Estimation of Elastic Parameters of Sandwich Composite Plates Using a Gradient Based Finite Element Model Updating Approach

The dynamic behavior of sandwich composite structures needs to be predicted as accurately as possible for ensuring safety and serviceability. A properly converged finite element model can accurately predict such behavior, if the current material properties are determined within very close ranges to their actual values. The initial nominal values of material properties are guessed from established standards or from manufacturer’s data, followed by verification through quasi-static characterization tests of extracted samples. Such structures can be modal tested to determine the dynamic responses very accurately, as and when required. A mathematically well posed inverse problem can thus be formulated to inversely update the material parameters accurately from initial guesses through finite element model updating procedures. Such exercise can be conveniently used for condition assessment and health monitoring of sandwich composite structures. The method is capable of determining the degradation of material properties, hence suitable for damage detection. The in-plane as well as out-of-plane elastic moduli can be determined to predict the actual responses which can be verified by physical measurement. In the present investigation, the in-plane and out-of-plane elastic parameters of the face sheets made of glass fiber reinforced plastics, i.e. E₁, E₂, G₁₂, G₁₃, G₂₃ of the face sheet and the Young’s modulus (E) of the core of a sandwich composite plate has been determined inversely from available modal responses. The method is based on the correlation between the dynamic responses as predicted using finite element model and those measured from modal testing to form the objective function, sensitive enough to the in-plane and out-of-plane material constants. A gradient based Inverse Eigensensivity Method (IEM) has been implemented to identify these material parameters of a rectangular sandwich composite plate from natural frequencies. It may be noted that the initial characterization test data may not be useful in predicting accurate dynamic responses of existing degraded sandwich structures, if the material constants have changed substantially. Destructive characterization test on existing structure is mostly not permitted as samples need to be extracted which may damage the otherwise intact structure.