Titanium Metal Matrix Composites - Essay Example

Fracture and Fatigue of Titanium Metal Matrix Composites Literature Review Titanium Metal Matrix Composites (Ti MMCs In general before venturing into the discussion of Titanium Metal Matrix Composites Lets just give basic introduction of a metal matrix composite. A metal matrix composite also known as MMC is understood as a type of composite material with the basic consideration of having twp parts. One part must be a metal and other part being any other materials like organic compounds. If we look at metal matrix composite composition structure, we can say that they are made by dispersing a reinforcing material into a metal matrix. Usually the surface is coated to prevent chemical reaction with matrix. The metals which are usually used are Aluminum, Magnesium, Titanium and Copper. In case of titanium metal matrix composite the principal metal used is titanium. Hence the metal matrix composite is Titanium metal matrix composite. In the MMC the metal is the monolithic material into which the reinforcement is embedded and it is completely continuous. The reinforcement can be continuous which can be monofilament or multifilament or it can be discontinuous which can be a particle, whisker or short fiber. They have lot of applications in commercial workspace. Metal-matrix composites are either in use or prototyping for the Space Shuttle, commercial airliners, electronic substrates, bicycles, automobiles, golf clubs, and a variety of other applications. It is widely perceived that Titanium Metal Composites have lot of potential in space propulsion applications. If we look at Titanium and its alloys we can say that they have good corrosion resistance, fatigue properties, and high strength-to-weight ratios. Products differ in terms of composition, grade, shape, dimensions, and features. Commercially pure, unalloyed or very low alloy titanium does not contain or contains only very small amounts of alloying elements. By contrast, titanium alloys contain significant amounts of added elements or constituents. Clad or bimetal titanium alloys consist of two different alloys that are bonded integrally together. Metal matrix composites have a composite or reinforced metal or alloy matrix filled with a second component, which may be in particulate, chopped fiber, continuous filament, or fabric form. Other unlisted, specialty or proprietary titanium and titanium alloys are also available. These materials are often based on a unique alloy system, use a novel processing technology, or have properties tailored for specific applications.    While selecting titanium and titanium alloys, we also need to check an analysis of dimensions, production processes, and performance features. Outer diameter, inner diameter, overall length, and overall thickness are important dimensions. Most materials are cast, wrought, extruded, forged, cold-finished, hot-rolled, or formed by compacting powdered metals or alloys. Performance features for titanium and titanium alloys include resistance to corrosion, heat, and wear. Ti MMCs offer provide potential advantages for structural applications, where they combine the high strength, high temperature capability, and oxidation resistance of titanium with an increase in stiffness provided by the ceramic reinforcement. Another thing is that they have the advantage of being isotropic in behavior, cheaper to manufacture and more amenable to subsequent processing and component forming operations. Of all the potential reinforcing phases for titanium which includes TiB, TiB2, SiC, Al2O3, and TiC, TiB offers the best balance of stiffness, stability, and similarity of thermal expansion coefficients. Other properties, such as the strength of metal matrix composites, depend in a much more complex manner on composite microstructure. The strength of a fiber-reinforced composite, for example, is determined by fracture processes, themselves governed by a combination of micro structural phenomena and features. These include plastic deformation of the matrix, the presence of brittle phases in the matrix, the strength of the interface, the distribution of flaws in the reinforcement, and the distribution of the reinforcement within the composite. Thus in this way the properties of Ti MMCs have been taken into consideration. Now let’s consider the idea of reinforcements. Basically reinforcements used in metal matrix composites fall under the following categories. They are continuous fibers, short fibers, whiskers, equiaxed particles, and interconnected networks. In continuous fibers when we consider the case of monofilaments, silicon carbide monofilament comes into picture .Silicon carbide monofilaments are made by a process called chemical vapor deposition in which tungsten or carbon core is used. If we look at the commercial product, a Japanese multifilament yarn which is designated as silicon carbide by the manufacturers is available. However this is made of pryolosis of organo- metallic precursor fiber and thus it is not a pure silicon carbide. This can be attributed by the fact that the properties differ significantly from those of monofilament silicon carbide. Now we will consider the concept of linear elastic concept mechanics. Linear elastic Fracture mechanics assumes that the material is isotropic and linear elastic in nature. On the basis of the assumption the stress field near the crack tip is calculated by using the concept of elasticity. When the stresses near the crack tip outgrow the material fracture toughness, the crack will star growing. In this concept of linear elastic fracture mechanics there are three modes of loadings on a cracked body. They are opening, sliding and tearing. One thing to be noted is that the linear elastic fracture mechanics is valid only when the inelastic deformation is small