Transition Criteria - From a Pit to a Crack

A brief review of pitting, which is a localized corrosion process resulting from pitting and fatigue interaction has been conducted in this paper. The transition of a pit to a crack is a very important phenomenon occurring under PCF conditions. Various theories for transition of a pit to a crack were examined within the limits of present understanding and limitations of the models. The transition of a pit to a crack was also discussed for a variety of pit shapes utilizing the pit aspect ratios, defect size and linear elastic fracture mechanics, crack tip characterizing parameters such as Mode I stress intensity factor range. Pitting corrosion and fatigue interaction is presented in terms of a four-stage model hypothesized within electrochemical and mechanical components. Limited data are presented to support the hypotheses in this paper, which need to be validated with more data

The effects of pitting on fatigue crack nucleation in 7075-T6 aluminum alloy

A high-strength aluminum alloy, 7075-T6, was studied to quantitatively evaluate chemical pitting effects of its corrosion fatigue life. The study focused on pit nucleation, pit growth, and fatigue crack nucleation. Pitting corrosion fatigue experiments were conducted in 3.5 percent NaCl aqueous solution under constant amplitude sinusoidal loading at two frequencies, 5 and 20 Hz. Smooth and unnotched specimens were used in this investigation. A video recording system was developed to allow in situ observation of the surface changes of the specimens during testing. The results indicated that pitting corrosion considerably reduces the fatigue strength by accelerating fatigue crack nucleation. A metallographic examination was conducted on the specimens to evaluate the nature of corrosion pits. First, the actual shapes of the corrosion pits were evaluated by cross-sectioning the pits. Secondly, the relation between corrosion pits and microstructure was also investigated. Finally, the possibility of another corrosion mechanism that might be involved in pitting was explored in this investigation. The fractography of the tested specimens showed that corner corrosion pits were responsible for fatigue crack nucleation in the material due to the associated stress concentration. The pits exhibited variance of morphology. Fatigue life for the experimental conditions appeared to be strongly dependent on pitting kinetics and the crack nucleation stage

Determination of Flaw Growth Characteristics of Ti-6Al-4V Sheet in the Solution-Treated and Aged Condition

The specific experimental investigation undertaken was designed to answer these questions on Ti-6Al-4V in the solution treated and aged condition. The defect growth and fracture characteristics were studied in parent (unwelded) and welded sheet material. The results of the study indicate that cryogenic proof testing will screen smaller size defects than proof testing at ambient conditions. However some unusual crack growth behavior during the proof test simulation suggests that some further study be made of stress and time duration effects

Microstructurally based variations on the dwell fatgue life of titanium alloy IMI 834

An experimental study was undertaken to determine the role of microstructure on the fatigue life reduction observed in titanium alloy IMI 834 under dwell loading conditions. The wave forms compared were a trapezoid with 15 and 30 second hold times at the maximum test load and a baseline, 10 Hertz, haversine. The stress ratio for both loading wave forms was 0.10. The fatigue loading of each specimen was conducted in a vacuum within a scanning electron microscope chamber which minimized the possibility that the laboratory environment would adversely affect the material behavior. Two microstructural conditions were investigated in the experimental program. The first involved standard 'disk' material with equiaxed alpha in a transformed beta matrix. The second material was cut from the same disk forging as the first but was heat treated to obtain a martensitic alpha prime microstructure. Tensile tests were performed prior to the onset of the fatigue loading portion of the study, and it was determined that the yield strengths of the specimens from both material conditions were within ten percent. The maximum fatigue loads were chosen to be 72 percent of the average yield strength for both materials as determined from the tensile tests. It was found that the cycles to failure from the 10 Hertz loading wave form were reduced by a factor of approximately five when the loading was changed to the trapezoidal wave form for the standard 'disk' material. The fatigue life reduction for the martensitic structure under identical test conditions was approximately 1.75. The improvement observed with the martensitic structure also was accompanied by an increase in overall fatigue life for the wave forms tested. This paper will review the results and conclusions of this effort

The role of fretting corrosion and fretting fatigue in aircraft rivet hole cracking

