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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.

This thesis details an investigation into the properties and applications of shape memory alloy (SMA) composites. SMA-composites are a new material which have the possibility of having a large impact on what the structures as we know today, are constructed with. SMA-composites are adaptive materials which can be used to control the shape and frequencies of vibration of a structure. In order to determine the effectiveness of such a material, research into the functional properties of SMAs and SMA-composites was conducted. As an initial step, the transformation behaviour of constrained SMAs was investigated in order to obtain a better understanding into the recovery stress generation of these wires when embedded into a composite material. It is known that the transformation is based on two types of martensite within the alloy; self accommodating and preferentially oriented martensite. The amounts of each type and how they vary with differing pre-strain were determined through DSC measurements and an explanation for why preferentially oriented martensite is not observed during DSC testing was made. The next step was to investigate the effectiveness of embedding SMA wires into composites and the thermomechanical properties of the SMA wires and the SMA-composites were determined. This was completed using a specially designed tensile testing machine which was capable of having the whole specimen immersed into an oil bath and heated and cooled repeatedly. The stress-strain, strain-temperature, stress-temperature, resistance-strain and cyclic properties of various wires were obtained, giving a better understanding of the behaviour of SMA wires under different test conditions. NiTiCu SMA wires were embedded into kevlar composite materials and the recovery stress generation (stress-temperature), stress-strain, and strain-temperature behaviour was determined. If SMA-composites are to be used as new materials for structural applications, verification that the embedment of pre-strained SMA wires into the material doesn't adversely affect the impact behaviour needs to be carried out. SMA-composite specimens with varying volume fractions of superelastic SMA wires, pre-strain and position through the thickness were made up for impact damage characterisation. These specimens were impacted at three different energy levels. The results showed that by embedding SMA wires into composite materials there is a reasonably low damage accumulation after impact. There is also no adverse impact effect on the structure compared with structures without wires as well as structures with other types of wires such as steel and martensitic SMA wires. The SMA-composites showed good damping and energy absorption capabilities. A novel application of SMA-composites is their use as a SMA patch in order to repair damage in existing cracked metallic structures. An analytical study and finite element modelling showing the closure stresses obtainable for use as patches was made.

This thesis details an investigation into the properties and applications of shape memory alloy (SMA) composites. SMA-composites are a new material which have the possibility of having a large impact on what the structures as we know today, are constructed with. SMA-composites are adaptive materials which can be used to control the shape and frequencies of vibration of a structure. In order to determine the effectiveness of such a material, research into the functional properties of SMAs and SMA-composites was conducted. As an initial step, the transformation behaviour of constrained SMAs was investigated in order to obtain a better understanding into the recovery stress generation of these wires when embedded into a composite material. It is known that the transformation is based on two types of martensite within the alloy; self accommodating and preferentially oriented martensite. The amounts of each type and how they vary with differing pre-strain were determined through DSC measurements and an explanation for why preferentially oriented martensite is not observed during DSC testing was made. The next step was to investigate the effectiveness of embedding SMA wires into composites and the thermomechanical properties of the SMA wires and the SMA-composites were determined. This was completed using a specially designed tensile testing machine which was capable of having the whole specimen immersed into an oil bath and heated and cooled repeatedly. The stress-strain, strain-temperature, stress-temperature, resistance-strain and cyclic properties of various wires were obtained, giving a better understanding of the behaviour of SMA wires under different test conditions. NiTiCu SMA wires were embedded into kevlar composite materials and the recovery stress generation (stress-temperature), stress-strain, and strain-temperature behaviour was determined. If SMA-composites are to be used as new materials for structural applications, verification that the embedment of pre-strained SMA wires into the material doesn't adversely affect the impact behaviour needs to be carried out. SMA-composite specimens with varying volume fractions of superelastic SMA wires, pre-strain and position through the thickness were made up for impact damage characterisation. These specimens were impacted at three different energy levels. The results showed that by embedding SMA wires into composite materials there is a reasonably low damage accumulation after impact. There is also no adverse impact effect on the structure compared with structures without wires as well as structures with other types of wires such as steel and martensitic SMA wires. The SMA-composites showed good damping and energy absorption capabilities. A novel application of SMA-composites is their use as a SMA patch in order to repair damage in existing cracked metallic structures. An analytical study and finite element modelling showing the closure stresses obtainable for use as patches was made.