compared to the size of the crack. (Suresh, 1998) Now let’s move our discussion to fatigue. It is known that most of the failures of the engineering materials are caused by fatigue. Fatigue failure can be defined as the tendency of a material to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of intensity considerably below the normal strength. A good example of fatigue failure is breaking a thin steel rod or wire with your hands after bending it back and forth several times in the same place. Another example is an unbalanced pump impeller resulting in vibrations that can cause fatigue failure. One thing we need to know is that fundamental requirements during design and manufacturing for avoiding fatigue failure are different for different cases and should be considered during the design phase. Some of the characteristics common to fatigue in all materials are as mentioned as below. When the stress applied is greater the life of material shortens up. Another thing is damage is cumulative. Materials do not recover when rested. Fatigue failure depends on factors like temperature and surface. Some materials like steel exhibit fatigue limit which can be defined as a limit below which repeated stress does not induce failure, theoretically, for an infinite number of cycles of load. The major share of the fatigue life of the component may be taken up in the propagation of crack. By applying fracture mechanics principles it is possible to predict the number of cycles spent in growing a crack to some specified length or to final failure. (Suresh, 1998) Fig 1. Extended service life of a cracked component In another figure the crack length, a, is plotted versus the corresponding number of cycles, N, at which the crack was measured. Fig 2 Constant amplitude crack growth data The crack growth rate, da/dN, is obtained by taking the derivative of the above crack length, a, versus cycles, N, curve. Two of the most common approaches for obtaining the derivative are spline fitting method and polynomial method Values of log da/dN can then be plotted versus log K, for a given crack length, using the equation Fracture & fatigue with respect to Ti metal matrix composite. One thing to understand is that metal matrix composite is susceptible to fatigue like thermal fatigue. This can happen as a consequence of difference in properties and if we consider Ti matrix composite we can see that titanium alloys reinforced with particles or whiskers of TiB exhibit a decent near threshold fatigue crack growth characteristics than the unreinforced matrix alloy if fatigue failure occurs predominantly within ductile matrix. Deflection of the fatigue crack by the brittle particles, enhanced crack closure and crack tapping contribute to this improved fatigue resistance in the composite by lowering the effective crack tip opening displacement. We can consider the concept of spatially varied interfaces while considering fatigue crack growth. It is a design concept for composite materials where the interface mechanical properties are different for every fiber/matrix interface. These interfaces can be used to modify titanium metal matrix composite properties like transverse tensile strength and fatigue crack growth resistance. It can also look at interface failure mechanisms for understanding complex mechanical phenomena. So here single Ti-6Al-4V matrix composite containing strongly bonded SiC fibers were fabricated both in the as-received condition and with a weak longitudinal stripe along the sides of the fibers. The striped SVI composites exhibited an increase in the overall fatigue crack growth life of the specimens compared to the unmodified specimens. (Suresh, 1998) This improvement was caused by an increased extent of debonding and cracks bridging in spatially varied interface composites. Another thing to consider in discussion is local failures of fiber reinforced Ti matrix composite. We need to determine the spatial location which can be detected by using acoustic emission sensors .also a calculation method need to be developed to take into account the different acoustic wave velocities in the composite specimen and grips. Thus the location of the failure can be known by using fiber probing technique. (Suresh, 1998) Thus local failures can be detected to avoid problems. Fracture behavior of the matrix composite also needs to look into. Whenever we are looking at fracture behavior we need to take biaxial strength into account. It is measured by anticlastic bending test for the metal matrix composite. According to observation of the composite micro cracks were first generated and then they are propagated and connected to form macroscopic cracks on the surface. Thus in this way we have mentioned about fatigue crack growth with regard to titanium metal matrix composite. References: 1. Suresh S (1998) Fatigue of Materials. London: Cambridge University Press 2. Moura Branco (1999) Mechanical behavior of materials at High temperatures. Dallas: Springer 3. H.O.Fuchs (2001) Metal Fatigue in engineering. Washington :Wiley 4. Robert Piaschik (1998) Fatigue & Fracture Mechanics. London: ASTM International 5. Shankar Mall (2000) Titanium Matrix Composites. New York: CRC Press 6. Taylor D and Tilmans S (2004) Stress intensity variations in bone micro cracks during the repair process. J.Theor. Bio.l 229 169-177. 7. Crupi G and Taylor D (2004) Residual stresses and fatigue prediction using the theory of critical distances. Ibid 8. Bellett D and Taylor D. (2005) The effect of crack shape on the fatigue limit of three-dimensional stress concentrations. Int.J.Fatigue IN PRESS 9. T.W Clyne (2001) An introduction to Metal matrix composite. Manchester: Cambridge University Press 10. Steven Johnson (1997) Metal Matrix Composite: testing Analysis and failure modes. Chicago : ASTM International Read More

