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Composite laminates are increasingly being used in more complex structural applications where edges and cut outs are inevitable. These applications include wing skins of military and civil aircraft, further aerospace applications as well as automotive panels and critical structures. Composite components in such applications are highly susceptible to damage. Composites behave in a different manner to conventional metallic materials, which has introduced several design problems not previously encountered. One such problem has been the susceptibility of the material to accidental low energy impacts which frequently leave no visible mark on the impacted surface but considerable internal damage. Investigation of the residual strength and stiffness of composites after edge impact has become important for the design of aerospace components. Previously, the research work involved central impact of composite laminates but in this research we are investigating edge impact behaviour of composite laminates as parts of composite structures are particularly vulnerable to impacts, including near the edge of an inspection port or other aperture. Furthermore, impacts to such areas may lead to more severe damage near the edge of the laminate rather than the surface. Thus the present work extends these investigations to impact on the edge of composite laminates. The thesis includes both experimental investigations and finite element simulations of impact damage on the plane of the laminate near the edge (near-edge), and on the edge (on-edge) of composite laminates. A comparison with centre impact with on and near-edge impact is done to understand the damage on the edges and away from the edges. A new design has been developed and implemented to perform edge impact experiments. The research investigated the effects of various parameters like thickness, absorbed energies, force-time histories and damage behaviour of composite laminate. The damage size and mechanisms have been explored. Impact simulation was carried out using finite element code Abaqus. Explicit solution technique of the code was used to analyse the edge impact phenomenon. Results of the finite element analysis were compared with experiments. The residual strength of the laminates under compressive and tensile loading has been measured. Tensions after impact (TAI) tests were conducted to evaluate the residual load carrying capacity. The effect of edge impact on the low velocity impact response and the residual tensile strength is discussed via the test results. This thesis also includes computed tomography as the main technique for micro level damage characterisation and investigates the study of damage mechanisms of glass/epoxy laminates subjected to edge impact with varying energy levels and thickness. Computed Tomography aims to provide damage behaviour such as internal damage state, delaminations during different types of edge impact.

Asymmetric composite laminates can have a bistable response to loading. The potentially large structural deformations which can be achieved during snap-through from one stable state to another with small and removable energy input make them of interest for a wide range of engineering applications. After 30 years of research effort the shapes and response to applied loads of laminates of general layup can be quantitatively predicted. With attention switching to the incorporation of bistable laminates for practical applications, tools for the design and optimisation of actuated bistable devices are desirable. This thesis describes the analytical and experimental studies undertaken to develop novel modelling and optimisation techniques for the design of actuated asymmetric bistable laminates. These structures are investigated for practical application to morphing structures and the developing technology of piezoelectric energy harvesting. Existing analytical models are limited by the need for a numerical solver to determine stable laminate shapes. As the problem has multiple equilibria, convergence to the desired solution cannot be guaranteed and multiple initial guesses are required to identify all possible solutions. The approach developed in this work allows the efficient and reliable prediction of the stable shapes of laminates with off-axis ply orient at ions in a closed form manner. This model is validated against experimental data and finite element predictions, with an extensive sensitivity study presented to demonstrate the effect of uncertainty and imperfections in the laminate composition. This closed-form solution enables detailed optimisation studies to tailor the design of bistable devices for a range of applications. The first study considers tailoring of the directional stiffness properties of bistable laminates to provide resistance to externally applied loads while allowing low energy actuation. The optimisation formulation is constrained to guarantee bistability and to ensure a useful level of deformation. It is demonstrated that 'cross-symmetric' layups can provide stiffness in an arbitrary loading direction which is five times greater than in a chosen actuation direction.

In this research, through-transmission ultrasonic (TTU) Acoustography was applied to measure and quantify porosity levels in carbon fiber reinforced polymer (CFRP) composite laminates. This study employed several CFRP specimens with wide ranges of porosity prepared by altering the curing pressure during the manufacturing process. The Acoustography method, operating at 5 MHz, was able to show contrast in ultrasonic images obtained for composite laminates with varied porosity levels. Porosity levels in composite laminates were quantified using destructive methods: acid digestion and microscopy. Also, strength analysis tests were conducted to investigate the effect porosity has on the laminate’s structural integrity. From the results obtained, it was demonstrated that the mechanical properties, interlaminar shear strength (ILSS), and flexural strength of CFRP decreased with the increasing void content. In addition, Acoustography absorption coefficient measurements were related to varied porosity levels in the composite laminates. As the porosity content increased within the laminates, the acoustic absorption coefficient increased. These findings are significant because Acoustography is being developed as a faster alternative to traditional ultrasonic inspection of composites and porosity is an important anomaly to quantify utilizing NDE methods.

Efficient and optimal use of composites in structures requires tools to monitor and capture the complex degradation that can occur within the laminates over time. Structural health monitoring (SHM) techniques uses sensors/actuators on the structure to progressively monitor the health of the structure with minimal manual intervention. Electrical tomography (ET) is a SHM technique that uses voltage measurements from the surface of the laminate to reconstruct a conductivity map of the structure. Since damage has been shown to modify the conductivity of the laminate, the conductivity map can provide an indirect measure of the damage within the material. Studies have shown the capability of ET to identify macroscale damage due to impact. But, little has been done to quantitatively assess damage using ET. In this work, we present a theoretical framework to link degradation mechanisms occuring at the microscale to the conductivity at the mesoscale through damage indicators. The mesoscale damage indicators are then shown to be intrinsic to the ply. Next, we use the knowledge obtained through mesoscale homogenization to study the detectability of transverse cracks. Last, we show how the mesoscale homogenization participates in regularization of the inverse problem and in the quantitative assessment of the reconstructed conductivity map. This is as such the first step towards turning ET into a viable quantitative health monitoring technique.