Impact damage resistance is an essential requirement of lightweight structural components for high-performance applications. The aim of this thesis is to study the impact damage and perforation behaviour of lightweight materials including thin aluminium alloy plates and carbon fibre reinforced epoxy composites. The focus of this investigation is on the stress state and strain rate dependence of failure, and the effect of microstructural modifications on indentation and impact response. The thesis is divided into three parts. In the first part (Chapter 2) the impact response of thin monolithic ductile aluminium alloy plates is investigated. Impact perforation experiments are performed using different projectile nose shapes to span a wide range of stress states at the onset of ductile fracture. Impact perforation behaviour, ballistic limit velocity, energy absorption capability and sensitivity to projectile tip geometry are evaluated. Modes of deformation and failure during impact are assessed experimentally. It is shown that modelling the stress state and strain rate dependence of plasticity and failure is crucial to accurately predict ductile fracture initiation in thin metal plates. In the second part (Chapters 3 and 4), the stress state and strain rate dependent yield and failure behaviour of epoxy resin is investigated. An iterative numerical-experimental approach is shown to be essential to develop a material model capable of predicting the failure behaviour of epoxy for a wide range of stress triaxialities across different regimes of failure. The influence of microstructural modifications in epoxy, through two different toughening strategies, on its failure behaviour is investigated. The effect of increasing the applied strain rate on the stress state dependent response of epoxy is investigated to provide an insight into the impact damage resistance of carbon fibre reinforced epoxy composites. In the third part (Chapter 5), experimental studies are conducted on the quasi-static indentation and impact perforation response of plain weave carbon fibre reinforced epoxy composites to investigate the effect of toughening the epoxy matrix to improve resistance to indentation and impact. The nose shape sensitivity of failure initiation in carbon/epoxy composite targets is assessed by considering indenters with different tip geometries. Conclusions and suggestions for future work are presented in Chapter 6.

This thesis investigates the effects of damage characteristics on residual compressive strength (RCS) of 4-mm thick preconditioned carbon/epoxy quasi-isotropic panels through the study of their compressive behaviour. Results of 2-mm thick preconditioned panels mostly from a previous study are also analysed. The preconditions of varying sizes include impact damage, quasi-static damage, single and multiple artificial delaminations of circular and elliptical shapes embedded at different through-the-thickness (TTT) locations, hemispherical-shaped domes of different curvature and depth and open holes. The mechanisms of impact damage and the characteristics of energy absorption were dependent on panel thickness and incident kinetic energy (IKE). A damage threshold for compressive strength (CS) reduction was found at 455-mm2 and 1257 mm2 for 2- and 4-mm thick panels, respectively. Panels affected by the presence of internal delaminations followed a sequence of prebuckling, local and global buckling (mode I) and postbuckling (mode II) in both the longitudinal and transverse directions. Their compressive failure was related to mode I to II transition. Possibility of delamination propagation was examined using response characteristics on the basis of the sequences. Evidence of delamination propagation was found only in panels with large damages and was not sensitive to RCS. For low and intermediate IKEs the effect of impact damage could be simulated with a single delamination (2-mm thick panels) and 3 delaminations of medium size (4-mm thick panels). For high IKEs, the additional effect of local curvature change was significant. The combined effect of delamination number, size and curvature change determines the RCSs. It was demonstrated that the present method of embedding artificial delaminations proves to be very useful for studying RCS of impact-damaged panels via the establishment of response characteristics and their links to the effects of the preconditions on them. This thesis also presents two analytical models, one for deflection of transversely loaded panels and the other one for the prediction of compressive strength retention factor (CSRF) based on the correlation between the ratio of maximum transverse force to initial threshold force and the CSRF, observed experimentally in thick panels.

