of a flexible suppor t structure in the form of a simply supported beam and a rigid body mo unted on the support structure by a multi-dimensional non-linear ''har dening'' spring element. The non-linear elastic element is made of a v ery thin beam clamped between tapered ends. Multi-harmonic, amplitude- dependent, frequency-sweep-direction-dependent periodic responses to s lowly swept harmonic excitation were measured by using order tracking with a dynamic signal analyzer. It was found that the static load indu ces the ''hardening'' stiffness element to behave like a ''softening'' spring under certain conditions. The cause of this is explained via t heoretical studies of a simple single-degree-of-freedom non-linear osc illator and a more complex model of the experimental system itself. Ex perimental and theoretical studies of the multi-degree-of-freedom, mul ti-dimensional test system also showed that the local non-linearity ha s a broad spectral and spatial influence on the dynamic behavior of th e overall system. For instance, it alters the characteristics of sever al system resonances and modes. (C) 1996 Academic Press Limited