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The mechanical properties of material are ascertained by performing carefully designed laboratory experiment that replicate as nearly as possible the service conditions. Identify and describe three experimental testing techniques that can be used to identify the mechanical properties of a material. The description should cover the following aspect:
Machines and test equipment
In manufacturing process, before engineers begin to do project, there are some properties that are important to made note of. One of important criteria of these properties is mechanical properties. To determine the mechanical properties of material, there are several key factors that have to be considered first. Ductility, toughness, hardness and tensile strength are example of the mechanical properties of material. Tensile strength is the ability of a material to stretch without breaking or snapping and ductility is the ability of a material to change shape (deform) usually by stretching along its length. The toughness of a material is the ability of a metal to deform plastically and to absorb energy in the process before fracture. Hardness is resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or to resistance to scratching, abrasion, or cutting. There are several tests to determine these mechanical properties of material such is:
Tensile test are performed for several reasons. The results of tensile tests are used in selecting materials for engineering applications. Tensile properties frequently are included in material specifications to ensure quality. Tensile properties often are measured during development of new materials and processes, so that different materials and processes can be compared (Davis, 2004). By doing tensile test, the data and the result that we get from this test, we can predict, decide and choose select the best material to use to make a product.
A test-piece of known cross-sectional area is gripped in the jaws of a testing machine, and is subjected to a tensile force which is increased by suitable increments. For each increment of force, the amount by which the length of a known 'gauge length' on the test-piece is measured using a suitable extensometer. When the test-piece begins to stretch rapidly, the extensometer is removed (rapid extension is a sign that the fracture is imminent, and failure to remove the extensometer from the test-pieced would probably lead to the destructive of the extensometer). The maximum force applied to the test-pieced before fracture is measured (Higgins, 1988). This is a brief of the procedure of tensile test and how the test is going to be conduct.
There are two type of specimen that we can use in this test which is flat or round. According to (Gourd, 1988) flat strips can be cut directly from sheet or plate material. Round test-pieces are machined from thick plates, bar stack and material of irregular shape such as casting or moulded components. With both flat and round specimens, the central portion is machined with parallel sides. A gauge length is marked in this portion and is used to measure strain. It is important that the fracture occurs within the gauge length if a realistic measurement is to obtained.
Figure 1.1 Example of a specimen. The end of the specimen is bigger compare to the gauge length to avoid end effect. (An Introduction to Engineering Materials, 1988)
According to (Davis, 2004) the gauge length is the region over which measurements are made and is centre within the reduced section. The distances between the ends of the gauge section and the shoulders should be great enough so that the larger ends do not constrain deformation within the gauge section, and the gauge length should be great relative to its diameter. Otherwise, the stress state will be more complex than simple tension localized in this region. The gauge length is the region over which measurements are made and is centred within the reduced section. The distances between the ends of the gauge section and the shoulders should be great enough so that the larger ends do not constrain deformation within the gauge section, and the gauge length should be great relative to its diameter. Otherwise, the stress state will be more complex than simple tension.
Figure 1.2 Typical tensile specimens, showing a reduced gauge section and enlarged shoulders. To avoid end effects from the shoulders, the length of the transition region should be at least as great as the diameter, and the total length of the reduced section should be at least four times the diameter. (Davis, 2004)
There are many types of griping to the specimen and depends of the specimen. According to (Davis, 2004) the end may be screwed into a threaded grip, or it may be pinned, butt ends may be used, or the grip section may be held between wedges. The most important concern in the selection of a gripping method is to ensure that the specimen can be held at the maximum load without slippage or failure in the grip section. Bending should be minimized.
