In this paper, we show that the Bauschinger effect is intimately tied not only to plasticity as is historically understood but to the damage state as well. We illustrate the plasticity and damage ...influence on the Bauschinger effect by employing different definitions (Bauschinger stress parameter, Bauschinger effect parameter, the ratio of forward-to-reverse yield, and the ratio of kinematic-to-isotropic hardening) for two differently processed aluminum alloys (rolled and cast) in which specimens were tested to different prestrain levels under tension and compression. Damage progression from second phase particles and inclusions that were generally equiaxed for the cast A356-T6 Al alloy and elongated for the rolled 7075 Al alloy was quantified from interrupted experiments. Observations showed that the Bauschinger effect had larger values for compression prestrains when compared to tension. The Bauschinger effect was also found to be a function of damage to particles/inclusions, dislocation/particle interaction, and the work hardening rate.
•The local optimal process parameter set for AFS-D AA2219 is presented.•The monotonic tensile response, microhardness, grain size, and precipitate existence are presented.•The cyclic plastic response ...of AFS-D AA2219 is compared to the wrought AA2219 response.
This work characterizes aluminum alloy (AA) 2219 created by use of the solid-state additive friction stir-deposition (AFS-D) process. Specifically, it determines the “goodness” for use in particular circumstances of as-deposited material created by varied process parameter sets as a function of microhardness and grain size measurements. It presents ideal process parameters for AFS-D AA2219 and characterizes the local optimal as-deposited material by cyclic plastic response, monotonic tensile response, microhardness, grain size, and precipitate existence. Ultimately, this work provides the foundation for the AFS-D technology to be used in construction of NASA’s Space Launch System (SLS) vehicle.
► Crack incubation and growth rates could be calculated from replica observations. ► These rates were approximated by grain orientation-based crack growth equations. ► These updated equations ...accurately estimated the fatigue life of independent results. ► Previous atomistic simulations validated the orientation effects from this work.
The objective of this paper is to quantify the microstructurally small fatigue crack growth of an extruded AZ61 magnesium alloy. Fully reversed and interrupted load-controlled tests were conducted on notched specimens that were taken from the material in the longitudinal and transverse orientations with respect to the extrusion direction. In order to measure crack growth, replicas of the notch surface were made using a dual-step silicon-rubber compound at periodic cyclic intervals. By using microscopic analysis of the replica surfaces, crack initiation sites from numerous locations and crack growth rates were determined. A marked acceleration/deceleration was observed to occur in cracks of smaller length scales due to local microheterogeneities consistent with prior observations of small fatigue crack interaction with the native microstructure and texture. Finally, a microstructure-sensitive multistage fatigue model was employed to estimate the observed crack growth behavior and fatigue life with respect to the microstructure with the most notable item being the grain orientation. The crack growth rate and fatigue life estimates are shown to compare well to published findings for pure magnesium single crystal atomistic simulations.
We examine the dependence of fatigue properties on the different size scale microstructural inclusions of a cast A356 aluminum alloy in order to quantify the structure-property relations. Scanning ...electron microscopy (SEM) analysis was performed on fatigue specimens that included three different dendrite cell sizes (DCSs). Where past studies have focused upon DCSs or pore size effects on fatigue life, this study includes other metrics such as nearest neighbor distance (NND) of inclusions, inclusion distance to the free surface, and inclusion type (porosity or oxides). The present study is necessary to separate the effects of numerous microstructural inclusions that have a confounding effect on the fatigue life. The results clearly showed that the maximum pore size (MPS), NND of gas pores, and DCS all can influence the fatigue life. These conclusions are presumed to be typical of other cast alloys with similar second-phase constituents and inclusions. As such, the inclusion-property relations of this work were employed in a microstructure-based fatigue model operating on the crack incubation and MSC with good results.
Novel diagnostic techniques incorporating focused ion beam (FIB) micro-milling, scanning electron microscopy (SEM), and in-situ SEM nanomechanical load frames performed micro-cantilever beam ...experiments on bio-inspired nanocomposites. Video of fracture behavior during the experiments correlated to load frame data indicated a hardening mechanism similar to the deformation mechanisms found in nacre. These methods developed for nanocomposites are applicable to a wide range of material systems for providing a unique understanding of deformation and failure mechanisms at lower length scales.
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•In-situ SEM nanomechanical testing of nanocomposite cantilever beam•Correlated video of experimental data to crack initiation and fracture.•Tablets in nanocomposite provide similar crack deflection as aragonite in nacre.•Tests provide new methodology to quantify material behavior at lower length scale.
