Total Number Of Dislocations Research Articles

Исследование частоты и вероятных причин дислокаций ИОЛ после факоэмульсификации возрастной катаракты

Purpose. Analysis of the frequency and structure of dislocations of the «IOL – capsular bag» complex at various times of the postoperative period of minimally invasive age-related cataract surgery. Material and methods. All cases of dislocation of the «IOL – capsular bag» complex over a 5-year period were selected using the continuous sampling method. Their severity was assessed, the probable causes of dislocations were clarified. A clinical assessment of the outcomes of dislocations was carried out, and the possibilities of their surgical correction were assessed. Results. A total of 480 cases of dislocation of the «IOL – capsular bag» complex (480 eyes) occurred during this period. The patients’ age averaged 72 ± 2 years. There were 268 men and 212 women. In all 480 cases, there was a dislocation leading to a decrease in visual functions. In 94 eyes, there were contusion injuries of the eyeball, in 254 eyes – the presence of pseudoexfoliation syndrome of various degrees, in 79 eyes – primary open-angle glaucoma of II–III stages, in 84 eyes – high myopia. In a number of eyes there was a combination of the above conditions. In 71 cases, it was not possible to establish the causes of dislocations on the part of the ophthalmic status. Conclusion. The total number of dislocations over a 5-year period was significant – 480 cases. In 14.7% of cases, it was not possible to identify the underlying causes that led to the dislocation of the complex. Keywords: age-related cataract, Zinn’s ligament, IOL, ultrasound biomicroscopy

Unconventional cracks with dislocation-free zones

Using the method of continuously distributed dislocations, we study the dislocation-free zones for unconventional internal cracks in an infinite isotropic elastic medium under uniform remote anti-plane shear stress. The unconventional cracks are represented by a symmetric lip crack with two cusp tips and a symmetric airfoil crack with a single cusp tip. For each case, the equilibrium condition is formulated in terms of a single singular integral equation in the image plane. The resulting singular integral equation is solved numerically to arrive at the dislocation distribution function and the total number of dislocations in the plastic zone, the condition on remote loading and the local mode III stress intensity factor at the cusp tip. In the absence of the dislocation-free zones, the results are valid for the Dugdale model of these unconventional cracks.

Эффективность современных методик оперативного лечения передней рецидивирующей нестабильности плечевого сустава в зависимости от анатомо‑функциональных изменений

Introduction Traumatic anterior dislocation of the shoulder usually results in chronic recurrent shoulder instability, especially in young adults. To reduce the rate of recurrent dislocations, various options of osteoplastic operations have been proposed. The purpose of our study is to determine the significance of anatomical and functional disorders in patients with anterior shoulder joint instability; to evaluate the results of the methods for surgical treatment of patients with anterior recurrent instability of the shoulder joint, depending on the anatomical and functional disorders. Materials and methods The results of diagnostic examination and treatment of 169 patients with recurrent post-traumatic dislocation of the shoulder who were operated on at the Federal State Budgetary Organization “Federal Center for Traumatology, Orthopedics and Arthroplasty” (Cheboksary) were analyzed. The average age of patients was 31.1 ± 13.1 years; the average period of postoperative follow-up was 49.5 ± 24.0 months. Clinical data were assessed by angulometry, using the ROWE, ASES, UCLA, VAS scales. The size of bone defects was determined using radiological methods. 99 patients (58.6 %) underwent arthroscopic Bankart repair, 24 patients (14.2 %) with more than 15 % anterior glenoid bone defect underwent Latarjet operation. Bankart + Remplissage surgery was performed in 39 patients (23.1 %), Latarjet + Remplissage in 7 (4.1 %) patients. Results Bone defect of the glenoid of more than 15 % of the area was detected in 32 patients. Hill-Sachs grade 3 injury was reported in 24 patients. Most of these patients had a history of more than 20 dislocation episodes. Hyperelasticity of the shoulder joint was found in 34 patients. In the postoperative period, there was a slight improvement in the mean indicators of flexion and abduction in the shoulder joint from 174.2 ± 15.9° and 170.6 ± 17.8° to 175.6 ± 9.3° and 173.0 ± 12.3°, respectively. The mean external rotation worsened from 61.6 ± 25.7° to 55.9 ± 14.4°. Indicators of the ROWE, ASES, UCLA rating scales improved significantly from 38.5 ± 8.5, 76.0 ± 13.2 and 20.8 ± 3.0 to 89.9 ± 17.5, 94.0 ± 7.1 and 32.7 ± 3.7 points, respectively. Poor results were observed in 7 patients (4.1 %), of which three had a relapse. Conclusions Critically significant anatomical and functional disorders in patients with anterior instability of the shoulder joint are the defect of the anterior rim of the articular process of the scapula more than 15 %, the defect of the posterior surface of the humeral head (Hill-Sachs defect) of grade 3, hyperelasticity of the soft tissues of the shoulder joint. The severity of bone defects is directly proportional to the total number of dislocations. The effectiveness of surgical techniques for stabilizing the shoulder joint, depending on the above anatomical and functional disorders, was 95.9 %.

