Buckling and stability analysis of sandwich beams subjected to varying axial loads

Mohamed A. Eltaher,Salwa A Mohamed 국제구조공학회 2020 Steel and Composite Structures, An International J Vol.34 No.2

This article presented a comprehensive model to study static buckling stability and associated mode-shapes of higher shear deformation theories of sandwich laminated composite beam under the compression of varying axial load function. Four higher order shear deformation beam theories are considered in formulation and analysis. So, the model can consider the influence of both thick and thin beams without needing to shear correction factor. The compression force can be described through axial direction by uniform constant, linear and parabolic distribution functions. The Hamilton’s principle is exploited to derive equilibrium governing equations of unified sandwich laminated beams. The governing equilibrium differential equations are transformed to algebraic system of equations by using numerical differential quadrature method (DQM). The system of equations is solved as an eigenvalue problem to get critical buckling loads and their corresponding mode-shapes. The stability of DQM in determining of buckling loads of sandwich structure is performed. The validation studies are achieved and the obtained results are matched with those. Parametric studies are presented to figure out effects of in-plane load type, sandwich thickness, fiber orientation and boundary conditions on buckling loads and mode-shapes. The present model is important in designing process of aircraft, naval structural components, and naval structural when non-uniform in-plane compressive loading is dominated.

Static and stress analyses of bi-directional FG porous plate using unified higher order kinematics theories

Salwa Mohamed,Amr E. Assie,Nazira Mohamed,Mohamed A Eltaher 국제구조공학회 2022 Steel and Composite Structures, An International J Vol.45 No.3

This article aims to investigate the static deflection and stress analysis of bi-directional functionally graded porous plate (BDFGPP) modeled by unified higher order kinematic theories to include the shear stress effects, which not be considered before. Different shear functions are described according to higher order models that satisfy the zero-shear influence at the top and bottom surfaces, and hence refrain from the need of shear correction factor. The material properties are graded through two spatial directions (i.e., thickness and length directions) according to the power law distribution. The porosities and voids inside the material constituent are described by different cosine functions. Hamilton’s principle is implemented to derive the governing equilibrium equation of bi-directional FG porous plate structures. An efficient numerical differential integral quadrature method (DIQM) is exploited to solve the coupled variable coefficients partial differential equations of equilibrium. Problem validation and verification have been proven with previous prestigious work. Numerical results are illustrated to present the significant impacts of kinematic shear relations, gradation indices through thickness and length, porosity type, and boundary conditions on the static deflection and stress distribution of BDFGP plate. The proposed model is efficient in design and analysis of many applications used in nuclear, mechanical, aerospace, naval, dental, and medical fields.

Nonlinear buckling and free vibration of curved CNTs by doublet mechanics

Mohamed A. Eltaher,Nazira Mohamed,Salwa A. Mohamed 국제구조공학회 2020 Smart Structures and Systems, An International Jou Vol.26 No.2

In this manuscript, static and dynamic behaviors of geometrically imperfect carbon nanotubes (CNTs) subject to different types of end conditions are investigated. The Doublet Mechanics (DM) theory, which is length scale dependent theory, is used in the analysis. The Euler-Bernoulli kinematic and nonlinear mid-plane stretching effect are considered through analysis. The governing equation of imperfect CNTs is a sixth order nonlinear integro-partial-differential equation. The buckling problem is discretized via the differential-integral-quadrature method (DIQM) and then it is solved using Newton's method. The equation of linear vibration problem is discretized using DIQM and then solved as a linear eigenvalue problem to get natural frequencies and corresponding mode shapes. The DIQM results are compared with analytical ones available in the literature and excellent agreement is obtained. The numerical results are depicted to illustrate the influence of length scale parameter, imperfection amplitude and shear foundation constant on critical buckling load, post-buckling configuration and linear vibration behavior. The current model is effective in designing of NEMS, nano-sensor and nano-actuator manufactured by CNTs.

Energy equivalent model in analysis of postbuckling of imperfect carbon nanotubes resting on nonlinear elastic foundation

Nazira Mohamed,Mohamed A. Eltaher,Salwa A. Mohamed,Laila F. Seddek 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.70 No.6

This paper investigates the static and dynamic behaviors of imperfect single walled carbon nanotube (SWCNT) modeled as a beam structure by using energy-equivalent model (EEM), for the first time. Based on EEM Young’s modulus and Poisson’s ratio for zigzag (n, 0), and armchair (n, n) carbon nanotubes (CNTs) are presented as functions of orientation and force constants. Nonlinear Euler-Bernoulli assumptions are proposed considering mid-plane stretching to exhibit a large deformation and a small strain. To simulate the interaction of CNTs with the surrounding elastic medium, nonlinear elastic foundation with cubic nonlinearity and shearing layer are employed. The equation governed the motion of curved CNTs is a nonlinear integropartial- differential equation. It is derived in terms of only the lateral displacement. The nonlinear integro-differential equation that governs the buckling of CNT is numerically solved using the differential integral quadrature method (DIQM) and Newton's method. The linear vibration problem around the static configurations is discretized using DIQM and then is solved as a linear eigenvalue problem. Numerical results are depicted to illustrate the influence of chirality angle and imperfection amplitude on static response, buckling load and dynamic behaviors of armchair and zigzag CNTs. Both, clamped-clamped (C-C) and simply supported (SS-SS) boundary conditions are examined. This model is helpful especially in mechanical design of NEMS manufactured from CNTs.

