Generalized Model for IGC Growth in Aluminum Alloys

A generalized brick wall model is developed to describe intergranular corrosion in Equi-axedAA7178-T6 and Wingskin AA7178-T6 aluminum alloys. The intergranular corrosion rate is highly related to grain size and shape. High strength aluminum alloys are often elongated and anisotropic, with the fastest nominal IGC growth rate in the longitudinal direction (L) or long transverse direction (T) and the slowest in the short transverse direction (S). We propose a three-way intersection model and use it to simulate the corrosion kinetics for each direction. With a proper combination of model parameters, the generalized IGC model provides a good fit to experimental data developed by the foil penetration technique

NASA-UVA light aerospace alloy and structures technology program

The report on progress achieved in accomplishing of the NASA-UVA Light Aerospace Alloy and Structures Technology Program is presented. The objective is to conduct interdisciplinary graduate student research on the performance of next generation, light weight aerospace alloys and associated thermal gradient structures in close collaboration with researchers. The efforts will produce basic understanding of material behavior, new monolithic and composite alloys, processing methods, solid and fluid mechanics analyses, measurement advances, and a pool of educated graduate students. The presented accomplishments include: research on corrosion fatigue of Al-Li-Cu alloy 2090; research on the strengthening effect of small In additions to Al-Li-Cu alloys; research on localized corrosion of Al-Li alloys; research on stress corrosion cracking of Al-Li-Cu alloys; research on fiber-matrix reaction studies (Ti-1100 and Ti-15-3 matrices containing SCS-6, SCS-9, and SCS-10 fibers); and research on methods for quantifying non-random particle distribution in materials that has led to generation of a set of computer programs that can detect and characterize clusters in particles

Doctor of Philosophy

dissertationIt is well known that corrosion and simultaneous cyclic loading have a detrimental impact in the integrity of devices or structures. Understating these mechanisms is critical to ensure safety of aircraft. This work presents an extensive literature review on issues of corrosion mechanisms including pitting, exfoliation and intergranular attack. Moreover, models for phases of life and pitting corrosion are presented. Relevant definitions related to these failure modes are presented. The nucleation of fatigue cracks from corrosion pits was investigated by evaluating the effects of two variables on the fatigue life of dog-bone specimens of aluminum alloys 7075-T6 and 2024-T3. The specimens were exposed to different levels of corrosion in an acidified saline solution of 3.5% NaCl. In addition, the specimens were exposed to concomitant fatigue and corrosion until failure by fracture occurred. SEM analysis indicated that fatigue cracks formed/nucleated from each pit, and subsurface mechanisms of degradation were identified associated with the pitting nucleation sites including subsurface pitting, cracking, tunneling and intergranular attack. Failure data were analyzed by ANOVA methods and three transformations were evaluated to minimize the variance, including natural log, inverse square root and power with a lambda of 1/3. Contour and surface plots were developed to show how these variables impact the response of cycles to failure for the conditions evaluated. The effects of stress are more detrimental than corrosion time on the fatigue life of the specimens for the values previously defined by the DOE matrix. The research reported herein presents a methodology for accelerated corrosion fatigue of high strength aluminum alloys in an acidified saline environment. Subsequently a statistical based methodology to assess the impact of multiple variables into the fatigue life of specimens is presented. Statistical models are developed to assess the effect of two variables, stress and corrosion time into the fatigue life of the specimens. Stress levels were chosen to simulate conditions of current aircraft, such as fuselage bulkheads in the F-16. Development of statistical models to predict the behavior of materials will increase our ability to predict and prevent catastrophic structural failures thereby increasing the safety of our aircraft structures

THREE-DIMENSIONAL MICROSTRUCTURE-BASED MODELS FOR FATIGUE CRACK NUCLEATION AND FATIGUE CRACK BRANCHING IN HIGH STRENGTH AL ALLOYS

