An extensive research program was performed to investigate the load interaction effect of the combined action of small amplitude high R ratio cycles and large amplitude low R ratio underloads on the crack growth of large cracks. The study was driven by the needs of the damage tolerance approach in the helicopter structures, which requires robust knowledge on the crack growth behaviour of the advance high strength alloys under the characteristic helicopter spectra loading. The study was conducted on three metallic alloys, Ti-10V-2Fe-3Al, Al8090 T852 and Al7010 T76351 using compact tension specimens (w=70mm, t=17mm). The potential drop technique was used for the measurements of the crack length. The crack opening loads were determined from the applied load versus crack opening mouth displacement curve using a curve fitting technique and crack opening displacement gauge. The experimental results show that cracks can grow faster than the life predictions with no load interaction effects under spectra containing tensile underloads. The acceleration effects are different depending on the number of the small cycles, the Kmax, the R ratio of the small cycles, the underload cycle and the material. Significant closure observations on the underloads and on the small cycles of variable amplitude loading spectra were made. Based on the test finding and on the studies of other researchers, it is suggested that the acceleration effects are mainly due to the reduction of crack opening point of the tensile underloads comparing with the Constant Amplitude Loading (CAL) data. An extensive evaluation of the ability of FASTRAN model to predict the fatigue lives under the tested loading spectra was carried out. The evaluation focuses on the influence of the constraint factor a and the ∆Keff curve inputs on the predictions. The model produces very good and consistent predictions for the three alloys, when the inputs represent adequately the actual fatigue mechanism. The model predicts the measured acceleration effects by reducing the closure level of the underloads.

In this thesis the validation of a computational fluid dynamics (CFD) code against published experimental data and alternative CFD results is explored. Six case studies are presented with flow speeds ranging from mid supersonic to hypersonic. The studies include complex compressible flow phenomena such as the interaction of shockwaves with boundary layers, vortices and other shockwaves. Two of the six validation cases also consider complex thermodynamic behaviour such as vibrational and chemical nonequilibrium. The problems chosen are limited in scope to flows which are planar or axisymmetric and remain laminar throughout the region of flow studied. This allows the studies to focus on simulating the fundamental behaviour of the flow without the added complication of turbulence or the need for large scale computing facilities. The problems which arose during the validation studies and a background to why they occurred and, wherever possible, strategies to avoid their occurrence are discussed. General modelling strategies in compressible CFD are also discussed. In this thesis the software suite CFD-FASTRAN is used to construct and analyse the CFD models. The material presented in this thesis is however general such that users of similar packages should find the thesis beneficial.

In this thesis the validation of a computational fluid dynamics (CFD) code against published experimental data and alternative CFD results is explored. Six case studies are presented with flow speeds ranging from mid supersonic to hypersonic. The studies include complex compressible flow phenomena such as the interaction of shockwaves with boundary layers, vortices and other shockwaves. Two of the six validation cases also consider complex thermodynamic behaviour such as vibrational and chemical nonequilibrium. The problems chosen are limited in scope to flows which are planar or axisymmetric and remain laminar throughout the region of flow studied. This allows the studies to focus on simulating the fundamental behaviour of the flow without the added complication of turbulence or the need for large scale computing facilities. The problems which arose during the validation studies and a background to why they occurred and, wherever possible, strategies to avoid their occurrence are discussed. General modelling strategies in compressible CFD are also discussed. In this thesis the software suite CFD-FASTRAN is used to construct and analyse the CFD models. The material presented in this thesis is however general such that users of similar packages should find the thesis beneficial.

