Modification in the resin-rich region between plies, also known as the interlaminar region, was investigated to increase the toughness of laminate composites structures. To achieve suitable modifications, the complexities of the physical and chemical processes during the resin curing procedure must be studied. This includes analyses of the interactions among the co-dependent microstructure, process parameters, and material responses. This dissertation seeks to investigate these interactions via a series of experimental and numerical analyses of the geometric- and temperature-based effects on locally interleaving toughening methods and further interlaminar synergistic toughening without interleaf. Two major weaknesses in composite materials are the brittle resin-rich interlaminar region which forms between the fiber plies after resin infusion, and the ply dropoff region which introduces stress concentration under loads. To address these weaknesses and increase the delamination resistance of the composite specimens, a dual bonding process was explored to alleviate the dropoff effect and toughen the interlaminar region. Hot melt bonding was investigated by applying clamping pressure to ductile thermoplastic interleaf and fiber fabric at an elevated temperature, while diffusion bonding between thermoplastic interleaf and thermoset resin is performed during the resin infusion. This method increased the fracture energy level and thus delamination resistance in the interlaminar region because of deep interleaf penetration into fiber bundles which helped confining crack propagation in the toughened area. The diffusion and precipitation between thermosets and thermoplastics also improved the delamination resistance by forming a semi-interpenetration networks. This phenomenon was investigated in concoctions of low-concentration polystyrene additive modified epoxy system, which facilitates diffusion and precipitation without increasing the viscosity of the system. Additionally, chemical reaction induced phase separation, concentration of polystyrene, and various curing temperatures are used to evaluate their effects on diffusion and precipitation. These effects were directly investigated by performing attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The diffusivity and curing kinetics experiments are performed to quantify the diffusivity coefficient of epoxy, hardener and thermoplastics, as well as the reaction rate constant of curing epoxy at various temperatures. Finally, mechanical testing and fracture surface imaging were used to quantify the improvements and characterize the toughening mechanism. Further improvement on delamination resistance was studied through the synergistic effect of combining different modification methods without the interleaf. Polysulfone molecules are end-capped with epoxide groups. Fiber surface is functionalized with amino groups to generate micro-mechanical interlocks. The interaction between two individual modifications chemically links the modified semi-interpenetration networks to the improved interfacial strength between fiber and epoxy to. The impact of the additive on the crosslinking density was examined through glass transition temperatures, and the chemical modification was characterized by Raman spectroscopy. Mode I and II fracture tests were performed to quantify the improvement of delamination resistance under pure opening and shear loads. The mechanism of synergistic effect was explained based on the fracture surface morphology and the interactions between the modification methods.

