Use Of Composite Associated With Shear Thickening Fluid Biology Essay

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Application of polymer based retrofitting methods for the unreinforced masonry (URM) buildings to sustain earthquakes has recently gained attention due of their advantages such as low cost, good sustainability, environmental friendly, ease in application and efficient performance. However, the loss of shear strength and stiffness of polymer reinforcement due to non-equally distributed stresses still needs to be addressed and resolved. In this paper, the application of shear-thickening fluid (STF) is proposed for polymer based seismic retrofitting of the URM buildings. The main objective of the research discussed in this proposal is to eliminate high stressed areas in the polymer reinforcement by enhancing its strength, stiffness and energy absorption/dissipation properties. The shear stresses created by the seismic loading will be transferred from the URM buildings to the composite and then to STF leading to the increase in viscosity of STF. Eventually, the dissipation of energy in the composite will be compensated by the increase in elastic modulus of STF. Two methods are chosen to prepare polymer-STF composite: (1) STF impregnated microporous geotextile being sandwiched between two geomenbranes, and (2) STF coated or filled fibers to be incorporated into the polymer matrix. Rheological properties, surface morphology, dispersion, thermal stability, durability, and mechanical properties of STFs and composites will be characterized using rheometer, SEM, TEM, TGA, accelerated weathering tests and DMA respectively. Shear wall testing will be used to test the final polymer-STF composite in order to qualify for retrofitting of the URM buildings.

Introduction

Unreinforced masonry (URM) buildings

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Non-engineered and unreinforced masonry buildings are prevalent in developing countries due to low construction and material costs. In fact, 40% to 50% of the world's population lives in these types of buildings. URM buildings generally experience a sudden collapse during earthquakes resulting in large population and economic losses1. The major cause for this sudden collapse was the use of rigid and brittle construction materials. Common failure modes of these buildings include shear walls cracks, out-of-plane failure, diagonal cracks, separation of roofing from walls, and separation of wall corners. These failures occur mainly because of higher magnitudes of lateral seismic forces over the ultimate tensile strength of the mortar joints2.

The past two decades have seen rapid development in construction materials with emphasis on retrofitting methods. Some of the retrofitting methods include grout-epoxy injections, shotcrete overlay coatings, natural fiber reinforcement, steel mesh cage, post-tensioning, and polymer based retrofitting. All these retrofitting methods have shown significant improvement in lateral resistance and ductility of the URM structures3.

Among these methods, polymer based retrofitting methods that include polymer reinforcement either externally or internally have many advantages over other methods in terms of performance, sustainability, economic feasibility and applicability. Additionally, these methods have shown to provide better shear resistance to the structural elements compared to non-polymer based methods. The most commonly used polymer based retrofitting methods are fiber reinforced polymers (FRP), polypropylene strip and polymer mesh4-7.

Problem statement

Polymer based retrofitting methods have limitations on their ability to withstand the non-uniform distribution of stresses generated by the seismic loadings3, 5. This non-uniform stress distribution creates highly concentrated stresses in both the structure and polymer material, which leads to a shear failure. The common shear failures for polymer based retrofitting are diagonal tensile rupture (Figure 1a) and peeling-off or debonding (Figure 1b)8, 9.

Figure 1. (a) Shear failure due to tensile rupture, (b) shear failure to debonding8, 9

The shear failure of polymer reinforcement due to non-equally distributed stresses is related to the shear strength of polymer reinforcement. The total shear strength of reinforced structure (Vt) can be expressed in terms of individual shear strengths of unreinforced structure (Vm) and polymer reinforcement (Vp)10.

Vt = Vm + Vp

In general, truss model is widely used to describe the shear behavior of various structures. According to this model, Vp is defined as

Vp = η εcr Ep t r z (Cosθ/Sinα)

Where η represents the non-uniform distribution of stains/stresses, εcr represents maximum allowable strain, Ep represents elastic modulus of reinforcement, t represents thickness of reinforcement and r represents arrangement, z represents the height of structural element to be reinforced. And α and θ are the inclination of crack and the angle between the strain and reinforcement arrangement respectively10.