In this study, a combined computational and experimental particle tracking investigation was performed for a solid-state additive manufacturing and repair process, Additive Friction Stir Deposition ...(AFSD). Specifically, smoothed particle hydrodynamics (SPH) simulations of AFSD were conducted in-order to elucidate deposition mechanics. The particle tracking of the SPH AFSD simulations was validated using experimental depositions of two feedstock varieties, including anodized AA6061-T6 feedstock to track external particles and AA6061-T6 copper wire core feedstock to track internal particles, to represent flow behavior from different regions of the feedstock. The X-Ray computed tomography (CT) experimental results revealed that the anodized oxides on the outside of the feedstock flowed to the retreating side, whereas the copper wire in the center of the feedstock migrated to the advancing side. Particle tracking results from the SPH simulations showed that, in general, particle movement is limited to directly beneath the feedstock. The rotational, radial, and traverse flow interactions visualized by AFSD simulations explained the advancing and retreating side biases experienced by the internal copper wire and surface oxides on the anodized feedstock. This work demonstrates the ability to predict AFSD material distributions, which has a significant impact on as-deposited material quality.
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► Failure loads in 6061-T6 aluminum resistance spot welded joints were investigated. ► Force, displacement, and time were captured prior complete failure. ► Nugget and microstructure characteristics ...were quantified. ► We used laser beam profilometry and electron back scatter diffraction techniques. ► Process sensitivity was captured and optimal welding conditions were established.
This study offers a novel research approach to compare weld quality for different welding conditions in order to achieve optimal end-product results. Using electron back scatter diffraction (EBSD) scanning, tensile testing, and laser beam profilometry (LBP) measurements along with optical microscopy (OM) images, failure loads and deformation of 6061-T6 aluminum alloy, resistance spot welded (RSW) joints were experimentally investigated. Three welding conditions, nugget and microstructure characteristics were quantified according to predefined process parameters. Quasi-static tensile tests were used to characterize the failure loads in specimens based upon these same process parameters. Profilometer results showed that the larger the applied welding current, the deeper the weld imprints. In addition, good correlation was obtained between the EBSD scans and the welding conditions. A strong dependency was found between the grain size and orientation and the welding parameters.
Additive friction stir deposition (AFSD) is a solid-state additive manufacturing and repair process that provides a way to rapidly repair and/or fabricate components using similar feedstocks as the ...repaired substrate without melting the feedstock or substrate material. In AFSD, the material is deposited using a high-shear thermomechanical deposition process resulting in a near-net shape product. In this work, we evaluate the viability of the AFSD process for depositing stainless steel 304L (SS304L) feedstock onto the same stainless steel substrate. This is considered as a potential crack repair or cladding repair methodology, especially at high deposition rates, for dry cask storage containers (DCSCs) used for spent nuclear fuel. The SS304L DCSCs are susceptible to chloride-induced pitting and stress corrosion cracking, which demands periodic maintenance and repair. Therefore, AFSD, due to its non-melting deposition process provides low heat input and wrought-like mechanical properties, is evaluated here for SS304L through a combined microstructural and mechanical analysis of the deposited layers and substrate material. Specifically, electron backscatter diffraction identified equiaxed grains with a mean size of ~ 3–4 µm in the deposit, similar to the average grain size of 3.5 µm in the wrought substrate, with regions of refined grains near the tool/deposit and deposit/substrate interface. The microhardness at the mid-thickness of the deposit (280 Hv) is comparable to the wrought substrate hardness of 278 Hv. The dense, wrought-like SS304L deposit suggests further applying the promising AFSD technique to repair structures like the DCSCs.
In this work, Additive Friction Stir Deposition (AFSD) was employed for ballistic repair of AA7075-T6511 plates. After penetration with 7.62 × 51 mm FMJ rounds, the AA7075-T6511 plates were repaired ...by AFSD using the same AA7075-T6511 feedstock material. The repaired plates were impacted and penetrated with the same 7.62 × 51 mm FMJ rounds, and the surface damage characteristics including the initial and residual velocities were compared against the control wrought plates. The AFSD process successfully repaired the damaged control plates with the same alloy, without any observable defects such as large cracks or pores prior to impact tests. Although the surface appeared pristine other than milling marks, the surface damage characteristics of the repaired plates were significantly different than the control plates. The increase of spalling and petalling with the repaired material can be attributed to the thermomechanical processing of AFSD, which would alter the control T6511 temper of the feedstock due to coarsening of strengthening precipitates. A cross-sectioned repaired plate was analyzed using microhardness plots and optical microscopy to illustrate the effectiveness of the AFSD process for ballistic repair by depositing the same material into the damaged area. Despite the surface damage discrepancy, the repaired plates performed similarly to the control plates with respect to initial and residual velocities.
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In this work, optimal modeling parameters for self-pierce riveting (SPR) were determined using a factorial design of experiments (DOE). In particular, we show statistically how each of the ...calibrating parameters used in modeling the SPR process through nonlinear finite element modeling can drastically change the geometry of the joint. The results of this study indicate that the degree of interlock, which is a key feature of a sound joint, is largely influenced by the friction between the die and bottom sheet as well as the friction between the rivet and top sheet. Furthermore, this numerical study also helped elucidate the role of friction in SPR and sheds light on how coatings with diverse friction coefficients can affect material deformation and ultimately structural integrity of the joint.
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