Tensile Tests on Metals and Plastics - Assignment Example

Tensile Tests on Metals and Plastics Name: Course: College: Tutor: Date: Activity 1 and 2 – Tensile Tests on Metals and Plastics Discussion of Results and comparisons between different metals When the specimen is loaded, it starts behaving like a spring with an elongation following the Hooke’s Law which states that F=kx, according to the equation, “k” is the spring constant which correspond with Young’s Modulus law (Leeming andHartley, 1981). The stress-strain curve is found to be linear in the elastic region of the graph. The yield point in any material is the point in which the element start to loose elasticity and the curve start to bend. When the load exceeds the yield point, the material will go through plastic deformation and it will end up getting permanently deformed even if the load is removed. There are different shapes of stress-strain curves for different metals as obtained in the experiment. The magnitude and shape of curves depends on composition, temperature, treatment of heat, the rate of strain, and the state of stress imposed when carrying out the test(Leeming andHartley, 1981). The length of steel is compared with an untested original specimen to allow for relative measurements to be made. Low Carbon Steel Fracture Surface has a dimpled interior structure indicating ductile failure. The surrounding surface is smoother and this shows a brittle failure across crystal planes. Different metals will exhibit this mixed failure pattern in varying degrees (Leeming andHartley, 1981). The constraints that are used to explain the stress-strain curve of a metal include the tensile strength, yield strength or yield point, percent elongation, and reduction of area. The initial 2 are the parameters of strength and the last two shows ductility of the metal. Brittle materials, like cast iron, are weak in tension test because of the presence of submicroscopic cracks and faults (Leeming andHartley, 1981). There is a relatively permanent increase in length and large extent of necking of the medium carbon specimen compared to that of the high carbon specimen. Ductility indicate the flow of plastic before fracture, changes in impurity level or processing conditions, also the extend to which a metal can be deformed without any crack in metalworking operations like rolling and extrusion. Discussion of Results and comparison between different polymers It has been shown that the extension curve is not accurately linear for a small strain. This was anticipated as it occurs in viscoelastic materials. The basic property of polymer is stress/strain ratio which is called elastic modulus. The ratio is obtained from most stiff part of the load curve. The elastic modulus of a material is used to show the stiffness of a given material as compared to other material types. The stiffness can found by use of the strain of the material specimen and its gauge length (Leeming andHartley, 1981). Generally, the stiffness of polymers is relatively low, which means that the machine’s elastic scale will deflect slightly. Comparison between Metals and Plastics Metals are brittle while plastics are ductile (Leeming andHartley, 1981). Plastics exhibit amorphous properties while metals are show crystalline properties. In addition, plastics behave as liquids while metals flow along the slip planes through slip mechanism. On the other hand, metals have randomly oriented grains. This means that there are many slip planes that makes the metals behave like plastic flow in a fluid. Finally, plastics show different mechanical behavior under tensile test, although it does not show brittleness like steel or Gray Cast Iron. Activity 3 Connecting rod (Leeming andHartley, 1981) Functions of a connecting rod Connecting rod is connected to the crankshaft and it transmits thrust from the piston to the crankshaft. The thrust comes from the reciprocating transverse motion in the engine then converts it into rotary motion of the crankshaft. The piston should be strong enough to remain rigid with the repeated loadings and light to minimize the inertia forces produced when the rod stops and change the direction at the end of each stroke. The engineers should design the rod with great reliability that is able to transmit both axial tension and axial compression forces, and bending stresses caused by the pull and thrust on the piston without twisting or bending (Mahadeo and Uday, 2008). The rod should be designed in a way that even under huge amount of load pressure; it should be in a position of not failing prematurely. The edges that are sharp are smoothened with materials like sand paper; this will help in reducing the stress from rising on the connecting rod. To increasing the strength against cracking, the rod is also shot-peened or hardened. In applications where performance required is high, the rod is well balanced to get rid of unwanted harmonics from forming excessive wear. Material Properties of a connecting rod The material mostly used to produce the connecting rod is a special steel alloy. This alloy is strong, tough and can readily be forged into different shapes and finished by machining. The sharp edges are