Models based on the Continuum Damage Mechanics principle are increasingly used for predicting the initiation and growth of damage in materials. The growing reliance on 3-D finite element (FE) virtual structural testing demands implementation and validation of robust material models that can predict the material behaviour accurately. The use of these models within numerical analyses requires suitable material data. EU aerospace companies along with Cranfield University and other similar research institutions have created the MUSCA (non-linear MUltiSCale Analysis of large aero structures) project to develop virtual structural testing prediction. The MUSCA project focuses on static failure testing of large aircraft components. It aims to reduce laboratory tests using advanced numerical analysis to predict failure in order to save overall cost and development time. This thesis aims to improve the current capability of finite element codes in predicting orthotropic material behaviour, primarily damage. The Chow and Wang damage model has been implemented within ABAQUS as a VUMAT subroutine. This thesis presents the development of a numerical damage prediction model and an experimental study to develop a damage material characterisation process that can easily be performed using standard tensile test specimen and equipment already available in the aerospace industry. The proposed method makes use of Digital Image Correlation (DIC), a non-contact optical strain field measurement technique. Experiments were conducted at Cranfield University material testing facility on aerospace aluminium alloy material AA-2024-T3 and AA-7010-T7651. After thorough literature survey a complete new method was formulated to implement Chow and Wang damage model in Abaqus Explicit numerical code. The damage model was successfully implemented for isotropic and orthotropic behaviour using single element model, multi-element coupon test model and a simple airframe structure. The simulation results were then verified with the similar experimental results by repeating the experimental procedure using simulation for each material type and found matching results. The model is then compared with experimentally determined orthotropic material parameter for AA2024 and AA7010 for validation and found agreeable results for practical use. The material characterisation of damage parameters from standard tensile specimen using DIC technique was also demonstrated and the procedures were established. In this research the combination of experimental work and numerical analysis with clear and simpler calibration strategy for damage model is demonstrated. This is the important contribution of this research work and the streamlined procedures are vital for the industry to utilise the new damage prediction tools. The damage model implementation and test procedures developed through this research provide information and processes involved in fundamentally predicting the ductile damage in metals and metal alloys. The numerical damage model developed using the well-defined verification and validation procedures explained in this research work with new streamlined damage material characterisation using recent contact less DIC technique has wider implication in the material model development for ductile metals in general. The thesis ultimately delivered a fully verified, validated robust damage model numerical simulation code with a new DIC damage characterisation procedure for practical application. The model is now used by the aerospace industry for predicting damage of large aircraft structures.

This thesis describes a series of experimental and computational studies carried out on Xl00 pipeline steel with the objective to characterise the tearing resistance of the material. A Cellular Automata-Finite Element(CAFE) technique was used in this work to develop a 3D numerical model to provide a more realistic description of the ductile damage mechanisms of the pipeline steel. In this model, the Rousselier micro-mechanisms damage theory and an appropriate cell size in a CA array represent the material behaviour. The experimental work consisted of laboratory tensile specimens in four different orientations of the material to determine the properties of the pipeline steel. Two novel designs were conducted to measure the deformation behaviour when loaded in the through wall direction. Compact C(I) and tear specimens were also tested to capture the crack growth, and the flat and shear fracture characteristics. The experimental data of laboratory samples were used to calibrate the continuum damage models. SEM (Scanning Electron Microscope) micrograph observations were carried out in tensile tests, standard C(f), slant notch C(f) and tear specimens. These observations revealed that spacing between large dimples of flat fracture are of the order of five times larger than shear fracture. It is important since the transfer of the material model parameters is made by modifying the cell size according to the average spacing between large voids in the material, d. Therefore 3D CAFE models for flat and shear fracture were created according to the microstructural characteristics to interpret the experimental findings. The main aim of the research reported here is to investigate transferability of the damage model parameters to gas pipelines steels from laboratory scale samples, and then to predict the fracture response of real structures. The CAFE technique has been shown to be a powerful tool in reducing simulation time whilst maintaining good predictions of shear damage and material resistance in terms of CTOA criterion. This was not achieved by classical FE methods where a very fine mesh is required to represent the characteristic dimension of ductile fracture. Similar reasonable results were obtained when anisotropic flat fracture was predicted but transferability of the damage parameters to CT specimens needs still further investigation.