Figure 1.3 Systems for gripping tensile specimens. For round specimens, these include threaded grips (a), serrated wedges (b), and, for butt end specimens, split collars constrained by a solid collar (c). Sheet specimens may be gripped with pins (d) or serrated wedges (e). (Davis, 2004)
Machines and test equipment
As we progress into advanced technology, there are many types of tester that have being produced. This include tensile tester. The most common testing machines are universal testers, which test materials in tension, compression, or bending. Their primary function is to create the stress-strain curve in particularly. Testing machines are either electromechanical or hydraulic. The principal difference is the method by which the load is applied. Electromechanical machines are based on a variable-speed electric motor; a gear reduction system; and one, two, or four screws that move the crosshead up or down. This motion loads the specimen in tension or compression. Crosshead speeds can be changed by changing the speed of the motor. A microprocessor-based closed-loop servo system can be implemented to accurately control the speed of the crosshead
In this test, we will use hydraulic testing machines. Hydraulic testing machines are based on either a single or dual-acting piston that moves the crosshead up or down. However, most static hydraulic testing machines have a single acting piston or ram. In a manually operated machine, the operator adjusts the orifice of a pressure-compensated needle valve to control the rate of loading. In a closed-loop hydraulic servo system, the needle valve is replaced by an electrically operated servo valve for precise control. In general, electromechanical machines are capable of a wider range of test speeds and longer crosshead displacements, whereas hydraulic machines are more cost-effective for generating higher forces. Davis (2004)
Figure 1.4 Components of hydraulic machines. (Davis, 2004)
During the testing, some important mechanical properties can be obtained. It's crucial to analyse the data before select the materials. The plastic strain is expressed as a permanent elongation measured over a predetermined gauge length. Percentage elongation can be used as a measure of tensile ductility. The total elongation can be determined from force-elongation curve. Another approach to tensile ductility lies in measuring the reduction in cross-sectional area which occurs at the point of fracture in a tensile test. Expressed as a percentage of the original area, the reduction in area provides a measure of tensile ductility.
1.4 Data analysis
After the test is done, we will analyse some of the mechanical properties that gain from the test.
Stress is depending on the cross-sectional area. The force is uniformly distributed through the bar, hence the smaller the diameter, the greater is the intensity of the force. Engineering stress is defined as force divided by original area:
Original area of test specimen
SI unit=M Pa (Pascal)
Strain is changed in length. Engineering strain is defined as displacement divided by original length.
e= Engineering strain
L¯= Original length of specimen
SI unit= Unitless or no unit
1.4.3 Young's Modulus
Based on (Gourd, 1988) the ratio of stress to strain is constant for a given material. But if we plot stress-strain curve for a number of different materials it will be seen that the slope of the linear alters from one another. It is also referred to simplify as the elastic modulus. Young's modulus is a measure of the stiffness of a material and it's important factor in material selection and design.
Young's modulus (E) =
1.4.4 Stress-Strain curve
Figure 1.5 A Stress-Strain curves. (An Introduction to Engineering Materials, 1988)
As we can see the limit of proportionality, there is a change from straight line to a curve. At this rate, the stress is no longer proportional to strain. But it will remain elastic if the stress is removing and it will returns to its original length.
The elastic limit is the maximum of the stress level that can be absorbed by the metal. It's slightly above the limit of proportionality, the material ceases to become elastically and some permanent deformation or elongation is taking place. It can return to its original shape or length if the stress is removed.
At the point of Yield stress, the stress strength is required to produce a permanent deformation and starts to deform plastically. According to (Gourd, 1988) materials do not always behave in some manner at yield. A few, especially low carbon steel, show an appreciable increase in strain with no significant increases in stress level.
Figure 1.6 Example of the process of deformation
Ts= Tensile strength. (It's the maximum stress level before deform)
The Tensile strength is the maximum point on the engineering stress-strain curve. This corresponds to the maximum stress that can be sustained by a structure in tension. If this stress is applied and maintained, fracture will occur.
Ductility is the ability of a material to plastically deform without fracture. There is two way to measure ductility. One is percent of elongation and another is percent of reduction.
Percent of elongation
L¯= The initial length gauge length
Lð‘“= The length of the gauge section at fracture
Percent of reduction
%RA= Ã- 100
A¯= The initial area of cross-sectional area
Að‘“= The cross-sectional area at fracture
Before engineers select the material, hardness is another aspect that we should consider also. Hardness is one of the basic mechanical properties of engineering materials. Hardness test is practical and provide a quick assessment and convenient as well. The result can be used as a good indicator for material selections. The good hardness means that the material is resistant to scratching, wear or penetration. Hardness test also good to ensure the quality in parts which require high wear resistance such as gears and cutting tools. . Most tooling item should be hard for scratch and wear resistance. As the hardness tester goes deeper or wider indentation, it indicates a less resistance to plastic deformation of the material being tested and resulting in a lower hardness value.
There are many types of tester for the hardness test. The most known hardness test is Brinell test (HB), Rockwell test (HR), Vickers test (HV) and Knoop test (HK). This hardness test also known as non-destructive test. It is also a universal test to be carried out. The test is very popular because it is simple and inexpensive. For this test, we will use Brinell test (HB) as hardness tester.
