Vibration, Buckling and Parametric Resonance Characteristics of Delaminated Composite Plates Subjected to In-plane Periodic Loading

摘要

The composite materials have significant applications over metallic materials in different fields of structural engineering. Structural elements subjected to in-plane periodic forces may lead to parametric or dynamic instability under certain combination of load parameters which caused resonant transverse vibrations. The spectrum of values of parameters causing unstable motion is referred to as the regions of dynamic instability or parametric resonance. Delamination is a very serious concern to composite applications and it arises as a consequence of impact loading, stress concentration near geometrical or material discontinuity or manufacturing defects. The study of dynamic stability itself requires a special investigation of basic problems of vibration and static stability. So the present investigation deals with the study of vibration, static and dynamic stability of delaminated composite plates. However, some studies on static analysis of delaminated composites involving the effects of different parameters on interlaminar shear strength (ILSS) are studied for completeness. The influence of various parameters on the free vibration and static stability (buckling) behavior of delaminated composite plates are investigated both experimentally and numerically. The parametric instability behaviour of delaminated composite plates is examined using finite element method. For numerical analysis, a finite element model is developed with an eight noded two dimensional quadratic isoparametric element having five degrees of freedom per node based on first order shear deformation theory (FSDT). Element elastic stiffness, geometric stiffness and mass matrices are derived using the principle of Stationery potential energy. A simple two dimensional single delamination model proposed earlier for vibration is extended in the present analysis for the vibration, static and dynamic stability analysis of delaminated composite panels under in-plane uniaxial periodic forces by multiple delamination modelling. A general formulation for parametric resonance characteristics of delaminated composite plates under in-plane periodic loading is presented. Experimental investigations are conducted for ILSS, vibration and buckling analysis of delaminated composite plates. Materials used for fabrication of laminates are woven roving glass fiber as reinforcement, epoxy as resin, hardener, polyvinyl alcohol as a releasing agent and Teflon foil for introduction of artificial delamination. Fiber and matrix are used in 50:50 proportion by weight. Material constants are determined from the tensile test as per relevant ASTM standards. The FFT analyzer B&Kñ3560 is used for modal testing of composite plates. To obtain the buckling result, INSTRON 1195 machine of 100 KN capacities is used. There is a very good agreement between numerical result and experimental result in case of natural frequency and critical buckling loads of woven fiber composite plates with delaminations. Both the results revealed that the fundamental natural frequency and critical buckling load of delaminated composite plates decrease with the increase in area of delaminations and fiber orientations. The instability studies showed a good agreement with the results available in the open literature. The onset of instability occurs at lower exciting frequency with the increase in delamination size and static load factor. It is also observed that with the increase in number of layers, aspect ratio and degree of orthotropy of delaminated plates, the dynamic instability occurs at higher excitation frequency. Thus the instability behavior of delaminated plates is influenced by the geometry, material, ply lay-up, ply orientation and size of delamination. This can be used to the advantage of tailoring during the design of delaminated composite structure. This study can be used as a tool for structural health monitoring for identification of delamination, its location and extent of damage in composites and helps in assessment of structural integrity of composite structures.