A Study on Effect of Residual Stress on Crack Growth of Aluminum Plate
One technique used to enhance the fatigue strength of a fastener hole is to introduce a compressive residual stress field around the hole. An applied load must overcome this residual stress before the crack can grow, thus leading to a longer fatigue life. The cold expansion of the holes is largely employed to obtain improvement in fatigue performance of structures with initially uncracked holes. In this research, considered is a fatigue crack growth from a hole under the influence of residual stress. The entire cold working processing- including hole expansion, elastic recovery, and finish reaming of two aluminum alloys 2024-T351 and 7075-T6 is simulated. This method of fatigue life prediction is compared with a superposition technique and experimental data. The crack closure level is computed from a finite element simulation and a fatigue life is predicted. The conclusions obtained here are beneficial to the understanding of the phenomenon of fatigue crack initiation and growth at the perimeter of cold worked holes.
A New Method of Predicting Residual Stresses in Autofrettaged Gun Barrels.
Abstract : The determination of residual stresses in autofrettaged gun barrels has been considered by many investigators using different mathematical methods and material models. Most of the earlier solutions were based on the assumption that the material behaves elastically on the release of the autofrettaged pressure. However, many materials, particularly the quenched and tempered, low alloy steels generally used for high pressure vessels, exhibit a significant Bauschinger effect. In a recent paper, this author presented a closed-form solution of residual stresses in autofrettaged tubes based on a theoretical model considering the Bauschinger and hardening effects during unloading, but neglecting the strain-hardening during loading. A more general theoretical model without this restriction was proposed earlier, but only part of the final results were shown. In the present report, the complete method of stress and deformation analysis based on the general theoretical model is stated. The new model is a better representation of the actual loading/unloading behavior is a high strength steel. The Bauschinger effect factors is treated as a function of overstrain. The strain-hardening effect is taken into account with different parameters used for loading and unloading processes. The formulas for calculating stresses, strains, and displacements are given. The new results indicate that the influence of the Bauschinger and hardening effects on residual stresses is significant. A comparison with two experimental results has been made.
Prevention of Barite Sag in Oil-Based Drilling Fluids Using a Mixture of Barite and Ilmenite as Weighting Material
Drilling high-pressure high-temperature (HPHT) wells requires a special fluid formulation that is capable of controlling the high pressure and is stable under the high downhole temperature. Barite-weighted fluids are common for such purpose because of the good properties of barite, its low cost, and its availability. However, solids settlement is a major problem encountered with this type of fluids, especially at elevated downhole temperatures. This phenomenon is known as barite sag, and it is encountered in vertical and directional wells under static or dynamic conditions leading to serious well control issues. This study aims to evaluate the use of barite-ilmenite mixture as a weighting agent to prevent solids sag in oil-based muds at elevated temperatures. Sag test was conducted under static conditions (vertical and inclined) at 350 °F and under dynamic conditions at 120 °F to determine the optimum ilmenite concentration. Afterward, a complete evaluation of the drilling fluid was performed by monitoring density, electrical stability, rheological and viscoelastic properties, and filtration performance to study the impact of adding ilmenite on drilling fluid performance. The results of this study showed that adding ilmenite reduces sag tendency, and only 40 wt.% ilmenite (from the total weighting material) was adequate to eliminate barite sag under both static and dynamic conditions with a sag factor of around 0.51. Adding ilmenite enhanced the rheological and viscoelastic properties and the suspension of solid particles in the drilling fluid, which confirmed sag test results. Adding ilmenite slightly increased the density of the drilling fluid, with a slight decrease in the electrical stability within the acceptable range of field applications. Moreover, a minor improvement in the filtration performance of the drilling fluid and filter cake sealing properties was observed with the combined weighting agent. The findings of this study provide a practical solution to the barite sag issue in oil-based fluids using a combination of barite and ilmenite powder as a weighting agent to drill HPHT oil and gas wells safely and efficiently with such type of fluids.