According to (Higgins, 1988) the Brinell test, devised by a Swede, Dr. J. A. Brinell, is probably the best known of the hardness test. A hardened steel ball is forced into the surface of a test-piece by means of a suitable standard load. The diameter of the impression is then measured, using some form of calibrated microscope, and the Brinell Hardness Number (H) is found form:
Brinell hardness =
D= diameter of ball
d= diameter of impression
Figure 2.1 The relationship between ball diameter (D), depth of impression (h), diameter of impression (d) and dimension of the test piece in the Brinell test. (Materials for the Engineering Technician, 1988) (Higgins, 1988)
Before the test is carried out, the depth of the indentation must not be to great relative to the thickness of the test piece. If the ball diameter is too big, the load applied to the test-piece is going to be to great and makes the result not accurate. If the ball diameter is too small, the load applied to the test piece will not have the sufficient amount of force to penetrate the test-piece, hence the result cannot be obtain.
Figure 2.2 This is example of the necessity of using the correct ball diameter in the relationship of the thickness of the test-pieces. (Materials for the Engineering Technician, 1988) (Higgins, 1988)
2.2 Machines and Test Equipment
Figure 2.2 Example of Brinell hardness machine. It's a portable Brinell hardness tester. (http://www.ukcalibrations.co.uk/common_files/images/htm_images/prod_36img_1.jpg, Accessed 14.3.2010)
For the indenter of the Brinell hardness test, the tester will use the tungsten carbide ball as the indenter. Tungsten carbide is a very hard material and it is suitable to check the indentation of the test-piece. Tungsten carbide is containing of tungsten and carbon atom. The ball diameter is available at 10mm, 5mm and 1mm. It is widely used for testing metals and non-metals of low to medium hardness. The tungsten carbide or hard ball of steel is pressed into the specimen surface with a load of 500, 1500 and 3000 Kg.
2.3 Data collection
After the testing of material is done, the data can be obtained. Some automatic or digital tester machine, the data can be obtained easily because the data is recorded in the computer and the computer will interpret the data easily. The hardness result can be obtained using the Brinell Hardness Number (H). We can obtain a several data by doing the test several times again and to compare the data for each the test is conducted.
2.4 Data analysis
From the data, we can analysis the data we have obtained. The data we obtained can be a guide line before we select the material to use. Some other mechanical properties also can be measure such as toughness. The toughness of a material must be distinguished from it is ductility. The toughness is the combine of strength and ductility. This Brinell hardness test can be converting into other hardness test as well.
Figure 2.3 Conversion chart for hardness test. Because of the many factors involved, these conversions are approximate. (Manufacturing Engineering and Technology, 2010)
According to (Higgins, 1988) these tests are used to indicate the toughness of a material and particularly its capacity for resisting to mechanical shock. Brittleness resulting from a variety of causes and is often not revealed during a tensile test. However the unsatisfactory material would prove to be extremely brittle compared with the corrected treated one after the heat treatment which would be tough.
Toughness is a measure of the amount of energy a material can absorb before fracturing. It becomes of engineering importance when the ability of a material to withstand an impact load without fracturing is considered. Impact test conditions were chosen to represent those most severe relative to the potential for fracture such as deformation at a relatively low temperature, a high strain level and a triaxial stress state.
Ductile material will absorb sufficient energy and as the area of around the notch will experience a deformation during the fracture period. Besides that, the brittle material will experience a little on the deformation and absorb the energy. There are two types of test that can be done which is Izod and Charpy test. These tests are quite similar to one another and the difference between the Izod and Charpy test is the arrangement of the specimen. For the Izod test, the specimen is placed vertically while the Charpy test is placed horizontally.
For both of the impact test, the specimen is placed vertically in the path of the heavy pendulum. The V-notch is machined into a bar specimen with a square cross section. Upon release the heavy pendulum, a small knife is mounted at the edge at the pendulum and strikes to it as it act as a point of stress concentration.
Figure 3.1 Example of the specimen (http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/anal/yue/img00007.jpg, Accessed 23.2.2010)
3.2 Machines and Test equipment
Figure 3.2 The test impact machine (http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/anal/yue/img00007.jpg, Accessed 23.2.2010)
According to (Gourd, 1988) the machine is used for the test consist of a hammer which swings about a pivot situated above an anvil holding the test-pieced. The hammer is raised at the pre-set position thus acquiring potential energy. When released, the hammer swings down to hit the specimen. It continue to swings past the anvil after the fracturing the test-pieced. The height it reaches is a function of the energy remaining.