In this thesis an investigation into the behaviour of thin walled lipped channel columns under combined end compression and bending is reported. The thesis begins with a short introduction, followed by a review of the relevant published literature. In the introduction an outline is given of the different buckling characteristics associated with Euler, local and coupled Euler-local modes of behaviour. The scope of the literature review spans from the early development work on coupled mode buckling through to the wide variety of interaction problems covered in present day studies. An introduction to the basic differential equations and elastic strain energy expressions used in the theoretical approach of the lipped channel problem is then briefly outlined. Theoretical analyses of local instability and post-local buckling interaction behaviour are presented. The local buckling analysis is undertaken for the section using the Rayleigh-Ritz method. In this, the cross-sectional deflected shape is taken as a series of algebraic polynomials and each polynomial set in the series is arranged to satisfy all compatibility and equilibrium boundary conditions at the plate junctions and at the free edges of the lip. The analysis is carried out for various values, of section length, to obtain that value for which the lowest buckling load is induced. From this multi-term buckling analysis a very accurate approximation is used as the first term in the subsequent semi-energy post-buckling (ii) analysis, which incorporates also an additional set of algebraic polynomials to take account of changes in the deflected form in the post-buckling range. Thus, the deflection functions postulated are extremely accurate at the point of buckling and have a built-in facility to change in the post-buckling range. The numerical work involved in the analysis was carried out in the University's ICL 1904S computer for which a program was written using the FORTRAN IV programming language. Details are given of an experimental investigation which was carried out to obtain information on the interactive buckling behaviour of lipped channel columns and to provide experimental results to authenticate the validity of the theoretical solutions. A description of the loading rig and its operation is presented in conjunction with a full set of assembly and component design drawings. In general, the procedure used in the investigation was to load columns of various section dimensions and with various loading eccentricities to collapse, recording strain measurements and overall deflections as loading progressed. A strain investigation was carried out to obtain knowledge of the stress variations at the centre of the column at various stages of loading. A comparison of experimental results with theoretical predictions is given. The agreement between theory and experiment is good in general, and in particular for the comparisons of stress variations with progressive loading. A summary of the nvestigation is included, together with conclusions and suggestions for further research and extension of the analysis. The publications discussed in the literature review appear in a chronological bibliography list at the end of the text. The thesis is concluded by six short appendices which supplement the main text and enlarge on some aspects of the investigation.

Current defect assessment procedures assume all flaws to be sharp, a conservative assumption that can lead to pessimistic structural integrity assessment of structures and under-estimation of their safety margin against fracture. This study has developed an engineering assessment procedure for predicting cleavage fracture resistance of structures containing non-sharp defects. The new procedure can be incorporated into existing defect assessment methods via a modification of the Failure Assessment Diagram (FAD). The new procedure was developed and validated using a combination of testing and finite element analysis. A heat-treated structural steel formed the basis for the experimental work. A large test programme of 135 Single Edge Notch Send SEeS) specimens with sharp cracks and U-notches was used to quantify the increase in apparent toughness that occurs with increasing notch root radius p. The Weibull stress-based toughness scaling model was used to predict the increase in cleavage fracture resistance that occurs with increasing p up to a notch radius of 2mm. In the new assessment procedure several new parameters are utilised that are analogous to parameters used in existing constraint correction methods. The notch tip loading severity is described by the elastic notch tip stress ON , analogous to the use of the elastic T-stress used to quantify crack tip constraint in constraint correction. The notch geometry is measured using a loadindependent non-dimensional parameter {3N analogous to the {3r parameter used in constraint correction. Material parameters r and I define the sensitivity of the material toughness to the notch effect; these are analogous to the material parameters a and k used for constraint correction. The parameters r and I can either be obtained from test data or from a series of look-up tables developed using the Wei bull stress-based toughness scaling model. The study also showed that for a given level of constraint defined by the T-stress, the notch and constraint effects are independent of each other. This enables the toughness benefit due to the notch to be applied in conjunction with existing constraint correction methods, e.g. for the assessment of blunt, shallow flaws.

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