Krishna Shankar

Vibration-based delamination detection in composite beams through frequency changes

Delamination is a common damage in fibre reinforced composite laminates, usually hidden from external view, that can substantially reduce the structural stiffness which changes the dynamic response of the structures such as natural frequencies. Natural frequencies are the most reliable parameters for detecting damage while they do not directly provide information regarding its location and severity. To determine the location and severity of damage, it is necessary to solve the inverse problem using frequency shifts in multiple modes. In this paper, the graphical approach, which was previously employed for estimating two variables of crack (location and size) in isotropic beams, is extended in the current work to estimate the three variables of delamination (interface, span-wise location and size) in anisotropic composite beams from measured frequency shifts. Compared to the use of optimisation or neural network for detection, graphical technique is computationally inexpensive and quick since it solves the inverse problem without iterations or network training. The present approach has been validated using numerical simulation as well as experimental data from modal testing conducted on quasi-isotropic simply supported and cantilever beams. Results show that the proposed graphical technique can be used to assess the location and severity of delamination in composite beams with a high degree of accuracy.

Validation of algorithms for delamination detection in composite structures using experimental data

This research work deals with aspects concerned with delamination detection in composite structures as revealed by an approach based on vibration measurements. Variations in vibration characteristics generated in composite laminates indicate the existence of delaminations because degradation due to delamination causes reduction in flexural stiffness and strength of the material and as a result vibration parameters like natural frequency responses are changed. Hence, it is possible to monitor the variation in natural frequencies to identify the presence of delamination, and assess its size and location for online structural health monitoring (SHM). The approach to this paper, therefore, typically depends on undertaking the analysis of structural models implemented by finite element analysis (FEA). The numerical solutions using FE models known as the simulator computes the natural frequencies for the delaminated and undelaminated specimens of composite laminates. However, these FE models are computationally expensive, and surrogate (approximation) models are introduced to curtail the computational expense. The simulator is employed to solve the inverse problem using algorithms based on computational intelligence concepts. An artificial neural network (ANN) model is developed to also solve the inverse problem for delamination detection directly and to provide surrogate models integrated with optimization algorithms (the gradient-based local search and non-dominated sorting genetic algorithm-II) to contain the computationally expensive simulations by FEA. This approach is termed as surrogate assisted optimization and it is seen that the engagement of surrogate models in lieu of the FE models in the optimization loop greatly enhances the accuracy of delamination detection results within an affordable computational cost and provides control over handling different variables. Meanwhile, to aid with the building of effective surrogate models using substantial number of training datasets, K-means clustering algorithm is harnessed and this effectively reduces the large training datasets usually required for ANN training. This paper demonstrated that ANN and optimization algorithms with surrogates show immense potentialities for use in delamination damage detection scenarios. Prediction errors of the algorithms were quantified and they were shown to be satisfactory when applied to previously experimental data. The algorithms in their inverse formulations are capable of predicting accurately delamination parameters. Hence, these algorithms should be employed for application in the domain of SHM where their small computational requirements could be exploited for online damage detection.

Experimental, Theoretical and Numerical Investigation of the Flexural Behaviour of the Composite Sandwich Panels with PVC Foam Core

This study presents the main results of an experimental, theoretical and numerical investigation on the flexural behaviour and failure mode of composite sandwich panels primarily developed for marine applications. The face sheets of the sandwich panels are made up of glass fibre reinforced polymer (GFRP), while polyvinylchloride (PVC) foam was used as core material. Four-point bending test was carried out to investigate the flexural behaviour of the sandwich panel under quasi static load. The finite element (FE) analysis taking into account the cohesive nature of the skin-core interaction as well as the geometry and materials nonlinearity was performed, while a classical beam theory was used to estimate the flexural response. Although the FE results accurately represented the initial and post yield flexural response, the theoretical one restricted to the initial response of the sandwich panel due to the linearity assumptions. Core shear failure associate with skin-core debonding close to the loading points was the dominant failure mode observed experimentally and validated numerically and theoretically.

Active control of swept shock wave/turbulent boundary-layer interactions

We look at active control of the swept shock wave/turbulent boundary-layer interaction using smart flap actuators. The actuators are manufactured by bonding piezoelectric material to an inert substrate to control the bleed/suction rate through a plenum chamber. The cavity provides communication of signals across the shock, allowing rapid thickening of the boundary-layer approaching the shock, which splits into a series of weaker shocks forming a lambda shock foot, reducing wave drag. Active control allows optimum control of the interaction, as it would be capable of positioning the control region around the original shock position and control the rate of mass transfer. The actuators are modelled using classical composite material mechanics theory, namely analysis of a laminate with a uniform cross section. Furthermore a finite element modelling program (ANSYS 5.7) was used to produce improved results

Surrogate-assisted optimisation design of composite riser

The risers made out of advanced composite materials would be significantly lighter than their metallic counterparts. In addition, they would also offer better fatigue and corrosion resistance. Their use can lead to increased production capacities, reduced operational costs of existing offshore platforms and the possibility of extraction of oil and gas from greater depths. The use of advanced composite materials also offers the possibility of tailoring composite riser’s design to meet specific load requirements. In the first phase of this research, the optimisation of the parameters of the composite tubing was attempted through a process of parametric analysis, manual inspection and selection. In the current paper, the optimisation of the composite riser for minimum weight is accomplished through implementation of a more rigorous mathematical optimisation technique. A population-based surrogate-assisted evolutionary algorithm is employed to identify the optimum design. The objective is to achieve the design with minimum structural weight while satisfying critical load cases, both local and global. The approach and the results of the design optimisation of the composite riser are presented. The optimal design results are verified using finite element analysis under local and global design requirements and compared with different design methods.

