Buckling analysis of linearly tapered micro-columns based on strain gradient elasticity

Bekir Akgöz,Ö mer Civalek 국제구조공학회 2013 Structural Engineering and Mechanics, An Int'l Jou Vol.48 No.2

The buckling problem of linearly tapered micro-columns is investigated on the basis of modified strain gradient elasticity theory. Bernoulli-Euler beam theory is used to model the non-uniform micro column. Rayleigh-Ritz solution method is utilized to obtain the critical buckling loads of the tapered cantilever micro-columns for different taper ratios. Some comparative results for the cases of rectangular and circular cross-sections are presented in graphical and tabular form to show the differences between the results obtained by modified strain gradient elasticity theory and those achieved by modified couple stress and classical theories. From the results, it is observed that the differences between critical buckling loads achieved by classical and those predicted by non-classical theories are considerable for smaller values of the ratio of the micro-column thickness (or diameter) at its bottom end to the additional material length scale parameters and the differences also increase due to increasing of the taper ratio.

사회통합 - 우리는 어떤 도전 앞에 직면 했는가?

Lale Akgün 한독사회과학회 2011 한독사회과학회 국제학술대회 Vol.2011 No.1

Disorders Center, Fujieda Heisei Memorial Hospital, Fujieda, Shizuoka, Japan

Turgut Akgül,Kerim Sarıyılmaz,Murat Korkmaz,Okan Özkunt,Özcan Kaya,Fatih Dikici 대한척추외과학회 2018 Asian Spine Journal Vol.12 No.1

Study Design: Retrospective analysis of adolescent idiopathic scoliosis. Purpose: This study aimed to investigate the influence of distinct distal fusion levels on spinopelvic parameters in patients with adolescent idiopathic scoliosis (AIS) who underwent posterior instrumentation and fusion surgery. Overview of Literature: The distal fusion level selection in treatment of AIS is the one of milestone to effect on surgical outcome. Most of the paper focused on the coronal deformity correction and balance. The literature have lack of knowledge about spinopelvic changing after surgical treatment and the relation with distal fusion level. We evaluate the spinopelvic and pelvic parameter alteration after fusion surgery in treatment of AIS. Methods: A total of 100 patients with AIS (88 females and 12 males) were retrospectively reviewed. Patients were assigned into the following three groups according to the distal fusion level: lumbar 2 (L2), lumbar 3 (L3), and lumbar 4 (L4). Using a lateral plane radiograph of the whole spine, spinopelvic angular parameters such as thoracic kyphosis (TK), lumbar lordosis (LL), pelvic incidence (PI), sacral slope (SS), and pelvic tilt (PT) were radiologically assessed. Results: The mean age was 15±2.4 years, and the mean follow-up period was 24.27±11.69 months. Regarding the lowest instrumented vertebra, patients were categorized as follows: 30 patients in L2 (group 1), 40 patients in L3 (group 2), and 30 patients in L4 (group 3). TK decreased from 36.60±13.30 degrees preoperatively to 26.00±7.3 degrees postoperatively in each group (p =0.001). LL decreased from 52.8±9.4 degrees preoperatively to 44.30±7.50 degrees postoperatively (p =0.001). Although PI showed no difference preoperatively among the groups, it was statistically higher postoperatively in group 3 than in the other groups (p <0.05). In addition, in group 2, PI decreased from 50.60±8.74 degrees preoperatively to 48.00±6.84 degrees postoperatively (p =0.027). SS decreased from 35.20±6.40 degrees preoperatively to 33.40±5.80 degrees postoperatively (p =0.08, p >0.05). However, mean SS was significantly higher in group 3 (p =0.042, p <0.05). PT decreased from 15.50±7.90 degrees preoperatively to 15.2±7.10 degrees postoperatively. The positive relationship (28.5%) between LL and PI measurements was statistically significant (r =0.285; p =0.004, p <0.01). Furthermore, the positive relationship (36.5%) between LL and SS measurements was statistically significant (r =0.365; p =0.001, p <0.01). Conclusions: When the distal instrumentation level in AIS surgery is below L3, a significant change in PT and SS (pelvic parameters) is anticipated.