Multilayered materials and structures are of special interest to both academic researchers and industrial engineers as they have been used in an increasing number of applications, such as micro- electromechanical system (MEMS) on polymer substrate, protective coating on metal structure for anti-corrosion, ceramic coating on metal substrate for abrasion resistance, thermal barrier coatings, metal or metal oxide coating on metal-coated polymer for reflectance tuning and protection, as well as laminated composites and structures, etc. However, the surface layer and joint interface are commonly prone to premature failures because of cracking and delamination due to their different thermal and mechanical properties. In return, the premature failures will affect the performance and structural integrity of the device and eventually cause the failure of the multilayered structures to perform their functions. Therefore, it is essential to understand the mechanical behavior and corresponding failure mechanisms of the multilayered systems. This Ph.D. dissertation focuses on the study of opening-mode fracture (OMF) behavior and interfacial delamination in different types of multilayered structures, such as the aluminum plate or wire with alumina protective coating, multilayered advanced polymeric reflectors, and asphalt pavements. In addition, another failure mechanism, i.e. the material degradation and aging, is also studied. Firstly, a two-dimensional (2D) elasoplastic fracture model in Cartesian coordinates is developed to study the OMF in the thin alumina film fully bonded to an aluminum plate undergoing large- scale yielding. The stress field in the coating layer is described by one section between two adjacent cracks. The 2D plane strain formulations are employed to analyze the elastic field in the thin film, while a one-dimensional (1D) linear hardening plastic model is applied to account for the large plastic deformations in the substrate under substantial yielding. An elastoplastic shear lag model is established to transfer the tensile stress in the substrate to the thin film. General formulations and explicit expressions of the elastic/elastoplastic solutions of the thin film/substrate system under different loading stages have been presented. The elastic field in the thin film is then verified with the finite element (FE) results. The fracture energy release rate (ERR) is calculated and corresponding elastoplastic fracture analysis is conducted. Experimental characterization is further conducted to validate the present model; the results show that this fracture modle can capture the fracture initiation, infilling, and saturation in the thin film successfully. This model has been extended to cylindrical coordinates, where the alumina coating is fully bonded to an aluminum wire/rod. Due to the axial symmetry, the proposed 2D elasoplastic fracture model has been reproduced in polar coordinates and be used to study the OMF behavior of alumina coating fully bonded to an aluminum wire. For the case where thermal loading is applied on coating/substrate structures, neither the plane strain nor plane stress assumption can be applied because the thermal loading would introduce multi axial normal stress, therefore, a three-dimensional (3D) elastic fracture model is then developed to study the OMF in the coating fully bonded to the elastic substrate. When the temperature change reaches a certain level, block cracking will initiate in the surface layer to release the accumulated thermal stress. For simplicity, we assume the top surface of the coating would keep flat and in a rectangular shape after deformation. Then the elastic field in both coating and substrate is analyzed and verified with the FE results. The fracture ERR is then obtained based on the solved elastic field and used to analyze the fracture initiation, infilling, and saturation. In order to verify the fracture analysis, the theoretical fracture analysis results are compared with FE simulation results based on the cohesive zone model (CZM) and experimental data from the literature. The good agreement demonstrates the accuracy of the proposed 3D fracture model. In addition, this model for coating/substrate system is extended to study multilayered structures with arbitrary number of layers. In order to verify this extended model, the predicted elastic field in an advanced polymeric solar reflector is compared with FE simulation results and parametric studies are conducted to investigate the effect of geometry on the accuracy of this model. Furthermore, the fracture behavior of the surface layer in the advanced polymeric solar reflector is studied using the calculated fracture ERR. Additionally, the delamination behavior, as another common failure mode of the coating/substrate structures, is then studied. The delamination fracture energy of a multilayered glass solar reflec- tor is tested by employing the width-tapered beam method. The testing results indicate that the weakest interface of the multilayered solar reflector would be the glass-copper interface with a de- lamination fracture energy 4.4 J/m2. Using the tested fracture energy as an input, an FE model is built based on the CZM and the returned peeling force from simulation is then compared with the test results to verify the accuracy of the test method. The good agreement between the simulation and test results demonstrates that the width-tapered beam method is accurate enough to measure the delamination fracture energy of this multilayered solar reflector. Additionally, the effect of aging on the delamination fracture energy is investigated by measuring the delamination fracture energy after 50 hrs’ accelerated aging test. The results show that the aging has minor effect of delamination fracture energy for samples with alumina (Al2O3) protective coating, while it reduces the delamination fracture energy for samples with titania (TiO2) protective coating. As another failure mechanism, the material degradation or aging behavior is studied in this dissertation. The weight percentage of oxygen (WPO) in three types of asphalt binders extracted from reclaimed asphalt pavements (RAPs) and one extracted from fresh Hot Mix Asphalt (HMA), that have been aged under continuous ultraviolet (UV) or UV/moisture/condensation exposure for different period, is measured using a energy-dispersive X-ray spectroscopy (EDX). Then the tested data are fitted based on two classic aging models, namely the fast-rate constant-rate (FRCR) and nonlinear differential dynamic (NDD) models. The good fitted results show that both FRCR and NDD models can capture the aging behavior of asphalt binders extracted from both RAPs and fresh HMA under continuous UV or UV/moisture/condensation exposure. Meanwhile, although exposed under UV for the same time, the WPO in samples after UV/moisture/condensation aging are lower than those in samples after continuous UV aging, which indicates that condensation and moisture reduce the UV-induced photo-oxidative aging rate.

Polymeric composites exhibit time-dependent behavior, which raises a concern about their long term durability and leads to a viscoelastic study of these materials. Linear viscoelastic analysis has been found to be inadequate because many polymers exhibit nonlinear viscoelastic behavior. Classical laminate theory is commonly used in the study of laminated composites, but due to the plane stress/strain assumption its application has been limited to solving two dimensional, simple plate problems. A three dimensional analysis is necessary for the study of interlaminar stress and for problems involving complex geometry where certain local effects are important. The objective of this research is to develop a fully three-dimensional, nonlinear viscoelastic analysis that can be used to model the time-dependent behavior of laminated composites. To achieve this goal, a three-dimensional finite element computer program has been developed. In this program, 20-node isoparametric solid elements are used to model the individual plies. The three-dimensional constitutive equations developed for numerical calculations are based on the Lou-Schapery one-dimensional nonlinear viscoelasticity model for the uniaxial stress case. The transient creep compliance in the viscoelastic model is represented as an exponential series plus a steady-flow term, which allows for a simplification of the numerical procedure for handling hereditary effects. A cumulative damage law for three dimensional analysis was developed based on the Brinson-Dillard two-dimensional model to predict failure initiation. Calculations were performed using this program in order to evaluate its performance in applications involving complex structural response. IM7/5260-H Graphite/Bismaleimide and T300/5208 Graphite/Epoxy were the materials selected for modeling the time-dependent behavior. The cases studied include: 1) Tensile loading of unnotched laminates; 2) bending of a thick laminated plate; and 3) tensile loading of notched laminates. The analysis emphasized the study of the traction-free edge-effect of laminated composites, stress distribution around a circular hole, and stress redistribution and transformation in the layers. The results indicate that the stress redistributions over time are complicated and could have a significant effect on the long-term durability of the structure. / Graduation date: 1994

