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K Srikanth - One of the best experts on this subject based on the ideXlab platform.
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Dislocation Mechanism based model for portevin le chatelier like instability in microindentation of dilute alloys
Social Science Research Network, 2018Co-Authors: G Ananthakrishna, K SrikanthAbstract:While the topic of intermittent plastic flow manifesting as load fluctuations or displacement jumps in nanoindentation (depths less than 100 nm) has attracted considerable attention, the existence of steps on load-indentation (F-z) curves reported in microindentation (depths of several microns) of samples of dilute alloys has received little attention from a modeling point of view. There are no simulations either. Following our earlier approaches to nanoindentation instabilities and indentation size effect, we develop a model that predicts all the reported characteristic experimental features. We develop time evolution equations for the mobile, the forest, Dislocations with solute atmosphere and the geometrically necessary Dislocation densities. The model includes all the relevant Dislocation Mechanisms such as collective pinning and unpinning of Dislocations from solute atmosphere, Dislocation-solute interaction resulting in strengthening of the alloy samples in addition to multiplication, storage and recovery Mechanisms. We model the growth of the geometrically necessary Dislocation density by the number of loops that can be activated under the contact area and the mean strain gradient based on recent experimental observations that show small misorientation at small depths suggesting limited geometrically necessary Dislocation density. The equations are then coupled to the load rate equation. The model predicts all the characteristic features of experiments such as (a) the stepped response of the F-z curves, (b) the existence of a critical load and critical indentation depth for the onset of the instability, (c) the decreasing dependence of the maximum indentation depth of the F-z curves with increasing concentration of the alloying element, (d) the mean critical indentation depth z* for the onset of the instability increases with decreasing concentration with a concomitant increase in levels of fluctuations, (e) the decreasing power law dependence of critical indentation depth with concentration, (f) the manifestation of intermittent stepped response in a window of load rates, and (g) The magnitude of the load steps scales linearly with the load.
Marc Schnetzke - One of the best experts on this subject based on the ideXlab platform.
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injury patterns following simple elbow Dislocation radiological analysis implies existence of a pure valgus Dislocation Mechanism
Archives of Orthopaedic and Trauma Surgery, 2021Co-Authors: Marc Schnetzke, Alexander Ellwein, Dirk Maier, Ferdinand Christian Wagner, Paul-alfred Grützner, Thorsten GuehringAbstract:The aim of the present study was to analyze the injury pattern and thus the Dislocation Mechanism after simple elbow Dislocation using radiographs and magnetic resonance imaging (MRI) data sets. The MRI data sets of 64 patients with a mean age of 44 years (18–77 years) were analyzed retrospectively. The inclusion criteria for the study were (1) radiograph with confirmed simple elbow Dislocation, (2) low-energy trauma, (3) MRI of the affected elbow ≤ 3 weeks after trauma. The Dislocation direction was determined using radiographs. The integrity of the lateral collateral ligament complex (LCLC), common extensor origin (CEO), anterior capsule (AC), medial collateral ligament (MCL), and common flexor origin (CFO) as well as the joint congruity were assessed based on MRI. 34 patients (53%) had a posterolateral, 26 patients (41%) a posterior, and 4 patients (6%) a posteromedial Dislocation. LCLC and AC were affected in 64 out of 64 patients (100%). MCL was affected in 58 patients (91%). CEO were affected in 25 patients (39%) and the CFO in 20 patients (31%). In 11 patients (17%) the injury pattern was more pronounced medially than laterally (MCL, CFO, LCLC), with 2 of these patients exhibiting only a partial LCLC tear. All cases with joint incongruency (n = 12, 19%) showed CEO and/or CFO involvement. Simple elbow Dislocation leads to a very heterogeneous spectrum of soft tissue injury pattern. A small proportion of patients showed medially pronounced injury patterns. These findings strongly indicate existence of a “reversed Horii circle” with an underlying valgus Mechanism (medial force induction) originating and continuing from medial to anterior.
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Injury patterns following simple elbow Dislocation: radiological analysis implies existence of a pure valgus Dislocation Mechanism
Archives of Orthopaedic and Trauma Surgery, 2020Co-Authors: Marc Schnetzke, Alexander Ellwein, Dirk Maier, Ferdinand Christian Wagner, Paul-alfred Grützner, Thorsten GuehringAbstract:Introduction The aim of the present study was to analyze the injury pattern and thus the Dislocation Mechanism after simple elbow Dislocation using radiographs and magnetic resonance imaging (MRI) data sets. Materials and methods The MRI data sets of 64 patients with a mean age of 44 years (18–77 years) were analyzed retrospectively. The inclusion criteria for the study were (1) radiograph with confirmed simple elbow Dislocation, (2) low-energy trauma, (3) MRI of the affected elbow ≤ 3 weeks after trauma. The Dislocation direction was determined using radiographs. The integrity of the lateral collateral ligament complex (LCLC), common extensor origin (CEO), anterior capsule (AC), medial collateral ligament (MCL), and common flexor origin (CFO) as well as the joint congruity were assessed based on MRI. Results 34 patients (53%) had a posterolateral, 26 patients (41%) a posterior, and 4 patients (6%) a posteromedial Dislocation. LCLC and AC were affected in 64 out of 64 patients (100%). MCL was affected in 58 patients (91%). CEO were affected in 25 patients (39%) and the CFO in 20 patients (31%). In 11 patients (17%) the injury pattern was more pronounced medially than laterally (MCL, CFO, LCLC), with 2 of these patients exhibiting only a partial LCLC tear. All cases with joint incongruency ( n = 12, 19%) showed CEO and/or CFO involvement. Conclusions Simple elbow Dislocation leads to a very heterogeneous spectrum of soft tissue injury pattern. A small proportion of patients showed medially pronounced injury patterns. These findings strongly indicate existence of a “reversed Horii circle” with an underlying valgus Mechanism (medial force induction) originating and continuing from medial to anterior.
