The primary objective of this work is to experimentally examine the effect of various geometric parameters on the impact damage and damage tolerance of thin carbon/epoxy plate and panel structures. Due to the number of parameters involved in a low velocity impact event and the complexity of the damage created, determining what effect individual parameters have is extremely demanding, especially when some of the parameters may even be coupled. However, by experimentally simulating in a controlled manner, either the impact event or the damage created, the effect of individual geometric parameters can be isolated and determined. A quasi-static indentation test has been used to simulate an impact event. The parameters of indenter size, nose shape, plate size and boundary condition, were examined. Four different plate failure modes were identified. Indenter nose shape was found to be the dominant geometric parameter, as a change in nose shape resulted in a change in failure mode and hence maximum load. From this work, a set of geometric parameters was selected for impact testing. Impact testing at various Incident Kinetic Energies (IKE) was performed on an instrumented drop weight impact rig. From examination of internal and external damage, the development of damage to increasing IKE was determined and shown to have four distinctive phases. Coupled with strike and rebound velocity measurementsa, non-linear relationship between IKE-damage area was established and a delamination threshold energy level of 1.1 J was calculated. Damaget olerance assessmenot f impacted panels was then performed in a Compression-After- Impact (CAI) rig. Strain gauge responses allowed global and local behaviour to be compared to intact specimens. It was found that once a critical damage size was surpassed, a gradual nonlinear degradation in compressive strength was observed until a point was reached where no further degradation in performance was attained. Furthermore, propagation of internal damage in a stable and unstable manner was directly linked to the nature of sublaminate buckling behaviour. Damaget olerance assessmenot f artificially delaminatedp anels loaded in compressionw as then performed. A single artificial delamination of various size, shape and orientation, embedded at the centre of a panel was examined. Delamination width was found to be the dominant geometric parameter. Hence, when comparing a circular delamination to an elliptical one of the same area, the effect of orientation and shape is aspect ratio dependent. Finally, a comparison of impacted and artificially delaminated panels was made.

Sandwich panels using fibre-reinforced composite skins and low-density cores are being increasingly used in the aerospace industry due to their superior specific strength and stiffness, and increased design flexibility over traditional metallic and composite structures. However, it is well-known that sandwich panels are highly vulnerable to the effects of impact damage, with even low-energy impacts potentially causing very severe reductions in the in-plane compressive strength of these structures. The objective of this project was to produce a faithful and reliable numerical model for the simulation of the compression-after-impact strength of composite sandwich panels. An in-depth literature review revealed that delamination within the skins of a sandwich panel is a damage mechanism that has gone almost entirely neglected in previous efforts at modelling this problem, despite the proven significance of this mechanism in the failure of impact damaged sandwich panels in compression. Consequently, the use of the cohesive zone model for delamination initiation and propagation is the key unique feature of this model, with Hashin s criteria being used for intra-laminar damage formation, and a simple plasticity response capturing core crushing. An experimental study is performed to produce a thorough dataset for model validation, featuring differing levels of damage induced via quasi-static indentation, and novel asymmetric panels with skins of unequal thickness (the thinner skin being on the unimpacted side). The experimental study revealed that the use of a thinner distal (undamaged) skin could improve the strength of mildly damaged sandwich panels over undamaged sandwich panels using the same asymmetric configuration. It is believed that this effect is due to the movement of the neutral plane of the sandwich panel caused by the reduction in the stability of the damaged skin through stiffness reduction and geometric imperfections. This removes the eccentricity of the compressive loading that exists in the undamaged asymmetric panels, which has mismatched axial stiffness between the indented skin and the thinner distal skin, and thus a noticeably lower ultimate strength than the undamaged symmetric panels. The sandwich model is developed using pre-existing experimental and material data, and trialled for a variety of different skin lay-ups, core thicknesses and indenter sizes. The numerical model generally agreed well with the ultimate stress found in the experiments for these different configurations, but is quite poor at estimating the magnitude of the damage induced by the indentation. When used to model the experimental study, the model gave generally good, conservative estimates for the residual compressive strength of both the symmetric and asymmetric panels. The tendency of the asymmetric panels to become stronger with mild damage was not captured by the model per se, with the numerical results instead showing an insensitivity to damage in the asymmetric panels, which was not shared by the symmetric panels. However, the numerical model did exhibit erroneous strain-stress responses for both panel configurations, particularly for the undamaged and mildly damaged cases. Investigations revealed that this erroneous behaviour was caused by inconsistency in the material data, which had been collected partially via experimentation and partly from literature sources. Overall, the model developed here represents a promising advancement over previous efforts, but further development is required to provide accurate damage states.