In recent impact and penetration mechanical tests, steel projectiles (AISI4340) were impacted into targets like concrete with striking velocities (1200 m/s to 1500 m/s). Results indicated a material removal from the nose of the projectile, phase changes of the projectile materials, a reduction in the length of the projectile, and a blunting of the nose shape. These observations cannot be explained by current theories and numerical integration code that are used to study impact and penetration mechanics. Thus, the objectives of the thesis research are to (a) formulate and characterize the mechanisms responsible for the material erosion of the impacting projectile and the mass loss from the nose region; and (b) to determine the physical properties of alloy steels that are important to penetration mechanics from ab initio methods. The results can be used to design new projectile materials that can provide the desired penetration characteristics. These objectives are accomplished by investigating two related problems. The first problem is to formulate simplified models that can explain the penetration mechanics. The new models include the varying cross-section nose, changes of yield stress behind the shock wave and high strain rate phase transitions. Nose erosion effects, and time-dependent penetration path can be determined by integrating ODEs. A cavity expansion theory model is used to obtain the target resistance that is responsible slowing and deforming the penetrating projectile. The second problem concerns the determination of the constitutive relations from ab initio methods. The equation of state (EOS) and magnetic moments for alloy steels are investigated by using a special quasirandom structure technique and ab initio methods. Specifically, EOS for an interstitial disordered alloy Fe1-x-yNixCy is developed. First, the EOS of iron and phase transition of iron are studied and validated. Second, Nickel is considered to investigate the substitutional disordered alloy Fe1-x-yNixCy. Third, Carbon is placed at an interstitial position in the substitutional disordered alloy. These investigations will form foundation for future work involving new projectile with steel nose and shank made of multifunctional structural energetic materials.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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The present study involves analysis and design optimisation of lattice composite structures using symbolic computation. The concept of a homogenised model is used to represent heterogeneous composite isogrid structure as a homogeneous structure with the stiffness equivalent to the original grid structure. A new homogenisation technique is developed and used in the present study. The configuration of a unit cell and the geometrical parameters of the ribs of a composite isogrid cylinder are optimised subject to a strength criterion in order to maximise externally applied loading to provide maximum strength and stiffness of the structure as a whole. The effects of tension and torsion on the optimum design are investigated. Special purpose computation routines are developed using the symbolic computation package Mathematica for the calculation of equivalent stiffness of a structure, failure analysis and calculation of optimum design parameters. The equivalent stiffness homogenisation approach, in conjunction with optimum search routines, is used to determine the optimal values of the design variables. The numerical approach employed in the present study was necessitated by the computational inefficiency and conventional difficulties of linking the optimiser and the FEM analysis package for calculating the stress resultants used in the optimisation process. These drawbacks were successfully overcome by developing special purpose symbolic computation routines to compute stress resultants directly in the program using a new homogenisation approach for the model with equivalent stiffness. In the design optimisation of cylindrical isogrids the computational efficiency of the optimisation algorithm is improved and good accuracy of the results has been achieved. The investigation on the basis of failure analysis shows that the difference in the value of the maximum load applied to the optimal and non-optimal isogrid structure can be quite substantial, emphasising the importance of optimisation for the composite isogrid structures. The computational efficiency of optimisation algorithms is critical and therefore special purpose symbolic computation routines are developed for its improvement. A number of optimal design problems for isogrid structures are solved for the case of maximum applied load design. / Thesis (Ph.D.)-University of Natal, Durban, 2000.

The purpose of this research is to make an investigation into the crashworthiness modelling of Sheet Moulding Compound (SMC) composite materials, and to study the response of SMC composite structures under dynamic loading. The primary research objectives are thus to review classical and advanced material failure models, and to perform numerical simulation of the crash of composite structures using already available material models. Additionally, a new material model is to be developed for implementation into a commercially available finite element package. In parallel with the numerical simulation of the crasrung of an SMC composite structure, experimentation is performed which is used as a source of validation and comparison with the simulation. For this purpose a testing regime is introduced, which may be mirrored in simulation. As any material model requires initial experimental inputs, the purpose of experimentation is twofold, with testing required both for the quantification of the required model inputs and the basic material characterisation before simulation may begin, as well as for the proposed validation and comparison after the simulation has been carried out. Thus the design of the testing methodology, as well as the design, selection and fabrication of the testing equipment and the composite specimens and demonstrators, as well the actual testing itself, are necessary secondary requirements of the project. Once the testing regime has been facilitated and carried out, numerical simulation validation using already available composite material models may then be carried out at various levels. The results are then analysed and validated with the resultant justification of a new model being developed. The critical viewpoint to be delivered throughout is the need for theoretical formulations for material modelling to be extensively researched and validated in terms of their implementabilty and practicality, a key analysis seemingly missing in most technical write-ups. Such analyses are performed and discussed here, rughlighting the volume of additional work that is encompassed by such a study. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2003.

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