Stick-slip failure in sheared fault with a variety of end constraint degrees: An experimental study

For the commonly-used laboratory frictional earthquake model, unstable slip usually occurs without any end constraint. This experimental design is contradictory to a simple fact: real faults are confined by their surroundings, and the relative sliding is not end-free. To investigate the effects of end constraint conditions on the stick-slip behaviors, direct shear tests are conducted on PMMA sheets with three kinds of pre-crack length, and high sampling rate strain gage array and digital image correlation method are used to monitor the detailed processes of dislocation(stick-slip). We find that pre-crack length has great influence on the intervals of dislocation (stick-slip), the total number of dislocations (stick-slip) that occurs before crack extension, the magnitude of stress-drop and rupture velocity. A transition of the dislocation propagation velocity from sub-Rayleigh to super-shear is observed for PMMA sheets with a long pre-crack, while super-shear velocity rupture cannot be obtained in PMMA sheets with a short pre-crack.

A Class of Rate-Independent Lower-Order Gradient Plasticity Theories: Implementation and Application to Disc Torsion Problem.

As the characteristic scale of products and production processes decreases, the plasticity phenomena observed start to deviate from those evidenced at the macroscale. The current research aims at investigating this gap using a lower-order gradient enhanced approach both using phenomenological continuum level as well as crystal plasticity models. In the phenomenological approach, a physically based hardening model relates the flow stress to the density of dislocations where it is assumed that the sources of immobile dislocations are both statistically stored (SSDs) as well as geometrically necessary dislocations (GNDs). In the crystal plasticity model, the evolution of the critical resolved shear stress is also defined based on the total number of dislocations. The GNDs are similarly incorporated in the hardening based on projecting the plastic strain gradients through the Burgers tensor on slip systems. A rate-independent formulation is considered that eliminates any artificial inhomogeneous hardening behavior due to numerical stabilization. The behavior of both models is compared in simulations focusing on the effect of structurally imposed gradients versus the inherent gradients arising in crystal plasticity simulations.

A continuum model for dislocation pile-up problems

A 2-d dislocation pile-up model is developed to solve problems with arrays of edge dislocations on one or multiple slip planes. The model developed in this work has four unique features: 1) As a continuum mechanics model, it captures the discrete behaviors of dislocations including the region near pile-up boundaries. 2) It allows for a general distribution of dislocations and applied boundary conditions. 3) The computational complexity does not quadratically scale with increased number of dislocations. 4) The effect of anisotropy and stacking fault energy can be naturally modeled. Pile-ups against a lock under shear load are extensively investigated, which shows the dependence of near-lock piles distribution on the total number of dislocations. The stacking fault energy effect is found to be positively correlated to the length of an equilibrated pile-up. The stress intensity near a bi-metallic interface is studied for both isotropic material and anisotropic materials. The model is validated by reproducing the solutions of problems for which analytical solutions are available. More complicated phenomena such as interlacing and randomly distributed dislocations are also simulated.

The effects of junction interdiffusion and misfit dislocations on the efficiency of highly mismatched heterojunction photovoltaic devices

A general modelling methodology has been developed to evaluate the effects of chemical interdiffusion and misfit dislocations on the performance of heterojunction solar cells made from highly mismatched materials. Results for the exemplar materials system CdS-CdTe are contrary to the widely held belief that such interdiffusion is beneficial to photovoltaic performance. In the model, recombination is presumed to take place at the cores of misfit dislocations, with the distribution of these dislocations in the interdiffused layer being calculated so as to minimise the total energy (an incidental result shows that the total number of dislocations is independent of the diffusion profile). The model takes calculated chemical profiles, optical absorption, and dislocation distributions from which the photovoltaic performance and recombination losses are evaluated. It was shown that for the realistic case in which the interdiffused region does not extend beyond the space charge region, the photovoltage losses dominate over any photocurrent gains. Methods to engineer mixed junctions that may increase solar conversion efficiency are discussed.

