RTM6 has for more than 20 years been the main commercial epoxy system for infusion processing qualified by the aerospace industry. In common with other aerospace-grade epoxy systems RTM6 is mechanically strong but brittle, producing carbon-fibre (CF) composites with relatively low impact resistance and damage tolerance. This thesis reports an approach to toughening epoxy-CF composites without modification of the resin. Thus, a T300 carbon fabric (ES-fabric) coated with 20 weight % of a poly (aryl ether ketone) (PAEK) was used to toughen the composite. The initial stage of the study was the manufacturing process. DSC and oscillatory-shear rheology were used to determine flow times and cure conditions, and to produce laminates with fibre volume fractions ≥55% a hybrid resin infusion/hot-press process was developed. Dynamic mechanical thermal analysis also showed that the PAEK coating produced relatively little plasticization of the epoxy matrix, with values of the matrix glass transition temperature shifting from 186±4.4 to 181± 1.4 ºC when using the ES-fabric. The main body of the study focussed on the toughening effect afforded by the PAEK coating relative to an uncoated fabric system as a reference. Mode I and Mode II interlaminar fracture toughness behaviour were studied using dual cantilever beam (DCB) and four-point end-notch flexure (4ENF) tests, respectively. The measured mode-I fracture energy, GIC, increased three-fold, from 216 ± 7.2 Jm-2 to 751 ± 105 Jm-2, due to the toughening effect of the PAEK coating; whereas the mode-II fracture energy, GIIC, increased almost four-fold from 857 ± 99 Jm-2 to 3316 ± 372 Jm-2. Damage resistance was studied using low-velocity impact testing and damage tolerance using a miniature compression-after-impact (CAI). A comparative study of damage tolerance was performed using open-hole compression (OHC) testing. The impact damage resistance significantly improved with the use of the PAEK-coated ES-fabric as well as the CAI and OHC behaviour. Impact testing showed the PAEK -toughened system exhibited higher energy abortion than the untoughened system, larger damage area was created in the T300/RTM6-2 after impacted with same energy. The CAI results indicated that the normalized CAI strength is major related that damage width rather than other factor. OHC results are predicted by using W-N criteria, for ES/RTM6-2: ASC a0 = 9.35 mm and PSC d0 = 2.72mm; whereas for T300/RTM6-2: ASC a0 = 7.95 mm and PSC d0 =2.43 mm, indicates that the compressive strength of T300/RTM6-2 is more sensitive to the size of the hole, thus ES/RTM6-2 perform better damage tolerance. The results from mechanical testing indicate that the PAEK coating toughened the composite system and significantly improved damage tolerance. Scanning electron microscopy indicated that these improvements in fracture behaviour were due to morphological changes induced by the PAEK coating in the matrix-CF interfacial region, where such changes can provide the maximum benefit. Small particles of RTM (approximately 1 µm in diameter) were observed imbedded within a continuous PAEK phase. Thus, during testing crack propagation was deflected (or bifurcated) by the RTM6 particles or stopped by shearing of the continuous PAEK phase of this multiphase region. This morphology is proposed to have formed in the interfacial region during processing by dissolution of the PAEK coating within the matrix resin system, followed by reaction-induced phase separation and then phase-inversion as the matrix cures.

