Understanding the mechanical behavior of composite materials requires extensive knowledge of fracture behavior as a crack approaches an interface between the bulk material and the reinforcement structure. Overall material toughness can be greatly influenced by the propensity of an impinging crack to propagate directly through the substrate or deflect along an interface boundary. As the basis for this thesis; the assertion that an impinging crack may encounter a reinforcement structure at various incident angles is explored. This requires the ability to predict crack penetration/ deflection behavior not only normal to the reinforcement, but at various incident angles. Previous work in the area of interface fracture mechanics has used a stress or energy based approach, with recent advances in the field of a combined cohesive-zone method. Work presented here investigates the interaction between strength and toughness when using the cohesive-zone method on the problem of an impinging crack not normally incident to the interface of a composite material. Computational mechanics methods using Abaqus and user-define cohesive elements will be applied to this angled incident crack problem. A circular model based on the displacement field equations for mode-I fracture loading is introduced and verified against well-established LEFM solutions. This circular model is used to study the effects of incident crack angle on the penetration vs. deflection behavior of an impinging crack at various angles of incidence. Additionally, the effects of angle on the load applied to the model at fracture are explored. Finally, a case study investigating how the interaction between strength and toughness found using the cohesive-zone method helps to explain some of the inconsistencies seen in the interface indentation fracture test procedure. / Graduation date: 2013

This thesis is concerned with the study of the crashworthiness of thin-walled composite structures. Composites are being used more and more in different fields of engineering, particularly, in aerospace and automotive industries because of their high strength-to-weight and stiffness-to-weight ratios, quality and cost advantages. More and more metal parts in cars for instance become or are already replaced by new advanced materials. Composite materials are included in these new advanced materials with the following advantages: weight reduction, corrosion resistance, aesthetics and style, isolation and the ability to integrate several parts into one single structural component. The introduction of new composite structural components (body panels, bumpers, crash absorbers, etc.) requires the development and implementation of new approaches to structural analysis and design. Crashworthiness is one of the foremost goals of aircraft and automotive design. It depends very much on the response of various components which absorb the energy of the crash. In order to design components for crashworthy structures, it is necessary to understand the effects of loading conditions, material behaviour, and structural response. Due to the complexity of the material structure (matrix reinforced with fibres) and specific mechanical properties the nature of transforming the collision kinetic energy into material deformation energy differs from that of conventional metal alloys. The energy absorption mechanics are different for the advanced composites and depend on the material structure (type of reinforcement) and structural design. The primary function of the energy absorption for the composites belongs to the progressive crushing of the materials themselves and structural components (beams, tubes, etc.) made of such materials. Since the mechanics of composite materials and structural components differs substantially from the conventional applications there is a need to develop an appropriate way of modelling and analysis relevant to this problem. Currently there are a large variety of design approaches, test results, and research investigations into the problem under consideration depending on the type of composite material and design geometry of the parts. It has been found that in general an application of fibre reinforced plastics (FRP) to vehicle compartments can satisfy the structural requirements of the passenger compartment including high strength and light weight. Implementation of new advanced composite materials provides the opportunity to develop designs of reliable structural composite parts in high volume for improved automotive fuel economy. Structural optimisation and crashworthiness of composite components should be incorporated into design calculations to control the mechanical performance. The introduction which follows describes the aims of the present study of the crashworthiness modelling and simulation of the structural response of thin-walled composite components which are subjected to various loading conditions relevant to vehicle design. The research programme undertaken within the framework of this project includes development and validation of the modelling and simulation methodology applicable to the crashworthiness analysis of thin-walled composite structures. Development of computerised dynamic modelling of structural components offers the capability of investigating the design parameters without building the actual physical prototypes. In this approach, the dynamic behaviour of the structure is simulated for specified external inputs, and from the corresponding response data the designer is able to determine its dynamic response characteristics, and estimate the crashworthiness of the structure in vehicle engineering applications. / Thesis (Ph.D.)-University of Natal, Durban, 2003.