In the Izod impact test, the test piece is a cantilever, clamped upright in an anvil, with a V-notch at the level of the top of the clamp. The test piece is hit by a hammer carried on a pendulum which is allowed to fall freely from a fixed height. For Charpy impact test, the test piece is tested as a beam supported at each end. A notch is cut across the middle of one face, and the hammer hits the opposite face directly behind the notch.
3.3 Data collection
The result of an impact test is collected in joules. Usually it's necessary to test three of more test-pieced and obtain an average result. Based on the difference between h and h', the energy absorption of the specimen is computed. The primary difference between the Charpy and Izod techniques lies in the manner of specimen support as is indicated in Figure 3.1. Variables including specimen size and shape, as well as notch configuration and depth can influence the test results. The temperature of the room also can determine the data collection for the impact test.
3.4 Data analysis
Figure 3.3 The nature of the fractured surface in the Izod test (Materials for the Engineering Technician, 1988)
According to (Higgins, 1988) examination of the fracture cross-section of the test piece reveals the further useful information. In the most ductile materials the fractured surface is likely to be of a 'fibrous' nature and will be rather dull and 'silky' in appearance as in the figure 3.3 since plastic flow of the crystalline structure has occurred. With very brittle metals the fractured surface will be relatively bright, sparkling and 'crystalline' since crystal have not been plastically deformed. As fracture has followed the crystal boundaries each small crystal reflect a bright point of light as in the figure 3.3... Many metals will fall somewhere between these extremes as the fractured surface will show a combination of ductile and 'crystalline' areas. For steels in particular it is possible to estimate quite accurately the % crystalline area of the fractured surface of the test piece and to use this as a measure of the notch ductility. A typical fracture appearance with crystallinity increases as the temperature is reduced.
Identify and describe five major considerations involved in selecting the candidate materials for piston for an in-line four-cylinder engine. These considerations should be covering the essential materials properties to provide the necessary performance in service, as well as the business factors that can affect the materials decisions.
Figure 3.1 Example of the piston (http://www.hope.edu/brain/engineering/ank/public_html/Labs/Car_Engine_Lab/Engine5/Piston.jpg, accessed on 1.3.2010)
The function of the piston is to absorb the energy released after the air or fuel mixture is ignited by the spark plug. The piston then accelerates producing useful mechanical energy and the engine to start. To accomplish this, the piston must be sealed so that it can compress the air or fuel mixture and not allow gases out of the combustion chamber. To make the piston to work, these several condition should be consider.
The material of the piston should preferably have a low coefficient of thermal expansion. The temperature in the engine is so hot. The pistons have to cope with high temperature to make it run. The piston should causes less heating of the air or fuel mixture during the induction stroke so that the mixture filling in the cylinder is cooler and dense and it will improved engine power output. If the pistons have a high thermal expansion, the piston cannot work for a long period of time during the process. Hence it will reduce the efficiency of the piston.
The car is moving at a fast rate, this is cause by piston which is moving very fast to make the job done. The density should be major consideration because it will permitting the engine run at higher speed and can develop much more power. The piston working for an in-line four-cylinder engine is moving in the fast rate. The material for the piston should be lighter to make the piston move up and down to produce power. If the piston is density of the piston is heavier, it working principle is slow hence it cannot produce much more power compare to the lighter piston. A lightweight piston specifically designed for high-performance use can withstand substantial cylinder pressures.
The strength of the piston should be high enough to absorb the energy by the piston. The piston should have a low expansion. The temperature in the engine is very high in temperature. It needs extra clearance between the piston and cylinder or otherwise the piston would be tight and seize under operating condition. However, this clearance usually gives rise to piston slap when the engine is cold and consequently rapid wear. It also has to resistance to high wear and tear under the working condition. The strength is important for the piston because it can avoid fracture failure. Since the piston is one of the most important components in an engine assembly, transferring combusted energy to mechanical movement, it must have enough strength to withstand severe temperature fluctuations and high pressure.
