Fatigue Life Fatigue Crack Propagation And Fracture Toughness Study Of 7075 Aluminum Alloy Subjected To Thermomechanical Processing

Due to its high strength/weight ratio, 7075 aluminum alloy has been widely used as an aerospace material. However, it has relatively low fatigue strength and low fracture toughness in the T6 condition. Thermomechanical processing, including pre-cyclic loading and stretching at high and ambient temperatures, has been investigated with the aim of improving these properties. Fatigue life tests show that all four thermomechanical processes investigated increase the fatigue life at low stress levels, but cause a reduction at high stress levels. The effect of the precipitate free zone (PFZ) width on the fatigue crack propagation rate and fracture toughness has also been investigated. The fatigue crack propagation rate of the alloy with a wide PFZ was found to be approximately the same as that of the alloy with a narrow PFZ under similar conditions of cyclic loading. The alloy with the wide PFZ demonstrated a greater value of fracture toughness than that shown by the alloy with the narrow PFZ. These experimental results have been analyzed taking recently proposed theoretical models into account. Transmission electron microscopy showed that a large number of dislocations were generated within the soft PFZ areas as a result of the cyclic pre-loading. This is believed to contribute to the increase in fatigue life at low applied stress levels. Scanning electron microscopy has shown an intergranular fracture mode for the alloy with the narrow PFZ and a dimple rupture mode for the alloy with the wide PFZ.

Axial load fatigue life, fatigue-crack propagation, and fracture toughness experiments were conducted on sheet specimens made of 7075-T6 aluminum alloy. These experiments were conducted at pressures ranging from atmospheric to 5 x 10 to the minus 8th torr. Analysis of the results from the fatigue life experiments indicated that for a given stress level, lower air pressures produced longer fatigue lives. At a pressure of 5 x 10 to the minus 8th torr fatigue lives were 15 or more times as long as at atmospheric pressure. Analysis of the results from the fatigue crack propagation experiments indicated that for small stress intensity factor ranges the fatigue crack propagation rates were up to twice as high at atmospheric pressure as in vacuum. The fracture toughness of 7075-T6 was unaffected by the vacuum environment. Fractographic examination showed that specimens tested in both vacuum and air developed fatigue striations. Considerably more striations developed on specimens tested at atmospheric pressure, however.

A series of fatigue tests with specimens subjected to constant amplitude and two-step axial loads were conducted on 12-inch-wide sheet specimens of 2024-T3 and 7075-T6 aluminum alloy to study the effects of a change in stress level on fatigue-crack propagation. Comparison of the results of the tests in which the specimens were tested at first a high and then a low stress level with those of the constant-stress-amplitude tests indicated that crack propagation was generally delayed after the transition to the lower stress level. In the tests in which the specimens were tested at first a low and then a high stress level, crack propagation continued at the expected rate after the change in stress levels.

Results are presented of a series of fatigue tests to study crack propagation and the resulting stress distributions in tension panels. The panels were all of the same general design, and configurations varied mainly in the relative amount of cross-sectional area in the skin, stiffeners, and flanges. The panels were constructed of 2024-T3 and 7075-T6 aluminum alloys. It was found that the average rate of crack growth was slower in panels made of 2024-T3 aluminum alloy than in panels made of 7075-T6 aluminum alloy. All cracks initiated in the skin, and the slowest crack growth was measured in configurations where the highest percentage of cross-sectional area was in the stiffeners. Strain-gage surveys were made to determine the redistribution of stress as the crack grew across the panels. As a crack approached a given point in the skin, the stress at that point increased rapidly. The stress in the stiffeners also increased as the crack approached the stiffeners. During the propagation of the crack the stress was not distributed uniformly in the remaining area.

This volume contains papers presented in the third international symposium titled Fatigue of Materials: Advances and Emergences in Understanding held during the Materials Science and Technology 2014 meeting in Pittsburgh, Pennsylvania, USA, in October 2014. The book contains contributions from engineers, technologists, and scientists from academia, research laboratories, and industries. The 19 papers are divided into five topical areas: Session 1: Aluminum Alloys Session 2: Ferrous Materials I Session 3: Ferrous Materials II Session 4: Composite Materials Session 5: Advanced Materials Session 6: Modeling The papers cover a broad spectrum of topics that represent the truly diverse nature of the subject of fatigue as it relates to the world of materials.

Its objective is to develop an understanding of the mechanisms involved in the initiation and propagation of fatigue cracks in metals in order to optimize the microstructure of high strength aluminum alloys for overall fatigue resistance. The research conducted during this year was divided into three tasks. Task I was concerned with the effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack growth resistance of 7475. Special thermal mechanical processing procedures were developed to control the microstructural features of interest. Task II was concerned with the use of the cyclic stress strain curve and a damage model for predicting fatigue crack growth thresholds. Fatigue crack initiation and fatigue crack propagation both involve the concept of cyclic accumulated damage. The details of the damage structure can be related to a material's cyclic stress strain response (CSSR). Task III is concerned with the effect of ion implantation on the low cycle fatigue response of 7475. Since fatigue crack initiation is a surface phenomenon and fatigue crack propagation is a bulk phenomenon, the fatigue properties may be optimized by production processes that develop microstructures resistant to FCI on the surface, and microstructures resistant to FCP throughout the bulk.