Thorsten Guehring - One of the best experts on this subject based on the ideXlab platform.
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injury patterns following simple elbow Dislocation radiological analysis implies existence of a pure valgus Dislocation Mechanism
Archives of Orthopaedic and Trauma Surgery, 2021Co-Authors: Marc Schnetzke, Alexander Ellwein, Dirk Maier, Ferdinand Christian Wagner, Paul-alfred Grützner, Thorsten GuehringAbstract:The aim of the present study was to analyze the injury pattern and thus the Dislocation Mechanism after simple elbow Dislocation using radiographs and magnetic resonance imaging (MRI) data sets. The MRI data sets of 64 patients with a mean age of 44 years (18–77 years) were analyzed retrospectively. The inclusion criteria for the study were (1) radiograph with confirmed simple elbow Dislocation, (2) low-energy trauma, (3) MRI of the affected elbow ≤ 3 weeks after trauma. The Dislocation direction was determined using radiographs. The integrity of the lateral collateral ligament complex (LCLC), common extensor origin (CEO), anterior capsule (AC), medial collateral ligament (MCL), and common flexor origin (CFO) as well as the joint congruity were assessed based on MRI. 34 patients (53%) had a posterolateral, 26 patients (41%) a posterior, and 4 patients (6%) a posteromedial Dislocation. LCLC and AC were affected in 64 out of 64 patients (100%). MCL was affected in 58 patients (91%). CEO were affected in 25 patients (39%) and the CFO in 20 patients (31%). In 11 patients (17%) the injury pattern was more pronounced medially than laterally (MCL, CFO, LCLC), with 2 of these patients exhibiting only a partial LCLC tear. All cases with joint incongruency (n = 12, 19%) showed CEO and/or CFO involvement. Simple elbow Dislocation leads to a very heterogeneous spectrum of soft tissue injury pattern. A small proportion of patients showed medially pronounced injury patterns. These findings strongly indicate existence of a “reversed Horii circle” with an underlying valgus Mechanism (medial force induction) originating and continuing from medial to anterior.
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Injury patterns following simple elbow Dislocation: radiological analysis implies existence of a pure valgus Dislocation Mechanism
Archives of Orthopaedic and Trauma Surgery, 2020Co-Authors: Marc Schnetzke, Alexander Ellwein, Dirk Maier, Ferdinand Christian Wagner, Paul-alfred Grützner, Thorsten GuehringAbstract:Introduction The aim of the present study was to analyze the injury pattern and thus the Dislocation Mechanism after simple elbow Dislocation using radiographs and magnetic resonance imaging (MRI) data sets. Materials and methods The MRI data sets of 64 patients with a mean age of 44 years (18–77 years) were analyzed retrospectively. The inclusion criteria for the study were (1) radiograph with confirmed simple elbow Dislocation, (2) low-energy trauma, (3) MRI of the affected elbow ≤ 3 weeks after trauma. The Dislocation direction was determined using radiographs. The integrity of the lateral collateral ligament complex (LCLC), common extensor origin (CEO), anterior capsule (AC), medial collateral ligament (MCL), and common flexor origin (CFO) as well as the joint congruity were assessed based on MRI. Results 34 patients (53%) had a posterolateral, 26 patients (41%) a posterior, and 4 patients (6%) a posteromedial Dislocation. LCLC and AC were affected in 64 out of 64 patients (100%). MCL was affected in 58 patients (91%). CEO were affected in 25 patients (39%) and the CFO in 20 patients (31%). In 11 patients (17%) the injury pattern was more pronounced medially than laterally (MCL, CFO, LCLC), with 2 of these patients exhibiting only a partial LCLC tear. All cases with joint incongruency ( n = 12, 19%) showed CEO and/or CFO involvement. Conclusions Simple elbow Dislocation leads to a very heterogeneous spectrum of soft tissue injury pattern. A small proportion of patients showed medially pronounced injury patterns. These findings strongly indicate existence of a “reversed Horii circle” with an underlying valgus Mechanism (medial force induction) originating and continuing from medial to anterior.
Huajian Gao - One of the best experts on this subject based on the ideXlab platform.