smoothened with sanded in order to reduce friction and stress rising on the connecting rod. To increase its strength against cracking the connecting rod is hardened. In most cases balancing is done on high performing applications to prevent unwanted harmonics from causing excessive wear. Another property is that, the material used for connecting rod should be light in weight to allow the engine move faster (Mahadeo and Uday, 2008). Materials used for a con rod The material which is mostly used to produce connecting rod is cast iron. Connecting rod is produced by pouring molten steel into a mold and the resulting product is finished through a machining process. Cast iron has been used successfully to produce less expensive engines of diesel and gasoline. Forged iron rod is also used to produce engines of high performance. They are create through milling of steel block. The material can withstand much greater loads and more engine vibrations as compared to cast iron. High-performance engines are produced using forged steel rod. The process of producing connecting rod through machining of billets from billets steel is costly and can only be used in high horse power engine. Aluminum material is also used to produce a high performing connecting rod. The aluminum rod is light in weight on the crankshaft and it allows the engine to accelerate much faster. Aluminum can absorbs shocks of the engine much better then cast iron however, it is not durable and has to be changed more frequently than steel (Mahadeo and Uday, 2008). Manufacturing processes to make a con rod and their comparisons The most common processes for manufacturing con rod are casting, powder-forged and drop-forged. Forging is a process of plastic deformation where the work piece is passed through two dies and compressed using either gradual or impact pressure (Mahadeo and Uday, 2008). The process can be done either at room temperature or at temperature. The product of this process is tough, resistant to impact and has a high strength to weight ratio. In Powder forging the powder of iron and copper are compacted, heated and then forged to increase its density to that of wrought steel. Die-casting is the process of forcing molten metal under high pressure to form metal dies. Directional solidification starts from the center of the arm to each. The connecting rods produced should stress as a result of high speed cycling that requires exacting tolerance and fits to components that are connected to like crankshaft or piston. Heat treatment This process involves a controlled heating and cooling of metals. It is done to change their properties to improve their performance and to make ease the processing. A common example of heat treatment is the hardening of high carbon steel. The steel rod is heated to a dull red heat and then plunged into cold water to cool it rapidly. This process is called quenching. The steel rod becomes hard and brittle but if the rod is heated to a dull red heat and allowed to cool it will becomes softer and less brittle. After the heat treatment on the material it can be passed through the process of flow forming, where the grains will be distorted and this will result in most metals becoming if flow formed at room temperature (Banabic, 2007). The material is the normalized to remove any locked in stresses resulting from the forming operations. Normalizing also prepare the material for machining. Aluminum alloys are heat treated through Heat solutionizing, quenching, and aging. During quenching process, the cooling rate of the material is controlled. This will cause the precipitate to nucleate heterogeneously at any defects in the α-aluminum matrix. This will reduces the super saturation of the solid and reduces the ability of the alloy to attain maximum strength as compared to the subsequent aging treatment. Alloying and Heat treatment alter the microscopic Structure of iron. Steel structures into which carbon spheroids are embedded have different properties according to the heat treatment they are subjected to (Banabic, 2007). Microscopic structures of iron before and after alloying (Banabic, 2007) Conclusions on connecting rods The microscopic nature of the material determines t its properties. The desired properties of steel are obtained by alloying with carbon. Carbon alloy elements disperse in the crystalline structure of iron. The properties of iron can also be changed using a controlled temperature. Such properties include hardness, toughness, brittleness and ductility which are important when producing connecting rods. Cast iron is weak in tension test because of brittleness materials and the presence of submicroscopic cracks and faults The two most competitive high volume manufacturing processes of connecting rods are forged steel and powder metal processes. This is because they are less costly and durable. References David J Leeming and Reg Hartley, 1981. Heavy vehicle technology, Leckhampton: Stanley Thomas Prakash Mahadeo Dixit and Uday S Dixit, 2008. Modeling of metal forming and machining processes: by finite element and soft computing methods, London: Springer D Banabic, 2007. Advanced methods in material forming, Berlin: Springer Read More