Impact Resistance of Single-Layer and Multi-Layered Materials

Use of impact resisting materials to prevent head injury and concussion is the subject of much study in protective equipment for sports and other activities. Understanding the mechanical response of impact resistant materials and how this response changes with geometric and material parameters is important when designing and optimizing new materials. This thesis summarizes the impact resistance of various material combinations using a twin wire drop tower. A database of the response of numerous samples subject to a step impact drop test was created. The maximum acceleration versus drop height, impact force versus displacement and time history of the impact impulse are presented for each sample tested. At a given impact height, the most optimal material response should have a stiffness allowing for maximum energy absorption which will decrease the forces due to the impact. The variation in material properties and geometry can be used to create a design criterion that can achieve a certain performance requirement. In this study, there were two types of impact resistant materials, urethane honeycomb and polymeric foam materials that were tested in various combinations including layer height, cell structure and material properties. The foam material which is classified mathematically as a hyperfoam material is categorized according to the material stiffness as P09, P15 and P25, based upon density parameters provided by the manufacturer. The honeycomb material is classified mathematically as hyperelastic and has a varying cellular structure where the cell wall shape and dimensions can be modified. The honeycomb material is classified according to material durometer (hardness) as H561, H781, H1036 and H1056 and according to cell geometric structure. Regular hexagonal shapes and irregular shapes were tested. One layer results of the foam material showed that a stiffer material is generally more optimal when the impact height increases. Stiffness in the materials tested is directly related to the density. Increasing the thickness and accordingly the deformation of the energy absorbing material allows the use of softer materials resulting in lesser impact forces and acceleration. Regarding the results of one layer of urethane honeycomb, increasing of the material thickness has the same net effect as in the foam. Overall stiffness of the honeycomb material is controlled by the material durometer and the solids ratio. In addition, the response is influenced by buckling of the cell walls which tend to limit forces imparted until the material is consolidated. Modification of the cell wall thickness or the cell size leads to changes in response that can be used to optimize the structure under impact. In addition, multilayered structures may be used to mitigate impact over a wider range of input energy than could a single layer material. Accordingly, the impact attenuation of several multi-layer samples was explored. Those samples consist of combinations from soft, moderate and stiff material to get reasonable values of the acceleration and displacement at every impact height

PERMEABILITY, STRUCTURAL CHANGES, FLOW REGIME TRANSITIONS IN FUEL CELL CATALYST LAYERS AND A MODEL FOR PREDICTING TRAPPED SATURATION FOR TWO-PHASE FLOW IN POROUS MEDIA

Proper water management in Proton Exchange Membrane (PEM) fuel cell is important to achieve high performance. Understanding the percolation of the produced water at the cathode catalyst layer (CL) is critical for any robust water management technique. In this study, an ex-situ experimental setup is used to study percolation in the CL at different injection rates and relative humidity (RH) conditions. The results show that increasing the flow rate force the liquid to flow through the bulk of the pores due to the dominant viscous effect. On the other hand, at low injection rates, the capillary becomes dominant and liquid flow along the roughness of pores surfaces. At low flow rates, a big wetted area is captured, compared to high flow rates tests, but the liquid saturation is lower. Another set of testing were done at a fixed flow rate and a varied RH conditions, where permeability is calculated based on the steady percolation pressure. The Permeability of the CL for both gas and liquid decreases as RH increases, and that more likely related to ionomer swelling. A correlation between the permeability and water content (λ) is derived. A sharp decrease in the permeability is observed at low water content (λ \u3c 3), beyond that there are no significant changes. Moreover, low and high RH conditions show significant effects on the structure of CL and flow regime. Static contact angle measurements at a range of RH also indicate possible morphological changes in the CL. In addition, fractional flow theory (FFT) model is adapted to study immiscible displacement of two-phase flow in a porous medium. The resulting model accurately predicts trapped saturation that occurs during imbibition and drainage of incompressible fluids for any capillary number. It also accurately predicts the fluid-fluid front displacement and critical capillary number at which trapped saturation begins to decrease. The unique aspect of this model is incorporation of a scaling factor, suggested by Médici and Allen (2016), that captures the propensity for gas or liquid holdup for any porous media and fluid pair

The Effect of Using Data Pre-Processing by Imputations in Handling Missing Values