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This model describes polymer reinforcement as linearly elastic, which indicates that different strains cause different stresses. When stresses localize and redistribution of the loads through the polymer reinforcement does not occur, the localized stress/strain will exceed the critical strain or maximum allowable strain (εcr) in the reinforcement leading to a sudden shear failure that is similar to brittle failure. Since this brittle and shear failure mechanism arises due to predominantly linear elastic nature of polymer reinforcement, the energy dissipation property that normally occurs from non-linear range is minimized in the polymer reinforcement10, 11.

Shear Thickening Fluid (STF)

Shear-thickening behavior is found in certain fluids which undergo transition from fluid to solid when stressed. The viscosity increase in these fluids due to shear-thickening behavior could be sometimes enormous. STFs including colloid dispersions possess unique properties such as increased energy dissipation and enhanced elastic modulus that make them one of the important materials for damping and shock-absorption applications12. Recently, these properties have been explored for various shock-absorbing applications such as sporting equipment, automotive and body armor where these materials reduce the vibrations and shocks without losing their strength and stiffness 13-15. As of today, the application of STFs in earthquake resistant building materials has not been tested or implemented.

As shown in Figure 2 (a), the hysteresis loops of force versus displacement for different frequencies in a STF filled damper can be used to estimate the energy dissipation corresponding to the area for each cycle. The behavior of effective stiffness and damping constant with respect to loading frequency of the STF damper is shown in Figure 2 (b). The damping constant approaches maximum value during the transition state and drops to relatively lower value in shear-thickened state16. At this state, the effective stiffness reaches the maximum value. It can be inferred that the improvement in the stiffness and energy dissipation properties can be achieved with shear thickening effect in the system.

Figure 2. (a) Damping force versus displacement at various frequencies, (b) the equivalent stiffness and damping coefficient versus frequency16.

Shear thickening in colloidal dispersions depends on various factors such as particle size, volume fraction, surface chemistry, interparticle and hydrodynamic forces, polydispersity, and the properties of suspending medium17. Several researchers have studied the shear thickening behavior in colloidal dispersions under steady and dynamic shear18-20. As shear thickening transition occurs at critical shear rate under steady shear and at critical frequency under dynamic shear respectively, it is imperative to control these parameters to achieve the required properties.

Proposed technique

The proposed technique will introduce polymer-STF composite as a novel retrofitting material in polymer reinforcement of the URM buildings. The incorporation of STF into polymer matrix is proposed to enhance the strength, stiffness and energy dissipation properties of the reinforcement during high shear stresses or deformations. When shear stresses increase during seismic loading, the increase in viscosity of STF will help the composite reinforcement to redistribute the loads and minimize the localization of stresses. In response to the ground motion during earthquakes, it is expected that the viscoelastic behavior of STF in the composite would change accordingly to achieve the shear thickening state depending on the nature of shear rate and thereby dissipate the energy being created by the shear stresses.

When reinforced structural elements experience the deformations due to lateral seismic forces, the stresses that are developed inside the walls will be transferred to the composite as a result of the bond between them. At higher stress levels, these stresses will be relocated to the STF at the interface between matrix and STF causing STF's viscosity increase. Hence, an advent degree of non-linear damping effect in the composite will lead to energy dissipation before the occurrence of shear failure in the polymer reinforcement.

The polymer-STF composite can be used in the form of a sheet, a mesh or bands. Examples of its possible application are shown in Figure 3, where the structural elements of the URM buildings such as walls, columns, roof edges and corners can be either externally or internally reinforced using the composite associated with STF.

Figure 3. Masonry walls and wall corners reinforced with the polymer-STF composite

Objectives and Hypotheses

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The goal of this research is to develop a polymer-STF composite as a novel retrofitting material that will help a structure to withstand seismic forces and prevent from sudden shear failures. The major objectives of this research proposal are:

To synthesize a polymer-STF composite.

To characterize the synthesized composite for various properties including (i) rheological properties of STF such as viscosity, stress-strain relationship, critical shear rate and critical frequency under steady and dynamic shear (ii) mechanical properties such as dynamic moduli, force-displacement curves for energy dissipation, shear strength and tensile strength (iii) morphological properties such as surface texture of composite, dispersion of particles in STF, and fibers in the polymer matrix, (iv) shear wall testing of the final composite.

To investigate performance of this technology in two different methods of polymer reinforcement: (i) without fiber for relatively low cost applications, (ii) with fiber technology for cost-effective applications.