The exploitation of sandwich structures as a means toachieve high specific strength and stiffness is relatively new.Therefore, the knowledge of its damage tolerance is limitedcompared to other structural concepts such as truss bars andmonocoque plate solutions. Several aspects of the damage tolerance of sandwichstructures are investigated. The influence of impact velocityonresidual strength is investigated. Sandwich panels withfaces of glass fiber reinforced vinylester are impacted bothwith very high velocity and quasi static. The residual strengthafter impact is found to be similar for both cases of impactvelocity. Curved sandwich beams subjected to opening bending momentare studied. Faceñcore debonds of varying size areintroduced between the compressively loaded face sheet and thecore. Finite element analysis in combination with a pointstress criterion is utilized to predict the residual strengthof the beams. It is shown that it is possible to predict thefailure load of the beams with face-core debond. Using fractography the governing mode of failure ofcompressively NCF-carbon is characterized. Sandwich panelssubjected to compression after impact are shown to fail byplastic micro buckling. The residual compressive strength after impact of sandwichpanels is investigated. Sandwich panels with face sheets ofnon-crimp fabric (NCF) carbon are subjected to different typesof impact damages. Predictions of residual strength are madeusing the Budiansky, Soutis, Fleck (BSF) model. The residualstrength is tested, and the results are compared topredictions. Predictions and tests correlate well, and indicatethat the residual strength is dependent on damage size and notthe size of the damaged panel. A study of the properties of a selection of fiberreinforcements commonly used in sandwich panels is conducted.The reinforcements are combined with two types of core materialand three types of matrix. Also the influence of laminatethickness is tested. Each combination materials is tested inuni-axial compression, compressive strength after impact andenergy absorption during quasi static indentation. Thespecimens which are tested for residual strength are eithersubjected to quasi-static or dynamic impact of comparableenergy level. Prediction of the residual strength is made andcorrelates reasonably whith the test results. The tests showthat if weight is taken into account the preferred choice offiber reinforcement is carbon.

<p>The exploitation of sandwich structures as a means toachieve high specific strength and stiffness is relatively new.Therefore, the knowledge of its damage tolerance is limitedcompared to other structural concepts such as truss bars andmonocoque plate solutions.</p><p>Several aspects of the damage tolerance of sandwichstructures are investigated. The influence of impact velocityonresidual strength is investigated. Sandwich panels withfaces of glass fiber reinforced vinylester are impacted bothwith very high velocity and quasi static. The residual strengthafter impact is found to be similar for both cases of impactvelocity.</p><p>Curved sandwich beams subjected to opening bending momentare studied. Faceñcore debonds of varying size areintroduced between the compressively loaded face sheet and thecore. Finite element analysis in combination with a pointstress criterion is utilized to predict the residual strengthof the beams. It is shown that it is possible to predict thefailure load of the beams with face-core debond.</p><p>Using fractography the governing mode of failure ofcompressively NCF-carbon is characterized. Sandwich panelssubjected to compression after impact are shown to fail byplastic micro buckling.</p><p>The residual compressive strength after impact of sandwichpanels is investigated. Sandwich panels with face sheets ofnon-crimp fabric (NCF) carbon are subjected to different typesof impact damages. Predictions of residual strength are madeusing the Budiansky, Soutis, Fleck (BSF) model. The residualstrength is tested, and the results are compared topredictions. Predictions and tests correlate well, and indicatethat the residual strength is dependent on damage size and notthe size of the damaged panel.</p><p>A study of the properties of a selection of fiberreinforcements commonly used in sandwich panels is conducted.The reinforcements are combined with two types of core materialand three types of matrix. Also the influence of laminatethickness is tested. Each combination materials is tested inuni-axial compression, compressive strength after impact andenergy absorption during quasi static indentation. Thespecimens which are tested for residual strength are eithersubjected to quasi-static or dynamic impact of comparableenergy level. Prediction of the residual strength is made andcorrelates reasonably whith the test results. The tests showthat if weight is taken into account the preferred choice offiber reinforcement is carbon.</p>

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.

A better understanding of the effect of impact damage on composite structures is necessary to give the engineer an ability to design safe, efficient structures. Current composite structures suffer severe strength reduction under compressive loading conditions, due to even light damage, such as from low velocity impact. A review is undertaken to access the current state-of-development in the areas of experimental testing, and analysis methods. A set of experiments on Nomex honeycomb core sandwich panels, with thin woven fiberglass cloth facesheets, is described, which includes detailed instrumentation and unique observation techniques. These techniques include high speed video photography of compression after impact (CAI) failure, as well as, digital image correlation (DIC) for full-field deformation measurements. The effect of nominal core density on the observed failure mode is described. A finite element model (FEM) is developed to simulate the experiments performed in the current study. The purpose of this simulation is to predict the experimental test results, and to conrm the experimental test conclusions. A newly-developed, commercial implementation of the Multicontinuum Failure Theory (MCT) for progressive failure analysis (PFA) in composite laminates, Helius:MCT, is included in this model. The inclusion of PFA in the present model gives it the new, unique ability to account for multiple failure modes. In addition, significant impact damage detail is included in the model as a result of a large amount of easily available experimental test data. A sensitivity study is used to assess the effect of each damage detail on overall analysis results. Mesh convergence of the new FEM is also discussed. Analysis results are compared to the experimental results for each of the 32 CAI sandwich panel specimens tested to failure. The failure of each specimen is accurately predicted in a high-fidelity, physics-based simulation and the results highlight key improvements in the understanding of honeycomb core sandwich panel CAI failure. Finally, a parametric study highlights the strength benefits compared to mass penalty for various core densities. / Ph. D.

