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- 저자 : Ahmed Najjar (검색결과 5 건)
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Fuzzy inference systems based prediction of engineering properties of two-stage concrete
Manal F. Najjar,Moncef L. Nehdi,Tareq M. Azabi,Ahmed M. Soliman 사단법인 한국계산역학회 2017 Computers and Concrete, An International Journal Vol.19 No.2
Two-stage concrete (TSC), also known as pre-placed aggregate concrete, is characterized by its unique placement technique, whereby the coarse aggregate is first placed in the formwork, then injected with a special grout. Despite its superior sustainability and technical features, TSC has remained a basic concrete technology without much use of modern chemical admixtures, new binders, fiber reinforcement or other emerging additions. In the present study, an experimental database for TSC was built. Different types of cementitious binders (single, binary, and ternary) comprising ordinary portland cement, fly ash, silica fume, and metakaolin were used to produce the various TSC mixtures. Different dosages of steel fibres having different lengths were also incorporated to enhance the mechanical properties of TSC. The database thus created was used to develop fuzzy logic models as predictive tools for the grout flowability and mechanical properties of TSC mixtures. The performance of the developed models was evaluated using statistical parameters and error analyses. The results indicate that the fuzzy logic models thus developed can be powerful tools for predicting the TSC grout flowability and mechanical properties and a useful aid for the design of TSC mixtures.
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Anterolateral Cervical Kyphoplasty for Metastatic Cervical Spine Lesions
Amer Sebaaly,Ahmed Najjar,Zhi Wang,Ghassan Boubez,Laura Masucci,Daniel Shedid 대한척추외과학회 2018 Asian Spine Journal Vol.12 No.5
Study Design: Retrospective case series. Purpose: To evaluate the clinical and radiological efficacy of anterolateral kyphoplasty for cervical spinal metastasis. Overview of Literature: Although the spine is the third most common site of tumor metastasis, the cervical spine is the least commonly affected (incidence, 10%–15%). Surgical decompression is highly challenging because of the proximity of neural and vascular elements. Kyphoplasty for cervical spine metastasis has been described in small case reports with promising results. Methods: Retrospective analysis of a prospective collected single-center spine metastasis database was done for cervical kyphoplasty cases. Data pertaining to age, sex, primary tumor diagnosis, modified Tokuhashi score, Spinal Instability Neoplastic Score (SINS), preoperative Visual Analog Scale (VAS) score, and analgesic medication were extracted. Postoperative data included VAS score at postoperative day 1, duration of hospitalization, self-reported functional outcome, and VAS score at the last follow-up. Results: Eleven patients (mean age, 62.5 years) with cervical spine metastases were treated with 15-level kyphoplasty. Mean Tokuhashi score was 8.1, and mean SINS was 7.85. Mean preoperative pain score was 7.1, and 82% of patients used opioid analgesics. Mean total bleeding volume was 100 mL. Mean complication-free length of stay was 2.6 days with a decrease in postoperative pain (VAS score=2.8, p <0.05). There was a 56% decrease in opioid dosage and the number of consumed analgesics (1.09, p =0.004). Eightytwo percent of the patients reported excellent improvement at the last follow-up self-assessment. Conclusions: To our knowledge, this case series represents the largest series of vertebral augmentation using balloon kyphoplasty for cervical spinal metastasis. This technique is associated with low postoperative complications as well as significant decrease in pain, use of opioids, and length of hospital stay. The main indications for vertebral kyphoplasty are lytic lesions of the cervical spine, painful lesions refractory to medical treatment, SINS score of 6–10, and absence of posterior wall defect.
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Experimental and numerical FEM of woven GFRP composites during drilling
Mohamed S. Abd-Elwahed,Usama A. Khashaba,Khaled I. Ahmed,Mohamed A. Eltaher,Ismael Najjar,Ammar Melaibari,Azza M. Abdraboh 국제구조공학회 2021 Structural Engineering and Mechanics, An Int'l Jou Vol.80 No.5
This paper investigates experimentally and numerically the influence of drilling process on the mechanical and thermomechanical behaviors of woven glass fiber reinforced polymer (GFRP) composite plate. Through the experimental analysis, a CNC machine with cemented carbide drill (point angles =118° and 6 mm diameter) was used to drill a woven GFRP laminated squared plate with a length of 36.6 mm and different thicknesses. A produced temperature during drilling “heat affected zone (HAZ)” was measured by two different procedures using thermal IR camera and thermocouples. A thrust force and cutting torque were measured by a Kistler 9272 dynamometer. The delamination factors were evaluated by the image processing technique. Finite element model (FEM) has been developed by using LS-Dyna to simulate the drilling processing and validate the thrust force and torque with those obtained by experimental technique. It is found that, the present finite element model has the capability to predict the force and torque efficiently at various drilling conditions. Numerical parametric analysis is presented to illustrate the influences of the speeding up, coefficient of friction, element type, and mass scaling effects on the calculated thrust force, torque and calculation’s cost. It is found that, the cutting time can be adjusted by drilling parameters (feed, speed, and specimen thickness) to control the induced temperature and thus, the force, torque and delamination factor in drilling GFRP composites. The delamination of woven GFRP is accompanied with edge chipping, spalling, and uncut fibers.
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Analysis of the machinability of GFRE composites in drilling processes
Usama. A. Khashaba,Mohamed S. Abd-Elwahed,Khaled I. Ahmed,Ismail Najjar,Ammar Melaibari,Mohamed A Eltaher 국제구조공학회 2020 Steel and Composite Structures, An International J Vol.36 No.4
Drilling processes in fiber-reinforced polymer composites are essential for the assembly and fabrication of composite structural parts. The economic impact of rejecting the drilled part is significant considering the associated loss when it reaches the assembly stage. Therefore, this article tends to illustrate the effect of cutting conditions (feed and speed), and laminate thickness on thrust force, torque, and delamination in drilling woven E-glass fiber reinforced epoxy (GFRE) composites. Four feeds (0.025, 0.05, 0.1, and 0.2 mm/r) and three speeds (400, 800, and 1600 RPM) are exploited to drill square specimens of 36.6x36.6 mm, by using CNC machine model “Deckel Maho DMG DMC 1035 V, ecoline”. The composite laminates with thicknesses of 2.6 mm, 5.3 mm, and 7.7 mm are constructed respectively from 8, 16, and 24 glass fiber layers with a fiber volume fraction of about 40%. The drilled specimen is scanned using a high-resolution flatbed color scanner, then, the image is analyzed using CorelDraw software to evaluate the delamination factor. Multi-variable regression analysis is performed to present the significant coefficients and contribution of each variable on the thrust force and delamination. Results illustrate that the drilling parameters and laminate thickness have significant effects on thrust force, torque, and delamination factor.
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