title: Nonlinear Effects in Three-minute Oscillations of the Solar Chromosphere. I. An Analytical Nonlinear Solution and Detection of the Second Harmonic

authors: Chae, Jongchul

subjects: hydrodynamics

abstract: The vertical propagation of nonlinear acoustic waves in an isothermal atmosphere is considered. A new analytical solution that describes a finite-amplitude wave of an arbitrary wavelength is obtained. Although the short- and long-wavelength limits were previously considered separately, the new solution describes both limiting cases within a common framework and provides a straightforward way of interpolating between the two limits. Physical features of the nonlinear waves in the chromosphere are described, including the dispersive nature of low-frequency waves, the steepening of the wave profile, and the influence of the gravitational field on wavefront breaking and shock formation. The analytical results suggest that observations of three-minute oscillations in the solar chromosphere may reveal the basic nonlinear effect of oscillations with combination frequencies, superposed on the normal oscillations of the system. Explicit expressions for a second-harmonic signal and the ratio of its amplitude to the fundamental harmonic amplitude are derived. Observational evidence of the second harmonic, obtained with the Fast Imaging Solar Spectrograph, installed at the 1.6 m New Solar Telescope of the Big Bear Observatory, is presented. The presented data are based on the time variations of velocity determined from the Na i D₂ and Hα lines.

text: Nonlinear Effects in Three-minute Oscillations of the Solar Chromosphere. I. An Analytical Nonlinear Solution and Detection of the Second Harmonic The vertical propagation of nonlinear acoustic waves in an isothermal atmosphere is considered. A new analytical solution that describes a finite-amplitude wave of an arbitrary wavelength is obtained. Although the short- and long-wavelength limits were previously considered separately, the new solution describes both limiting cases within a common framework and provides a straightforward way of interpolating between the two limits. Physical features of the nonlinear waves in the chromosphere are described, including the dispersive nature of low-frequency waves, the steepening of the wave profile, and the influence of the gravitational field on wavefront breaking and shock formation. The analytical results suggest that observations of three-minute oscillations in the solar chromosphere may reveal the basic nonlinear effect of oscillations with combination frequencies, superposed on the normal oscillations of the system. Explicit expressions for a second-harmonic signal and the ratio of its amplitude to the fundamental harmonic amplitude are derived. Observational evidence of the second harmonic, obtained with the Fast Imaging Solar Spectrograph, installed at the 1.6 m New Solar Telescope of the Big Bear Observatory, is presented. The presented data are based on the time variations of velocity determined from the Na i D₂ and Hα lines.

year: 2017

title: Analysis of fracture, fatigue, and metallurgical properties of dissimilar TIG & RFW Welds: S355J2/SS316L and Ti-64/Ti-1023