This thesis documents the findings of a three year experimental investigation into the impact damage resistance and damage tolerance of composite honeycomb sandwich panels. The primary area of work focuses on the performance of sandwich panels under quasi-static and low-velocity impact loading with hemispherical and flat-ended indenters. The damage resistance is characterised in terms of damage mechanisms and energy absorption. The effects of varying the skin and core materials, skin thickness, core density, panel boundary conditions and indenter shape on the transverse strength and energy absorption of a sandwich panel have been examined. Damage mechanisms are found to include delamination of the impacted skin, core crushing, limited skin-core de bonding and top skin fibre fracture at high loads. In terms of panel construction the skin thickness is found to dominate the panel strength and energy absorption with core density having a lesser influence. Of the external factors considered the indenter noseshape has the largest effect on both failure load and associated damage area. An overview of existing analytical prediction methods is also included and the most significant theories applied and compared with the experimental results from this study. The secondary area of work expands the understanding obtained from the damage resistance study and assesses the ability of a sandwich panel to withstand in-plane compressive loading after sustaining low-velocity impact damage. The importance of the core material is investigated by comparing the compression-after-impact strength of both monolithic carbon-fibre laminates and sandwich panels with either an aluminium or nomex honeycomb core. The in-plane compressive strength of an 8 ply skinned honeycomb sandwich panel is found to be double that of a 16 ply monolithic laminate, with the type of honeycomb also influencing the compressive failure mechanisms and residual compressive strength. It is concluded that under in-plane loading the stabilising effect of the core opposes the de-stabilising effect of any impact damage.

Les composites thermoplastiques sont de plus en plus privilégiées dans les structures aérospatiales au vue de leur tolérance aux dommages améliorée par rapport aux résines thermodurcissables. Néanmoins, ils restent sensibles à l’impact car il produit des endommagements complexes au sein du matériau, dont le délaminage est le plus critique. La propagation de ces endommagements en compression après impact (CAI) entraîne une réduction de la tenue résiduelle. D’abord, des essais ENF ont été menés afin de déterminer la ténacité interfaciale par le biais de la méthode de la complaisance et de la technique de thermographie infrarouge. Ensuite, l’essai « Short Beam Shear » est proposé afin d’investiguer le couplage entre la fissuration matricielle et le délaminage. L’effet de la vitesse de sollicitation a été également étudié. La valeur de ténacité mesurée semble indépendante à la vitesse de sollicitation car, lors des essais réalisés, la propagation est instable. Ensuite, le comportement d’une éprouvette académique été étudié à l’aide du « Discrete Ply Model » (DPM) permettant d’enchaîner la simulation d’impact et de CAI. Ce modèle est basé sur une approche semi-discrète modélisant le délaminage et la fissuration matricielle par des éléments cohésifs, permettant de prendre en compte le couplage entre ces deux endommagements. Une vaste campagne d'essais expérimentaux d’impact et de CAI a été mise en place sur quatre empilements différents impactés à trois niveaux d’énergie. Le modèle DPM a prouvé sa capacité à prédire correctement les endommagements d’impact et de CAI. Finalement, afin de se rapprocher des conditions de structures réelles, le comportement en compression après impact d’une plaque trouée a été investigué. / High-performance thermoplastic composite have been increasingly used in aerospace applications because of their advantageous mechanical properties. Nevertheless, impact damage leads to significant reduction in structure compressive strength although damage may remain unnoticed. Delamination is the most critical damage. Short Beam Shear (SBS) test has been proposed to reproduce impact damage chronology and characterize delamination toughness. Infrared thermography is used for local measuring of fracture toughness in this unclassical test showing unstable fracture growth. Mode II fracture toughness (GIIC) values are comprised between 0.9 and 1.7 N/mm and there was no influence of the loading rate in GIIC values. Discrete Ply Model (DPM) is therefore used to model impact and compression after impact tests on laminated composite structures. Tests have been conducted in order to validate DPM capacity to capture the effects of progressive damage and failure. Impact damage and specimen’s compressive strength is well predicted by DPM. CAI damage propagation is driven by the buckling of the structure. DPM is finally employed to study impact on an industrial sample with a large diameter hole. Impact damage correlates with tests but buckling is difficult to estimate, meaning that rupture of the specimen does not correlate to tests. Nevertheless, DPM shows a good ability to predict damage in thermoplastic composite.