Notre génome subit constamment les effets néfastes des agents endommageant de l'ADN. Afin de se protéger de ces effets délétères, les cellules disposent d’un système de détection des dommages à l’ADN (point de contrôle ou « checkpoint »). Certaines lésions peuvent persister quand les cellules entrent en phase S et inhiber ainsi la synthèse de l’ADN en interférant avec les ADN polymérases réplicatives. Ceci peut provoquer des arrêts prolongés des fourches de réplication ce qui fragilise l’ADN. Pour préserver l’intégrité de l’information génétique, les cellules ont développé une voie de tolérance qui implique des ADN polymérases spécialisées dans la réplication des lésions, appelées ADN Polymérases translésionnelles (Pols TLS). Dans ce processus, PCNA joue le rôle de facteur d’échafaudage pour de nombreuses protéines impliquées dans le métabolisme de l'ADN. Les mécanismes de régulation des échanges entre les différents partenaires de PCNA ne sont pas très bien compris. Parmi les protéines qui interagissent avec PCNA, CDT1, p21 ou encore PR-Set7/Set8 sont caractérisées par une forte affinité pour cette protéine. Ces dernières possèdent un motif d’interaction particulier avec PCNA, nommé « PIP degron », qui favorise leur protéolyse d'une manière dépendante de l’E3 ubiquitine ligase CRL4Cdt2. Après irradiation aux UV-C, le facteur d’initiation de la réplication CDT1 est rapidement détruit d’une manière dépendante de son PIP degron, Dans la première partie de mon travail, j’ai contribué à comprendre le rôle fonctionnel de cette dégradation. Les résultats obtenus ont fourni des évidences expérimentales qui montrent que l’inhibition de la dégradation de CDT1 par CRL4Cdt2 dans les cellules de mammifères compromet la relocalisation des TLS Pol eta et Pol kappaen foyers nucléaires induits par les irradiations UV-C. On a constaté que seules les protéines qui contiennent un PIP degron interfèrent avec la formation de foyers de Pol eta. La mutagenèse du PIP degron de CDT1 a révélé qu'un résidu de thréonine conservé parmi les PIP degrons est essentiel pour l'inhibition de la formation des foyers des TLS Polymérases. Les résultats obtenus suggèrent que l’élimination de protéines contenant des PIP degrons par la voie CRL4Cdt2 régule le recrutement de TLS Polymérases au niveau des sites des dommages induits par les UV-C.Dans un second temps, on s’est intéressé à l’étude du checkpoint des dommages à l’ADN au cours de l’embryogénèse. En effet, dans les embryons précoces, le checkpoint est silencieux jusqu'à la transition de mid-blastula (MBT), en raison de facteurs maternels limitants. Dans ce travail, nous avons montré, aussi bien in vitro qu’in vivo, que l’ubiquitine ligase de type E3 RAD18, un régulateur majeur de la translésion, est un facteur limitant pour l’activation du checkpoint dans les embryons de xénope. Nous avons montré que l'inactivation de la fonction de l’ubiquitine ligase RAD18 conduit à l'activation du checkpoint par un mécanisme qui implique l’arrêt des fourches de réplication en face des lésions produites par les UV-C. De plus, nous avons montré que l'abondance de RAD18 et de PCNA monoubiquitiné (PCNAmUb) est régulée au cours de l’embryogénèse. À l’approche de la MBT, l’abondance de l'ADN limite la disponibilité de RAD18, réduisant ainsi la quantité de PCNAmUb et induisant la dé-répression du checkpoint. En outre, nous avons montré que cette régulation embryonnaire peut être réactivée dans les cellules somatiques de mammifères par l'expression ectopique de RAD18, conférant une résistance aux agents qui causent des dommages à l'ADN. Enfin, nous avons trouvé que l'expression de RAD18 est élevée dans les cellules souches cancéreuses de glioblastome hautement résistantes aux dommages de l'ADN. En somme, ces données proposent RAD18 comme un facteur embryonnaire critique qui inhibe le point de contrôle des dommages de l’ADN et suggèrent que le dérèglement de l’expression de RAD18 peut avoir un potentiel oncogénique inattendu / Our genome is continuously exposed to DNA damaging agents. In order to preserve the integrity of their genome, cells have evolved a DNA damage signalling pathway known as checkpoint. Some lesions may persist when cells enter the S-phase and halt the progression of replicative DNA polymerases. This can cause prolonged replication forks stalling which threaten the stability of the genome. To preserve the integrity of genetic information, cells have developed a tolerance pathway which involves specialized DNA polymerases, called translesion DNA polymerases (TLS Pols). These polymerases have the unique ability to accommodate the damaged bases thanks to their catalytic site. In this process, PCNA acts as a scaffold for many proteins involved in DNA metabolism. The mechanisms governing the exchanges between different PCNA partners are not well understood. Among the proteins that interact with