Nonlinear stability of bio-inspired composite beams with higher order shear theory

Nazira Mohamed,Salwa A Mohamed,Alaa A. Abdelrhmaan,Mohamed A Eltaher 국제구조공학회 2023 Steel and Composite Structures, An International J Vol.46 No.6

This manuscript presents a comprehensive mathematical model to investigate buckling stability and postbuckling response of bio-inspired composite beams with helicoidal orientations. The higher order shear deformation theory as well as the Timoshenko beam theories are exploited to include the shear influence. The equilibrium nonlinear integro-differential equations of helicoidal composite beams are derived in detail using the energy conservation principle. Differential integral quadrature method (DIQM) is employed to discretize the nonlinear system of differential equations and solve them via the Newton iterative method then obtain the response of helicoidal composite beam. Numerical calculations are carried out to check the validity of the present solution methodology and to quantify the effects of helicoidal rotation angle, elastic foundation constants, beam theories, geometric and material properties on buckling, postbuckling of bio-inspired helicoidal composite beams. The developed model can be employed in design and analysis of curved helicoidal composite beam used in aerospace and naval structures.

Pectoral nerve blocks for transvenous subpectoral pacemaker insertion in children: a randomized controlled study

Elhaddad Ahmed Mohamed,Hefnawy Salwa Mohamed,El-Aziz Mohamed Abd,Ebraheem Mahmoud Mostafa,Mohamed Ahmed Kareem 대한마취통증의학회 2023 Korean Journal of Anesthesiology Vol.76 No.5

Background: Postoperative pain management after pacemaker insertion routinely requires opioid agents, nonsteroidal anti-inflammatory drugs, or paracetamol. However, interest in opioid-sparing multimodal pain management to minimize postoperative narcotic use has increased recently. This study aimed to assess the pectoral nerve (PECS) block versus standard treatment on postoperative pain control and opioid consumption in pediatric patients after transvenous subpectoral pacemaker insertion.Method: In this randomized controlled study, 40 pediatric patients underwent transvenous subpectoral pacemaker insertion with either congenital or postoperative complete heart block. Patients were randomly assigned to two groups: Group C (control) received conventional analgesic care without any block and Group P (pectoral) received a PECS block. Demographics, procedural variables, postoperative pain, and postoperative opioid consumption were compared between the two groups.Results: In children undergoing transvenous subpectoral pacemaker insertion, the PECS block was associated with a longer procedure time; however, the cumulative dose of fentanyl and atracurium was reduced and the hemodynamic profile was superior in Group P compared with Group C intraoperatively. Postoperatively, the PECS block was associated with lower postprocedural pain scores, which was reflected by the longer interval before the first call for rescue analgesia and lower postoperative morphine consumption, without an increase in the rate of complications.Conclusion: Ultrasound-guided PECS blocks are associated with a good intraoperative hemodynamic profile, reduced postoperative pain scores, and lower total opioid consumption in children undergoing transvenous subpectoral pacemaker placement.

Nonlinear thermal vibration of pre/post-buckled two-dimensional FGM tapered microbeams based on a higher order shear deformation theory

Asmaa A. Hendi,Mohamed A Eltaher,Salwa A Mohamed,Mohamed A. Attia,A.W. Abdalla 국제구조공학회 2021 Steel and Composite Structures, An International J Vol.41 No.6

The size-dependent nonlinear thermomechanical vibration analysis of pre- and post-buckled tapered two-directional functionally graded (2D-FG) microbeams is presented in this study. In the context of the modified couple stress theory, the formulations are derived based on the parabolic shear deformation beam theory and von Karman nonlinear strains. Different thermomechanical material properties are assumed to be temperature-dependent and smoothly vary in both length and thickness directions using the power law and the physical neutral axis concept is employed. The nonlinear governing equations are derived using the Hamilton principle and the resulting variable coefficient equations of motion are solved using the differential quadrature method (DQM) and iterative Newton’s method for clamped-clamped and simply supported boundary conditions. Comparison studies are presented to validate the derived model and solution procedure. The impacts of induced thermal moments, temperature power index, two gradient indices, nonuniform cross-section, and microstructure length scale parameter on the frequency-temperature configurations are explored for both clamped and simply supported microbeams.

