Optimisation of stacking sequence of fibre reinforced plastic laminated composite structures subjected to buckling

The objective of the present work is to develop mathematical/finite element based optimization techniques for fibre reinforced polymer (FRP) laminated composite structures subjected to buckling. Many issues arise when a laminated FRP composite structure subjected to compressive load and ultimately fail under buckling. Issues such as understanding FRP composite materials, buckling and post-buckling behaviour of the structure, delamination and detection of the crack front need attention. Hence, in the present research works were carried out in each of these areas. Various experimental studies were carried out to study material characterisation, the delamination fracture toughness in mode I, mode II and mixed-mode l/ll using DCB, ENF and MMB tests and buckling of FRP composite plates. From these series of test GIC, GIIC and fracture envelope under different mode mixity ratio were determined. Also the buckling tests of the plates with optimum and non-optimum stacking sequence were performed to verify the optimisation results. The effect of damage on buckling load was studied by tests on buckling of plates with pre-existing centrally located delamination patch at the plate mid- plane and on plate with a hole at the centre of the plate to investigate the effect of cut-out and damage on buckling load. Finally, IR thermography and CT-Scan non-destructive tests (NDT) were used for plates with pre-existing centrally located delamination patch to study the direction and the extent of delamination crack propagation after the buckling tests. In the case of plates with pre-existing centrally located delamination patch with diameter less than 32mm, the critical buckling load has not changed. But when the delamination patch diameter reached to 48mm (at around 60% of plate width), there was significant reduction in the critical buckling load. in the case for plate with cut-out a noticeable reduction on the critical buckling load was observed when the diameter of the hole was more than 25% of the plate width. IR thermography and CT-Scan images analysis of the plates after buckling tests showed that in plates with pre-existing centrally located delamination patch with a diameter of D=16 mm, the plate failure occurs near the loading edge. In the case of plates with delamination patch of D=32mm, some plates failed near the loading edge and in some plates crack propagated along the i45° fibre direction around delamination patch. However, for plates with delamination patch with diameter of D=48mm, in all samples the delaminated area propagated along the fibre direction around the delaminated area and no failure observed near the loading edge. Inherent to the use of FRP composite materials is the inclusion of ply angles and stacking sequence as design variables. These design variables are discrete in nature. The optimization of these models is typically difficult due to their combinatorial nature and potential existence of multiple local minima in the search space. In this research bottom-up enumeration search optimisation approach was developed for optimum design of stacking sequence of laminated composite structure for maximum critical buckling load above the required target load using MATLAB software. The optimised results were verified by buckling experiments and FE simulations. The developed programme is flexible to use for other loading condition. For the case of uniaxial compressive loading with preselected target buckling load, the optimum number of layers and orientation for 0/90 biaxial fabrics and unidirectional plies were determined. The percentage of difference between analytical buckling load and FEA eigen solution with experiments are about -13.1% and -3.2%, respectively. Depending on the properties and arrangement of the skin and stiffener, different buckling modes and failure loads can occur in a stiffened plate. For shape optimisation of blade- stiffened plate subjected to buckling, Sequential Quadratic Programming (SQP), Genetic Algorithms (GA) and Simulated Annealing (SA) techniques were used in MATLAB optimisation programme in conjunction with ANSYS finite element software. The developed techniques are tested for minimum weight of a blade-stiffened plated with predefined stacking sequence of stiffener and the plate where the geometry parameters were design variables. In this work, the size of the stiffener height and the distance between the stiffener for a required target buckling load and optimum weight were determined.