The present study involves the analysis and design optimization of thin and thick laminated composite structures using symbolic computation. The fibre angle and wall thickness of balanced and unbalanced thin composite pressure vessels are optimized subject to a strength criterion in order to maximise internal pressure or minimise weight, and the effects of axial and torsional forces on the optimum design are investigated. Special purpose symbolic computation routines are developed in the C programming language for the transformation of coordinate axes, failure analysis and the calculation of design sensitivities. In the study of thin-walled laminated structures, the analytical expression for the thickness of a laminate under in-plane loading and its sensitivity with respect to the fibre orientation are determined in terms of the fibre orientation using symbolic computation. In the design optimization of thin composite pressure vessels, the computational efficiency of the optimization algorithm is improved via symbolic computation. A new higher-order theory which includes the effects of transverse shear and normal deformation is developed for the analysis of laminated composite plates and shells with transversely isotropic layers. The Mathematica symbolic computation package is employed for obtaining analytical and numerical results on the basis of the higher-order theory. It is observed that these numerical results are in excellent agreement with exact three-dimensional elasticity solutions. The computational efficiency of optimization algorithms is important and therefore special purpose symbolic computation routines are developed in the C programming language for the design optimization of thick laminated structures based on the higher-order theory. Three optimal design problems for thick laminated sandwich plates are considered, namely, the minimum weight, minimum deflection and minimum stress design. In the minimum weight problem, the core thickness and the fibre content of the surface layers are optimally determined by using equations of micromechanics to express the elastic constants. In the minimum deflection problem, the thicknesses of the surface layers are chosen as the design variables. In the minimum stress problem, the relative thicknesses of the layers are computed such that the maximum normal stress will be minimized. It is shown that this design analysis cannot be performed using a classical or shear-deformable theory for the thick panels under consideration due to the substantial effect of normal deformation on the design variables. / Thesis (Ph.D.)-University of Natal, Durban, 1994.

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.

Flush repairs to military aircraft are expected to become more prevalent as more thick skin composites are used, particularly on the surface of the fuselage, wings and other external surfaces. The use of these repairs, whilst difficult to manufacture provide an aerodynamic, ???stealthy??? finish that is also more structurally efficient than overlap repairs. This research was undertaken to improve the design methodology of scarf repairs with reduced material removal and to investigate the damage tolerance of scarf repair to low velocity impact damage. Scarf repairs involve shallow bevel angles to ensure the shear stress in the adhesive does not exceed allowable strength. This is important when repairing structures that need to withstand hot and humid conditions, when the adhesive properties degrade. Therefore, considerable amounts of parent material must be machined away prior to repair. The tips of the repair patch and the parent laminate are very sharp, thus a scarf repair is susceptible to accidental damage. The original contributions include: ??? Developed analytic means of predicting the stresses within optimised scarf joints with dissimilar materials. New equations were developed and solved using numerical algorithms. ??? Verified using finite element modelling that a scarfed insert with dissimilar modulus subjected to uniaxial loading attracted the same amount of load as an insert without a scarf. As such, the simple analytic formula used to predict load attraction/diversion through a plate with an insert may be used to predict the load attraction/diversion into a scarf repair that contains a dissimilar adherend patch. ??? Developed a more efficient flush joint with a doubler insert placed near the mid line of the parent structure material. This joint configuration has a lower load eccentricity than external doubler joint. ??? Investigated the damage tolerance of scarf joints, with and without the external doubler. The results showed that scarf joints without external doublers exhibited a considerable strength reduction following low velocity impact. Based on the observations, the major damage mechanics in the scarf joint region following impact have been identified. These results demonstrated that it is important to incorporate damage tolerance in the design of scarf repairs.

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Three test methods were employed to characterize the response of unidirectional Boron/Aluminum metal matrix composite material under monotonic and cyclic loading conditions, namely: Iosipescu Shear, Off-Axis Tension and Compression. The characterization of the elastic and plastic response includes the elastic material properties, yielding and subsequent hardening of the unidirectional composite under different stress ratios in the material principal coordinate system. The elastic response is compared with the prediction of the transformation theory, based on the far field stress ōₓₓ, the Pagano-Halpin Model, and finite element analysis. Yield loci were generated for different stress ratios and were compared for the three different test methods, taking into account residual stresses and specimen geometry. The yield locus for in-plane shear was compared with the prediction of an analytical micromechanical model. The influence of the scatter in the experimental data on the predicted yield surface was also analyzed. Likewise the experimental material strength in tension and compression was compared with the Maximum Stress and the Tsai-Wu failure criterion. / M.S.

This investigation was aimed at understanding the global and local strain and deflection responses of stiffened skins. Global deformations of the stiffened skins, under load, produce high local stresses in the interface region between the stiffener and skin. Test panels were designed to study the stiffener and skin interactions using parameters typical of stiffened skins for aircraft fuselages. A total of six panels were tested. Two skin laminates, both 0.04 in. thick, and three stiffener configurations were studied. The panels, having clamped edge boundary conditions, were subjected to pressure loads of up to 14.5-14.8 psi. Out-of-plane deflections and longitudinal and transverse strains were measured in several locations. The deflection responses showed a strongly nonlinear behavior at pressure loads of less than 5 psi. In addition relatively severe gradients of both longitudinal and transverse strains were measured in the interface region of the stiffener and skin. Finite element models incorporating geometric nonlinearities were made of four of the panels. Results of these models substantiated the overall findings of the experimental measurements. / Master of Science / incomplete_metadata

M.S.