Cost is a major consideration in the manufacturing. It has to satisfy specific design production, operation and maintenance. It will produce a component or assembly at acceptance cost and should involve considerations of design and manufacturing aspect while view specific properties. Engineers have to design and make products which will function adequately or efficiently for the whole design product life. Many cases purchase cost of material is accounts of the total work cost. For example, the expensive material may permit the use of relatively simple and low cost processing while the cheap material may require long period of process, complex and expensive production method. If the material is complex to produce or manufacture, the cost of the material is gone to be expensive. The cost of a particular material is subject to fluctuation caused by factors such as supply and demand or as complex as geopolitics. If a product is no longer cost competitive, alternate and cheaper material is likely to be choose. The cost of a material is slightly decreased as the quantity of the purchased is increase. The unit cost of a raw material depends not only the material itself, but also shape, size and condition.
The material should have a mass availability in the production process. If the availability is less hence the cost of the material is higher. The engineer should consider this availability as well. If the materials are not available are desired quantities, shape, dimension or surface texture, substitute materials or addition processing is required on a particular material maybe required and eventually will contribute to the overall cost. Based on (Kalpakjian & Schmid) reliability of supply is important in order to meet the production schedule. Reliability of supply also important considering most countries imports numerous raw materials. Sometimes, geopolitics should be considered also since several countries did not get along with each other.
Recognize and discuss potential causes of failure of the piston used in-line four-cylinder engine.
The piston used in-line four-cylinder engine is working in extreme condition such as high temperature, huge amount of pressure, long period working mechanism and many more. Thus, this could lead several failures as piston working in the car for a very long time. Failures as in term in engineering are commonly applied to the manufacturing process that provides the component which did not meet the requirement specification. There are several causes that will make the piston to fail such as fracture, fatigue and creep.
Fracture is the separation of a body into two or more pieces in the conjunction to an imposed stress which is static at a low temperature. There are related to the low melting temperature of the material. There are two types of fracture, brittle and ductile fracture. Brittle fracture is caused by a little or no plastic deformation with low energy absorption whereas ductile fracture is a plastic deformation with high energy absorption. As the test-pieced elongate, the extension is depends on some factor such as the hardness, shape, distribution of stress and many more.
Figure 3.1 Schematic illustrations of the types of fracture in tension: (a) brittle fracture in polycrystalline metals; (b) shear fracture in ductile single crystals; (c) ductile cup-and-cone fracture in polycrystalline metals; (d) complete ductile fracture in polycrystalline metals, with 100% reduction of area. (Manufacturing Engineering and Technology, 2010)
Ductile and brittle is related to one another, it is a particular force is acting and other causes are depending on the situation. Ductility is the percent of elongation and the reduction of area. Ductile is a slow process and takes time to fracture whereas brittle is faster to fracture when the crack length is elongate. The measured of fracture strength is lower than the theoretically calculation due to many factors such as temperature, hardness and distribution of stress.
Figure 3.2 Sequence of events in the necking and fracture of a tensile-test specimen: (a) early stage of necking; (b) small voids begin to form within the necked region; (c) voids coalesce, producing an internal crack; (d) the rest of the cross-section begins to fail at the periphery, by shearing; (e) the final fracture surfaces, known as cup- (top fracture surface) and cone- (bottom surface) fracture. (Manufacturing Engineering and Technology, 2010)
For ductile material, the plastic deformation in order the maximum stress is exceeds the yield strength. This will ensure the stress is equally distributed to surrounding area of the stress raiser. The stress raiser is not occurring to the brittle material hence the fracture is fast.
The piston may break because of fracture. This could happen when the engineers design the piston in the early stage and their assumption is wrong. The fracture arises as a consequence of excessive load. An incipient crack in the piston is formed due to the excessive load will then spread to the entire piston even with under normal loads and will eventually causes the piston to split or fracture. The stress imposed to the piston is high which the piston material selection could not absorb the stress better. The temperature in the piston is another factor for the piston to fracture. The temperature in the piston is high. There are related to the low melting temperature of the material.
Creep is a permanent deformation of a material under a static load for a period of time. This creep usually occurs to the metals and some kind of non-metals material. Creep can occur in any temperature. Creep is important for the high temperature appliance. The piston working condition is in high temperature and creep is definitely will be the reasons for the piston failure. By determine the creep value, engineers can predict the lifetime of a component. For metals, it becomes important when the temperature is high than 0.4 (= absolute melting point). To determine the creep value by doing the creep test. The creep test specimen is put under a constant load and the temperature is kept constant. The length deformation is measure at various times.
Figure 3.3 The creep curve of strain against time. (Manufacturing Engineering and Technology, 2010)
In the primary stage, the material is experiencing an increase of strain but in a slow pace. The secondary part is crucial to determine the specific creep life. This also the longest period of creep to deformed. The constant load and temperature to the material will make the material to become softer but still to retain its ability to experience deformation. As engineer, the design parameter is too considered for a long-life appliance or if fail, the failure is at minimum rate. Finally in the tertiary stage, the material will rupture. The creep rate increases with material temperature and load is constant.