PCNA, CDT1, p21 and PR-Set7/set8 are characterized by a high binding affinity. These proteins have a particular interaction domain with PCNA, called "PIP degron", which promotes their proteasomal degradation via the E3 ubiquitin ligase CRL4Cdt2. After UV-C irradiation, the replication initiation factor CDT1 is rapidly degraded in a PIP degron-dependent manner. During the first part of my work, we wanted to understand the functional role of this degradation. Our results have shown that inhibition of CDT1 degradation by CRL4Cdt2 in mammalian cells, compromises the relocalisation of TLS Pol eta and Pol kappato nuclear foci after UV-C irradiation. We also found that only the proteins which contain a PIP degron interfere with the formation of Pol eta foci. Mutagenesis experiments on CDT1 PIP degron revealed that a threonine residue conserved among PIP degrons is essential for inhibiting foci formation of at least two TLS polymerases. This results suggest that CRL4Cdt2-dependent degradation of proteins containing PIP degrons regulates the recruitment of TLS polymerases at sites of UV-induced DNA damage.During the second part of my thesis, we studied DNA damage checkpoint regulation during embryogenesis. Indeed, in early embryos, the DNA damage checkpoint is silent until the mid-blastula transition (MBT) due to maternal inhibiting factors. In this work, we have shown, both in vitro and in vivo, that the E3 ubiquitin ligase RAD18, a major regulator of translesion DNA synthesis, is a limiting factor for the checkpoint activation in Xenopus embryos. We have also shown that RAD18 depletion leads to the activation of DNA damage checkpoints by inducing replication fork uncoupling in front of the lesions. Furthermore, we showed that the abundance of RAD18 and PCNA monoubiquitination (PCNAmUb) is regulated during embryonic development. Near the MBT, the increased abundance of DNA limits the availability of RAD18, thereby reducing the amount of PCNAmUb and inducing the de-repression of the checkpoint. Moreover, we have shown that this embryonic-like regulation can be reactivated in somatic mammalian cells by ectopic expression of RAD18, conferring resistance to DNA damaging. Finally, we found high RAD18 levels in glioblastoma cancer stem cells highly resistant to DNA damage. All together, these data propose RAD18 as a critical factor that inhibits DNA damage checkpoint in early embryos and suggests that dysregulation of RAD18 expression may have an unexpected oncogenic potential

No description available.

This thesis utilized a linear analytical approach to explore the damage tolerance or residual strength as a function of increasing damage in traditional single and hybrid-grid IsoTruss® structures. Residual strength was studied for structures subjected to axial compression, torsion and flexural bending, independently. Carbon/epoxy material properties were applied in all load cases, and fiberglass/epoxy material properties were also applied in the flexural bending case. Prior to imposing damage conditions, the IsoTruss® structures were parametrically optimized to achieve the highest strength-to-weight ratios. Typical compression strut, driveshaft, and utility pole specifications governed the design strength dimensions and boundary conditions. Damage growth was achieved by removing members from IsoTruss® structures progressively about the circumference in a symmetrical manner. The sequence of member removal, beginning with primary or secondary members, was examined, and is described as primary and secondary analyses. ABAQUS finite element analysis was employed to quantify the residual strength of each IsoTruss® configuration. Reduction in residual strength trends are compared to net section strength, which assumes a linear relationship between damage size and residual strength. Results indicate that the 6-node IsoTruss® configuration is the most damage tolerant structure in the sense that the 6-node configuration deviates the least from the net section strength. As more nodes are added, IsoTruss® structures behave more like a composite tube, exhibiting a brittle behavior characterized by an increase in strength reduction for a given damage size. Bending results reveal that carbon fiber IsoTruss® structures are more damage tolerant under primary bending conditions than fiberglass poles. On the other hand, fiberglass IsoTruss® poles prove to be more damage tolerant under secondary bending conditions than carbon fiber structures. Most importantly, however, hybrid-grid IsoTruss® poles are definitively more optimal structures than single-grid poles in terms of both strength-to-weight ratio and damage tolerance. The results and conclusions from this thesis provide benchmark capacities for mechanically manufactured IsoTruss® structures. Also included in this thesis is documentation of a special program written to analyze IsoTruss® structures.

