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From class Notes
18 Mechanical Properties Which Every Mechanical Engineer Should Know
Selection of material is an important aspect for manufacturing industries . The quality of product is highly depends upon its material properties. These properties are used to distinguish materials from each other. For Example: A harder material is used to make tools.A ductile material is used to draw wires. So the knowledge of mechanical properties of material is desirable for any mechanical student or for any person belongs to mechanical industries. This post brings top 18 mechanical properties. Mechanical properties of material: There are mainly two types of materials. First one is metal and other one is non metals. Metals are classified into two types : Ferrous metals and Non-ferrous metals. Ferrous metals mainly consist iron with comparatively small addition of other materials. It includes iron and its alloy such as cast iron, steel, HSS etc. Ferrous metals are widely used in mechanical industries for its various advantages. Nonferrous metals contain little or no iron. It includes aluminum, magnesium, copper, zinc etc. Most Mechanical properties are associated with metals. These are #1. Strength: The ability of material to withstand load without failure is known as strength. If a material can bear more load, it means it has more strength. Strength of any material mainly depends on type of loading and deformation before fracture. According to loading types, strength can be classified into three types. a. Tensile strength: b. Compressive strength: 3. Shear strength: According to the deformation before fracture, strength can be classified into three types. a. Elastic strength: b. Yield strength: c. Ultimate strength: #2. Homogeneity: If a material has same properties throughout its geometry, known as homogeneous material and the property is known as homogeneity. It is an ideal situation but practically no material is homogeneous. #3. Isotropy: A material which has same elastic properties along its all loading direction known as isotropic material. #4. Anisotropy: A material which exhibits different elastic properties in different loading direction known as an-isotropic material. #5. Elasticity: If a material regain its original dimension after removal of load, it is known as elastic material and the property by virtue of which it regains its original shape is known as elasticity. Every material possess some elasticity. It is measure as the ratio of stress to strain under elastic limit. #6. Plasticity: The ability of material to undergo some degree of permanent deformation without failure after removal of load is known as plasticity. This property is used for shaping material by metal working. It is mainly depends on temperature and elastic strength of material. #7. Ductility: Ductility is a property by virtue of which metal can be drawn into wires. It can also define as a property which permits permanent deformation before fracture under tensile loading. The amount of permanent deformation (measure in percentage elongation) decides either the material is ductile or not. Percentage elongation = (Final Gauge Length – Original Gauge Length )*100/ Original Gauge Length If the percentage elongation is greater than 5% in a gauge length 50 mm, the material is ductile and if it less than 5% it is not. #8. Brittleness: Brittleness is a property by virtue of which, a material will fail under loading without significant change in dimension. Glass and cast iron are well known brittle materials. #9. Stiffness: The ability of material to resist elastic deformation or deflection during loading, known as stiffness. A material which offers small change in dimension during loading is more stiffer. For example steel is stiffer than aluminum. #10. Hardness: The property of a material to resist penetration is known as hardness. It is an ability to resist scratching, abrasion or cutting. It is also define as an ability to resist fracture under point loading. #11. Toughness: Toughness is defined as an ability to withstand with plastic or elastic deformation without failure. It is defined as the amount of energy absorbed before actual fracture. #12. Malleability: A property by virtue of which a metal can flatten into thin sheets, known as malleability. It is also define as a property which permits plastic deformation under compression loading. #13. Machinability: A property by virtue of which a material can be cut easily. #14. Damping: The ability of metal to dissipate the energy of vibration or cyclic stress is called damping. Cast iron has good damping property, that’s why most of machines body made by cast iron. #15. Creep: The slow and progressive change in dimension of a material under influence of its safe working stress for long time is known as creep. Creep is mainly depend on time and temperature. The maximum amount of stress under which a material withstand during infinite time is known as creep strength. #16. Resilience: The amount of energy absorb under elastic limit during loading is called resilience. The maximum amount of the energy absorb under elastic limit is called proof resilience. #17. Fatigue Strength: The failure of a work piece under cyclic load or repeated load below its ultimate limit is known as fatigue. The maximum amount of cyclic load which a work piece can bear for infinite number of cycle is called fatigue strength. Fatigue strength is also depend on work piece shape, geometry, surface finish etc. #18. Embrittlement: The loss of ductility of a metal caused by physical or chemical changes, which make it brittle, is called embrittlement.
Types of Welding
Welding is a process of joining similar and dissimilar metals or other material by application of heat with or without application of pressure and addition of filler material. It is used as permanent fasteners. Welding is essential process of every manufacturing industries. In fact, the future of any new metal may depend on how far it would lend itself to fabrication by welding.