Study of acoustic emission in glass upon exposure to infrared laser pulses

The previously published hypothesis of the acoustic emission (AE) occurrence due to recombination of defects transferred to the surface by an elastic wave is analyzed. The dynamics of the rate of the defect appearance on the surface, controlling the AE level, is determined. The effect of such factors as the character of the dependence of the number of transferred defects on the wave intensity, their fraction in the total number of dislocations, and the wave damping rate in the sample is studied. The results are compared with experimental data.

Nonlinear scattering in photonic crystals having dislocations with fractional topological character and multiple dislocations

The spectrum of the second harmonic signal generated in quadratic nonlinear photonic crystals, having different types of edge dislocations, was studied theoretically and experimentally. In the case of a dislocation with a fractional topological charge, we observed an asymmetric spectral conversion efficiency response, where the degree of asymmetry depends on the value of the fractional charge. Moreover, we have found that the conversion efficiency spectrum exhibits a periodic dependence on the topological charge value. In addition, nonlinear photonic crystals with multiple edge dislocations were studied. We show that for any number of dislocations characterized by even topological charge, the nonlinear spectral response will be identical to the response of the ideal, dislocation-free structure. This is a generalization of a previous observation that was made for crystals with a single even charge dislocation. Furthermore, for any number of dislocations with odd topological charge, two new peaks of maximal efficiency are observed in the second harmonic spectrum, in addition to a series of local efficiency peaks that are governed by the total number of dislocations. This is also a generalization of a previous observation that was made for a single dislocation case having an odd topological charge.

Continuum Dislocation Dynamics Based on the Second Order Alignment Tensor

ABSTRACTIn the current paper we present a continuum theory of dislocations based on the second-order alignment tensor in conjunction with the classical dislocation density tensor (Kröner-Nye-tensor) and a scalar dislocation curvature measure. The second-order alignment tensor is a symmetric second order tensor characterizing the orientation distribution of dislocations in elliptic form. It is closely connected to total densities of screw and edge dislocations introduced in the literature. The scalar dislocation curvature density is a conserved quantity the integral of which represents the total number of dislocations in the system. The presented evolution equations of these dislocation density measures partly parallel earlier developed theories based on screw-edge decompositions but handle line length changes and segment reorientation consistently. We demonstrate that the presented equations allow predicting the evolution of a single dislocation loop in a non-trivial velocity field.

Towards a further understanding of dislocation pileups in the presence of stress gradients

This study is an essential complement and extension to the stress-gradient concept recently proposed by Hirth. An analytic method is presented for studying the behaviour of double-ended dislocation pileup in the presence of various stress gradients by solving a singular integral equation based on the continuous approximation of dislocations. Four special cases of double-ended pileup in the presence of stress gradients are discussed in detail. The corresponding dislocation distribution, the length of pileup, the total number of dislocations within the pileup and the force on the leading dislocations at the pileup ends are derived, respectively. It is shown that both the number of dislocations and the force on the leading dislocation in a pileup are sensitive to the relative magnitude of stress near the dislocation source and both are less than that in constant stress case. Of particular importance, it is indicated that the small-scaled materials subjected to a stress involving a gradient would be stronger than that under a constant stress. Applied to wire torsion and foil bending, the stress gradient model predicts an increase in the initial yielding, which is in reasonable agreement with the recent experimental data. The proposed stress gradient concept may provide a new physical insight into the size-dependent plasticity phenomena at small length scale.

Higher order alignment tensors for continuum dislocation dynamics

ABSTRACTDislocation density based modeling of crystal plasticity remains one of the central challenges in multi scale materials modeling. A dislocation based theory requires sufficiently rich dislocation density measures which are capable of predicting their own evolution. Continuum dislocation dynamics is based on a higher dimensional dislocation density tensor comprised of two distribution functions on the space of local orientations, which are the density of dislocations per orientation and the density of dislocation curvature per orientation. We propose to expand these functions into series of symmetric tensors (alignment tensors), to be used in dislocation based theories without extra dimensions. The first two terms in the expansion of the density define the total dislocation density and the Kröner-Nye tensor. The first term in the expansion of the curvature density, the scalar total curvature density, turns out to be a conserved quantity; the integral of which corresponds to the total number of dislocations. The content of the next higher order tensors is discussed.