authors: Das, Rajkumar

abstract: Weld joints comprising dissimilar metals with varying compositions are widely utilized across numerous contemporary industrial sectors such as aerospace, automotive manufacturing, shipbuilding, oil and gas, power generation, and medical device production. They offer cost efficiency, performance enhancement, weight reduction, design flexibility, and functionality. However, welding dissimilar metals with varying compositions poses complexity due to compositional gradient and differences in microstructure, leading to variations in properties across the joint. Gaining insight into the influence of parameters in welding on material properties for manufacturing defect-free welded joints is crucial. Nonetheless, there is a considerable gap in knowledge in understanding the relationship between welding parameters, microstructure, fracture toughness, and fatigue in dissimilar material joints.
This study focuses on investigating how parameters in welding influence the microstructure and fracture toughness of welds between two dissimilar materials i.e. structural steel (S355J2) and stainless steel (SS) (316L) using Rotary Friction Welding (RFW) and Tungsten Inert Gas (TIG) welding. Additionally, the study explores how parameters in welding affect the microstructure and fatigue characteristics of welds between two dissimilar titanium alloys i.e. Ti-6Al-4V (Ti-64) and Ti-10V-2Fe-3Al (Ti-1023) using RFW. To achieve the project objectives, dissimilar welding preforms of S355J2 and SS316L are meticulously prepared using corresponding pipe pup pieces and welded via RFW and TIG processes. A comprehensive series of tests are conducted, including microstructural analysis, tensile, hardness, and fracture toughness characteristics are accomplished using compact tension (CT) specimens extracted from various zones of both dissimilar weld joints. For titanium, dissimilar weld joints are fabricated using a round bar of Ti-64 and Ti-1023 via RFW, and tests, including microstructural analysis, tensile, hardness, and stress-controlled high cycle fatigue (HCF), are performed.
Results reveal significant microstructural variations in both dissimilar welding processes for the S355J2 and SS316L as well as the Ti-64 and Ti-1023 alloys. RFW joint between S355J2 and SS316L shows less instability in hardness across the weld with the highest hardness of 208 HV1 in the thermo-mechanically affected zone (TMAZ) of S355J2. Noticeable changes in hardness are observed across the TIG weld with 419 HV1 at the weld centre line (WCL) due to chromium carbide precipitation and 284 HV1 at the TIG heat affected zone (HAZ) of S355J2 is attributed to martensite formation. Both welds exhibit superior tensile characteristics, with the RFW weld displaying ultimate tensile strength (UTS) of 540 MPa and yield strength (YS) of 367 MPa, while the TIG weld shows UTS of 526 MPa and YS of 300 MPa. Tensile failures predominantly occur on the S355J2 side, exhibiting ductile features. Both welds show nearly identical fracture toughness (KQ) values, closely matching those of the parent metal, whereas, in terms of crack tip opening displacement (CTOD), RFW-WCL shows superior performance with 0.35 mm compared to TIG-WCL's 0.32 mm. Notably, the HAZ of S355J2 in the TIG weld exhibits the lowest CTOD of 0.31 mm. Fractography exhibits ductile failure, except for the TIG WCL, which displays cleavage fracture.
In the RFW joint between Ti-64 and Ti-1023, the highest hardness of 342 HV1 occurs at the WCL, gradually diminishing on either side of the weld, attributed to formation of hard microstructural phases and the growth of secondary alpha (αs) phases. The weld exhibits remarkable tensile properties, comparable with those of the parent metal, with a UTS of 826 MPa and an YS of 792 MPa. Tensile failures primarily manifest on the Ti-1023 side, demonstrating ductile characteristics. The HCF test, conducted at a frequency of 40 Hz with a R value of 0.1, reveals a fatigue strength of the weld at 550 MPa, surpassing that of the Ti-64 parent metal. In all HCF specimens, crack initiates at the surface, with those under higher stress exhibiting a narrow crack propagation area characterized by coarse fatigue striation marks indicative of dominant tensile overloading, while specimens under lower stress reveal a wider crack propagation area with quasi-cleavage facets and very fine fatigue striations.
Overall, the study findings establish that the RFW process yields a robust dissimilar weld joint between different materials S355J2 / SS316l and Ti-64 / Ti-1023.