The major hypotheses of this proposal are:

Use of STFs in the polymer reinforcement can produce better performance when STF shows shear thickening behavior in the frequency range of seismic shear rates.

Higher shear and tensile strengths are obtained with the combination of polymer matrix and STF when STF reaches its maximum stiffness.

Shear failure such as tensile rupture and debonding cracks will be minimized in the polymer reinforcement with the effect of increased energy dissipation via shear thickening behavior in STF.

Experimental

Synthesis of STF polymer reinforcement

This section describes the selection of materials for the synthesis of various STFs, synthesis of polymer-STF composite without fiber, and synthesis of polymer-STF composite with STF coated and filled fibers.

Selection of materials

Materials choice will depend upon various factors such as cost, toxicity, compatibility, physical, rheological and chemical properties. As seen in the previous literature reports for energy dissipation applications, either silica or titania dispersions have been commonly used. Moreover, dispersions of silica and titania have been investigated for various other applications in great detail. It is because the variety of their preparation schemes using sol-gel processing is enormous. They can be obtained relatively for lower costs ($0.105/1gm). Though the present project will begin with the use of silica and titania particles for the preparation of STFs, several other types of particles such as alumina, zirconia, and oxides of tin and zinc may also be considered for further research. Various sizes of the particles and different grades will be used to obtain a range of data for final rheological properties.

Rheology of dispersions of hydrophilic silica in different polar organic media has been investigated by S.R.Raghavan21. It was found that the colloids behaved as sols when there were strong hydrogen bonding interactions between hydroxyl groups on silica particle surface and the suspending medium. The viscosity of colloids at 25°C was found to increase with the increase in the molecular weight of polyethylene glycol (PEG) and polypropylene glycol (PPG). In our study, various molecular weights of PEG (250 and 500g/mol) and PPG (400-3000g/mol) will be investigated.

Figure 5: Schematic representation of processes used to prepare composites with STFs

After the preparation of STF, the synthesis of polymer-STF composite can be performed in two ways as shown in Figure 5: (1) without fibers: STF impregnated geotextile sandwiched between two geomembranes, and (2) with fibers: STF coated or filled fibers incorporated into a polymer matrix.

When choosing materials for the preparation of polymer-STF composite without fiber, it is highly essential to consider factors such as compatibility of geotextile with STF, cost and durability. STF being polar liquid should have good adhesion with the walls of microporous geotextile. Thus, polyester, polyurethane and polyamide can be considered as geotextiles for impregnation with STF. As geomembranes are used to prevent leakage of STF from geotextile, polyethylene and polypropylene will be used to encapsulate the geotextile.

For fiber reinforced polymer composite, short discontinuous fibers can be classified according to four characteristics: (1) fiber material such as natural and man-made, (2) physical/chemical properties such as density, surface roughness, non-reactivity and chemical stability, (3) mechanical properties such as tensile strength, elastic modulus, stiffness and surface adhesion properties, and (4) geometric properties such as length, diameter, and cross-sectional shape. Therefore even with a single fiber material such as glass, there can be many possible combinations to tailor the final properties of composite. In order to make this fiber technology cost-effective, it is likely that the material choice can be very tricky22.

The preparation of STFs using various materials and their characterization using various techniques will be discussed in the following sections.

Synthesis of polymer-STF composite without fiber

In the sonochemical process, different weight percentages of silica (40% and 50%) and polyethylene glycol (PEG) (60% and 50%) will be mixed with ethyl alcohol and sonicated under ultrahigh sonication with Misonix sonicator for 5h at 10°C. The resultant mixture will be used as the STF to impregnate the fabric. 300mm X 300mm layers of Kevlar and Nylon fabrics will be used for impregnation with STF. Each sheet will be soaked in STF solution for 1min and then squeezed with a rod to remove excess amount of STF. The composite will be hanged and dried at room temperature for 48h as shown in Figure 6.

Figure 6. Procedure for making composite having colloidal dispersions as STF23

Experimental Design for STF of colloidal dispersions is shown in Table 1. After preliminary tests, different grades and particle sizes of SiO2 and TiO2 will be used. If required, surface modification of particles will also be tried. Different polymers such as PEG and PPG with various molecular weights will be used as suspending media for dispersions.