Unidirectional basalt fiber rods consolidated with an aramid sleeve were measured for compression strength after impact at various energy levels and compared to undamaged control specimens. These structural elements represent local members of open three-dimensional composite lattice structures (e.g., based on isogrid or IsoTruss® technologies) that are continuously fabricated using advanced three-dimensional braiding techniques. The unidirectional core specimens, nominally 8 mm (5/16") and 11 mm (7/16") in diameter, were manufactured using bi-directional braided sleeves or unidirectional spiral sleeves with full or partial (approximately half) coverage of the core fibers. The 51 mm (2") specimens were shorter than the critical buckling length, ensuring the formation of kink bands, typical of strength-controlled compression failure of unidirectional composites. The test results indicate an approximate decrease in the average undamaged compression strength of approximately 1/3 and 2/3 when impacted with 5 J (3.7 ft-lbs) and 10 J (7.4 ft-lbs) for the 8 mm (5/16") diameter specimens and 10 J (7.4 ft-lbs.) and 20 J (14.8 ft-lbs.) for the 11 mm (7/16") diameter specimens, respectively. The aramid sleeves improved the damage tolerance of the composite members, with the amount of coverage having the greatest effect; full coverage exhibiting up to 45% greater strength than partial coverage. Braided sleeves improved compression strength after impact by up to 23% over spiral sleeves, but generally had little effect on damage tolerance. Larger diameter specimens tend to be more resistant to damage than those specimens of a smaller diameter. The compressive material properties for undamaged basalt composites are also presented with the average compressive strength being 800 MPa (116 ksi).

The majority of aircraft structures are designed to be damage-tolerant such that safe operation can continue in the presence of minor damage. It is necessary to schedule inspections so that minor damage can be found and repaired. It is generally not possible to perform structural inspections prior to every flight. The scheduling is traditionally accomplished through a deterministic set of methods referred to as Damage Tolerance Analysis (DTA). DTA has proven to produce safe aircraft but does not provide estimates of the probability of failure of future flights or the probability of repair of future inspections. Without these estimates maintenance costs cannot be accurately predicted. Also, estimation of failure probabilities is now a regulatory requirement for some aircraft. The set of methods concerned with the probabilistic formulation of this problem are collectively referred to as Probabilistic Damage Tolerance Analysis (PDTA). The goal of PDTA is to control the failure probability while holding maintenance costs to a reasonable level. This work focuses specifically on PDTA for fatigue cracking of metallic aircraft structures. The growth of a crack (or cracks) must be modeled using all available data and engineering knowledge. The length of a crack can be assessed only indirectly through evidence such as non-destructive inspection results, failures or lack of failures, and the observed severity of usage of the structure. The current set of industry PDTA tools are lacking in several ways: they may in some cases yield poor estimates of failure probabilities, they cannot realistically represent the variety of possible failure and maintenance scenarios, and they do not allow for model updates which incorporate observed evidence. A PDTA modeling methodology must be flexible enough to estimate accurately the failure and repair probabilities under a variety of maintenance scenarios, and be capable of incorporating observed evidence as it becomes available. This dissertation describes and develops new PDTA methodologies that directly address the deficiencies of the currently used tools. The new methods are implemented as a free, publicly licensed and open source R software package that can be downloaded from the Comprehensive R Archive Network. The tools consist of two main components. First, an explicit (and expensive) Monte Carlo approach is presented which simulates the life of an aircraft structural component flight-by-flight. This straightforward MC routine can be used to provide defensible estimates of the failure probabilities for future flights and repair probabilities for future inspections under a variety of failure and maintenance scenarios. This routine is intended to provide baseline estimates against which to compare the results of other, more efficient approaches. Second, an original approach is described which models the fatigue process and future scheduled inspections as a hidden Markov model. This model is solved using a particle-based approximation and the sequential importance sampling algorithm, which provides an efficient solution to the PDTA problem. Sequential importance sampling is an extension of importance sampling to a Markov process, allowing for efficient Bayesian updating of model parameters. This model updating capability, the benefit of which is demonstrated, is lacking in other PDTA approaches. The results of this approach are shown to agree with the results of the explicit Monte Carlo routine for a number of PDTA problems. Extensions to the typical PDTA problem, which cannot be solved using currently available tools, are presented and solved in this work. These extensions include incorporating observed evidence (such as non-destructive inspection results), more realistic treatment of possible future repairs, and the modeling of failure involving more than one crack (the so-called continuing damage problem). The described hidden Markov model / sequential importance sampling approach to PDTA has the potential to improve aerospace structural safety and reduce maintenance costs by providing a more accurate assessment of the risk of failure and the likelihood of repairs throughout the life of an aircraft. / Statistics

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.