Another example is an unbalanced pump impeller resulting in vibrations that can cause fatigue failure. One thing we need to know is that fundamental requirements during design and manufacturing for avoiding fatigue failure are different for different cases and should be considered during the design phase. Some of the characteristics common to fatigue in all materials are as mentioned as below. When the stress applied is greater the life of material shortens up. Another thing is damage is cumulative.

Materials do not recover when rested. Fatigue failure depends on factors like temperature and surface. Some materials like steel exhibit fatigue limit which can be defined as a limit below which repeated stress does not induce failure, theoretically, for an infinite number of cycles of load. The major share of the fatigue life of the component may be taken up in the propagation of crack. By applying fracture mechanics principles it is possible to predict the number of cycles spent in growing a crack to some specified length or to final failure.

(Suresh, 1998) Fig 1. Extended service life of a cracked component In another figure the crack length, a, is plotted versus the corresponding number of cycles, N, at which the crack was measured. Fig 2 Constant amplitude crack growth data The crack growth rate, da/dN, is obtained by taking the derivative of the above crack length, a, versus cycles, N, curve. Two of the most common approaches for obtaining the derivative are spline fitting method and polynomial method Values of log da/dN can then be plotted versus log K, for a given crack length, using the equation Fracture & fatigue with respect to Ti metal matrix composite.

One thing to understand is that metal matrix composite is susceptible to fatigue like thermal fatigue. This can happen as a consequence of difference in properties and if we consider Ti matrix composite we can see that titanium alloys reinforced with particles or whiskers of TiB exhibit a decent near threshold fatigue crack growth characteristics than the unreinforced matrix alloy if fatigue failure occurs predominantly within ductile matrix. Deflection of the fatigue crack by the brittle particles, enhanced crack closure and crack tapping contribute to this improved fatigue resistance in the composite by lowering the effective crack tip opening displacement.

We can consider the concept of spatially varied interfaces while considering fatigue crack growth. It is a design concept for composite materials where the interface mechanical properties are different for every fiber/matrix interface. These interfaces can be used to modify titanium metal matrix composite properties like transverse tensile strength and fatigue crack growth resistance. It can also look at interface failure mechanisms for understanding complex mechanical phenomena. So here single Ti-6Al-4V matrix composite containing strongly bonded SiC fibers were fabricated both in the as-received condition and with a weak longitudinal stripe along the sides of the fibers.

The striped SVI composites exhibited an increase in the overall fatigue crack growth life of the specimens compared to the unmodified specimens. (Suresh, 1998) This improvement was caused by an increased extent of debonding and cracks bridging in spatially varied interface composites. Another thing to consider in discussion is local failures of fiber reinforced Ti matrix composite. We need to determine the spatial location which can be detected by using acoustic emission sensors .also a calculation method need to be developed to take into account the different acoustic wave velocities in the composite specimen and grips.

Thus the location of the failure can be known by using fiber probing technique. (Suresh, 1998) Thus local failures can be detected to avoid problems. Fracture behavior of the matrix composite also needs to look into. Whenever we are looking at fracture behavior we need to take biaxial strength into account.

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