The weldability has been defined as the capacity of being welded into inseparable joints having specified properties such as definite weld strength proper structure. The weldability of any metal depends on five major factors. These are melting point, thermal conductivity, thermal expansion, surface condition, and change in microstructure. Types of welding: Basically welding may be classified into three types. 1. Plastic welding: In plastic welding or pressure welding process, the pieces of metal to be joined are heated to a plastic state and then forced together by external pressure. These welding are also known as liquid-solid welding process. This procedure is used in forge welding and resistance welding. 2. Fusion welding: In the fusion welding or no pressure welding process, the material at the joint is heated to a molten state and allowed to solidify. These welding are also known as liquid state welding process. This includes gas welding, arc welding, thermite welding etc. 3. Cold welding: In this welding process, the joints are produced without application of heat, but by applying pressure which results diffusion or inter-surface molecular fusion of the parts to be joined. It is also known as solid state welding process. This process is mainly used for welding nonferrous sheet metal, particularly aluminum and its alloys. This includes ultrasonic welding, friction welding, Explosive welding etc. 4 Main Welding Processes: 1. Arc Welding (Fusion Welding): In this type of welding process, weld metal melted from the edges to be joined and allow to solidifies from the liquid state and usually below the recrystallization temperature without any applied deformation. Arc welding is most extensively employed method of joining metal parts by fusion. In this welding the arc column is generated between an anode, which is the positive pole of power supply, and the cathode, the negative pole. When these two conductors of an electric circuit are brought together and separated for a small distance such that the current continues to flow through a path of ionized particles called plasma, an electric arc is formed. This ionized gas column acts as a high resistance conductor that enables more ions to flow from the anode to the cathode. Heat is generated as the ions strike the cathode. This heat used as melting of metal to be joined or melting the filler metal which further used as joining material of welding metal. The electrode is either consumable or non-consumable as per welding requirement. The temperature at the center of the arc being 6000 OC to 7000OC 2. Gas Welding: The gas welding is done by burning of combustible gas with air or oxygen in a concentrated flame of high temperature. As with other welding methods, the purpose of the flame is to heat and melt the parent metal and filler rod of a joint. It can weld most common materials 3. Gas Metal arc welding (MIG): This welding is also known as metal inert gas welding. In this type of welding a metal rod is used as one electrode, while the work being welded is used as another electrode. It is a gas shielded metal arc welding which uses the high heat of an electric arc between a continuously fed, consumable electrode wire and the material to be welded. Metal is transferred through protected arc column to the work. In this process the wire is fed continuously from a reel through a gun to constant surface imparts a current upon the wire. In this welding the welding area is flooded with a gas which will not combine with the metal. The rate of flow gas is sufficient to keep the oxygen of the air away from the hot metal surface while welding is being done. 4. Gas Tungsten Arc Welding (TIG): This welding is also known as tungsten inert gas welding is similar to the MIG in that is uses the gases for shielding. This arc welding process uses the intense heat of an electric arc between a no consumable tungsten electrode and the material to be welded. In this process the electrode is not consumable during welding process and gas is used to protect the weld area form atmospheric air.
Difference between Hot Working and Cold Working
They both are the metal forming processes. When plastic deformation of metal is carried out at temperature above the recrystallization temperature the process, the process is known as hot working. If this deformation is done below the recrystallization temperature the process is known as cold working. There are many other differences between these processes which are described as below. Difference between Hot Working and Cold Working: S.No. Cold working Hot working 1 It is done at a temperature below the recrystallization temperature. Hot working is done at a temperature above recrystallization temperature. 2. It is done below recrystallization temperature so it is accomplished by strain hardening. Hardening due to plastic deformation is completely eliminated. 3. Cold working decreases mechanical properties of metal like elongation, reduction of area and impact values. It increases mechanical properties. 4. Crystallization does not take place. Crystallization takes place. 5. Material is not uniform after this working. Material is uniform thought. 6. There is more risk of cracks. There is less risk of cracks. 7. Cold working increases ultimate tensile strength, yield point hardness and fatigue strength but decreases resistance to corrosion. In hot working, ultimate tensile strength, yield point, corrosion resistance are unaffected. 8. Internal and residual stresses are produced. Internal and residual stresses are not produced. 9.
Cold working required more energy for plastic deformation.
It requires less energy for plastic deformation because at higher temperature metal become more ductile and soft. 10.
More stress is required.
Less stress required. 11.
It does not require pickling because no oxidation of metal takes place.
Heavy oxidation occurs during hot working so pickling is required to remove oxide. 12.
Embrittlement does not occur in cold working due to no reaction with oxygen at lower temperature.
There is chance of embrittlement by oxygen in hot working hence metal working is done at inert atmosphere for reactive metals.
CLASSIFICATION OF FLUIDS
Ideal fluid: A fluid, which is incompressible and having no viscosity, is known as an ideal fluid. Ideal fluid is only an imaginary fluid as all the fluids, which exist, have some viscosity. Real fluid: A fluid, which possesses viscosity, is known as real fluid. All the fluids, in actual practice, are real fluids. Example : Water, Air etc. Newtonian fluid: A real fluid, in which shear stress in directly proportional to the rate of shear strain or velocity gradient, is known as a Newtonian fluid. Example : Water, Benzine etc. Non Newtonian fluid: A real fluid, in which shear stress in not directly proportional to the rate of shear strain or velocity gradient, is known as a Non Newtonian fluid. Example : Plaster, Slurries, Pastes etc. Ideal plastic fluid: A fluid, in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain or velocity gradient, is known as ideal plastic fluid. Incompressible fluid: A fluid, in which the density of fluid does not change which change in external force or pressure, is known as incompressible fluid. All liquid are considered in this category. Compressible fluid: A fluid, in which the density of fluid changes while change in external force or pressure, is known as compressible fluid. All gases are considered in this category. Graphical representation of different fluids: Tabular representation of fluid types: Types of fluid Density Viscosity Ideal fluid Constant Zero Real fluid Variable Non zero Newtonian fluid Constant/ Variable T = u(du/dy) Non Newtonian fluid Constant/ Variable T ≠ u(du/dy) Incompressible fluid Constant Non zero/zero Compressible fluid Variable Non zero/zero CLASSIFICATION OF FLOWS ON THE BASIS OF MACH NUMBER. Incompressible flow-M less than 0.3 Compressible subsonic flow-M between 0.3 and 1 Transonic flow-M ranging between values less than 1 and more than 1 Supersonic flow-M greater than 1 but less than 5 Hypersonic flow - M greater than 5
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