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CONSTRAINED DIFFUSIONAL CREEP IN ULTRA THIN COPPER FILMS DEPOSITED ON SUBSTRATES
2016Co-Authors: Markus J. Buehler, Er Hartmaier, Huajian GaoAbstract:In a recent study of diffusional creep in polycrystalline thin films deposited on substrates, we have discovered a new class of defects called the grain boundary diffusion wedges (Gao et al., Acta Mat. 47, pp. 2865-2878, 1999). These diffusion wedges are formed by stress driven mass transport between the free surface of the film and the grain boundaries during the process of substrate-constrained grain boundary diffusion. The mathematical modeling involves solution of integro-differential equations representing a strong coupling between elasticity and diffusion. The solution can be decomposed into diffusional eigenmodes reminiscent of crack-like opening displacement along the grain boundary which leads to a singular stress field at the root of the grain boundary. We find that the theoretical analysis successfully explains the difference between the mechanical behaviors of passivated and unpassivated copper films during thermal cycling on a silicon substrate. An important implication of our theoretical analysis is that Dislocations with Burgers vector parallel to the interface can be nucleated at the root of the grain boundary. This is a new Dislocation Mechanism in thin films which contrasts to the well known Mathews-Freund-Nix Mechanism of threading Dislocation propagation. Recent TEM experiments at the Max Planck Institute for Metals Research have shown that, while threading Dislocations dominate in passivated metal films, parallel glide Dislocations begi
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3153 constrained diffusional creep in ultra thin copper films deposited on substrates
ICF11 Italy 2005, 2013Co-Authors: Markus J. Buehler, Alexander Hartmaier, Huajian GaoAbstract:In a recent study of diffusional creep in polycrystalline thin films deposited on substrates, we have discovered a new class of defects called the grain boundary diffusion wedges (Gao et al., Acta Mat. 47, pp. 2865-2878, 1999). These diffusion wedges are formed by stress driven mass transport between the free surface of the film and the grain boundaries during the process of substrate-constrained grain boundary diffusion. The mathematical modeling involves solution of integro-differential equations representing a strong coupling between elasticity and diffusion. The solution can be decomposed into diffusional eigenmodes reminiscent of crack-like opening displacement along the grain boundary which leads to a singular stress field at the root of the grain boundary. We find that the theoretical analysis successfully explains the difference between the mechanical behaviors of passivated and unpassivated copper films during thermal cycling on a silicon substrate. An important implication of our theoretical analysis is that Dislocations with Burgers vector parallel to the interface can be nucleated at the root of the grain boundary. This is a new Dislocation Mechanism in thin films which contrasts to the well known Mathews-Freund-Nix Mechanism of threading Dislocation propagation. Recent TEM experiments at the Max Planck Institute for Metals Research have shown that, while threading Dislocations dominate in passivated metal films, parallel glide Dislocations begin to dominate in unpassivated copper films with thickness below 400 nm. This is consistent with our theoretical predictions. We have developed large scale molecular dynamics simulations of grain boundary diffusion wedges to clarify the nucleation Mechanisms of parallel glide in thin films. Such atomic scale simulations of thin film diffusion not only show results which are consistent with both continuum theoretical and experimental studies, but also revealed the atomic processes of Dislocation nucleation, climb, glide and storage in grain boundaries. The study should have far reaching implications for modeling deformation and diffusion in microand nanostructured materials.
G Ananthakrishna - One of the best experts on this subject based on the ideXlab platform.
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Dislocation Mechanism based model for portevin le chatelier like instability in microindentation of dilute alloys
Social Science Research Network, 2018Co-Authors: G Ananthakrishna, K SrikanthAbstract:While the topic of intermittent plastic flow manifesting as load fluctuations or displacement jumps in nanoindentation (depths less than 100 nm) has attracted considerable attention, the existence of steps on load-indentation (F-z) curves reported in microindentation (depths of several microns) of samples of dilute alloys has received little attention from a modeling point of view. There are no simulations either. Following our earlier approaches to nanoindentation instabilities and indentation size effect, we develop a model that predicts all the reported characteristic experimental features. We develop time evolution equations for the mobile, the forest, Dislocations with solute atmosphere and the geometrically necessary Dislocation densities. The model includes all the relevant Dislocation Mechanisms such as collective pinning and unpinning of Dislocations from solute atmosphere, Dislocation-solute interaction resulting in strengthening of the alloy samples in addition to multiplication, storage and recovery Mechanisms. We model the growth of the geometrically necessary Dislocation density by the number of loops that can be activated under the contact area and the mean strain gradient based on recent experimental observations that show small misorientation at small depths suggesting limited geometrically necessary Dislocation density. The equations are then coupled to the load rate equation. The model predicts all the characteristic features of experiments such as (a) the stepped response of the F-z curves, (b) the existence of a critical load and critical indentation depth for the onset of the instability, (c) the decreasing dependence of the maximum indentation depth of the F-z curves with increasing concentration of the alloying element, (d) the mean critical indentation depth z* for the onset of the instability increases with decreasing concentration with a concomitant increase in levels of fluctuations, (e) the decreasing power law dependence of critical indentation depth with concentration, (f) the manifestation of intermittent stepped response in a window of load rates, and (g) The magnitude of the load steps scales linearly with the load.