Analysis of Dynamic Fracture Parameters in Functionally Graded Material Plates with Cracks by Graded Finite Element Method and Virtual Crack Closure Technique

Based on the finite element software ABAQUS and graded element method, we developed a dummy node fracture element, wrote the user subroutines UMAT and UEL, and solved the energy release rate component of functionally graded material (FGM) plates with cracks. An interface element tailored for the virtual crack closure technique (VCCT) was applied. Fixed cracks and moving cracks under dynamic loads were simulated. The results were compared to other VCCT-based analyses. With the implementation of a crack speed function within the element, it can be easily expanded to the cases of varying crack velocities, without convergence difficulty for all cases. Neither singular element nor collapsed element was required. Therefore, due to its simplicity, the VCCT interface element is a potential tool for engineers to conduct dynamic fracture analysis in conjunction with commercial finite element analysis codes

A hybrid exact strip and finite element method for modelling damage in composite plates

Delamination is a frequent cause of failure in laminated structures, particularly under compressive loads. The presence of delaminations in composite laminates reduces their overall stiffness. In addition, delaminations tend to grow rapidly under postbuckling loads, causing further reductions in the structural strength and leading ultimately to a sudden structural failure. Recently, many studies have been carried out to investigate the effects of delaminations on the buckling and vibration behaviour of composite structures. Finite element analysis is often used to perform these due to its ability to model complex geometries, loading and boundary conditions, but this comes at a high computational cost. The exact strip method provides an efficient alternative approach using an exact dynamic stiffness matrix based on a continuous distribution of stiffness and mass over the structure, so avoiding the discretization to nodal points that is implicit in finite element analysis. However due to its prismatic requirements, the exact strip method can model damaged plates directly only if the damaged region extends along the whole length of the plate. This thesis introduces a novel combination of exact strip and finite element analysis which can be used to model more complex cases of damaged plates. Comparisons with pure finite element analysis and a previous technique based on the exact strip method demonstrate the capability and efficiency of this hybrid method for a range of isotropic and composite plates

Shape Memory Alloy Applications on Control of Thermal Buckling, Panel Flutter and Random Vibration of Composite Panels

Shape Memory Alloy (SMA) has a unique ability to recover large prestrain (up to 8∼10% elongation for Nitinol, a typical SMA material) completely when the alloy is heated (e.g. aerodynamic heating) above the austenite finish temperature Af. An innovative concept is to utilize the large recovery stress by embedding the prestrained SMA in a traditional fiber-reinforced laminated composite plate, which is called SMA hybrid composite (SMAHC) plate. In this research, static thermal and aerothermal deflections, dynamic panel flutter and random response are investigated for traditional composite plates and SMAHC plates under combined aerodynamic, random and thermal loads by employing nonlinear finite element method. System equations are derived and based on classical laminated plate theory, von Karman nonlinear strain-displacement relation, first-order piston theory aerodynamics and quasi-steady thermal stress theory. Newton-Raphson iterative method is adopted for solving the static thermal and aerothermal buckling deflections. Both normal modes and new proposed aeroelastic modes are employed separately in solution procedures to transform the equations of motion in structural node degree-of-freedom (DOF) into modal equations of motion. Time domain numerical integration technique is adopted for the dynamic analysis under the combined aerodynamic, random and thermal loads. Numerical results of isotropic, traditional composite plates and SMAHC plates are determined, compared and discussed. Various plate behaviors are studied in detail. It is demonstrated that SMAHC plates can greatly suppress or reduce thermal buckling and panel flutter as compared with the traditional composite plates. While the SMAHC plates exhibit better performance at low levels of acoustic excitations, however, the SMAHC plates do not effectively suppress random response at high levels of acoustic excitations

A hybrid method for modelling damage in composites and its effect on natural frequency

Delamination is a frequent cause of failure in laminated structures, reducing their overall stiffness and hence their critical buckling loads. Delaminations tend to grow rapidly in postbuckling, causing further reductions in structural strength and leading ultimately to sudden structural failure. Many studies have investigated the effects of delaminations on buckling and vibration of composite structures. Finite element analysis is often used to model complex geometries, loading and boundary conditions, but incurs a high computational cost. The exact strip method provides an efficient alternative approach using an exact dynamic stiffness matrix based on a continuous distribution of stiffness and mass over the structure, so avoiding the implicit discretization to nodal points in finite element analysis. However due to its prismatic requirements, this method can model damaged plates directly only if the damaged region extends along the whole length of the plate. This paper introduces a novel combination of exact strip and finite element analysis to model more complex cases of damaged plates. Comparisons with pure finite element analysis and a previous smearing method demonstrate the capability and efficiency of this hybrid method for a range of isotropic and composite plates. The effect of damage on the lowest natural frequency is studied

Finite element simulation of plates under non-uniform blast loads using a point-load method: Buried explosives

There are two primary challenges associated with assessing the adequacy of a protective structure to resist explosive events: firstly the spatial variation of load acting on a target must be predicted to a sufficient level of accuracy; secondly, the response of the target to this load must also be quantified. When a high explosive is shallowly buried in soil, the added confinement given by the geotechnical material results in a blast which is predominantly directed vertically. This imparts an extremely high magnitude, spatially non-uniform load on the target structure. A recently commissioned experimental rig designed by the authors has enabled direct measurements of the blast load resulting from buried explosive events. These direct measurements have been processed using an in-house interpolation routine which evaluates the load acting over a regular grid of points. These loads can then be applied as the nodal-point loads in a finite element model. This paper presents results from a series of experiments where a free-flying plate was suspended above a shallow buried explosive. Dynamic and residual deformations are compared with finite element simulations of plates using the experimentally recorded, and interpolated, nodal point-loads. The results show very good agreement and highlight the use of this method for evaluating the efficacy of targets subjected to non-uniform blast loads

Buckling and vibration analysis of laminated composite plate/shell structures via a smoothed quadrilateral flat shell element with in-plane rotations

This paper presents buckling and free vibration analysis of composite plate/shell structures of various shapes, modulus ratios, span-to-thickness ratios, boundary conditions and lay-up sequences via a novel smoothed quadrilateral flat element. The element is developed by incorporating a strain smoothing technique into a flat shell approach. As a result, the evaluation of membrane, bending and geometric stiffness matrices are based on integration along the boundary of smoothing elements, which leads to accurate numerical solutions even with badly-shaped elements. Numerical examples and comparison with other existing solutions show that the present element is efficient, accurate and free of locking

Metallic tube type energy absorbers: a synopsis

This paper presents an overview of energy absorbers in the form of tubes in which the material used is predominantly mild steel and/or aluminium. A brief summary is also made of frusta type energy absorbers. The common modes of deformation such as lateral and axial compression, indentation and inversion are reviewed. Theoretical, numerical and experimental methods which help to understand the behaviour of such devices under various loading conditions are outlined. Although other forms of energy absorbing materials and structures exist such as composites and honeycombs, this is deemed outside the scope of this review. However, a brief description will be given on these materials. It is hoped that this work will provide a useful platform for researchers and design engineers to gain a useful insight into the progress made over the last few decades in the field of tube type energy absorbers

Dynamic behavior of rectangular plates and cylindrical shells

Numerical integration technique for determining dynamic response of rectangular plates and cylindrical shells subjected to stationary and constant velocity load