The characteristics of carbon fiber composites have enabled these materials to be accepted as replacements for metal parts in industry. However, due to their unsymmetrical material properties, carbon fiber composites are susceptible to damage, such as a delamination, which can cause premature failure in the structure. This has resulted in the need for nondestructive testing methods that can provide quick, reliable results so that these parts can be tested while in service. In this study, an approach was examined that involved a pencil lead break to excite multiple wave modes in a composite plate in an effort to identify key characteristics based on the wavespeed and frequency. These characteristics were then compared to models based on boundary conditions to generate dispersion curves using the transfer matrix method for whole composite plates that were either undamaged or damaged. To first test this approach, experiments were performed on multilayer isotropic plates and then on a composite plate. The results for all cases showed that modes could be excited by the pencil lead break in the undamaged region of the plates that were not theoretical possible in a delaminated region. Also modes that were specific to the delaminated region were excited and this allowed for a clear comparison between the two regions. This approach could be placed into practice to provide routine testing to detect delamination for in-service, carbon fiber composite parts. / Master of Science / The physical properties of high strength and low weight and the economic benefits of carbon fiber composites has resulted in these materials replacing metals in several industries. It is important, however, to be aware that the change in materials used impacts the different types of damage composites experience compared to conventional metals. One type of damage that could cause a composite part to fail is a delamination or a separation of layers. In order to identify if this damage has occurred, it is beneficial to have an inspection technique that will not damage the part. In this study, a technique was tested that involved breaking a piece of pencil lead on a plate in order to generate multiple wave modes that would propagate in the plate. Based on boundary conditions caused by the damage in the plate, the speed of the wave and frequency content could be compared to an undamaged plate to identify a delamination. A model was created to compare experimental results and demonstrated that using wavespeed and frequency could identify a delamination. The experimental results compared well with the model dispersion curves for a plate with and without a delamination suggesting this approach could be placed into practice to provide routine testing to detect delamination for in-service, carbon fiber composite parts.

A major concern affecting the efficient use of composite laminates in aerospace industry is the lack of understanding of the effect of low-velocity impact (LVI) damage on the structural integrity. This project aims to develop further knowledge of the response and damage mechanisms of composite laminates under LVI, and to explore the feasibility of assessing the internal impact damage with a visually inspectable parameter. The response and damage mechanisms of composite laminates under LVI have been investigated experimentally and numerically in this project. Various parameters including the laminates thickness, lay-up configuration, repeated impact, and curing temperature have been examined. The concept and the phenomena of delamination threshold load (DTL) have been assessed in details. It was found that DTL exists for composite laminates, but the determination of the DTL value is not straightforward. There is a suitable value of range between the impact energy and the laminates stiffness/thickness, if the sudden load drop phenomenon in the impact force history is used to detect the DTL value. It is suggested that the potential menace of the delamination initiation may be overestimated. The composite laminates tested in this project demonstrate good damage tolerance capacity due to the additional energy absorption mechanism following the delamination initiation. As a result, the current design philosophy for laminated composite structure might be too conservative and should be reassessed to improve the efficiency further. To explore the feasibility of linking the internal damage to a visually inspectable parameter, quasi-static indentation (QSI) tests have been carried out. The dent depth, as a visually inspectable parameter, has been carefully monitored and assessed in relation to the damage status of the composite laminates. It is proposed that the damage process of composite laminates can be divided into different phases based on the difference in the increasing rate of dent depth. Moreover, the internal damage has been examined under the optical microscope (OM) and the scanning electron microscope (SEM). Residual compressive strength of the damaged specimen has been measured using the compression-after-impact (CAI) test. The results further confirm the findings with regard to the overestimated potential menace of the delamination initiation and the proposed damage process assumption. The proposed damage process assumption has great potential to improve the efficiency and accuracy of both the analytical prediction and the structural health monitoring for damages in composite laminates under low-velocity impact.

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The aim of this thesis is to investigate Ti/APC-2 nanocomposite laminates mechanical properties after low velocity impact. The finite element analysis with software ANSYS/LS-DYNA is used to analyze the size of damage and plastic zone and internal energy of laminates during low velocity impact. Finally, the numerical results and experimental data are in good agreement. The work can be divided into two parts: the first is to fabricate the hybrid composite laminates and place the samples on the floor, subjected to the free drop of a rigid steel ball of 1m and 2m high. Then, the samples after impact were due to static tensile and fatigue tests to obtain mechanical properties. Using the optical microscopy the impact defects of laminate surface were measured. The second, ANSYS/LS-DYNA was used to simulate a laminate impacted by a steel ball. The energy change of steel ball impact and internal energy of laminates during impact were also discussed. From the experimental data, the mechanical properties, such as ultimate strength and stiffness, of virgin samples are better than those of impacted samples due to free drop. In addition, no matter the laminates were added nanoparticles SiO2 or not, the strength of laminates reduces after impact, however, the fatigue resistance of impacted samples does not lose much. Compare with the data of penetration depth and plastic zone due to free drop. The errors of numerical results are 5.4%~12.4% for the penetration depth and the errors 5.21%~8.98% for plastic zone respectively. That is acceptable. The numerical method ology provides a reference to realize the energy change in laminates after impact. Also, from the experimental measurement it is obvious to see damage area after impact and the mechanical properties do not reduce significantly due to low velocity impact generally in Ti/APC-2 composite laminates.

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