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SHORT FIBER REINFORCEMEN

TS

Sayan Baidya12407EN014

Department of Metallurgical EngineeringIIT BHU,Varanasi

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INTRODUCTION Unidirectional reinforcements tend to be very stiff and strong

in fiber direction, but very weak in the transverse direction. Their weakness in transverse direction is attributable to presence of significant stress concentration at the interface of matrix and fiber.

Given these attributes, unidirectional composites are very useful in applications where state of stress is well known.

However, if externally applied loads are omni‐directional, or if their direction can vary in time, then such laminates fabricated by stacking up unidirectional laminae may not necessarily meet our design needs.

We may still be able design a laminate for such cases which is equally strong in all directions, the top and bottom layers will be weak in transverse directions, and failure could get initiated from here.

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INTRODUCTION The lamina should have in‐plane isotropy, ie.

uniform properties in all direction.

One way to produce such lamina is by using short fibers which are randomly oriented. Such composites, in general are significantly less expensive than unidirectional composites

Such composites are used extensively in general purpose applications, such as car body panels, boats, household goods, etc.

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FIBER REINFORCEMENTS CAN BE CLASSIFIED INTO: Continuous Fiber: They have very high aspect

ratio(length/diameter). Any further increase in length does not increase the elastic modulus of the fiber. Eg: Mono/Multi-filament fiber

These fibers can be used as filler material, to increase the bulk and reduce the cost.

Discontinuous Fiber: They have aspect ratio of 0.2 – 1. Any change in its dimension will alter it structural properties. This includes Whiskers and particulates and short fiber reinforcements.

It is mostly used in Metal Matrix Composite.

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Material Diameter(μm)

Length (μm)

Fabrication Route

Examples

Mono-filament

100-150

∞ Chemical vapour deposition

Nextel 610

Multi-filament

7-30 ∞ Precursor Stretching/Melt spinning

Carbon Glass, Kevlar

Short Fibers 1-10 50-5000

Spinning of slurries

Saffil (δ-Al2O3+ 4% SiO2)

Whiskers 0.1-1 5-100 Vapor Phase Growth

SiC, Al2O3

Particulate 5-20 5-20 By product ofSteel making

B4C, TiB2

CLASSIFICATION OF REINFORCEMENTS

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TYPES OF FIBER REINFORCEMENTS

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MICROSTRUCTURE OF SHORT FIBER REINFORCED COMPOSITE ( ALUMINA)

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Mechanical properties of short fiber reinforced composites depend critically on the fiber length distribution (FLD) and the fiber orientation distribution (FOD). 

The strength of short fiber reinforced composites increases with the increase of the mean fiber length and with the decrease of the mean fiber orientation angle (angle between the fiber axis and the loading direction).

The composite elastic modulus increases with the decrease of the mean fiber orientation angle and with the increase of the fiber orientation coefficient.

Elastic modulus increases with the increase of mean fiber length when the mean fiber length is small. When the mean fiber length is large, it has nearly no influence on the elastic modulus of short fiber reinforced composites.

MECHANICAL PROPERTIES OF SHORT FIBER REINFORCEMENT

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LOAD TRANSFER MECHANISM IN SHORT‐FIBER COMPOSITES A part of this load gets transferred to fibers at their

ends, while remaining portion of this load gets transferred to fibers through their external cylindrical surfaces.

For unidirectional composites with continuous fibers, transfer of load at fiber ends will be very small.

This is because fibers are very long, and hence their cylindrical surfaces, across which load gets transferred through shear‐mechanism, are sufficiently long.

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LOAD TRANSFER MECHANISM IN SHORT‐FIBER COMPOSITES In continuous fibers, the effect of load transfer

through fiber ends may not significantly affect overall mechanics of load transfer

However, in short‐fiber composites the same may not be necessarily true. In such composites, the length of the fiber is not sufficiently long to transfer load across cylindrical surfaces of fibers.

Thus, in such fibers, both the ends, as well as external cylindrical surfaces of fibers play a significant role in matrix‐to‐fiber load transfer.

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Flow diagram of alumina-silica fiber production

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PRODUCTION TECHNIQUE A type of alumina + silica (tradename SAFFIL) is used

to produce short fibers.

It has 96% δ-AL2O3 and 4% SiO2 and a very fine diameter (3 μm).

It is made by sol-gel process. The aqueous phase consists of an oxide sol and an organic polymer.

The sol is extruded into a filament into a coagulating bath in which the extruded shape gels.

The gelled fiber is then dried and calcined to produce the final oxide fiber.

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PRODUCTION TECHNIQUE Aluminium oxychloride is mixed with 2% polyvinyl

alcohol. Which is evaporated in a rotatory evaporator till its viscosity is 800 Poise.

This solution is then extruded through a spinneret, and fibers wound on a drum at 800°C.

The organic material is burned away. Fine grained alumina fiber with 5-10 % porosity and 3-5 μm diameter is obtained.

At this stage it can be used for filtering purposes.

Finally heating to 1400°C which causes 3 4% linear shrinkage . This leaves us with refractory alumina fiber suitable for reinforcement.

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PHYSICAL PROPERTIES OF SAFFIL ALUMINA FIBERS Density 3500 Kg/m3

Melting Point  >2000 °C

Specific Heat  1 kJ/kg deg C

Thermal Conductivity 20.5 W/m.K

Tensile Strength  1545 MPa

Young's Modulus  315 GPa

Shear Modulus 125 GPa

Median Diameter  3.0-3.5 microns

Alumina Content  95-97 %

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PHYSICAL PROPERTIES OF SAFFIL ALUMINA FIBERS Stability to 1750°C Resistance to chemical attack High resilience Low linear shrinkage Lightweight Low thermal conductivity Immunity to thermal shock Ease of fabrication High strength & modulus of elasticity Uniform fiber diameter

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SPECIAL APPLICATIONS Catalyst Support

Saffil Fiber Catalytic Grade mat with fiber surface areas up to 150m2/g are excellent for catalyst support systems or flameless heaters due to the very efficient utilization of expensive precious metal catalysts.

Filtration

Filters made from Saffil Fibers are highly efficient due to the fiber's uniformity and absence of defects'. The filters remain effective in aggressive conditions and so are excellent candidates for applications involving either hot gases or corrosive liquids.

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SPECIAL APPLICATIONS Electrical Appliances

Due to its high purity and chemical inertness products such as Saffil papers, felts and mat make excellent separator materials in battery and electrical equipment.

Protective Clothing

Saffil Fiber mat and felt forms make excellent inner liners for protective clothing. The liners are lightweight, resilient and offer smooth handling characteristics. This allows easy manufacture of composite textiles using thermal ´resistant facing fabrics for use in high temperature and chemical splash protection.

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REFERENCES http://www.saffil.com

www.springer.com

COMPOSITE MATERIALS by K.K.Chawla

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