Unlike traditional vibration testing that involves driving a single axis, Multi-Input/Multi-Output (MIMO) testing has become increasingly popular due to its ability to more accurately replicate field responses and failure modes. Quantifying the mismatch between test response and field response is critical to understanding whether the field environment was adequately replicated by the vibration test. Ideally, a vibration test would replicate the field response in terms of deflection shape and magnitude and therefore also the stresses in the test article. However, a clear and concise process or metric to quantify the difference with respect to stress between the test and field environment does not exist. This paper first considers how the Cross Power Spectral Density (CPSD) metrics are affected by part to part variability between the field and the test. Basic properties of an analytical beam model, such as damping and stiffness, are incrementally varied and the effect on the metrics is observed. A more complex model is used to study the correlation between the CPSD metrics and failure mechanisms such as stress and fatigue. A synthetic field environment is generated so that the field stresses and fatigues are known. Many imperfect MIMO tests are constructed as samples for comparison, and several CPSD metric methods are computed for each MIMO case. The calculated CPSD metrics are correlated to the stress and fatigue differences, and the metrics that best correlate to the failure criteria are identified.
Accelerometers are commonly installed on aerospace vehicles to monitor structural response. Robust ways of identifying the fluid pressure loading associated with those measurements would be useful in understanding the fundamental environments the vehicle is being exposed to during operation. A study is performed to compare multiple inverse methods for their applicability to identify fluid pressure loading of a slender conical structure subject to hypersonic flow in a wind tunnel. The wind tunnel experiment consists of a structure with an instrumented, thin, flexible panel that was excited by the turbulent boundary layer. The turbulent loading was measured using multiple types of pressure sensors. To limit the number of unknowns and ensure a well-posed problem, the first inverse method demonstrated consists of a random vibration analysis computed in the frequency domain and subject to a parameter-ized pressure distribution. The second approach is a generic force identification of pressures in the time domain. In the study, the influence of the methods on the characteristics of the identified pressures are discussed as well as the computational costs and implementational considerations.
Engineering designers are responsible for designing parts, components, and systems that perform required functions in their intended field environment. To determine if their design will meet its requirements, the engineer must run a qualification test. For shock and vibration environments, the component or unit under test is connected to a shaker table or shock apparatus and is imparted with a load to simulate the mechanical stress from vibration. A difficulty in this approach is when the stresses in the unit under test cannot be generated by a fixed base boundary condition. A fixed base boundary condition is the approximate boundary condition when the unit under test is affixed to a stiff test fixture and shaker table. To aid in correcting for this error, a flexible fixture needs to be designed to account for the stresses that the unit under test will experience in the field. This paper will use topology optimization to design a test fixture that will minimize the difference between the mechanical impedance of the next level of assembly and the test fixture. The optimized fixture will be compared to the rigid fixture with respect to the test’s ability to produce the field stresses.
Structural dynamic testing is a common method for determining if the design of a component of a system will mechanically fail when deployed into its field environment. To satisfy the test's goal, the mechanical stresses must be replicated. Structural dynamic testing is commonly executed on a shaker table or a shock apparatus such as a drop table or a resonant plate. These apparatus impart a force or load on the component through a test fixture that connects the unit under test to the apparatus. Because the test fixture is directly connected to the unit under test, the fixture modifies the structural dynamics of the system, thus varying the locations and relative levels of stress on the unit under test. This may lead to a false positive or negative indication if the unit under test will fail in its field environment depending on the environment and the test fixture. This body of research utilizes topology optimization using the Plato software to design a test fixture that attaches to the unit under test that matches the dynamic impedance of the next level of assembly. The optimization's objective function is the difference between the field configuration and the laboratory configuration's frequency response functions. It was found that this objective function had many local minima and posed difficulties in converging to an acceptable solution. A case study is presented that uses this objective function and although the results are not perfect, they are quantifiably better than the current method of using a sufficiently stiff fixture.
Microstructural variabilities are among the predominant sources of uncertainty in structural performance and reliability. We seek to develop efficient algorithms for multiscale calcu- lations for polycrystalline alloys such as aluminum alloy 6061-T6 in environments where ductile fracture is the dominant failure mode. Our approach employs concurrent multiscale methods, but does not focus on their development. They are a necessary but not sufficient ingredient to multiscale reliability predictions. We have focused on how to efficiently use concurrent models for forward propagation because practical applications cannot include fine-scale details throughout the problem domain due to exorbitant computational demand. Our approach begins with a low-fidelity prediction at the engineering scale that is sub- sequently refined with multiscale simulation. The results presented in this report focus on plasticity and damage at the meso-scale, efforts to expedite Monte Carlo simulation with mi- crostructural considerations, modeling aspects regarding geometric representation of grains and second-phase particles, and contrasting algorithms for scale coupling.
Structural dynamics models with localized nonlinearities can be reduced using Hurty/Craig-Bampton component mode synthesis methods. The interior degrees-of-freedom of the linear subcomponents are reduced with a set of dynamic fixedinterface modes while the static constraint modes preserve the physical coordinates at which the nonlinear restoring forces are applied. For finite element models with a highly refined mesh at the boundary, a secondary modal analysis can be performed to reduce the interface down to a truncated set of local-level characteristic constraint modes. In this research, the cost savings and accuracy of the interface reduction technique are evaluated on a simple example problem involving two elastic blocks coming into contact.
The purpose of this document is to give further details on modeling than requested in the Excel sheets To analyze the results and draw any conclusion with respect to the vibration modeling purpose.