In-plane shear behaviour of composite sandwich panel incorporated with shear keys methodology at different orientations: finite element study

The effect of introducing semi-circular shear keys in at the skin-core interface of the composite sandwich panels is illustrated numerically in the current study using ABAQUS software. Particularly, the effect of the shear keys orientation (shear grid), namely ±15°, ±30°, ±45°, ±60° and 90/0°, on the shear response of the sandwich panel is introduced. Composite sandwich panels consisting of polyurethane foam core sandwiched between stiff glass fibre reinforced polymer skins were used, while chopped strand glass fibre impregnated with epoxy resin was utilized for the keys. The nonlinear finite element model was built to capture the shear response and the damage mode of the sandwich panel with the shear grid. The finite element model nominated the model with ±60° grid orientation to be the most sustainable one among the other investigated models. In comparison to the model with shear keys in one direction (uni-axial model), the model with shear grid showed a significant reduction in the shear strength.

Tensile Behaviour of Nano-Particulate Reinforced Aluminium Matrix Composites at Elevated Temperatures

The tensile behaviour of nanometric SiC particulate (SiCp) reinforced aluminium matrix composites (AMCs) was examined at room temperature, 215°C and 350°C. These AMCs were produced via powder metallurgy (P/M) using Al 7075 powder reinforced with different volume fractions (1 vol.%, 3 vol.% and 5 vol.%) of nano-SiCp. The experimental results exhibit that at room temperature un-reinforced Al has both maximum strength and ductility whereas the 5 vol.% SiCp/Al composite has only maximum stiffness. Similar trends were obtained for tests performed at 215°C. However at 350°C, the 1 vol.% SiCp/Al composite has the highest stiffness. Optical microscopy and scanning electron microscopy were performed for microstructure study, examination of the SiCp distribution in the Al matrix and fractography.

Normal shock wave/turbulent boundary-layer interaction control using ‘smart’ piezoelectric actuators

This paper looks at active control of the normal shock wave/turbulent boundary layer interaction (SBLI) using smart flap actuators. The actuators are manufactured by bonding piezoelectric material to an inert substrate to control the bleed/suction rate through a plenum chamber. The cavity allows rapid thickening of the boundary-layer approaching the shock, which splits into a series of weaker shocks forming a lambda shock foot, thus reducing wave drag. Active control allows optimisation of the interaction, as it would be capable of either positioning the control region around the original shock position using a series of unimorph flaps or fixing the shock position by controlling the rate of mass transfer. The level of control achieved by unimorph piezoelectric actuators is not large because of small amounts of deflection possible. It is believed that to provide optimal control a piezoelectric material, which can provide greater strain and hence higher amounts of deflection is needed. However, currently such a piezoelectric material is not commercially available.

A review of the design and analysis of reinforced thermoplastic pipes for offshore applications

The development and recent applications of reinforced thermoplastic pipes for offshore oil and gas applications are reviewed. The design and materials of reinforced thermoplastic pipes are presented. Reinforced thermoplastic pipes have been increasingly accepted as an important alternative to traditional metallic offshore pipes due to their distinct advantages such as a higher stiffness to weight ratio, improved fatigue resistance and better corrosion resistance. Their potential applications can be extended to deep-water risers. Loading conditions which could be experienced by them for offshore applications are described. Existent studies and analyses of offshore pipes under these loading conditions are discussed. Based on this discussion, this article outlines the limitations of the current studies of reinforced thermoplastic pipes and future work to improve the analysis and design of reinforced thermoplastic pipes is recommended.

Agent based damaged detection in composite laminates

In this paper, we present undamaged agent based, an effective damage identification technique along with Eigen-system Realization Algorithm (ERA) for damage assessment in laminated composite structures. Unlike the use of frequency measurements, this technique has the potential to readily pinpoint the location and the extent of damage, in addition to alerting to its occurrence. The proposed technique is implemented through numerical simulation and experimentation on laminated beams with and without delaminations. For numerical simulation, a Sate-space based finite element system model (SS-FESM) is employed to generate transient impulse response signals. In this study, we employ ERA for system identification of fibre reinforced composite laminates to identify the presence of delaminations and assess their location and severity. The impulse response and the input signals are feed into ERA to estimate the normalized dynamic properties, namely the stiffness and damping coefficients of all elements in the structure. Variations in the dynamic properties readily identify the presence of damage, however, the location and severity of the damage is assessed or determined through undamaged agent based iterating technique. The numerical simulation of 8 ply quasi-isotropic Carbon Fiber Reinforced Polymer (CFRP) laminated beams demonstrate the delaminations as small as one percent of the beam length in size can be identified and assessed using the present method. Experimental simulations indicate that noise in the measured impulse signals need to be eliminated for successful practical implementation of the technique.