Introduction to Composite Materials and Structures

Nachiketa Tiwari Indian Institute of Technology Kanpur Lecture 6 Aramid, Metallic and Other Fibers Lecture Overview

• Aramid Fibers

• Metallic Fibers

• Other Types of Fibers Aramid Fibers

• Aramid is short for “aromatic‐polymide”. Aramids are a class of polymers, where self repeating units contain large phenyl rings, linked together by amide groups.

• As per US based FTC, aramid fibers are manufactured fibers where“the fiber‐forming substance is a long‐chain synthetic polyamide in which at least 85% of the amide linkages, (‐CO‐NH‐) are attached directly to two aromatic rings” .

• Important properties of these fibers are: – High resistance to abrasion – High resistance to organic solvents – Tough as well as strong – Non‐conductive – No melting point (they start degrading at 500 C) – Low flammability – Sensitive to acids, and solvents Properties of Aramid Fibers

• Kevlar is a very well known and widely used aramid fiber. –Invented by DuPont – Widely used in ballistic applications –Comes in different flavors.

Important Properties of Kevlar Fibers Property Kevlar 29 Kevlar 49 Kevlar 129 Kevlar 149 Diameter (microns) 12 12 Specific gravity 1.45 1.45 1.5 1.45 Tensile modulus (GPa) 62 124 96.0 186 Tensile strength (MPa) 2760 3620 3380.0 3440 Elongation (%) 3.4 2.8 3.3 2.5 Axial CTE (per million per C) ‐2 ‐2 ‐2 ‐2 Radial CTE (per million per C) ‐60 ‐60 Boron Fibers

• Boron fibers are relatively more popular in composites, vis‐à‐vis other fibers (aluminum, steel, etc.).

• These fibers are made using a chemical vapor deposition (CVD) process. o Here, boron tri‐chloride is chemically reduced in a hydrogen environment on a tungsten or carbon filament substrate. o The tungsten or carbon filament is resistively heated at temperatures in excess of 1500 C. Due to application of temperature, boron‐tri‐chloride interacts with hydrogen, and reduces to pure boron. o This boron gets deposited on the tungsten or carbon filament. As the filament is continuously pulled out of reduction chamber, a well controlled boron layer deposits on the substrate wire. These wires have a boron “outside” and a tungsten or carbon core.

Important Properties of Boron‐Tungsten Fibers PtProperty Dia = 100 microns Dia = 140 microns Dia = 200 microns Specific gravity 2.61 2.47 2.4 Tensile modulus (GPa) 400 400 400.0 Tensile strength (MPa) 3450 3450 3450.0 CTE (per million per C) 4.9 4.9 4.9 Ceramic Fibers

• Ceramic fibers are used in high temperature applications. These fibers have high strength, high elastic modulus, as well as the ability to withstand high temperatures without getting chemically degraded.

• Commonly used fibers for such applications are made from alumina, and SiC.

• Alumina fibers are made spinning a slurry of alumina and firing of the slurry. These fibers retain their strength up to 1370 C.

• Silicon carbide fibers are produced either by a chemical vapor deposition ()(CVD) process, or through pyrolysis.

• SiC fiber retain their tensile strength up to 650 C.

• Alumina and SiC fibers work well in metal matrices, unlike carbon and boron fibers, since the llttatter react with mettlal matitrices. FthFurther, due to the ir resiitstance to hig h temperatures, these fibers are also used in turbine blades. HPPE Fibers

• HPPE stands for High Performance Polyethylene .

• HPPE fibers are have a density slightly less than that of water. Thus, even though their modulus and strength are slightly less than Kevlar fibers, on a specific strength, and specific modldulus are 30‐40% more than that for Kevlar fibers.

• HPPE fibers have very high energy absorption characteristics. Thus they are widely used in ballistic armor applications.

• HPPFE fiber’s modulus and strength increases significantly with increasing strain rates. Thus HPPFE composites work very well when subjected to high‐velocity impacts.

• HMPE (hig h modldulus polhl)lyethylene) and ECPE (dd(extended chihain polyethylene) are other materials with chemical structure similar to HPPE material. Their fibers are also used in composites. Properties of Ceramic and HPPE Fibers

Important Properties of Ceramic and HPPE Fibers SiC Property Alumina SIC (CVD) (Pyrolysis) HPPE

Diameter (microns) 15‐25 140 10‐20 38

Specific gravity 3953.95 333.3 262.6 0970.97

Tensile modulus (GPa) 379 430 180 62‐120

Tensile strength (MPa) 1380 3500 2000 2180‐3600

Elongation (%) 2.8‐4.4 RfReferences

1. Analysis and Performance of Fiber Composites, Agarwal, B.D.and BtBroutman, L. J., JhJohn Wiley & Sons.

2. MhiMechanics of CiComposite MilMaterials, Jones, R. M., Mc‐Graw Hill.

3. Engineering Mechanics of Composite Materials, Daniel, I. M. and Ishai, O., Oxford University Press.