Nonlinear vibration analysis of laminated plates restingon nonlinear two-parameters elastic foundations

Bekir Akgöz,Ömer Civalek 국제구조공학회 2011 Steel and Composite Structures, An International J Vol.11 No.5

In the present manuscript, geometrically nonlinear free vibration analysis of thin laminated plates resting on non-linear elastic foundations is investigated. Winkler-Pasternak type foundation model is used. Governing equations of motions are obtained using the von Karman type nonlinear theory. The method of discrete singular convolution is used to obtain the discretised equations of motion of plates. The effects of plate geometry, boundary conditions, material properties and foundation parameters on nonlinear vibration behavior of plates are presented.

A Diagnosis to Consider in an Adult Patient with Facial Features and Intellectual Disability: Williams Syndrome

Özlem Akgün Doğan,Pelin Özlem Şimşek Kiper,Gülen Eda Utine,Mehmet Alikaşifoğlu,Koray Boduroğlu 대한가정의학회 2017 Korean Journal of Family Medicine Vol.38 No.2

Williams syndrome (OMIM #194050) is a rare, well-recognized, multisystemic genetic condition affecting approxi-mately 1/7,500 individuals. There are no marked regional differences in the incidence of Williams syndrome. The syndrome is caused by a hemizygous deletion of approximately 28 genes, including ELN on chromosome 7q11.2. Prenatal-onset growth retardation, distinct facial appearance, cardiovascular abnormalities, and unique hyperso-cial behavior are among the most common clinical features. Here, we report the case of a patient referred to us with distinct facial features and intellectual disability, who was diagnosed with Williams syndrome at the age of 37 years. Our aim is to increase awareness regarding the diagnostic features and complications of this recognizable syn-drome among adult health care providers. Williams syndrome is usually diagnosed during infancy or childhood, but in the absence of classical findings, such as cardiovascular anomalies, hypercalcemia, and cognitive impair-ment, the diagnosis could be delayed. Due to the multisystemic and progressive nature of the syndrome, accurate diagnosis is critical for appropriate care and screening for the associated morbidities that may affect the patient’s health and well-being.

Application of strain gradient elasticity theory for buckling analysis of protein microtubules

Bekir Akgöz,Ömer Civalek 한국물리학회 2011 Current Applied Physics Vol.11 No.5

In this paper, size effect of microtubules (MTs) is studied via modified strain gradient elasticity theory for buckling. MTs are modeled by Bernoulli―Euler beam theory. By using the variational principle, the governing equations for buckling and related boundary conditions are obtained in conjunctions with the strain gradient elasticity. The size effect for buckling analysis of MTs is investigated and results are presented in graph form. The results obtained by strain gradient elasticity theory are discussed through the numerical simulations. The results based on the modified couple stress theory, nonlocal elasticity theory and classical elasticity theories have been also presented for comparison purposes.

Static stability analysis of axially functionally graded tapered micro columns with different boundary conditions

Bekir Akgöz 국제구조공학회 2019 Steel and Composite Structures, An International J Vol.33 No.1

In the present study, microstructure-dependent static stability analysis of inhomogeneous tapered micro-columns is performed. It is considered that the micro column is made of functionally graded materials and has a variable cross-section. The material and geometrical properties of micro column vary continuously throughout the axial direction. Euler-Bernoulli beam and modified couple stress theories are used to model the nonhomogeneous micro column with variable cross section. Rayleigh-Ritz solution method is implemented to obtain the critical buckling loads for various parameters. A detailed parametric study is performed to examine the influences of taper ratio, material gradation, length scale parameter, and boundary conditions. The validity of the present results is demonstrated by comparing them with some related results available in the literature. It can be emphasized that the size-dependency on the critical buckling loads is more prominent for bigger length scale parameter-to-thickness ratio and changes in the material gradation and taper ratio affect significantly the values of critical buckling loads.