Laminados compósitos com matrizes poliméricas, respectivamente termorrígida e termoplástica, reforçadas com fibras contínuas de carbono foram submetidos a impacto único transversal com diferentes níveis de energia. Os danos imprimidos aos materiais estruturais foram avaliados por termografia ativa infravermelha na modalidade transmissão. Em geral, os termogramas do laminado termoplástico apresentaram indicações mais claras e bem definidas dos danos causados por impacto, se comparados aos do compósito termorrígido. O aquecimento convectivo das amostras por fluxo controlado de ar se mostrou mais eficaz que o realizado por irradiação, empregando-se lâmpadas incadecentes. Observou-se também que tempos mais longos de aquecimento favoreceram a visualização dos danos. O posicionamento da face impactada do espécime, relativamente à câmera infravermelha e à fonte de calor, não afetou a qualidade dos termogramas no caso do laminado termorrígido, enquanto que influenciou significativamente os termogramas do compósito termoplástico. Os resultados permitiram concluir que a termografia infravermelha é um método de ensaio não-destrutivo simples, robusto e confiável para a detecção de danos por impacto inferior à 5 Joules em laminados compósitos poliméricos reforçados com fibras de carbono. / Continuous carbon fiber-reinforced thermosetting and thermoplastic composite laminates were exposed to single transversal impact with different energy levels. The damages marked to the structural materials were evaluated by active infrared thermography in transmission mode. In general, the thermoplastic laminate thermograms showed more clear and delineated damage indications when compared to the ones from thermosetting composite. The convective heating of the samples by controlled hot air flow was more efficient than via irradiation using lamp. It was also observed that longer heating times improved the damage visualization. The positioning of the specimen´s impacted face regarding the infrared camera and the heating source did not affect the thermo-imaging of thermosetting specimens, whereas it substantially influenced the thermograms of thermoplastic laminates. The results allow concluding that infrared thermography is a simple, robust and trustworthy methodology for detecting impact damages as light as 5 Joules in carbon fiber composite laminates.

This MEng (Master of Engineering) research thesis evaluates the capabilities and limitations of line-scan thermography for the non-destructive evaluation of composite structures containing hidden defects. In simple terms, line-scan thermography is a state-of-the-art technique in which a focused line of thermal energy is transmitted into a material. Line-scan thermography has great potential for the rapid and low cost non-destructive inspection of composite structures for aircraft, automobiles and ships. In this project, theoretical research exploring the heat transfer physics was undertaken in conjunction with experimental studies to develop an optimum inspection regime for line-scan thermography. The capability of line-scan thermography to detect impact damage in carbon/epoxy laminates was experimentally investigated in Chapter 3. From the impact side, in all materials, line-scan thermography overestimated the size of the impact damage whereas flash thermography underestimated the size. There was a close relationship between the ultrasonic profile and the line-scan thermographic thermal response curve. New experimental data has been produced and analysed for the ability of line-scan thermography to determine the defect as well as the defect size. It was found that line-scan thermography was able to distinguish back drilled holes, but it was not possible to determine accurate defect sizing due to the depth of the holes from the inspected surface and the limitations associated with the line-scan thermographic apparatus itself. There was excellent correlation between the C-scan ultrasonics intensity curves and the line-scan thermographs as well as excellent correlation with the theoretical results. The relationship between line-scan thermography and foreign body objects were experimentally investigated for carbon/epoxy composites. A major limitation found with line-scan thermography is its limited depth penetration, which is highlighted in the foreign object study using 6 mm and 13 mm diameter Teflon® discs and 13 mm Teflon® strips embedded in carbon/epoxy laminates. Depth penetration allowed only 2 mm resolution for the 13 mm diameter discs and 1.5 mm resolution for the 6 mm discs in a composite panel. The results of the investigation of stainless steel shim objects in carbon/epoxy laminates reveal that line-scan thermography is capable of determining their presence and size close to the surface. There was also excellent correlation between the ultrasonic response curve and the line-scan thermographic intensity curve. The results of the investigation of thermoplastic film foreign body objects in carbon/epoxy laminates show that at present line-scan thermography does not have the capability to determin e such defects. Experimental results show that line-scan thermography is capable of detecting large voids, back drilled holes, some foreign body objects, and impact damage. However, the ability of line-scan thermography to measure the defect dimensions is dependent on the size and type of damage, the distance from the line source, the depth of the defect, and the type of composite material.