This study is concerned with using numerical three-dimensional microstructure-based models to quantify the multi-site fatigue crack initiation behaviors by simulating the effects of pores in a cast aluminum alloy, and to analyze the mechanism of fatigue crack branching in thick aluminum alloy plates. It has been recently recognized the three-dimensional effects of pores on fatigue crack initiation, which provide opportunity to quantitatively identify the fatigue weak-link density and strength distribution in aluminum alloy. The stress and strain fields around a micro-pore in an A713 aluminum alloy (an elasto-plastic media) under cyclic loading were quantified as a function of pore position in depth on surface using a 3-D finite element model. The incubation life for the fatigue crack from a pore in surface could be estimated using a micro-scale Manson-Coffin equation. By matching to the experimentally measured fatigue weak-links, the minimum critical pore size for fatigue crack initiation was determined to be 11 μm in diameter at the cyclic maximum stress of 100% yield strength of the alloy. A quantitative model which took into account the 3-D effect of a pore on the local stress/strain fields was developed to quantify the fatigue weak-link density and strength distribution in the A713 Al alloy. In the model, a digital pore structure was first constructed using single-sized (15 μm in diameter) and multi-sized pores, respectively, that had a total volume fraction same as that of the pores measured experimentally in the alloy. In the sample surface randomly selected by cross-sectioning the simulated pore structure, the size and position in depth of each pore were know in the surface. The rate of fatigue crack initiation at these pores was found to be a Weibull function of the applied stress, which was consistent with the results experimentally measured in the alloy. The density and strength distribution of fatigue weak-links could then be derived and used to evaluate the fatigue crack initiation properties of the alloy. The simulated fatigue weaklink density and strength distribution in the multi-sized pore model were in a good agreement with the experimental results. The difference in the peak of strength distribution between experimentally measured and simulated results was only ~1.8%. It was also found that the average crack incubation life was linearly increased with decrease in the applied cyclic stress. The stochastic behaviors of the multi-site fatigue crack initiation and the reliability of fatigue crack initiation at each applied cyclic stress were quantified in the A713 cast Al alloy. The probability of fatigue crack initiation was characterized by a two-parameter Weibull function, i.e., with 10,000 cyclic loading, the survival probability for crack initiation was 72% at 110% yield strength, and was 99% at 50% yield strength. A detailed fractographic and microstructural study, using stereo optical and scanning electron microscopy was accomplished to characterize the behavior and mechanisms of fatigue crack branching in a thick commercial 7050-T7651 aluminum plate in the L-S orientation. The SEM fractographies of the middle tension specimen failed in crack growth experiments showed that the transition of the lead crack from a crystallographic (fatigue fracture) to non-crystallographic mode (overloading fracture) of growth at a ΔK of ~17 MPa√m with few non-through thickness branches at the mid-thickness plane. An interior branched crack grew into a through crack at a ΔK greater than 30 MPa√m. The intergranular fracture was observed in the branched cracks, indicating a relationship between crack branching and over-aging state in the alloy. A 3-D finite element model which took into account the effect of both a non-through and through thickness branched crack at the lead crack was consequently developed to simulate the growth behavior with branching. The simulated results demonstrated that the driving force, ΔK, for the lead crack was reduced significantly due to branching at the crack tip, which was responsible for the reduced crack growth rate of the lead crack as observed experimentally. The driving force for crack growth with branching was increased with increasing the lead crack length, which explained the reason why crack branching occurred relatively easily when the lead crack was long. The precipitate-free-zones along grain boundaries were responsible for crack branching parallel to the rolling (L) direction, since the grain structure was stretched in L direction in the Al alloy plate due to hot rolling

Time-dependent corrosion fatique crack propagation in 7000 series aluminum alloys

The goal of this research is to characterize environmentally assisted subcritical crack growth for the susceptible short-longitudinal orientation of aluminum alloy 7075-T651, immersed in acidified and inhibited NaCl solution. This work is necessary in order to provide a basis for incorporating environmental effects into fatigue crack propagation life prediction codes such as NASA-FLAGRO (NASGRO). This effort concentrates on determining relevant inputs to a superposition model in order to more accurately model environmental fatigue crack propagation

Fatigue of friction stir welded lap joints with sealants

A lack of understanding of corrosion fatigue in friction stir welded aluminum joints prevents friction stir welding from being implemented in aerospace applications. Fatigue testing reveals a 60-75% reduction in the fatigue life of friction stir welded aluminum lap joints immersed in 3.5% NaCl solution (corrosion fatigue) compared with that of lap joints tested in ambient air. The loss in fatigue life is attributed to accelerated fatigue cracking due to hydrogen environment embrittlement. Two polymer sealant candidates are investigated: silicone rubber and nylon-11. Both sealant candidates can be applied prior to welding and seal the faying surface gaps in lap joints upon welding. The rubber sealant cures at room temperature after welding and can be welded with the same parameters as without the sealant. The 50% sample population corrosion fatigue life is increased by 22% with the use of the rubber sealant, but the effectiveness of the rubber sealant is limited by its cohesive mechanical properties, e.g. elongation to failure. In ambient fatigue, the nylon sealed welds exhibit twice the 50% sample population fatigue life of other welds. Finite element modeling predicts a reduction in the stresses in the weld due the stiffness contribution of the nylon sealant. The effectiveness of the nylon sealant is limited by its adhesive bond strength. When immersed in water, as in corrosion fatigue, the adhesive bond strength is reduced, the sealant bond fails within 500 fatigue cycles, and the mechanical benefits of the nylon sealant are negated. The corrosion fatigue life of nylon sealed welds is 26% less than that of welds without sealant because of the more severe hook defect associated with hotter welding conditions required to melt the nylon. Finite element modeling results indicate an increase in stress intensity factors of about 10% in welds with more severe hook defects--Abstract, page iii