Microstructure-based multistage fatigue modeling of aluminum alloy 7075-T651

The multistage fatigue model for high cycle fatigue of a cast aluminum alloy developed by McDowell et al. is modified to consider the structure-property relations for cyclic damage and fatigue life of a high strength aluminum alloy 7075-T651 for aircraft structural applications. The multistage model was developed as a physically-based framework to evaluate sensitivity of fatigue response to various microstructural features to support materials process design and component-specific tailoring of fatigue resistant materials. In this work, the model is first generalized to evaluate both the high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes for multiaxial loading conditions, with appropriate modifications introduced for wrought materials. The particular microstructural features of relevance to fatigue in aluminum alloy 7075-T651 include micron-scale Fe-rich intermetallic particles and rolling textures. The model specifically addresses the role of local constrained cyclic microplasticity at fractured inclusions in fatigue crack incubation and microstructurally small crack growth, including the effect of crystallographic orientation on crack tip displacement as the driving force. The model is able to predict lower and upper bounds of the fatigue life based on measured inclusion sizes.     

Microstructure based fatigue life predictions for thick plate 7050-T7451 airframe alloys

The purpose of this work was to test a microstructure based fatigue life prediction Monte-Carlo model for potential use in quantifying fatigue quality and reliability of metallic structural alloys. The model used was of the crack growth type with the sizes of crack initiating pores, local crack geometry and crack tip texture as random variables. The model was verified using data for 7050-T7451 plate alloy fatigue tested in smooth sample configuration at σ_max of 241 and 276 MPa and R=0.1. The mechanical testing was supplemented with characterizations of the size distributions of the fatigue performance limiting bulk porosity and measurements of the actual sizes of the fatigue crack initiating pores. Predicted fatigue lives were in good agreement with the experimental results and the model identified the size distribution of the crack initiating pores as the dominant variable controlling fatigue performance. The distributions of those pores were predicted from the bulk pore size data using the statistics of extremes. The developed approach proved effective in incorporating microstructural information in modeling fatigue and could be used in ranking fatigue quality and reliability of materials based on microstructural data.     

Typical fatigue-initiating discontinuities in metallic aircraft structures

A fatigue lifing framework using a lead crack concept, based on years of detailed inspection and analysis of fatigue cracks in many specimens and airframe components, has been developed by the DSTO for metallic primary airframe components. This framework is an important additional tool for determining aircraft component fatigue lives in the Royal Australian Air Force fleet. Like the original Damage Tolerance concept, developed by the United States Air Force, this framework assumes that fatigue cracking begins as soon as an aircraft enters service. However, there are major and fundamental differences. Instead of assuming initial crack sizes and deriving early crack growth behaviour from back-extrapolation of growth data for long cracks, the framework uses data for real cracks growing from small discontinuities inherent to the material and the production of the component. To this end, this paper examines the types of discontinuities that initiate fatigue cracks in typical metallic airframe structures. These discontinuities and the fatigue cracks that have grown from them are taken from coupon, component and full-scale tests, and also from service aircraft, including commercial transport aircraft and high performance military aircraft.     

Hot Sheet Metal Forming Strategies for High-Strength Aluminum Alloys: A Review—Fundamentals and Applications

In the past decade, aluminum alloys have become important structural materials in the automotive industry, thanks to their low density, high strength, high fracture toughness, and good fatigue performance. However, an important limitation of aluminum alloys is their poor formability at room temperature; as a result, numerous studies have been conducted with the aim of developing forming techniques to overcome this and facilitate the forming of more complex-shaped components. Following an overview on the metallurgical background of aluminum alloys, this article reviews recent developments in forming processes for aluminum alloys. The focus is on process variants at room temperature and at higher temperatures and on a new hot forming technique promising considerable improvements in formability. This review summarizes the influence of different process parameters on microstructures and mechanical properties. Particular emphasis is given to process design and to the underlying microstructural phenomena governing the strengthening mechanisms.     

Identification of Particle Stimulated Nucleation during Recrystallization of AA 7050

Mechanical properties of polycrystalline metals are dependent upon the arrangement of microstructural features in the metal. Recrystallization is an important phenomenon that often occurs during thermo-mechanical processing of metals. This work focuses upon aluminum alloy 7050 and uses crystallographic texture and pair correlation functions of recrystallized grains to characterize the dominance of particle stimulated nucleation in the recrystallization process. The randomization of the recrystallization texture and similar pair correlation functions for the particle distribution and the recrystallization nuclei distribution indicate that particle stimulated nucleation controls the recrystallization behavior in this alloy.     

Microstructure history effect during sequential thermomechanical processing

The key to modeling the material processing behavior is the linking of the microstructure evolution to its processing history. This paper quantifies various microstructural features of an aluminum automotive alloy that undergoes sequential thermomechanical processing which is comprised hot rolling of a 150-mm billet to a 75-mm billet, rolling to 3 mm, annealing, and then cold rolling to a 0.8-mm thickness sheet. The microstructural content was characterized by means of electron backscatter diffraction, scanning electron microscopy, and transmission electron microscopy. The results clearly demonstrate the evolution of precipitate morphologies, dislocation structures, and grain orientation distributions. These data can be used to improve material models that claim to capture the history effects of the processing materials.     

Fatigue of rare‐earth containing magnesium alloys: a review

Lightweighting in ground vehicles is today considered as one of the most effective strategies to improve fuel economy and reduce anthropogenic environment‐damaging and climate‐changing emissions. Magnesium (Mg) alloy, as a strategic ultra‐lightweight metallic material, has recently drawn a considerable interest in the transportation industry to reduce the weight of vehicles due to their high strength‐to‐weight ratio, dimensional stability, good machinability and recyclability. However, the hexagonal close‐packed crystal structure of Mg alloys gives only limited slip systems and develops sharp deformation textures associated with strong mechanical anisotropy and tension–compression yield asymmetry. For the vehicle components subjected to dynamic loading, such asymmetry could exert an unfavourable influence on the material performance. This problem could be conquered through weakening the texture via addition of rare‐earth (RE) elements. Thus, a number of RE‐containing Mg alloys have recently been developed. To guarantee the structural integrity, durability and safety of highly loaded structural components, understanding the characteristics and mechanisms of cyclic deformation and fatigue fracture of such RE‐Mg alloys is of vital importance. In this review, the available fatigue properties including stress‐controlled fatigue strength, strain‐controlled cyclic deformation characteristics and fatigue crack propagation behaviour are summarized, along with the microstructural change and crystallographic texture weakening in the RE‐containing Mg alloys in different forms (cast, extruded and heat‐treated states), in comparison with those of RE‐free Mg alloys.     

Effect of laser shock processing on the fatigue crack initiation and propagation of 7050-T7451 aluminum alloy

The aim of this paper was to identify the effect of laser shock peening (LSP) on the fatigue crack initiation and propagation of 7050-T7451 aluminum alloy. The laser shocked specimen in which residual compressive stress is mechanically produced into the surface showed a very high dislocation density within the grains. This was evident throughout the LSP region. The spacing among the fatigue striations in the LSP region was narrow, which indicated that LSP had an obvious inhibitory action to fatigue crack initiation and growth. In contrast, the region without LSP exhibited an extremely low dislocation density. And LSP improved 7050-T7451 alloy specimens’ fatigue intensity.     

Low-Cycle Fatigue and Ratcheting Lifetime Estimation of a 7050-T6 Aluminum Alloy

Strength of Materials - Low-cycle fatigue properties and uniaxial ratcheting behavior of a 7050-T6 aluminum alloy were investigated by a non-symmetric cycle stress tests at the normal temperature....