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.

A weight function based modified strip-yield model is adapted to include the mode I effects of roughness induced crack closure. The weight function methodology is verified for arbitrary mixed mode loading conditions under load levels experienced in fatigue crack growth. A rough crack geometry is simulated as a sine wave fitted to a sawtooth defined by an asperity angle and period. Additional mode I crack closure due to mode II sliding of crack faces is calculated and incorporated into the strip-yield model by lengthening and shortening the crack face elements. Simulations of fatigue crack growth using the newly developed model are examined via a design and analysis of computer experiments. The effects of model parameters are identified. Combined roughness and plasticity induced crack closure for long cracks is studied.

Fatigue and crack-growth tests were conducted on 7050-T7451 aluminum alloy under a wide range of loading conditions. Crack-growth tests were conducted on compact, C(T), specimens under constant-amplitude loading, single-spike overloads, and a simulated aircraft spectrum loading. Fatigue tests were also conducted on single-edge-notch bend, SEN(B), specimens under constant-amplitude loading and three aircraft load spectra. The FASTRAN, life-prediction code, was used to make crack-growth predictions on the C(T) specimens; and to make fatigue-life calculations using a 12-micrometer initial flaw size at the center of the edge-notch on the SEN(B) specimens. The predictions agreed fairly well with most of the tests, except the model was unconservative on the single-spike overload tests and the severe spectrum Mini-TWIST+ Level 1 tests. The discrepancy was suspected to be caused by a low constraint factor and/or crack paths meandering around overload plastic zones. A roughness- and plasticity-induced crack-closure model would be needed to improve the model.

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[pt] A fluência em compósito cimentício reforçado com fibra (CRF) no estado pré-fissurado tornou-se um importante tópico de estudo recentemente. Isso se deve ao fato de que o comportamento dependente do tempo do CRF e a estabilidade a longo prazo de fissuras sob cargas de flexão sustentadas ainda são pouco compreendidas. Este trabalho busca explorar o uso de fibras de aço e PP para definir sua influência na fluência, analisando a evolução das aberturas de fissuras com o tempo. O material foi inicialmente caracterizado sob testes de flexão de três e quatro pontos em amostras prismáticas com entalhe. Para os testes de fluência, os corpos de prova foram pré-fissurados a 0,5 mm e testados sob carga constante durante 45 dias. Para entender os mecanismos relacionados, testes de fluência também foram realizados em fibras e em uma configuração de arrancamento. Analisando os resultados dos testes de fluência, verificou-se que a taxa de COD (crack opening displacement) é uma ferramenta interessante para avaliar o comportamento a longo prazo do CRF fissurado e para definir um critério de estabilidade. Além disso, verificou-se que o concreto incorporando fibras sintéticas apresenta maiores deformações de fluência do que o reforçado com fibras de aço. Isso pode ser explicado pelas diferentes características das ligações fibra-matriz, analisadas pelos testes de arrancamento monotônico e de carga sustentada, e pela resposta de compressão do compósito. Finalmente, as propriedades residuais das amostras ensaiadas por fluência foram determinadas por testes monotônicos de flexão. / [en] Creep in pre-cracked fiber reinforced cementitious composites has become an important topic of study recently. This is due to the fact that the time dependent behaviour of FRC and long term stability of cracks under sustained bending loads are still poorly understood. This work seeks to explore the use of steel and PP fibers in order to define their influence on creep, by analyzing the crack opening displacement rate in FRC specimens. The material was first characterized under three and four-point bending tests in notched prismatic specimens. For the creep tests, the specimens were pre-cracked to 0.5 mm, and then tested under constant load during 45 days. In order to better understand the related mechanisms, creep tests were also carried on single fibers and in a fiber pullout configuration. Analyzing the creep tests results, it was verified that the COD rate is an interesting tool to evaluate the long-term behaviour of the cracked FRC and to define a stability criterion. In addition, it was found that concrete incorporating macro synthetic fibers presents higher creep deformations and higher creep rate than concrete reinforced with steel fibers. This can be explained by the different fiber-matrix bond characteristics, analyzed by monotonic and sustained load pullout tests, and by the compression response of the composite. Finally, the residual properties of creep tested specimens were determined by monotonic flexural tests performed in the FRC specimens after the creep tests.