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**1 - 4**of**4**### The Effects of . . . Crack Growth of Aluminum Alloys

, 2000

"... The integrity of nearly all engineering structures are threatened by the presence of cracks. Structural failure occurs if a crack larger than a critical size exists. Although most well designed structures initially contain no critical cracks, subcritical cracks can grow to failure under fatigue load ..."

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The integrity of nearly all engineering structures are threatened by the presence of cracks. Structural failure occurs if a crack larger than a critical size exists. Although most well designed structures initially contain no critical cracks, subcritical cracks can grow to failure under fatigue loading, called fatigue crack growth (FCG). Because it is impossible or impractical to prevent subcritical crack growth in most applications, a damage tolerant design philosophy was developed for crack sensitive structures. Design engineers have taken advantage of the FCG threshold concept to design for long fatigue lives. FCG threshold (DK th ) is a value of DK (crack-tip loading), below which no significant FCG occurs. Cracks are tolerated if DK is less than DK th . However, FCG threshold is not constant. Many variables influence DK th including microstructure, environment, and load ratio. The current research focuses on load ratio effects on DK th and threshold FCG. Two categories of load ratio effects are studied here: extrinsic and intrinsic. Extrinsic load ratio effects operate in the crack wake and include fatigue crack closure mechanisms. Intrinsic load ratio effects operate in the crack-tip process zone and include microcracking and void production. To gain a better understanding of threshold FCG load ratio effects (1) a fatigue crack closure model is developed to consider the most likely closure mechanisms at threshold, simultaneously, and (2) intrinsic load ratio mechanisms are identified and modeled. An analytical fatigue crack closure model is developed that includes the three closure mechanisms considered most important at threshold (PICC, RICC, and OICC). Crack meandering and a limited amount of mixed-mode loading are also considered. The rough crack geometry, ...

### Advances in Fatigue and Fracture Mechanics Analyses for Metallic Aircraft Structures

, 2000

"... This paper reviews some of the advances that have been made in stress analyses of cracked aircraft components, in the understanding of the fatigue and fatigue-crack growth process, and in the prediction of residual strength of complex aircraft structures with widespread fatigue damage. Finite-elemen ..."

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This paper reviews some of the advances that have been made in stress analyses of cracked aircraft components, in the understanding of the fatigue and fatigue-crack growth process, and in the prediction of residual strength of complex aircraft structures with widespread fatigue damage. Finite-element analyses of cracked metallic structures are now used to determine accurate stress-intensity factors for cracks at structural details. Observations of small-crack behavior at open and rivet-loaded holes and the development of small-crack theory has lead to the prediction of stress-life behavior for components with stress concentrations under aircraft spectrum loading. Fatigue-crack growth under simulated aircraft spectra can now be predicted with the crackclosure concept. Residual strength of cracked panels with severe outof -plane deformations (buckling) in the presence of stiffeners and multiple-site damage can be predicted with advanced elastic-plastic finite-element analyses and the critical crack-tip-opening angle (CTOA) fracture criterion. These advances are helping to assure continued safety of aircraft structures.

### Characterization of Early

, 1996

"... New methods were developed and applied to extract and interpret information from the fracture surfaces of corrosion-fatigue specimens of clad and bare 2024-T3 aluminum sheet. We developed two fracture surface topography analysis methods that characterize the effects of environment and a fracture s ..."

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New methods were developed and applied to extract and interpret information from the fracture surfaces of corrosion-fatigue specimens of clad and bare 2024-T3 aluminum sheet. We developed two fracture surface topography analysis methods that characterize the effects of environment and a fracture surface analysis and finite element modeling approach to detect and quantify overloads that occurred during fatigue crack propagation. We used these methods to perform a detailed analysis of the fracture surface topography produced in the early stages of corrosion fatigue, and developed an understanding of the effect of environment on the crack nucleation mechanism based on fracture surface features.

### A Finite Element Calculation of Stress Intensity Factors of Cruciform and Butt Welded Joints for Some Geometrical Parameters

"... With welded joints, stress concentrations occur at the weld toe and at the weld root, which make these regions the points from which fatigue cracks may initiate. To calculate the fatigue life of welded structures and to analyze the progress of these cracks using fracture mechanics technique requires ..."

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With welded joints, stress concentrations occur at the weld toe and at the weld root, which make these regions the points from which fatigue cracks may initiate. To calculate the fatigue life of welded structures and to analyze the progress of these cracks using fracture mechanics technique requires an accurate calculation of the stress intensity factor SIF. The existing SIFs were usually derived for one particular geometry and type of loading. In this study, the finite element method (FEM) was used to calculate the SIF. The stress intensity factors during the crack propagation phase were calculated by using the software FRANC2D, which is shown to be highly accurate, with the direction of crack propagation being predicted by using the maximum normal stress criterion. In the current work, a new analytical approach for the weld toe crack in cruciform welded joints has been used. The SIF results from FRANC2D were compared with those from the International Institute of Welding-IIW, and literature. A good correlation was obtained and the work results have bench marked which made it possible to use FRANC2D to simulate different weld geometries. The results of these comparisons are shown and the agreement is clearly well.