Efficient Simulation of Structural Dynamic Systems with Discrete Nonlinearities

** Dynamic loading environments often involve complex loading and boundary conditions. For the case of externally mounted aircraft stores, qualification testing involves simulating this environment with relatively simple test fixtures and load application shakers. Simulation using Finite Element Analysis (FEA) is used to ensure that the test conditions are representative of the service environment. In this development, a method for efficiently simulating test and service conditions is presented. The component mode synthesis method 1,2 is used to significantly reduce the computational requirements of solving nonlinear store vibration problems while retaining a straightforward model structure at user specified boundary locations. While the component mode synthesis method provides an efficient coordinate transformation, the physical meaning of the degrees of freedom at the boundary are preserved. Nonlinear stiffness functions at the boundary degrees of freedom are added after the linear potion of the model has been reduced. The resulting set of nonlinear equations is integrated in time. Accuracy and computational expense are evaluated through comparison to full-order nonlinear solutions from existing commercial software. I. Introduction An important aspect of store qualification testing is tuning of laboratory tests to accurately represent the service use environment. This is done through simulation of both the test and service environments. These simulations result in quantification of the number of times critical values of load or stress are exceeded. This cyclic loading data can be combined with fatigue strength data for a given material to predict whether the structure can withstand the prescribed load spectrum. Note that while model tuning is performed by comparing test and analysis results for the same conditions, laboratory test tuning is performed on analytical results from the test and service environments (Figure 1). Comparison of the simulated cyclic loading data between test and service conditions provides valuable insight to engineers. This insight includes indication of whether the test is more or less severe than the service environment at various structural details. When combined with trade studies on the test configuration, this data enables tuning of laboratory tests to more accurately represent service loads. Nonlinear effects may have a significant effect on the accuracy of these findings. The interface of stores with the carrying aircraft is a well known source of nonlinearity. These interfaces are designed for effective release of the store and generally result in nonlinear stiffness at the interface. This development presents a new method for efficiently including that nonlinear stiffness.