Histopathologic evaluation of passive stabilizers in shoulder instability

The macroscopic pathomorphology in recurrent shoulder instability has been described. However, less is known regarding the histopathologic details of the affected structures. This study evaluates different histopathologic stages of shoulder instability by assessing biopsy specimens of static stabilizers for possible correlations with clinical parameters. Our hypothesis was that clinical parameters of shoulder instability correlate with histopathologic findings. Passive shoulder stabilizers (labrum, anterior bundle of the inferior glenohumeral ligament) were biopsied during arthroscopic shoulder stabilization. Samples were submitted to immunohistochemistry, in situ hybridization, and blinded evaluation. Clinical data, comprising age (<30 years or ≥30 years), total number of dislocations (1, 2-3, or >3), and period since initial dislocation (<6 months, 6months to 6 years, or >6 years), were tested for statistical correlation with the following histopathologic parameters: inflammation, lipomatous changes, vascular proliferation, tissue fragmentation, and cellularity. Standardized biopsies were performed in 30 consecutive patients (4 women and 26 men; mean age, 32.6 years) with anterior shoulder instability. Microscopic evaluation showed only small variations in histologic changes among all samples. Only limited variations in cell density, matrix swelling, and collagen fiber disruptions were found. Immunohistochemical analysis showed a similar expression of decorin in all samples. Clinical parameters (age, total number of dislocations, and period since initial dislocation) were statistically independent from histopathologic parameters (inflammation, lipomatous changes, vascular proliferation, tissue fragmentation, and cellularity). No correlation was found in patients with 1 dislocation versus those with more than 1 dislocation. In contrast to macroscopic findings among different grades of shoulder instability, this study detected no correlation between clinical items (age, total number of dislocations, and period since initial dislocation) and histopathologic parameters. These clinical items seem to be independent from the tissue status of static stabilizers of the shoulder.

Degeneration of patellar cartilage in patients with recurrent patellar dislocation following conservative treatment: evaluation with delayed gadolinium-enhanced magnetic resonance imaging of cartilage

To examine the characteristics of cartilage degeneration in patients with recurrent patellar dislocation (RPD) following conservative treatment using delayed gadolinium-enhanced magnetic resonance imaging (MRI) of cartilage (dGEMRIC). This study evaluated three groups of knees: group I, 35 knees from both knees of patients with bilateral RPD and dislocated side knees of patients with unilateral RPD; group II, 15 non-dislocated side knees of patients with unilateral RPD; and group III, 20 knees from both knees of healthy volunteers. Differences in post-contrast T1 [T1(Gd)] of cartilage at both medial and lateral facets between groups I, II and III were analyzed. For group I, possible relationships were evaluated between T1(Gd) of cartilage and patient age, length of time between the initial dislocation and MRI and the total number of dislocations between the initial dislocation and MRI for both medial and lateral facets. The mean T1(Gd) of cartilage at medial facets for groups I, II and III were 411+/-46ms, 465+/-38ms and 490+/-29ms, respectively; there were significant differences between these means (P<0.05). The mean T1(Gd) of cartilage at lateral facets for groups I, II and III were 426+/-53ms, 466+/-45ms and 510+/-36ms, respectively; there were also significant differences between these means (P<0.05). Significant correlations were observed between T1(Gd) of cartilage for both medial and lateral facets and length of time between the initial dislocation and MRI (P<0.05). No other correlations were significant. dGEMRIC may be a useful method to monitor glycosaminoglycan concentration in patients with RPD following conservative treatment.

