A recursive coupling-decoupling approach to improve experimental frequency based substructuring results

Substructure decoupling techniques allow identifying the dynamic behavior of a substructure starting from the dynamic behavior or the assembled system and a residual subsystem. Standard approaches rely on the knowledge of all FRFs at the interface DOFs between the two substructures. However, as these typically include also rotational DOFs which are extremely difficult and most of the time impossible to measure, several techniques have been investigated to overcome these limitations. A very attractive solution consists in defining mixed or pseudo interfaces, that allow to substitute unmeasurable coupling DOFs with internal DOFs on the residual substructure. Additionally, smoothing/denoising techniques have been proposed to reduce the detrimental effect of FRF noise and inconsistencies on the decoupling results. Starting from these results, some recent analysis on the possibility of combining coupling and decoupling FBS to validate the results and compensate for inconsistencies will be presented in this paper. The proposed method relies on errors introduced in the substructuring process when assuming that the interface behaves rigidly, while it is generally known that this assumption is seldom verified. Consequently, a recursive coupling-decoupling approach will be used to improve the estimation of the dynamic response of either the residual structure (for decoupling) or the assembly (for coupling). The method, validated on analytical data, will be here analyzed on a numerical example inspired by an experimental campaign used to validate the finite element models and on which standard substructuring techniques showed some limitations. The results discussed in this paper will be then used as guidelines to apply the proposed methodologies on experimental data in the future

Effects of Trade Cost on the Textile and Apparel Market: Evidence from Asian Countries

Global textile and apparel industry has since the 1950s been subjected to various forms of trade policy measures. Well noted among these are tariffs and non-tariff barriers (NTB)/policy indicators. Understanding the dynamics in such relevant policy indicators and the implications they yield for trade is a vital step toward informing relevant policy formulation and agribusiness investment decisions. With the textile and apparel industry being the primary grounds on which development in most Asian countries is founded, we for the first time in literature assess effects of various trade cost indicators on global textile and apparel imports from 37 Asian countries using a ‘cost-incorporated’ gravity model for the period 1988–2004. Estimates from this study affirm theory-based associations between trade, distance, cultural linkage, tariffs, and non-tariffs barriers. We however discovered quite interesting associations regarding effects of tariff increments and existence of NTB. Although both are primarily imposed/instilled to restrict trade flow, effect of tariff increments was consistently negative across all models, but that for NTB was consistently positive, although significant only in the case of apparel imports. Plausible reasons behind the implications for tariffs and NTB are elaborated on in this article. A keen discovery from this study, however, is that imports of apparels are more responsive than textile imports to dynamics in various trade-related cost, geographic and economic indicators

Using antiresonances to update finite element dynamic models

In this paper, an updating technique including antiresonances in the definition of the output residual is considered. Antiresonances are not a global system property, but are typical of each FRF, thus allowing to enlarge the residual vector with data identified from additional FRFs. However, antiresonance information is not independent of mode shape information, but it is rather an alternative, which is preferable for several reasons. In the implementation of the technique, matching problems arise whenever antiresonances identified from transfer FRFs are used: unlike in point FRFs, the distribution of antiresonances may be significantly altered by small changes in the structural model. Such problems may be circumvented by restricting the experimental database to point FRFs: in this case, very good results are obtained, but it is required to plan experimental testing differently from usual modal testing. To deal with transfer FRFs, matching between test and analysis FRFs at antiresonances can be sought through the FRAC (Frequency Response Assurance Criterion)

Identificazione parametrica di modelli dinamici agli elementi finiti mediante algoritmi genetici

The parametric identification/correction (updating) of dynamic finite element models is here tackled using genetic algorithms. The main advantage with respect to deterministic approaches is a lower sensitivity to measurement noise: this allows using less precise measurement techniques as well as to avoid sophisticated regularisation procedures. The goal of the formulation is to minimise an appropriate combination of the natural frequency error, the correlation error and the response residual. The method is tested by updating a three dimensional steel structure, fixed at one end, whose frequency response is previously measured

Natural frequency error versus response residual in dynamic model updating

Two different regularisation criteria for dynamic model updating are compared in this paper. Updating is performed by using a previously developed technique that solves the dynamic equilibrium equation in the least square sense, i.e. by minimising the force residual. However, the problem is ill-conditioned and sensitive to measurement errors: in order to control the error propagation, the use of some regularisation technique is required. The selected technique is based on the Truncated Singular Value Decomposition (TSVD). The two tested criteria use different methods for selecting the number of singular values to be truncated: in one case, the response residual is used; in the other case, the natural frequency error is chosen. The test structure is a steel frame with welded joints whose FRFs are determined using random excitation. Both techniques yield satisfactory results and can be even combined together to improve the robustness of the procedure

A comparison between the use of physical parameters and correction factors in dynamic model updating

The selection of quantities and/or variables that have to be corrected during the updating process is addressed in this paper. Among quantities, the major alternative is the choice between correction factors and physical parameters. The former represent scale factors used to adjust mass and stiffness submatrices of the analytical model, while the latter include parameters such as the elasticity modulus, mass density, geometrical dimensions, etc. Advantages and limitations in the process of updaring physical parameters instead of correction factors are highlighted: it can be shown that only a limited number of physical parameters can be simultaneously updated for each element. The two approaches are compared using a previously developed updating procedure to solve an experimental test case

Dynamic substructuring with a sliding contact interface

In this paper, the general framework for dynamic substructuring is extended to time-variant interfaces among connecting substructures. Specifically, a time-variant interface due to a sliding contact is considered. The contact can be without or with friction. The problem can be tackled in time domain using primal assembly and numerical time integration, and in time-frequency domain using dual assembly, thus obtaining a Time Dependent Frequency Response Function (TD-FRF) of the assembled structure. The method is applied to lumped parameter models of substructures, and, under some assumptions, it can be extended to simple finite element models

Experimental dynamic substructuring of the ampair wind turbine test bed

In a recent paper, the authors discussed the selection of a reduced set of interface DoFs in order to describe the coupling between the blades and the hub of the Ampair test bed wind turbine rotor. The study was conducted using simulated FRFs obtained from Finite Element model of the blades and the hub, but in view of using experimental FRFs. In this paper, test data measured on the turbine by the UW-Madison participants in the IMAC Focus Group on Experimental Dynamic Substructuring, and posted on the Wiki page of the group, are used for dynamic substructuring of the wind turbine test bed