text: Analysis of fracture, fatigue, and metallurgical properties of dissimilar TIG & RFW Welds: S355J2/SS316L and Ti-64/Ti-1023 Weld joints comprising dissimilar metals with varying compositions are widely utilized across numerous contemporary industrial sectors such as aerospace, automotive manufacturing, shipbuilding, oil and gas, power generation, and medical device production. They offer cost efficiency, performance enhancement, weight reduction, design flexibility, and functionality. However, welding dissimilar metals with varying compositions poses complexity due to compositional gradient and differences in microstructure, leading to variations in properties across the joint. Gaining insight into the influence of parameters in welding on material properties for manufacturing defect-free welded joints is crucial. Nonetheless, there is a considerable gap in knowledge in understanding the relationship between welding parameters, microstructure, fracture toughness, and fatigue in dissimilar material joints.
This study focuses on investigating how parameters in welding influence the microstructure and fracture toughness of welds between two dissimilar materials i.e. structural steel (S355J2) and stainless steel (SS) (316L) using Rotary Friction Welding (RFW) and Tungsten Inert Gas (TIG) welding. Additionally, the study explores how parameters in welding affect the microstructure and fatigue characteristics of welds between two dissimilar titanium alloys i.e. Ti-6Al-4V (Ti-64) and Ti-10V-2Fe-3Al (Ti-1023) using RFW. To achieve the project objectives, dissimilar welding preforms of S355J2 and SS316L are meticulously prepared using corresponding pipe pup pieces and welded via RFW and TIG processes. A comprehensive series of tests are conducted, including microstructural analysis, tensile, hardness, and fracture toughness characteristics are accomplished using compact tension (CT) specimens extracted from various zones of both dissimilar weld joints. For titanium, dissimilar weld joints are fabricated using a round bar of Ti-64 and Ti-1023 via RFW, and tests, including microstructural analysis, tensile, hardness, and stress-controlled high cycle fatigue (HCF), are performed.
Results reveal significant microstructural variations in both dissimilar welding processes for the S355J2 and SS316L as well as the Ti-64 and Ti-1023 alloys. RFW joint between S355J2 and SS316L shows less instability in hardness across the weld with the highest hardness of 208 HV1 in the thermo-mechanically affected zone (TMAZ) of S355J2. Noticeable changes in hardness are observed across the TIG weld with 419 HV1 at the weld centre line (WCL) due to chromium carbide precipitation and 284 HV1 at the TIG heat affected zone (HAZ) of S355J2 is attributed to martensite formation. Both welds exhibit superior tensile characteristics, with the RFW weld displaying ultimate tensile strength (UTS) of 540 MPa and yield strength (YS) of 367 MPa, while the TIG weld shows UTS of 526 MPa and YS of 300 MPa. Tensile failures predominantly occur on the S355J2 side, exhibiting ductile features. Both welds show nearly identical fracture toughness (KQ) values, closely matching those of the parent metal, whereas, in terms of crack tip opening displacement (CTOD), RFW-WCL shows superior performance with 0.35 mm compared to TIG-WCL's 0.32 mm. Notably, the HAZ of S355J2 in the TIG weld exhibits the lowest CTOD of 0.31 mm. Fractography exhibits ductile failure, except for the TIG WCL, which displays cleavage fracture.
In the RFW joint between Ti-64 and Ti-1023, the highest hardness of 342 HV1 occurs at the WCL, gradually diminishing on either side of the weld, attributed to formation of hard microstructural phases and the growth of secondary alpha (αs) phases. The weld exhibits remarkable tensile properties, comparable with those of the parent metal, with a UTS of 826 MPa and an YS of 792 MPa. Tensile failures primarily manifest on the Ti-1023 side, demonstrating ductile characteristics. The HCF test, conducted at a frequency of 40 Hz with a R value of 0.1, reveals a fatigue strength of the weld at 550 MPa, surpassing that of the Ti-64 parent metal. In all HCF specimens, crack initiates at the surface, with those under higher stress exhibiting a narrow crack propagation area characterized by coarse fatigue striation marks indicative of dominant tensile overloading, while specimens under lower stress reveal a wider crack propagation area with quasi-cleavage facets and very fine fatigue striations.
Overall, the study findings establish that the RFW process yields a robust dissimilar weld joint between different materials S355J2 / SS316l and Ti-64 / Ti-1023.

year: 2024-05-23

title: Māori education as justice and reckoning

authors: Hemi, Keakaokawai Varner

abstract: Hypothetical musings on what constitutes justice have been taking place at least since the ancient Greek philosopher Plato recorded the above debate between Socrates and Cephalus in Republic circa 380 BCE. Modem philosophical notions of justice have also often been heady stuff relying on heuristic "device[ s] of representation"², "thick and thin"³ theories and other abstract conceptions to untangle real-world inequalities. In comparison, current questions of access to justice are earthy, grunty and real. Increasingly vociferous access to justice claims cite the actual injustice-laden histories of particular groups, speak in terms of harm already inflicted and call for reparative, restorative or healing justice. Indigenous peoples, African-Americans and other disenfranchised, excluded and oppressed groups quote dates, casualty numbers and the legislative section or treaty responsible. For many indigenous people, any single incident is always just one among many in the collective memory; there are always more dates, cases,⁴ facts and figures in the "litany of injustices"⁵ which evidence denial of justice. Gritty statistics in one area of well-being flow into another and over generations. This is justice in the trenches, a place where numbers and statistics are what they are.

text: Māori education as justice and reckoning Hypothetical musings on what constitutes justice have been taking place at least since the ancient Greek philosopher Plato recorded the above debate between Socrates and Cephalus in Republic circa 380 BCE. Modem philosophical notions of justice have also often been heady stuff relying on heuristic "device[ s] of representation"², "thick and thin"³ theories and other abstract conceptions to untangle real-world inequalities. In comparison, current questions of access to justice are earthy, grunty and real. Increasingly vociferous access to justice claims cite the actual injustice-laden histories of particular groups, speak in terms of harm already inflicted and call for reparative, restorative or healing justice. Indigenous peoples, African-Americans and other disenfranchised, excluded and oppressed groups quote dates, casualty numbers and the legislative section or treaty responsible. For many indigenous people, any single incident is always just one among many in the collective memory; there are always more dates, cases,⁴ facts and figures in the "litany of injustices"⁵ which evidence denial of justice. Gritty statistics in one area of well-being flow into another and over generations. This is justice in the trenches, a place where numbers and statistics are what they are.

year: 2017