Challenges of Engineering Biomimetic Dental and Paradental Tissues

Grawish Mohammed E.,Grawish Lamyaa M.,Grawish Hala M.,Grawish Mahmoud M.,El-Negoly Salwa A. 한국조직공학과 재생의학회 2020 조직공학과 재생의학 Vol.17 No.4

Background: Loss of the dental and paradental tissues resulting from trauma, caries or from systemic diseases considered as one of the most significant and frequent clinical problem to the healthcare professionals. Great attempts have been implemented to recreate functionally, healthy dental and paradental tissues in order to substitute dead and diseased tissues resulting from secondary trauma of car accidents, congenital malformations of cleft lip and palate or due to acquired diseases such as cancer and periodontal involvements. Method: An extensive literature search has been done on PubMed database from 2010 to 2019 about the challenges of engineering a biomimetic tooth (BioTooth) regarding basic biology of the tooth and its supporting structures, strategies, and different techniques of obtaining biological substitutes for dental tissue engineering. Results: It has been found that great challenges need to be considered before engineering biomimetic individual parts of the tooth such as enamel, dentin-pulp complex and periodontium. In addition, two approaches have been adopted to engineer a BioTooth. The first one was to engineer a BioTooth as an individual unit and the other was to engineer a BioTooth with its supporting structures. Conclusion: Engineering of BioTooth with its supporting structures thought to be in the future will replace the traditional and conventional treatment modalities in the field of dentistry. To accomplish this goal, different cell lines and growth factors with a variety of scaffolds at the nano-scale level are now in use. Recent researches in this area of interest are dedicated for this objective, both in vivo and in vitro. Despite progress in this field, there are still many challenges ahead and need to be overcome, many of which related to the basic tooth biology and its supporting structures and some others related to the sophisticated techniques isolating cells, fabricating the needed scaffolds and obtaining the signaling molecules.

Demineralized Dentin Matrix for Dental and Alveolar Bone Tissues Regeneration: An Innovative Scope Review

Grawish Mohammed E.,Grawish Lamyaa M.,Grawish Hala M.,Grawish Mahmoud M.,Holiel Ahmed A.,Sultan Nessma,El-Negoly Salwa A. 한국조직공학과 재생의학회 2022 조직공학과 재생의학 Vol.19 No.4

BACKGROUND: Dentin is a permeable tubular composite and complex structure, and in weight, it is composed of 20% organic matrix, 10% water, and 70% hydroxyapatite crystalline matrix. Demineralization of dentin with gradient concentrations of ethylene diamine tetraacetic acid, 0.6 N hydrochloric acid, or 2% nitric acid removes a major part of the crystalline apatite and maintains a majority of collagen type I and non-collagenous proteins, which creates an osteoinductive scaffold containing numerous matrix elements and growth factors. Therefore, demineralized dentin should be considered as an excellent naturally-derived bioactive material to enhance dental and alveolar bone tissues regeneration. METHOD: The PubMed and Midline databases were searched in October 2021 for the relevant articles on treated dentin matrix (TDM)/demineralized dentin matrix (DDM) and their potential roles in tissue regeneration. RESULTS: Several studies with different study designs evaluating the effect of TDM/DDM on dental and bone tissues regeneration were found. TDM/DDM was obtained from human or animal sources and processed in different forms (particles, liquid extract, hydrogel, and paste) and different shapes (sheets, slices, disc-shaped, root-shaped, and barrier membranes), with variable sizes measured in micrometers or millimeters, demineralized with different protocols regarding the concentration of demineralizing agents and exposure time, and then sterilized and preserved with different techniques. In the act of biomimetic acellular material, TDM/DDM was used for the regeneration of the dentin-pulp complex through direct pulp capping technique, and it was found to possess the ability to activate the odontogenic differentiation of stem cells resident in the pulp tissues and induce reparative dentin formation. TDM/DDM was also considered for alveolar ridge and maxillary sinus floor augmentations, socket preservation, furcation perforation repair, guided bone, and bioroot regenerations as well as bone and cartilage healing. CONCLUSION: To our knowledge, there are no standard procedures to adopt a specific form for a specific purpose; therefore, future studies are required to come up with a well-characterized TDM/DDM for each specific application. Likely as decellularized dermal matrix and prospectively, if the TDM/DDM is supplied in proper consistency, forms, and in different sizes with good biological properties, it can be used efficiently instead of some widely-used regenerative biomaterials.

Diagnosis of Clostridium difficile infection in patients with hospital-acquired diarrhea

Ibrahim Afifi, Salwa Selim,Gomaa, Fatma Alzahraa M.,Fathi, Lamia Fouad,Rasslan, Fatma Salah,Hamdy, Ahmed Mohamed The Microbiological Society of Korea 2018 미생물학회지 Vol.54 No.3

Clostridium difficile infection (CDI) is a rapidly emerging infection that may have devastating consequences. Prompt and accurate diagnosis is crucial for management and control. The aim of this study was to determine the incidence of C. difficile associated diarrhea among hospitalized patients, and to compare different diagnostic laboratory methods for detection of toxin producing strains in clinical specimens. The study was conducted at a university hospital in Cairo during the period from May 2013 till June 2015. Subjects were under antibiotic therapy and presented with hospital-acquired diarrhea. Four hundred and sixty-five stool specimens were processed by different microbiological methods. C. difficile was recovered in culture in 51 of stool specimens. Of these, 86.3% to 98% were positive for toxin production by 2 different methods. This study showed that antibiotic intake is the major risk factor for development of hospital-acquired diarrhea. We evaluated different microbiological methods for diagnosis of C. difficile. We recommend the use of toxigenic culture as a gold standard for microbiological diagnosis of C. difficile.