This Thesis investigates the damage sensing capabilities of the electrical potential measurement technique in carbon fibre reinforced polymer composites. Impact damage was introduced in multidirectional laminates and its effect on potential distribution studied. It was found that delaminations and fibre breakages within the laminate can be detected and located by measuring potential changes on the external composite surface. The extent and size of potential changes were significantly affected by the position of the current electrodes in relation to the potential measurement probes. A numerical model was developed investigating the effect of different size delaminations, located in various positions within the lamina, on electrical potential distributions on the external ply, and a quantitative analysis of the numerical results is presented. The numerical simulations demonstrated that the measured potential changes on the external ply were in proportion to the delamination size. The numerical and experimental results were compared and the optimum configuration of current electrodes and potential probes for damage detection selected. The response of electrical potential to mechanical strain, in unidirectional and multidirectional samples was also investigated. It was found that the conductive medium, used for introducing the current, defines the piezo-resistance performance of the composite. A finite element model was developed able to predict the effect of inhomogeneous current introduction in unidirectional specimens on electrical potential and piezo-resistance. The effects of temperature and water absorption on potential measurements were also presented.

Foreign object damage (FOD) refers to the damage that generally takes place in aero engine fans and compressor blades, due to the ingestion of hard particles/debris during aeroplane take-off, taxiing, or landing. Such damage can reduce the fatigue life expectancy of the turbine engine components by 50%. Residual stresses and small microcracks induced by the high speed FOD impacts are two root causes that result in premature failure of these components. One way to mitigate the FOD related fatigue failure is to induce deep compressive residual stress into the surface. Among the available techniques that can induce such compressive residual stress, laser shock peening (LSP) has been found to be beneficial in improving the fatigue strength. In this study aerofoil-shaped Ti-6Al-4V leading edge specimens were laser shock peened. Subsequently, FOD was introduced onto the leading edge specimen through ballistic impacts of a cube edge at angles of 0° and 45° to the leading edge. The effect of foreign object damage (FOD) on the pre-existing compressive residual stress field associated with the laser shock peening (LSP), and its change upon solely low cycle fatigue (LCF) as well as combined low and high cycle fatigue cycling has been studied. The residual stress distribution and their redistribution upon fatigue cycling were mapped around the FOD notch, using synchrotron X-ray radiation and the contour method. The results suggest that under both impact angles, the FOD event superimposed a significant additional residual stress on top of the pre-existing stress associated with the LSP process. It has been observed that the FOD notch created by 45° impact was asymmetric in shape, and had differential notch depth between the entry and exit side. However, FOD damage that is created at 0° impact appeared as a sharp V notch. A higher amount of residual stresses were produced under 0° impact condition than at 45°. It has been found even though the FOD induced residual stresses relax, residual stresses due to LSP treatment remain highly stable even in the worst condition where a 7 mm long crack was grown from a 45° notch. The plastic zone sizes ahead of a crack tip was estimated for both 0° and 45° FOD impact, and the fatigue crack growth rates are predicted utilizing the measured residual stress distribution.