The evolution of big data analytics through machine learning and artificial intelligence techniques has caused organizations in a wide range of sectors including health, manufacturing, e-commerce, governance, and social welfare to realize the value of massive volumes of data accumulating on web-based repositories daily. This has led to the adoption of data-driven decision models; for example, through sentiment analysis in marketing where produces leverage customer feedback and reviews to develop customer-oriented products. However, the data generated in real-world activities is subject to errors resulting from inaccurate measurements or fault input devices, which may result in the loss of some values. Missing attribute/variable values make data unsuitable for decision analytics due to noises and inconsistencies that create bias. The objective of this paper was to explore the problem of missing data and develop an advanced imputation model based on Machine Learning and implemented on K-Nearest Neighbor (KNN) algorithm in R programming language as an approach to handle missing values. The methodology used in this paper relied on the applying advanced machine learning algorithms with high-level accuracy in pattern detection and predictive analytics on the existing imputation techniques, which handle missing values by random replacement or deletion..  According to the results, advanced imputation technique based on machine learning models replaced missing values from a dataset with 89.5% accuracy. The experimental results showed that pre-processing by imputation delivers high-level performance efficiency in handling missing data values. These findings are consistent with the key idea of paper, which is to explore alternative imputation techniques for handling missing values to improve the accuracy and reliability of decision insights extracted from datasets

Design, Quantification, and Strengthening of Interface Interaction Between Aramid Fibers and Polymer Matrix

Composites reinforced by aramid fibers may suffer poor interface interactions during mechanical loading due to the smooth and chemically inert surface of the fibers. Three main strategies have been used to improve the interface shear strength (IFSS) including increasing surface roughness, enriching fiber surface with functional groups, and surface coating with ceramic particles. Plasma treatment is usually employed to functionalize and roughen surface of the fibers, leading to strengthening of the interface between the fibers and polymer matrix. A concern for fibers reinforced polymers is that the IFSS would significantly decrease once the composite is exposed to elevated temperatures. To maintain the mechanical performance of the composite at elevated temperatures, a polymer-derived-ceramics (PDC) precursor additive is used to stabilize the interface of the fabricated composite. Testing has shown that the IFSS of composites reinforced by untreated fibers is 29.32 ± 1.00 MPa and 26.10 ± 5.23 MPa at 50 °C and 100 °C, respectively, which surpasses 23.46 ± 2.32 MPa, the IFSS without PDC stabilization at room temperature. Regarding the functional groups, they have been created on the surface of the fibers through plasma or chemical treatments to improve the mechanical performance of aramid fiber reinforced composites. Unfortunately, these functional groups may reorient toward the fiber and be passivated by polar molecules in air over time and then become inactive and unable to bond to the matrix. The active period of the functional groups after a plasma treatment has been carefully evaluated. It is found that the fiber surface can be reactivated by drying with and without water molecule pretreatment. With drying at 100 °C for 10 hours, the IFSS can be restored from ~23 MPa to ~30 MPa. Regarding the last method to enhance the IFSS, chemical vaper deposition (CVD) and physical vaper deposition (PVD) have been used as an alternative methods to improve the IFSS by synthesizing ceramic nanoparticles MoO3 on the aramid fibers. A few advantages include no damage to aramid fibers which will help preserve mechanical properties of the aramid fibers and no concern on passivation as ceramic nanoparticles are stable compared to functional groups. Contrast to the bare improvement on IFSS from the MoO3 grown with PVD, the IFSS reaches 31 MPa from the nanoparticles fabricated with CVD, which suggests CVD would deliver a better interface interaction between MoO3 and fibers. These projects will benefit the engineering of aramid fibers into composites by largely taking advantage of their superior mechanical properties. Further research into interactions between heat treatment and strengthening additives would help us fundamentally understand the interface of advanced composite materials, as well as provide theoretical guidance for designing the interface between reinforcement and matrix

The influence of the application of Shariy'a on crime and public security.

SIGLEAvailable from British Library Document Supply Centre- DSC:D98373 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Saving the Fundaments: Impact of a Military Coup on the Sudan Health System

Military coups are not uncommon occurrences, particularly in developing nations where political systems might be less firmly entrenched or still evolving. Developments of this nature can often have profound implications for the affected nation’s healthcare systems, both in the immediate aftermath and over the longer term. This paper narrates some notable consequences of political instability on the national health system, particularly placing them in the context of the military coup in October 2021 – emphasizing the context behind the political turbulence, its acute and direct consequences, and the possible long-term legacies of political shocks on the already overwhelmed health system. As a descriptive piece, this narrative does not only look at the impact of the military coup on hospitals, but considers the implications for the healthcare system as defined by the WHO, with particular emphasis on the impact of the coup on health funding from multi-laterals, service delivery, human resource availability, and supply chains in Sudan