While the structure and mechanical properties of natural cellular solids such as wood and trabecular bone have been extensively studied in the past, the structural design and underlying deformation mechanisms of natural cellular solids with very high mineral contents (> 90 wt%), which we term as natural ceramic cellular solids, are largely unexplored. Many of these natural ceramic cellular solids, despite their inherent brittle constituent biominerals (e.g., calcite or aragonite), exhibit remarkable mechanical properties, such as high stiffness and damage tolerance. In this thesis, by carefully selecting three biomineralized skeletal models with distinctly different cellular morphologies, including the honeycomb-like structure in cuttlefish bone (or cuttlebone), the stochastic open-cell structure in sea urchin spines, and the periodic open-cell structure in starfish ossicles, I systematically investigate the mechanical design strategies of these natural ceramic cellular solids. The three model systems are cuttlefish Sepia officinalis, sea urchin Heterocentrotus mammillatus, and starfish Protoreaster nodosus, respectively. By investigating the relationship between their mechanical properties and structural characteristics, this thesis reveals some novel structural design strategies for developing lightweight, tough, strong, and stiff ceramic cellular solids. The internal skeleton of S. officinalis, also known as cuttlebone, has a porosity of 93 vol% (constituent material: 90 wt% aragonite), which is a multichambered structure consisting of horizontal septa and thin vertical walls with corrugated cross-sectional profiles. Through systematic ex-situ and synchrotron-based in-situ mechanical measurements and collaborative computational modeling, we reveal that the vertical walls in the cuttlebone exhibit an optimal waviness gradient, which leads to compression-dominant deformation and asymmetric wall fracture, accomplishing both high stiffness (8.4 MN∙m/kg) and high energy absorption (4.4 kJ/kg). Moreover, the distribution of walls reduces stress concentrations within the horizontal septa, facilitating a larger chamber crushing stress and more significant densification. For the stochastic open-cell foam-like structure, also known as stereom (porosity: 60-80 vol%, constituent material: 99 wt% calcite) in H. mammillatus, we first developed a computer vision-based algorithm that allows for quantitative analysis of the cellular network of these structures at both local individual branch and node level as well as the global network level. This open-source algorithm could be used for analyzing both biological and engineering open-cell foams. I further show that the smooth, highly curved branch morphology with near-constant surface curvature in stereom results in low-stress concentration, which further leads to dispersed crack formation upon loading. Combined synchrotron in-situ analysis, electron microscopic analysis, and computational modeling further reveal that the fractured branches are efficiently jammed by the small throat openings within the cellular structure. This further leads to the formation of damage bands with densely packed fracture pieces. The continuous widening of the damage bands through progressive microfracture of branches at the boundaries contributes to the observed high plateau stress during compression, thereby contributing to its high energy absorption (17.7 kJ/kg), which is comparable and even greater than many synthetic metal- and polymer-based foams. Lastly, this thesis leads to the discovery of a unique dual-scale single-crystalline porous lattice structure (porosity: 50 vol%, constituent material: 99 wt% calcite) in the ossicles of P. nodosus. At the atomic level, the ossicle is composed of single-crystal biogenic calcite. At the lattice level, the ossicle's microstructure organizes as a diamond-triply periodic minimal surface (TPMS) structure. Moreover, the crystallographic axes at atomic and lattice levels are aligned, i.e., the c-axis of calcite is aligned with the [111] direction of the diamond-TPMS lattice. This single crystallinity co-alignment at two levels mitigates the compliance of calcite in the c-axis direction by utilizing the stiff <111> direction of the diamond-TPMS lattice. Furthermore, 3D in-situ mechanical characterizations reveal that the presence of crystal defects such as 60° and screw dislocations at the lattice level suppresses slip-like fracture along the {111} planes of the calcitic diamond-TPMS lattice upon loading, achieving an enhanced energy absorption capability. Even though the skeleton of echinoderm is made up of single-crystal calcite, the structure fractures in a conchoidal manner rather than along the clipping plane, which can continuously fracture the