Sushanta Chakraborty

Development of a finite element model updating technique for estimation of constituent level elastic parameters of FRP plates

A non-destructive alternative to evaluate elastic properties of constituent materials.Development of a program for quantification of elastic property degradation of constituent materials.Stability of the program against random noise.Effectiveness in modal and spatial spasity. A methodology has been developed combining the experimental modal testing and finite element technique to determine the constituent level fiber and matrix elastic properties of fiber reinforced plastics (FRP) composite plates. The methodology implements an inverse eigensensitivity algorithm based on the minimization of error function defined by weighted difference of eigenvalues and mode-shapes. The method is robust against modal and coordinate sparsity, even under random noise. The proposed model updating technique can be deployed for rapid assessment of degrading elastic properties at the constituent level for existing FRP structures in-situ; thus is suitable for health monitoring exercise.

Identification of non-proportional viscous damping matrix of beams by finite element model updating:

A methodology has been proposed to estimate non-proportional viscous damping matrix of beams from measured complex eigendata using finite element model updating technique. Representation of damping through a proportional damping matrix ignoring the complexity of eigenvectors may not be appropriate when external damping devices are employed. The current literature of determination of non-proportional damping matrix demands measurement of a large number of complex modes which is extremely difficult in practice. A gradient based finite element model updating algorithm implementing inverse eigensensitivity method has been presented through a series of numerically simulated cantilever beams. The method can accurately predict the non-proportional damping matrix even if the measured eigenvectors are polluted with random noise. The novelty of the current method is that it can sustain a high level of modal and coordinate sparsity in measurement. The method assumes prior determination or updating of the mass and stiffness matrices.

A new gradient based step size controlled inverse eigen sensitivity algorithm for identification of material and boundary parameters of plates

The dynamic performance of any structure is function of existing material properties and boundary stiffness parameters which may deteriorate or become more flexible due to prolonged use. These parameters are estimated inversely through optimization of a suitable objective function. The gradient based optimization methods are preferred due to their faster convergence from a set of initial guess points, but suffers mostly from lack of reliable methodology to select appropriate step sizes. Arbitrary selection of step sizes may sometimes work well, depending upon the judgment of the user, but is case specific. The present work describes the estimation of existing material properties and boundary stiffness of isotropic and orthotropic plates from measured frequencies and mode shapes using a new gradient based step size controlled inverse eigensensitivity algorithm. The method takes a strategy that the step sizes automatically become smaller when the change in gradient of objective function is having a high value and similarly, takes larger steps when the gradient is remaining fairly constants in subsequent iterations. The results obtained from the investigations are encouraging, as some convergences could be achieved by this new adaptive step size control only, whereas methods adopting arbitrary or no step size control diverged.

Fibre Bragg grating based accelerometer with extended bandwidth

We have shown experimentally that the operable bandwidth of a fibre Bragg grating (FBG) based accelerometer can be extended significantly, without compromising its sensitivity, using a post-signal processing technique which involves frequency domain weighting. It has been demonstrated that using the above technique acceleration can be correctly interpreted even when the operating frequency encroaches on the region where the frequency response of the sensor is non-uniform. Two different excitation signals, which we often encounter in structural health monitoring applications, e.g. (i) a signal composed of multi-frequency components and (ii) a sinusoidal excitation with a frequency sweep, have been considered in our experiment. The results obtained have been compared with a piezo accelerometer.

Damage Detection in Beams Using Frequency Response Function Curvatures Near Resonating Frequencies

Structural damage detection from measured vibration responses has gain popularity among the research community for a long time. Damage is identified in structures as reduction of stiffness and is determined from its sensitivity towards the changes in modal properties such as frequency, mode shape or damping values with respect to the corresponding undamaged state. Damage can also be detected directly from observed changes in frequency response function (FRF) or its derivatives and has become popular in recent time. A damage detection algorithm based on FRF curvature is presented here which can identify both the existence of damage as well as the location of damage very easily. The novelty of the present method is that the curvatures of FRF at frequencies other than natural frequencies are used for detecting damage. This paper tries to identify the most effective zone of frequency ranges to determine the FRF curvature for identifying damages. A numerical example has been presented involving a beam in simply supported boundary condition to prove the concept. The effect of random noise on the damage detection using the present algorithm has been verified.

Determination of fracture parameters of FRP composites: A combined experimental and numerical investigation

Fibre-reinforced plastic (FRP) composites are used in weight-sensitive aerospace type of applications as well as in infrastructure where durability is of real concern. FRP structures are prone to fracture at interfaces or within the matrix which may not be visible from outside. Thus, a thorough knowledge of initiation and propagation of crack in FRP composites is necessary. The present investigation focuses on a combined experimental and numerical investigation to understand the fracture behavior of FRP properly. The finite element modeling of FRP plate type of specimen is done using ABAQUS to estimate the fracture parameters, such as fracture toughness, fracture energy, etc. The same parameters are also measured experimentally by three-point bending tests as per American Society for Testing and Materials (ASTM) standards. The mode-I fracture behavior seems to be predicted very closely, whereas the observed mode II behavior does not match closely and the reasons for discrepancies are explored.

Inverse determination of local variations of constituent level elastic parameters of FRP composite plates

ABSTRACT A novel technique for the determination of local variations in the constituent level elastic parameters of fibre reinforced plastics composite plates has been presented from experimentally measured natural frequencies and mode shapes or frequency response functions (FRF) using finite element model updating technique. The constituent level elastic parameters, i.e. the fibre and matrix stiffnesses for a ‘patch area’ having different elastic properties from the rest of the structure have been estimated quantitatively. The location and extent of this patch can be predicted a-priori from careful comparison of mode shapes. The methodology uses the correlations between the experimentally measured frequencies and mode shapes or FRFs and their corresponding values from finite element predictions of the ‘patch plate’. The objective function, consisting of the weighted differences in the undamped eigenvalues as well as the normalized mode shapes or FRFs of the two models, is minimized in the least square sense using a gradient-based inverse eigen-sensitivity method implemented through a computer program. The methodology has been demonstrated through a numerically simulated example first, followed by a real experimental case study. Results indicated that the efficiency and the robustness of the algorithm depend upon the accuracy of the acquired modal data.

Critical factors influencing the dynamic characteristics of fibre-reinforced plastic bridges — a parametric study

Conventional bridge materials have certain environmental factors limiting their service life. These factors include termite or fungi attack on timber, chemical attacks on concrete and corrosion for steel. These facts point to the necessity of development of new construction materials and related technologies for use in bridges exposed to aggressive environmental conditions. Fibre-reinforced plastics have long been used in weight-sensitive applications such as aerospace, marine, biomedical and sports industries due to their high specific stiffness and strength. Its superior structural performance depends primarily upon its anisotropic and heterogeneous characteristics and can be optimized to the requirement of the user. However, its infrastructural application is somewhat recent. Since fibre-reinforced plastic can be moulded to various shapes, is corrosion resistant, has lower lifecycle cost and durability, it can be used in constructing bridges or its parts. However, the fibre-reinforced plastic bridge industry lacks simplified and practical design guidelines and specifications. A box girder bridge has been taken from available literature as an example and parametric studies have been carried out using general purpose finite element software ABAQUS to investigate the critical factors affecting the dynamic responses of such a fibre-reinforced plastic bridge.