Cyclic testing of chevron braced steel frames with IPE shear panels

Alirıza İ. Akgönen,Cavidan Yorgun,Cüneyt Vatansever 국제구조공학회 2015 Steel and Composite Structures, An International J Vol.19 No.5

The aim of this experimental research is to investigate the conformity of the four-bolt unstiffened moment end-plate connections consisting of European steel sections which do not meet the limitations specified for beam flange width and overall beam depth in ANSI/AISC 358-10 to the requirements of seismic application. However, the connections are satisfactory with the limitations required by Turkish Earthquake Code. For this purpose, four test specimens were designed and cyclic load was applied to three specimens while one was tested under monotonic loading to provide data for the calibration of the analytical models. The moment-rotation hysteresis loops and the failure modes for all test specimens are presented. A full three-dimensional finite element model is also developed for each test specimen for use to predict their behavior and to provide a tool for generating subsequent extensive parametric studies. The test results show that all specimens performed well in terms of rotation capacity and strength. Finite element models are found to be capable of approximating the cyclic behavior of the extended end-plate connection specimens.

Peripartum Cardiomyopathy, Hemolysis, Elevated Liver Enzymes, Low Platelet Values Syndrome, and an Incidental Atrial Septal Defect in a Multiparous Woman

Onur Furkan Akgün,Erhan Kılıç,Yavuzer Koza,Hakan Tas 아시아심장혈관영상의학회 2023 Cardiovascular Imaging Asia Vol.7 No.2

Peripartum cardiomyopathy (PPCM) and hemolysis, elevated liver enzymes, low platelet values (HELLP) syndrome are rare conditions with unknown etiologies and are associated with increased maternal and perinatal morbidity and mortality. However, survival chances can be increased and full recovery may be achieved in patients who receive an early diagnosis and treatment. We report the clinical follow-up of a multiparous woman who developed HELLP syndrome on the second day of the postpartum period and PPCM on the 40th day and was found to have an incidental secundum-type atrial septal defect.

Palliative Posterior Instrumentation versus Corpectomy with Cage Reconstruction Treatment for Thoracolumbar Pathological Fracture

Serkan Bayram,Turgut Akgül,Murat Altan,Tuna Pehlivanoğlu,Özcan Kaya,Mustafa Abdullah Özdemir,Cüneyt Şar 대한척추외과학회 2019 Asian Spine Journal Vol.13 No.2

Study Design: Single-center, retrospective cohort study. Purpose: We aimed to evaluate and compare the clinical outcomes in patients who underwent palliative posterior instrumentation (PPI) versus those who underwent corpectomy with cage reconstruction (CCR) for thoracolumbar pathological fracture. Overview of Literature: The requirement for anterior support after corpectomy has been emphasized in the treatment of pathological fractures of the vertebrae. However, for patients with a relatively short life expectancy, anterior reconstruction may not be required and posterior instrumentation alone may provide adequate stabilization. Methods: A total of 43 patients with metastases of the thoracolumbar spine underwent surgery in the department of orthopaedic and traumatology of Istanbul University Faculty of Medicine from 2003 to 2016. Surgical outcomes were assessed on the basis of survival status, pre- and postoperative pain, complication rate, and operation time. Results: PPI was performed for 22 patients and CCR was performed for 21 patients. In the PPI group, the follow-up period of the five surviving patients was 32 months. The remaining 17 patients died with a mean survival duration of 12.3 months postoperatively. In the CCR group, the five surviving patients were followed up for an average of 14.1 months. The remaining 16 patients died with a mean survival duration of 18.7 months postoperatively. No statistically significant difference (p=0.812) was noted in the survival duration. The Visual Analog Scale scores of the patients were significantly reduced after both procedures, with no significant difference noted on the basis of the type of surgical intervention (p>0.05). The complication rate in the CCR group (33.3%) was higher compared with that in the PPI group (22.7%); however, this difference was not noted to be statistically significant (p=0.379). The average operation time in the PPI group (149 minutes) was significantly shorter (p=0.04) than that in the CCR group (192 minutes). Conclusions: The PPI technique can decompress the tumor for functional improvement and can stabilize the spinal structure to provide pain relief.