Table 1: Experimental Design for STF of colloidal dispersions

STF

Particle Type

Liquid polymer

Particle Size

Particle Volume Fraction

Colloidal dispersions

SiO2,

TiO2(Anatase and Rutile)

Polyethylene glycol (PEG),

Polypropylene glycol (PPG)

20-100nm

15% to 60%

The composite of STF without fiber will be prepared by impregnating the porous geotextile such as polyester with above synthesized STF. In the work by Deshmukh et al., smart composites materials have been developed using cellular solids, a porous interpenetrating network or foam impregnated with field-responsive fluids such as Magneto-Rheological (MR) fluids or STFs24. The main advantage of using porous material is to prevent the sedimentation of STF and increase the rate of process of shear thickening. The cellular material with its absorptive properties is able to hold the STF intact on its porous walls. Figure 7 shows the porous foam material before and after the impregnation with MR fluid.

Figure 7. Elastomeric foam before and after impregnation with MR fluid24

After the impregnation, the porous geotextile will be encapsulated on both sides with two geomembranes such as polyethylene or polypropylene in order to prevent the leakage of STF out of the porous geotextile. Geomembranes will serve as the insulating layer or separation layer to protect geotextile from environmental effects such as moisture absorption and heat. Figure 8 shows the structure of polymer-STF composite containing geotexile and geomembrane layers.

Figure 8. Schematic of polymer-STF composite containing geotextile and geomembrane layers

Synthesis of polymer-STF composite with STF coated or filled fibers

Several researchers have attempted to improve the stiffness and damping properties of fibers in FRP composites by coating the fibers with various organic polymers25. These polymers such as styrene-maleic anhydride copolymers and methyl acrylate-acrylonitrile copolymers and polyamides containing functional groups can interact with both fibers and matrix. Surface treatment enhances the damping effect at the interface by creating the interfacial bonding between fibers and matrix.

STF associated fibers may enhance the performance to cost ratio of fiber composite by reducing the amount of required fibers with the incorporation of STF. Fischer et al. demonstrated that the incorporation of STF into dynamically loading structure improved the stiffness and damping properties of the structure. Sandwich structures containing STF sandwiched between polymer matrixes, for example polyvinyl chloride (PVC) beams, have also been investigated in order to improve the stiffness and damping properties of composites under dynamic stresses26.

Figure 9. (1) STF coated fibers incorporated into a reinforced composite; (2) STF filled fibers incorporated into a reinforced composite

The incorporation of hollow fibers filled with two pot epoxy resin into FRP composite was investigated as self-healing materials when the fibers were fractured during high stressed deformations. Recently, several self-repairing glass fiber reinforced composites have been developed and used to restore the mechanical properties of composite such as flexural, compression and tensile strength.

In the present proposal, the resultant STFs from colloidal dispersions in section 4.1.2 can be used to coat the solid fibers or fill the hollow fibers, and subsequently be incorporated into a polymer matrix as shown in Figure 9. Three types of fibers glass, carbon and Kevlar will be chosen by comparing their mechanical properties and durability. In general, epoxy resin is used as the composite matrix. Some of the established methods can be followed to prepare FRP composite involving STF27, 28.

Characterization

This section includes the characterization of STF for rheological, mechanical, and morphological properties, and thermal stability. It also includes the shear wall testing for the final polymer-STF composite.

Rheology

The STF samples will be tested for their rheological properties such as stress-strain behavior, energy dissipation and yield stress using the TA Instruments' ARES Rheometer available in CNSE, NDSU. Testing will be carried out at room temperature with various shear rate ramps (0-125/s) in steady-state strain sweeps, dynamic frequency sweeps and dynamic strain sweeps. The steady and dynamic viscosities of STFs will be determined at different shear rates and strain amplitudes respectively.

From the history of previous earthquake records, it was found that the range of frequency of most of the earthquakes that cause severe damage was around 1 - 2Hz and the average amplitude of acceleration was around 0.5g. Therefore, in order to match the frequency of the earthquake with the critical shear rate for shear thickening in STF, the viscosity increase in STF should be adjusted to occur at the frequency below 1 Hz. In the seismic active control systems where Electro-rheological (ER) fluids are used, the behavior of ER fluid changes from liquid to a yielding solid within milliseconds in response to seismic loading. Hence, it is required to determine the time period for shear thickening response of various STFs so as to ensure that the material undergoes the required viscosity change with respect to seismic vibrations.

Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) will be conducted using the Q500 Thermogravimetric Analyzer to determine the weight percentages of colloidal particles and suspending liquid in the as-prepared STF samples. Thermal stability of various STFs can also be determined using TGA.

Transmission electron microscopy (TEM)

Transmission electron microscopy will be performed using the JEOL JEM-100CX II transmission electron microscope to study the dispersion of colloidal particles in STF, morphology of coated and filled fibers, and interpenetrating networks of associating polymers.

Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) will be used to investigate the degree of impregnation of fabric with STF. The changes at the micro-level in the surface texture of fabric with impregnation would also help to determine the adhesion and uniformity of adsorption of STF onto the fabric surface.

Dynamic mechanical analysis (DMA)

Dynamic mechanical analysis (DMA) will be used to determine the dynamic properties of the composite such as dynamic moduli and damping energy. The results will provide an understanding of the effect of interfacial interactions among various ingredients in the composite (interactions between STF and polymer matrix, and STF and fibers).

Accelerated weathering tests

Accelerated weathering tests will be carried out to determine the stability of polymer-STF composites in terms of tensile strength and shear strength. They will be exposed to a high alkaline solution of pH close to 13.5 at a high temperature (60°C) for periods of 2-3 months, which corresponds to 50 years of exposure in a Northern climate. Tests will also be performed to evaluate the durability of polymer-STF composites under loading and immersion at 60°C.

Shear wall testing

For shear wall testing, the procedure used by Meguro et al. for retrofitting based on polypropylene (PP bands) will be followed29. The procedure describes the use of PP bands arranged in a mesh fashion and inserted in a cement mortar wall. Initially, the material properties including compression strength, young's modulus, and tensile strength of brick, mortar and cement will be measured.

In order to find out the efficiency of retrofitting method, it is one of the essential elements to determine the deformation resistance of masonry walls to in-plane and out-of-plane loading for both retrofitted and non-retrofitted samples. Our technique is supposed to provide the strength and stiffness to the walls in order to prevent it from cracking, separation from roof and separation of walls at the corners.

Figure 10. (a) Reinforced wall model before mortar overlay, (b) Experimental set-up for vertical and horizontal pre-compression load (c) In-plane and out-of-plane displacements29

Tests will be carried out according to ASTM C1314-07. Concrete masonry walls with and without reinforcement will be constructed using the blocks shipped from National Concrete Masonry Association. The wall dimensions will be estimated to be 985mm-1072mm-100mm consisting of 15 brick rows with 6 bricks each row. The force-displacement curves will be obtained by applying different forces (10kN to the failure) and monitoring the displacements.

Timeline, Budget and Possible Funding Agencies

The project is expected to continue for approximately 2 years. The proposed budget is shown below. Salaries are estimated for the research team that will consist of one Principal Investigator (PI), one full-time Graduate Student, and one part-time Undergraduate Student. Tuition fees waiver will be considered for the graduate students working on this project. Chemicals, materials and equipment for characterization and testing purpose have also been included in the budget. Instrumental charges are given for using instruments on and off the campus. Travel funding includes airfare, hotel and registration for two conferences each year for PI and the Graduate Student to present the research work. Funding for this project will be pursued from the National Science Foundation-Engineering Research Centre (NSF-ERC), the United States of Geological Survey (USGS), and the National Institute of Standards and Technology (NIST). Timeline for this project is shown in below Gantt Figure

Timeline

Figure 11. Gantt Chart of Timeline for this project

Table 3: Budget of the current proposal

S.NO

Category

1stYear

2ndYear

1.

Salary

Principal investigator (PI)

Graduate Student (1)

Undergraduate assistant (1)

$ 54,000

$ 22,000

$ 4,800

$ 54,000

$ 22,000

$ 4,800

2

Tuition fees for Graduate Students

$ 8,000

$ 8,000

3

Chemicals supplies and instruments charges

Chemicals and Materials

$6000

$6000

Instrumental charges (on and off campus)

$8000

$8000

Equipment for shear wall testing

$2000

$2000

4

Travel

$ 2,500

$ 2,500

Annual Total

$107,300

$ 107,300

Total for 2 years

$214,600