The piston is working under high temperature in engine. As the working period goes by, the heat generate in the piston will increase. The piston move up and down to produce power. It creates more heat. It will cause the piston to rupture if the material selection cannot resist the high temperature.
Fatigue is the main failure for piston to fail. Fatigue also known as brittle-like failure. Fatigue means the tendency of a material to fracture by means of progressive brittle cracking under repeated stress to the material. Almost 90% of material fails because of fatigue and it can happen without single warning.
The design of the piston can lead to fatigue failure. Stress concentration is caused by sharp corners or undercut from which fatigue crack can be spread. It also called as tension raiser. The surface finishes may lead to fatigue failure also. During manufacturing process, scratches in highly finish surface or tooling marks left when machining can becomes stress concentration and it will lead to fatigue failure. The changes of temperature at which the material is subsequently in used can have a significant effect upon the fatigue resistance of material.
Suggest and describe five measures that may be taken to increase the service life of the piston.
There many method to increase the service life of the piston. Since the material selection of the piston is important hence the material should be improve to increase the service life. The service life of a component is essentially important to determine how long the product will last.
5.1 Shot Peening
During the machining process, some scratch, tool marking and grooves are unwanted things that could decrease the service life by the cutting tools. These surfaces marking can limit the fatigue life. But this scratch and tool mark can be removed by polishing the material hence enhance the fatigue life. One of the methods is shot peening. Shot peening is blasting a metal part with a small steels ball of a range 0.125 mm to 5 mm and can penetrate until 1.25 mm at a controlled and constant velocity. Some of component after the machining process is not smooth surface and not uniform throughout the thickness of the component.
Figure 5.1 Sample of shot peening (http://www.engineersedge.com/manufacturing/images/peen-shot.jpg, accessed 14.3.2010)
The shot peening will improve the fatigue life of component by delaying the initial stage of crack of the component. Since the piston could fail because of the fatigue, this surface treatment could overcome the problem of fatigue failure.
5.2 Case hardening
Case hardening is method will improve the fatigue life, wear and the surface indentation at the same time. This is done through the nitriding process which the component is exposed to the nitrogen in a control temperature through the diffusion of nitrogen. The reaction of the nitrogen with the steel causes the formation of very hard iron and alloy nitrogen compound. This nitriding process can be harder compare to hardest tool steels or carburized steels. This nitriding process can eliminate the cracking and distortion since the process is elevated at low temperature and there is no subsequent quenching (Timming, 1991). The wear resistance also increases by this process. It is cheap if a large number if components are doing at the same time. The piston is moving up and down inside the engine. This nitriding can prevent the fatigue failure and wear failure.
5.3 Lower the temperature
The piston needs to have a small hole to allow the liquid oil or lubrication to enter the piston and cool down the piston. In the areas of affected in the piston, piston rings and cylinder running surface then run each other without any lubrication, which in the short time causes seizure with severely worn surface. This similar situation if there is lack of oil. This could generate the temperature faster and could wear the piston greater. It is essential to flow the lubricating oil into the piston to make sure the temperature is no generate faster than usual. It is important for the oil to transfer excess heat away from the pistons as quickly as possible. The lubricating oil also must prevent the metal-to-metal contact that would result in wear to the moving parts. This small hole cools down the piston. This can prevent the piston from fracture because of high temperature.
5.4 Geometrical design
In designing stage, the engineers have to avoid any sharp corners for the piston. This sharp corner can act as a stress raiser for the piston and eventually the piston will fracture. To prevent this to happen, engineers should use fillet to replace this sharp corner. The stress applied is less compare to any sharp corners. The piston working condition is extremely fast moving in the engine. This stress raiser cannot have to the piston and if this stress raiser has to the piston, the piston will fail because of fracture due to the stress raiser.
Polishing is process that will produce a shinny, smooth and lustrous surface of a component. Polishing is the removal of material to produce a scratch-free component. During the manufacturing process, some will leaves unwanted things to the component such as scratch or tool mark. This scratch and tool mark will act as a stress raiser to the component and this will lead to fracture. For the piston, this unnecessary stress raiser will make the piston fail. For this purpose, polishing can reduce this effect to the piston since piston moves up and down very fast.