Tilàn Dossogne

Experimental Assessment of the Influence of Interface Geometries on Structural Dynamic Response

Jointed interfaces are sources of the greatest amount of uncertainty in the dynamics of a structural assembly. In practice, jointed connections introduce nonlinearity into a system, which is often manifested as a softening response in frequency response, exhibiting amplitude dependent damping and stiffness. Additionally, standard joints are highly susceptible to unrepeatability and variability that make meaningful prediction of the performance of a system prohibitively difficult. This high degree of uncertainty in joint structure predictions is partly due to the physical design of the interface. This paper experimentally assesses the influence of the interface geometry on both the nonlinear and uncertain aspects of jointed connections. The considered structure is the Brake-Reus beam, which possesses a lap joint with three bolted connections, and can exhibit several different interface configurations. Five configurations with different contact areas are tested, identified, and compared, namely joints with complete contact in the interface, contact only under the pressure cones, contact under an area twice that of the pressure cones, contact only away from the pressure cones and Hertzian contact. The contact only under the pressure cone and Hertzian contact are found to behave linearly in the range of excitation used in this work. The contact area twice that of the pressure cone behaves between the complete contact and contact only under the pressure cone cases.

In Situ Measurements of Contact Pressure for Jointed Interfaces During Dynamic Loading Experiments

One of the greatest challenges for developing and validating models for the dynamics of jointed interfaces is measuring and characterizing the contact pressure within a joint. Previous approaches have focused on static measurements, typically taken separately from the dynamic testing of a jointed system. In this research, an electrical contact pressure measurement system is used to measure the contact pressures within a jointed interface during dynamic testing of the jointed system. These experiments invalidate a previously held modeling assumption: that the static pressure distribution is representative of the contact pressure during service of a jointed interface. In fact, for the measurements reported, the extent and magnitude of contact pressures dramatically change across the interface during sinusoidal excitation of the jointed system with more than a quarter of the interface oscillating between being in and out of contact during each period of excitation. While preliminary and scoping in nature, these experiments corroborate recent numerical studies that predict that the contact pressures across an interface significantly change over time as a function of the applied loading. The ramifications of these results are that the energy dissipation mechanisms within a jointed interface significantly evolve over time, resulting in more energy being dissipated in the interface away from the bolts than previously anticipated. This, in turn, necessitates a new constitutive modeling approach for reduced order modeling representations of joints in which the local kinematics are not regularized (such as in traditional Iwan models) and the normal contact forces are directly modeled and allowed to vary with load (contrary to most of the current modeling approaches).

Nonlinearities of an aircraft piccolo tube: identification and modeling

Piccolo tubes are parts of aircraft wings anti-icing system and consist of titanium pipes inserted into the internal structure of the slat. Due to differential thermal expansion, clearances between the tube and its support are unavoidable and cause the overall system to exhibit highly nonlinear behavior, resulting from impacts and friction. This paper addresses the identification and modeling of the nonlinearities present in the slat-Piccolo tube connection. The complete identification procedure, from nonlinearity detection and characterization to parameter estimation, is carried out based upon sine-sweep measurements. The use of several techniques, such as the acceleration surface method, enables to understand the complex dynamics of the Piccolo tube and build a reliable model of its nonlinearities. In particular, the parameters of nonsmooth nonlinear stiffness and damping mechanisms are estimated. The nonlinear model is finally validated on standard qualification tests for airborne equipments.

Identification of Complex Nonlinearities Using Cubic Splines with Automatic Discretization

One of the major challenges in nonlinear system identification is the selection of appropriate mathematical functions to model the observed nonlinearities. In this context, piecewise polynomials, or splines, offer a simple and flexible representation basis requiring limited prior knowledge. The generally-adopted discretization for splines consists in an even distribution of their control points, termed knots. While this may prove successful for simple nonlinearities, a more advanced strategy is needed for nonlinear restoring forces with strong local variations. The present paper specifically introduces a two-step methodology to select automatically the location of the knots. It proposes to derive an initial model, using nonlinear subspace identification, and incorporating cubic spline basis functions with fixed and equally-spaced abscissas. In a second step, the location of the knots is optimized iteratively by minimizing a least-squares cost function. A single-degree-of-freedom system with a discontinuous stiffness characteristic is considered as a case study.

A Comparison of Reduced Order Modeling Techniques Used in Dynamic Substructuring.

Experimental dynamic substructuring is a means whereby a mathematical model for a substructure can be obtained experimentally and then coupled to a model for the rest of the assembly to predict the response. Recently, several methods have been proposed that use a transmission simulator to overcome sensitivity to measurement errors and to exercise the interface between the substructures. This chapter compares the advantages and disadvantages of multiple reduced order modeling strategies for two dynamic substructuring problems. First, a simple system is investigated using two beams connected by means of a transmission simulator. With this simple system, multiple dynamic substructuring and model reduction techniques are considered including the traditional transmission simulator, Craig–Bampton, dual Craig–Bampton (DCB), Craig–Chang, and Craig–Mayes methods. The second system consists of a beam attached to a plate on one end of a cylinder that encases a pressed foam and metal assembly. This second example uses actual experimental measurements while the beam example is purely a numerical demonstration. By using a finite element model of the beam-plate-can assembly, an experimental model of the dynamics for the internal foam system can be described using dynamic substructuring. This is investigated using the traditional transmission simulator and Craig–Mayes techniques.