In this thesis the validation of a computational fluid dynamics (CFD) code against published experimental data and alternative CFD results is explored. Six case studies are presented with flow speeds ranging from mid supersonic to hypersonic. The studies include complex compressible flow phenomena such as the interaction of shockwaves with boundary layers, vortices and other shockwaves. Two of the six validation cases also consider complex thermodynamic behaviour such as vibrational and chemical nonequilibrium. The problems chosen are limited in scope to flows which are planar or axisymmetric and remain laminar throughout the region of flow studied. This allows the studies to focus on simulating the fundamental behaviour of the flow without the added complication of turbulence or the need for large scale computing facilities. The problems which arose during the validation studies and a background to why they occurred and, wherever possible, strategies to avoid their occurrence are discussed. General modelling strategies in compressible CFD are also discussed. In this thesis the software suite CFD-FASTRAN is used to construct and analyse the CFD models. The material presented in this thesis is however general such that users of similar packages should find the thesis beneficial.

In this thesis the validation of a computational fluid dynamics (CFD) code against published experimental data and alternative CFD results is explored. Six case studies are presented with flow speeds ranging from mid supersonic to hypersonic. The studies include complex compressible flow phenomena such as the interaction of shockwaves with boundary layers, vortices and other shockwaves. Two of the six validation cases also consider complex thermodynamic behaviour such as vibrational and chemical nonequilibrium. The problems chosen are limited in scope to flows which are planar or axisymmetric and remain laminar throughout the region of flow studied. This allows the studies to focus on simulating the fundamental behaviour of the flow without the added complication of turbulence or the need for large scale computing facilities. The problems which arose during the validation studies and a background to why they occurred and, wherever possible, strategies to avoid their occurrence are discussed. General modelling strategies in compressible CFD are also discussed. In this thesis the software suite CFD-FASTRAN is used to construct and analyse the CFD models. The material presented in this thesis is however general such that users of similar packages should find the thesis beneficial.

In this thesis the validation of a computational fluid dynamics (CFD) code against published experimental data and alternative CFD results is explored. Six case studies are presented with flow speeds ranging from mid supersonic to hypersonic. The studies include complex compressible flow phenomena such as the interaction of shockwaves with boundary layers, vortices and other shockwaves. Two of the six validation cases also consider complex thermodynamic behaviour such as vibrational and chemical nonequilibrium. The problems chosen are limited in scope to flows which are planar or axisymmetric and remain laminar throughout the region of flow studied. This allows the studies to focus on simulating the fundamental behaviour of the flow without the added complication of turbulence or the need for large scale computing facilities. The problems which arose during the validation studies and a background to why they occurred and, wherever possible, strategies to avoid their occurrence are discussed. General modelling strategies in compressible CFD are also discussed. In this thesis the software suite CFD-FASTRAN is used to construct and analyse the CFD models. The material presented in this thesis is however general such that users of similar packages should find the thesis beneficial.

In this thesis the validation of a computational fluid dynamics (CFD) code against published experimental data and alternative CFD results is explored. Six case studies are presented with flow speeds ranging from mid supersonic to hypersonic. The studies include complex compressible flow phenomena such as the interaction of shockwaves with boundary layers, vortices and other shockwaves. Two of the six validation cases also consider complex thermodynamic behaviour such as vibrational and chemical nonequilibrium. The problems chosen are limited in scope to flows which are planar or axisymmetric and remain laminar throughout the region of flow studied. This allows the studies to focus on simulating the fundamental behaviour of the flow without the added complication of turbulence or the need for large scale computing facilities. The problems which arose during the validation studies and a background to why they occurred and, wherever possible, strategies to avoid their occurrence are discussed. General modelling strategies in compressible CFD are also discussed. In this thesis the software suite CFD-FASTRAN is used to construct and analyse the CFD models. The material presented in this thesis is however general such that users of similar packages should find the thesis beneficial.