fragments into small pieces and enhance energy dissipation. / Doctor of Philosophy / The application of engineering ceramic cellular solids as structural components is limited by their brittleness and flaw sensitivity. In contrast, nature has evolved ceramic cellular materials such as sea sponge, sea urchin spine, and diatom shells that are simultaneously lightweight, strong, and damage-tolerant. These properties are thought to be achieved by the structure design of the component of those materials. Learning design strategies from these natural ceramic cellular solids is significant for developing lightweight bio-inspired ceramic materials with improved mechanical performance. In this thesis, I investigated mechanical design strategies from natural ceramic cellular solids in three model systems, i.e., cuttlebone from cuttlefish Sepia officinalis, spines from sea urchin Heterocentrotus mammillatus, ossicles from starfish Protoreaster nodosus. These three natural ceramic porous solids have high mineral content in the constituent materials (> 90 wt%) and have a highly porous structure (porosity: 50 vol%-93 vol%). These three model systems are selected to represent the analogs of three typical structure forms of synthetic cellular solids, i.e., honeycomb-like structures, stochastic and periodic open-cell structures, respectively. This thesis aims to establish a quantitative relationship between the 3D multiscale structure and deformation/toughening behavior for these selected natural ceramic cellular solids via a combination of different experimental and computational approaches.

Modern durability and damage tolerance predictions for composite material systems rely on accurate estimates of the local stress and material states for each of the constituents, as well as the manner in which the constituents interact. In this work, an number of approaches to estimating the stress states and interactions are developed. First, an elasticity solution is presented for the problem of a penny-shaped crack in an N-phase composite material system opened by a prescribed normal pressure. The stress state around such a crack is then used to estimate the stress concentrations due to adjacent fiber fractures in a composite materials. The resulting stress concentrations are then used to estimate the tensile strength of the composite. The predicted results are compared with experimental values. In addition, a cumulative damage model for fatigue is presented. Modifications to the model are made to include the effects of variable amplitude loading. These modifications are based upon the use of remaining strength as a damage metric and the definition of an equivalent generalized time. The model is initially validated using results from the literature. Also, experimental data from APC-2 laminates and IM7/K3B laminates are used in the model. The use of such data for notched laminates requires the use of an effective hole size, which is calculated based upon strain distribution measurements. Measured remaining strengths after fatigue loading are compared with the predicted values for specimens fatigued at room temperature and 350°F (177°C). / Ph. D.

Low velocity drop weight impact tests were conducted on carbon/epoxy laminates under various magnitudes of uniform tensile stress. The composite plates were 8 ply (+45,0,- 45,90)_s laminates supported in a clamped-clamped/free-free configuration. Tensile preloads from near zero to approximately 60% of ultimate breaking strength were applied to specimens which were impacted at energies of 3.4, 4.5 and 6 Joules (2.5, 3.3 and 4.4 ft- Ibs). The amount of damage induced into the specimen was evaluated using instrumented impact techniques, x-ray inspection and cross-sectional photomicroscopy. Some static indentation tests were performed to examine if the impact events utilized in this study were of a quasi-static nature and also to gain insight into the shape of the deflected surface at various preload/transverse load combinations. Load-displacement curves from these tests were compared to those of the impact tests as was damage determined from x-ray inspection. The finite element technique was used to model the impact event and determine the stress field within the laminae. Results showed that for a given impact energy level, more damage was induced into the specimen as the tensile preload was increased. The majority of damage observed consisted of back face splitting of the matrix parallel to the fibers in that ply, associated with delaminations emanating from these splits. Tensile preloads tended to increase the length of these splits. The analysis showed qualitatively the results of tensile preloads on maximum load of impact, maximum transverse deflection and first failure mode and location. / Ph. D.