Les impacts sur structures composites peuvent fortement diminuer leur résistance résiduelle sans laisser de marque visible sur la surface extérieure. Dans le domaine aéronautique, un seuil minimum de détection de l’endommagement d’impact est défini, basé sur l’indentation permanente laissée par l’impact. En deçà de ce seuil, la structure doit résister à un chargement défini : c’est la notion de tolérance aux dommages d’impact. Dimensionner numériquement une structure composite en tenant compte des aspects détectabilité et tolérance aux dommages nécessite donc de savoir modéliser à la fois l’impact, l’indentation permanente et la résistance résiduelle sous compression.Ces travaux se focalisent sur la modélisation numérique des composites stratifiés formés de plis unidirectionnels. L’objectif est d’établir un modèle prédictif de la tenue résiduelle après impact. Une étude expérimentale a été réalisée afin d’étudier le scénario d’endommagement à l’impact et sous compression après impact (CAI), et de fournir des résultats expérimentaux pour valider les modèles numériques.Une modélisation par éléments finis avec une approche de type Discrete Ply Model (DPM) est effectuée, basée sur des travaux précédents. Le modèle d'impact est amélioré et validé sur différentes séquences d'empilement pour assurer la robustesse du modèle. Des essais de flexion trois points spécifiques sont réalisés pour apporter une meilleure compréhension de la formation de l'indentation permanente. Un nouveau modèle d'indentation permanente est alors proposé et appliqué dans le modèle d'impact. Enfin, un modèle de CAI est construit pour prédire la résistance résiduelle. Les trois étapes : impact, indentation et CAI sont combinées au sein d’un unique modèle. / Impacts on composite structures can greatly reduce their residual strength without leaving a visible mark on the outer surface. In aeronautics, a minimum detection threshold of the impact damage is defined, based on the permanent indentation left after impact. Below this threshold, the structure must withstand a defined load: it is the notion of impact damage tolerance. The numerical design of a composite structure taking into account aspects such as detectability and damage tolerance thus requires to know how to represent impact, permanent indentation and residual strength under compression.This work focuses on the numerical modeling of composite laminates made of unidirectional plies. The objective is to develop a predictive model of post-impact residual strength. An experimental study was conducted to investigate the damage scenario during impact and compression after impact (CAI), and provide experimental data to validate the simulations.A finite element modeling with a Discrete Ply Model (DPM) approach is performed based on previous work done at the laboratory. The impact model is improved and validated on different stacking sequences to ensure the robustness of the model. Specific three-point bending tests are performed to have a better understanding of the formation of permanent indentation. A new model of permanent indentation is then proposed and applied in the impact model. Finally, a model is built to predict CAI residual strength. The three steps: impact, indentation and CAI are combined into a single model.

This thesis presents the work of an experimental investigation into the impact damage resistance and damage tolerance for symmetrical and unsymmetrical composite honeycomb sandwich panels through in-plane compression. The primary aim of this research is to examine the impact damage resistance of various types of primarily carbon/epoxy skinned sandwich panels with varying skin thickness, skin lay-up, skin material, sandwich asymmetry and core density and investigate the residual in-plane compressive strengths of these panels with a specific focus on how the core of the sandwich contributes to the in-plane compressive behaviour. This aim is supported by four specifically constructed preconditions introduced into panels to provide an additional physical insight into the loading-bearing compression mechanisms. Impact damage was introduced into the panels over a range of IKEs via an instrumented drop-weight impact test rig with a hemi-spherical nosed impactor. The damage resistance in terms of the onset and propagation of various dominant damage mechanisms was characterised using damage extent in both impacted skin and core, absorbed energy and dent depth. Primary damage mechanisms were found to be impacted skin delamination and core crushing, regardless of skin and core combinations and at high energies, the impacted skin was fractured. In rare cases, interfacial skin/core debonding was found to occur. Significant increases in damage resistance were observed when skin thickness and core density were increased. The reduction trends of the residual in-plane compressive strengths of all the panels were evaluated using IKE, delamination and crushed core extents and dent depth. The majority of impact damaged panels were found to fail in the mid-section and suffered an initial decline in their residual compressive strengths. Thicker skinned and higher density core panels maintained their residual strength over a larger impact energy range. Final CAI strength reductions were observed in all panels when fibre fracture in the impacted skin was present after impact. Thinner skinned panels had a greater compressive strength over the thicker skinned panels, and panel asymmetry in thin symmetrical panels appeared to result in an improving damage tolerance trend as IKE was increased due to that the impact damage balanced the in-plane compressive resistance in the skins with respect to the pre-existing neutral plane shift due to the uneven skin thickness.