This thesis investigates fatigue crack growth rate behavior in the threshold and near-threshold regimes for a Ti-6Al-4V (STOA) alloy using two proposed ASTM procedures- (1) load-shedding (LS) using a larger load-shed rate than the current ASTM Standard E647 load-reduction (LR) test procedure, and (2) compression pre-cracking constant-amplitude (CPCA) or load-increasing (CPLI) and load-shedding (CPLS). Tests were conducted at a low stress ratio (R = 0.1) on compact C(T) specimens of two different widths (W = 51 and 76 mm) and threshold fatigue crack growth rates were generated. These test data were compared to previous test data produced from the same batch of material using the current LR and the CPCA test procedure. While no test procedure provided an exact representation of the threshold value (?Kth), the compression pre-cracking (CP) procedures were the most promising. The LR, LS, and CPLS test procedures were influenced by prior loading-history and various crack-closure mechanisms, leading to higher ?Kth values and slower crack growths in the threshold regime. The LS tests (at shed-rates of -0.08,-0.32, and -0.95 mm-1) generated ?Kth values that were 15% to 32% higher than the estimated threshold stress-intensity factor range (?*Kth)R=0.1. The CP test procedures are a more accurate alternative for developing near-threshold and threshold fatigue crack growth rates. The CPLS test procedure produced a ?Kth value that was 10% higher than (?*Kth)R=0.1. LR and LS tests produced different ?Kth values as a function of the specimen width for the given load ratio. The CP test procedures produced consistent crack growth rates over the same range of ?K values examined, independent of the specimen width. Further research is required for developing test procedure(s) capable of providing a more definitive representation of the ?Kth value and closureree fatigue crack growth rates in the threshold regime.

In this thesis, store separation from two different configurations are solved using computational methods. Two different commercially available CFD codes / CFD-FASTRAN, an implicit Euler solver, and an unsteady panel method solver USAERO, coupled with integral boundary layer solution procedure are used for the present computations. The computational trajectory results are validated against the available experimental data of a generic wing-pylon-store configuration at Mach 0.95. Major trends of the separation are captured. Same configuration is used for the comparison of unsteady panel method with Euler solution at Mach 0.3 and 0.6. Major trends are similar to each other while some differences in lateral and longitudinal displacements are observed. Trajectories of a fueltank separated from an F-16 fighter aircraft wing and full aircraft configurations are found at Mach 0.3 using only the unsteady panel code. The results indicate that the effect of fuselage is to decrease the drag and to increase the side forces acting on the separating fueltank from the aircraft. It is also observed that the yawing and rolling directions of the separating fueltank are reversed when it is separated from the full aircraft configuration when compared to the separation from the wing alone configuration.

This study aims to investigate the mechanical and fatigue behavior of additively manufactured metallic materials. Several challenges associated with different metal additive manufacturing (AM) techniques (i.e. laser-powder bed fusion and direct laser deposition) have been addressed experimentally and numerically. Experiments have been carried out to study the effects of process inter-layer time interval – i.e. either building the samples one-at-a-time or multi-at-a-time (in-parallel) – on the microstructural features and mechanical properties of 316L stainless steel samples, fabricated via a direct laser deposition (DLD). Next, the effect of building orientation – i.e. the orientation in which AM parts are built – on microstructure, tensile, and fatigue behaviors of 17-4 PH stainless steel, fabricated via a laser-powder bed fusion (L-PBF) method was investigated. Afterwards, the effect of surface finishing – here, as-built versus machined – on uniaxial fatigue behavior and failure mechanisms of Inconel 718 fabricated via a laser-powder bed fusion technique was sought. The numerical studies, as part of this dissertation, aimed to model the mechanical behavior of AM materials, under monotonic and cyclic loading, based on the observations and findings from the experiments. Despite significant research efforts for optimizing process parameters, achieving a homogenous, defectree AM product – immediately after fabrication – has not yet been fully demonstrated. Thus, one solution for ensuring the adoption of AM materials for application should center on predicting the variations in mechanical behavior of AM parts based on their resultant microstructure. In this regard, an internal state variable (ISV) plasticity-damage model was employed to quantify the damage evolution in DLD 316L SS, under tensile loading, using the microstructural features associated with the manufacturing process. Finally, fatigue behavior of AM parts has been modeled based on the crack-growth concept. Using the FASTRAN code, the fatigue-life of L-PBF Inconel 718 was accurately calculated using the size and shape of process-induced voids in the material. In addition, the maximum valley depth of the surface profile was found to be an appropriate representative of the initial surface flaw for fatigue-life prediction of AM materials in an as-built surface condition.