Osteoarthritis in shoulders with traumatic anterior instability: Preoperative survey using radiography and computed tomography

The purposes of this report are to determine the incidence of preoperative osteoarthritis in shoulders with traumatic anterior instability and to detect the factors leading to its development. The radiographs and computed tomograms of shoulders in 282 patients with unilateral instability and without previous surgery, whose age was younger than 40 years and who had no possibility of nontraumatic secondary osteoarthritis, were examined by use of a simplified modification of Samilson and Prieto's criteria. Osteoarthritis was found in 32 joints (11.3%) (mild in 30 and moderate in 2) on the radiographs. Computed tomography (CT) revealed arthritic changes in 88 shoulders (31.2%), including all 32 radiographically osteoarthritic joints. The marginal spur of the humeral head initially occurred in the anteroinferior aspect and mainly extended downward. The total number of dislocations and/or subluxations and frequency of dislocation and/or subluxation of 32 radiologically osteoarthritic joints were significantly larger and higher, respectively, than those of 194 non-osteoarthritic joints (P < .02). The total number of dislocations and/or subluxations (P < .01) and the number of subluxations (P < .05) were significantly different between osteoarthritic joints and nonarthritic joints classified by CT findings. The rate detected by CT was higher than those of any other previous radiographic survey that may have underestimated the incidence rate. Most postoperative osteoarthritis cases recognized on radiographs in short-term or midterm follow-up studies are assumed to be preoperatively generated.

Effect of dislocation distribution in twin boundaries on the nucleation of microcracks at the twin tip

The effect of dislocation distribution in the boundaries of an arrested twin on the nucleation of microcracks at its tip is investigated. The twin is simulated by a double step pileup (cluster) of twinning dislocations located in adjacent slip planes. The equilibrium equations for dislocations are solved numerically. Clusters with different total numbers of dislocations and with different ratios of the numbers of dislocations at the upper and lower twin boundaries are considered. The formation of microcracks as a result of coalescence of head dislocations according to the force and thermally activated mechanisms is analyzed. The equilibrium configurations of a single twin boundary and of the twin are calculated. It is found that the condition for microcrack formation at the twin tip considerably depends on the ratio of the numbers of dislocations in twin boundaries. In the limit, this condition coincides with the condition of crack formation at the tip of a single twin boundary with the same total number of dislocations. It is shown that thermally activated formation of a microrack corresponds to lower values of the critical stress.

White microbeam diffraction from distorted crystals

We present a general description of white-beam (Laue) scattering from grains with dislocations. This approach is applied to examples with equal numbers of positive and negative Burger’s vectors (paired) and with unpaired dislocations of one sign (geometrically necessary). We find that streaking of the Laue reflections is sensitive to both long-range geometrical rotations introduced by unpaired edge dislocations and to local rotation fluctuations introduced by the total number of dislocations (paired and unpaired). We demonstrate the technique by analyzing the dislocation distribution in a nanoindented Cu single crystal.

The Behavior of Screw Dislocations Emitted from a Star Crack with a Central Hole

The behavior of screw dislocations emitted from a star crack with a central hole was investigated using discrete dislocation modeling. Cracks are uniformly distributed along the circumference of a circular hole. Dislocations are assumed to be emitted one by one from the crack tips along the radial direction. Each emitted dislocation moves along the radial direction and its velocity is proportional to the third power of the effective shear stress. A dislocation-free zone exists based on the assumption that the crack tip must overcome an energy barrier to emit a dislocation. The effects of the central hole, slit crack, number of cracks and applied stress on the plastic zone, total number of dislocations emitted from all crack tips and the first crack tip, and the dislocation-free zone were studied for a given friction stress.

Dislocation distribution under a microindentation into an iron-silicon single crystal

Abstract A transmission electron microscopy study of the dislocation distribution under an indentation has revealed first that a complex sequence of loop nucleation, cross-slip and renucleation results in a cross-like network at the free surface and a square-like network underneath. These dislocation distributions are made up of two types of dislocation: redundant dislocations responsible for general plasticity and work hardening, and shielding dislocations responsible for the equilibrium of forces. The former are roughly an order of magnitude more prevalent. The total number of dislocations are consistent with continuum plasticity theory while the shielding density is consistent with a proposed superdislocation pile-up model.

Production of highly perfect copper crystals with thermal cyclic annealing

Dislocation behavior in copper crystals in the course of annealing was studied, using an etch pit technique. Two annealing methods used were as follows: (1) isothermal annealing and (2) thermal cyclic annealing in which the temperature was changed between 1050° and 850°C with a period of 30 min. In the isothermal annealing, some of the isolated dislocations formed new sub-boundaries, without much reduction in the total number of dislocations. On the other hand, the thermal cyclic annealing was recognized to be conspicuously effective not only in the reduction of dislocations without forming new sub-boundaries but also in breaking-up of old sub-boundaries. With this thermal cyclic annealing highly perfect copper crystals were produced from crystals with initial dislocation densities as much as 1 × 10 6/cm 2.