Acoustical Properties of (Chitin/nano-TiO2) Bio ‎Polymer Composite Gel

درست الخصائص فوق الصوتية للمتراكب النانوي الجلاتيني (Chitin (Ch)/TiO2). تراكيز بوليمر كيتين المذاب في (500 مل) من N-N dimethyl- acetamide and (5% LiCl)  عند درجة حرارة الغرفة لمدة (نصف ساعة) كانت (0.1،0.2،...،0.8 غم/مل)% على التوالي، بعد ذلك اضيف TiO2 بنسب (0.25 و 0.5) غم لكل تراكيز البوليمر. تمت جميع القياسات عند التردد (25 كيلو هيرتز). اظهرت النتائج ان جميع الخصائص فوق الصوتية تحسنت بعد إضافة TiO2، واستخدمت هذه الخاصية في الأجهزة فوق الصوتية.Ultrasonic (U/S) properties of (Chitin (Ch)/TiO2) nano-composite gel have been studied. The concentrations of Chitin polymer that dissolved in (500 mL) from N-N dimethyl- acetamide and (5% LiCl) at RT. for (0.5 hrs.) were (0.1, 0.2,…, 0.8 g/mL)% respectively, then TiO2 added as (0.25 and 0.5) g for each concentrates of polymer. All U/S measurements have been made at (f =25 kHz) frequency.   The results show that all U/S properties enhanced after the addition of TiO2, and this property used in U/S device

Network Coded Modulation for Two Way Relay Networks

Modern wireless networks use cooperative relaying to expand its coverage area. Wireless relaying allows mobile terminals toparticipate in transmission even when these terminals are not being the initial source or final destination. This can be achieved in differentways ranging from simple conventional relaying to more sophisticated coding at terminal and relay nodes to improve transmissionperformance. The proposed work here studied such system by simulating different coding techniques to be used in two way relay networkcoding (TWRC). Some of these techniques were previously proposed to improve transmission performance in conventional datacommunication systems. BER performance of different arrangements of coding and relaying schemes are evaluated using computersimulation tests. Ideal AWGN channel model and standard wireless channel models for wireless networks are considered in the simulationtests. The results have been shown that coding schemes such as Trellis Coded Modulation (TCM) and Repeat Accumulate (RA) codes aremore suitable in two way relaying networks. Over AWGN, TCM-8PSK improves BER performance by at least 2 dB at high SNR fordifferent relaying arrangements. For fading channels, this advantage is slightly reduced. Regenerative repeating at relay node seems to be apromising arrangement. It seems that there is no sense in using RA codes over fading channels due to its poor performance and relativelylower speed caused by repetitions

Hardness, Corrosion Behavior, and Microstructure of Al-Cu-Mg Alloy as a Fuction of 0.3 wt.% Ti Addition and Treatment of Alloy by Polymer Solution PAG

      تم دراسة تأثير إضافة التيتانيوم (Ti) كعنصر سبك إلى سبيكة Al-Cu-Mg على صلادة وتآكل هذه السبيكة. أظهرت نتائج اختبار الصلادة والاستقطاب للعينات المعالجة لفترات مختلفة عن طريق التعتيق عند 423.15 كلفن لمدة 5 ساعات زيادة كبيرة في نسبة تحسين صلابة برينل (HBW) بنسبة 113.95٪ (من 43HBW إلى 92HBW) وانخفاض شديد في معدل التآكل في السبيكة بعد إضافة Ti تنخفض كثافة التيار بنسبة 77.54٪ (من 58.14 µA cm-2 إلى 13.06 µA cm-2) مع التعتيق لمدة 5 ساعات مقارنة بالسبيكة الأساسية, ينعكس تأثير إضافة (Ti) والمعالجة الحرارية للمحلول (في PAG) أيضًا في تقوية وإعادة بلورة وتعديل البنية المجهرية للحبوب. تم إثبات هذه التغييرات بوضوح من خلال الاختبار المجهري وتثبت أن إضافة Ti لها تأثير تآزري كبير مما يتسبب في تثبيط إعادة التبلور وصقل حجم الحبيبات.      The effect of adding titanium (Ti) as an alloying element to Al-Cu-Mg alloy on the hardness and corrosion of this alloy was investigated. The hardness and polarization test results of samples treated for various periods by aging at 423.15 K for 5hr showed a significant increment in the Brinell hardness (HBW) improvement ratio of 113.95% (from 43HBW to 92HBW) and an extreme reduction the corrosion rate of the alloy after Ti adding decrease in the current density by 77.54% (from 58.14 µA cm-2 to 13.06 µA cm-2) with aging for 5 hr compared to the base alloy. The impact of  addition (Ti) and solution heat  treatment (in PAG)  is also reflected in the strengthening, recrystallization, and modification of the grain microstructure. These changes were clearly demonstrated by microscopic testing and proves that the addition of Ti has a considerable synergistic effect causing inhibition of recrystallization and refinement of grain size