Personnel in the Quality and Integrity Design Engineering Center (QIDEC) at the University of Utah are working under a two year grant from the FAA to better understand the role of fretting corrosion and fretting fatigue in aircraft rivet hole cracking. The current program follows a one year grant program which was completed in 1993. This paper provides a status report on the results of these grant programs. Recent effort has been focused on developing basic fretting fatigue models which consider variation in the coefficient of friction with time and location within the fretting interface. This is a very important characteristic of the QIDEC model because coefficient of friction varies significantly during the fretting fatigue process. Copies of QIDEC documents discussed in this paper can be obtained by contacting the authors

Silencing Dkk1 expression rescues dexamethasone-induced suppression of primary human osteoblast differentiation

<p>Abstract</p> <p>Background</p> <p>The Wnt/β-catenin pathway is a major signaling cascade in bone biology, playing a key role in bone development and remodeling. The objectives of this study were firstly, to determine the effects of dexamethasone exposure on Wnt/β-catenin signaling at an intracellular and transcriptional level, and secondly, to assess the phenotypic effects of silencing the Wnt antagonist, Dickkopf-1 (Dkk1) in the setting of dexamethasone exposure.</p> <p>Methods</p> <p>Primary human osteoblasts were exposed in vitro to 10^-8M dexamethasone over a 72 h time course. The phenotypic marker of osteoblast differentiation was analyzed was alkaline phosphatase activity. Intracellular β-catenin trafficking was assessed using immunoflourescence staining and TCF/LEF mediated transcription was analyzed using a Wnt luciferase reporter assay. Dkk1 expression was silenced using small interfering RNA (siRNA).</p> <p>Results</p> <p>Primary human osteoblasts exposed to dexamethasone displayed a significant reductions in alkaline phosphatase activity over a 72 h time course. Immunoflourescence analaysis of β-catenin localization demonstrated a significant reduction in intracytosolic and intranuclear β-catenin in response to dexamethasone exposure. These changes were associated with a reduction of TCF/LEF mediated transcription. Silencing Dkk1 expression in primary human osteoblasts exposed to dexamethasone resulted in an increase in alkaline phosphatase activity when compared to scrambled control.</p> <p>Conclusions</p> <p>Wnt/β-catenin signaling plays a key role in regulating glucocorticoid-induced osteoporosis <it>in vitro</it>. Silencing Dkk1 expression rescues dexamethasone-induced suppression of primary human osteoblast differentiation. Targeting of the Wnt/β-catenin signaling pathway offers an exciting opportunity to develop novel anabolic bone agents to treat osteoporosis and disorders of bone mass.</p

Large-scale sequencing identifies multiple genes and rare variants associated with Crohn’s disease susceptibility

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Cold-working effects on previously damaged fasterner holes

posterMetal fatigue is microscopic damage/deformation that occurs each time a component is loaded. Metal fatigue is very common in aircraft structures, as they are often made as thin as possible for weight considerations. In order to continue flying older aircraft, cold expansion, or cold-working, was developed in the 1960's to improve the fatigue lives of fastener holes. Testing has shown that cold-working techniques improves the fatigue life of new fastener holes, but little work has been done to analyze the effects of cold-working a hole that already contains fatigue damage. This research will analyze and quantify cold-working effects for previously damaged fastener holes

Transition Criteria - From a Pit to a Crack

A brief review of pitting, which is a localized corrosion process resulting from pitting and fatigue interaction has been conducted in this paper. The transition of a pit to a crack is a very important phenomenon occurring under PCF conditions. Various theories for transition of a pit to a crack were examined within the limits of present understanding and limitations of the models. The transition of a pit to a crack was also discussed for a variety of pit shapes utilizing the pit aspect ratios, defect size and linear elastic fracture mechanics, crack tip characterizing parameters such as Mode I stress intensity factor range. Pitting corrosion and fatigue interaction is presented in terms of a four-stage model hypothesized within electrochemical and mechanical components. Limited data are presented to support the hypotheses in this paper, which need to be validated with more data