Mechanical Properties of Materials - Essay Example

Mechanical Properties of Materials Name Institution Lecturer Course Date Mechanical Properties of Materials A crucial task of engineers is the selection of the right material or combination of materials to serve the intended task while observing high levels of safety. Material selection is based on a number of factors such as aesthetics, mechanical properties, physical properties, chemical properties, electrical and electromagnetic properties of the material as well as the design limitations. The mechanical properties of a material play a fundamental role especially in civil, structural, mechanical, Agricultural and automotive engineering. The mechanical properties of materials describe how materials behave when subjected to a force/load. The mechanical properties of materials help engineers to have an idea of the type and extent of deformation that a material under question will undergo when a certain force is applied. In addition, knowledge of the mechanical properties of materials help engineers to determine the amount and type of force to apply to the material under question in order to deform it into the desired size and shape. The most important mechanical properties of materials include: tensile strength, yield strength, ductility, brittleness, bearing capacity, toughness, and hardness. These mechanical properties of materials will be described further alongside tests done to determine them. Material engineers are tasked with enhancing the properties of materials to suit different applications. Therefore, knowledge of the factors that affect the mechanical properties of materials is extremely crucial for material engineers in their work. In addition, knowledge of the methods used to determine and evaluate the mechanical properties of materials is extremely important for material engineers. This knowledge comes in handy when they need to determine if materials they have made have achieved the desired mechanical properties. Nevertheless, this knowledge is crucial for every engineer. This essay aims at enhancing the understanding of the mechanical properties of materials. It outlines tests done to determine and evaluate the mechanical properties of materials. Mechanical properties of materials are determined through destructive testing methods. The destructive testing methods described in this essay include tensile test, compression test, torsion test, shear test and hardness tests. The essay also explains the factors that affect the mechanical properties of materials. Determining the Mechanical Properties of Materials: Tests and Evaluations Mechanical properties of materials, unlike physical properties, cannot be determined through mere observation of the material. For example, an engineer cannot tell the elastic modulus of a material by simply observing it. Tests must be done to determine the mechanical properties of materials. Destructive tests are mainly used to determine the mechanical properties of materials (Kakani & Kakani, 2004). There are two types of engineering tests: destructive tests (DT) and non-destructive tests (NDT). In destructive tests, the specimen used to conduct the test fails (it is destroyed) while in NDT, the testing process does not cause damage to the material or specimen used (in other words, there is no destruction) (Kakani & Kakani, 2004). Some material properties can only be determined through destructive tests while other can only be determined through NDT. Generally, all the important mechanical properties of a material are determined through destructive tests, which provide more information than NDT (Kakani & Kakani, 2004). Tension Tests The most common test performed to determine the mechanical properties of materials is the tension test. In this test, a material specimen is obtained. The specimen is a cylindrical piece of the material with a given length (L) and diameter (D) so that its cross-sectional area (A) is known. The specimen is then subjected to a controlled force along its long axis until it breaks or fractures. The changes in dimensions (L and A) of the specimen, the mode of failure and the changes in applied load are used to determine mechanical strengths of the material. Tensile tests are primarily aimed at determining the strength of the material although the generated data is used to determine other mechanical properties of materials (ASM International, 2004). In order to get the best results and accurately determine the mechanical properties of a material, it is important to use the right specimen and methodology. Tensile testing specimens American Society for Metals (ASM) International gives the characteristics of a good tensile testing specimen. The specimen is a cylinder with two enlarged ends for gripping. The important part of the specimen is the middle, narrow section known as the gauge section (ASM International, 2004). This section is narrowed to ensure that deformation and failure will occur in the region but not in the grip section. The grip section must be designed in a way that the specimen will not slip during testing. Any slight slip would greatly interfere with the testing resulting to the generation of erroneous and unreliable results. To facilitate tight grip, the specimen may be screwed or pinned to the testing machine’s grip part. The specimen may also be tightly gripped between wedges; this is the most common gripping method. As such, the gripping section of the specimen is usually rough to ensure that specimen does not slip out of the wedges (ASM International, 2004). An example of a specimen is shown in appendix 1. Tensile testing machine Basically, the testing machine grips the specimen and then applies a controlled uni-axial tensile force to the specimen until it breaks. The testing machine then produces force and extension data used for the determination of the mechanical properties of the material. Due to advancement in technology, modern tensile testing machines now produce force-elongation curves showing how the specimen’s length changed with force application. Universal testers are the most common testing machines. They are capable of testing materials in compression, tension or bending (ASM International, 2004). Testing machines are classified into two based on the method of load application: electromechanical and hydraulic. Electromechanical testing machines use a variable speed electric motor to apply a controlled load to the specimen by moving the crosshead as desired. The motor speed can be changed to change the speed of movement of the crosshead and hence the rate at which the load is applied. Automatic machines are now in place that used a closed-loop feedback system that controls the speed of the crosshead accurately. On the other hand, hydraulic testing machines use fluids to apply load on the specimen. They use single or dual pistons to move the crosshead. They can be manually operated or automatic. Automatically operated hydraulic machines offer a precise control of the loading, which provides better quality results and more accurate mechanical properties of materials than manually operated machines (ASM International, 2004). An example of a universal testing machine showing the main components is shown in appendix 2. Test data Earlier machines were simply generating load-extension data that engineers used to plot load-extension and stress-strain curves. Drawing of load-extension data was simple and it involved plotting a curve using the generated data without further calculations. Testing machines were then improved to allow for the plotting of load-extension curve that gave a better insight of how load varied with the extension of the specimen until the specimen broke. A sample load-extension curve is shown in appendix 3. However, load-extension data makes very little, if any, sense with respect to the determination and understanding of the mechanical properties of materials. In order to determine the aforementioned mechanical properties of materials, a stress-strain curve is required. Stress is defined as the force applied per unit area; therefore, it is calculated by dividing force applied with the cross-sectional area of the test specimen. The initial cross-sectional area of the specimen is considered when calculating stress, and the resulting stress is known as nominal or engineering stress. Strain is defined as the ratio of extension to the original length. It is determined by dividing the extension by the original length of the specimen. The resulting strain is known as engineering strain or nominal strain (ASM International, 2004). Some testing machines eliminate the need for engineers to calculate nominal strain and nominal stress for every data value. These machines are programmed in such a way that once the initial cross-sectional area and length values of the specimen are fed into the system, they can plot stress-strain curves automatically as testing is performed. Nevertheless, by producing load and extension data, it is very easy to plot stress-strain curve. The stress-strain curve takes a similar shape to the load-extension curve as shown in appendix 3. However, the stress-strain curve is independent of the dimensions of the specimen; this is the reason why it is used in determining the material properties of the specimen. Each of the aforementioned mechanical property can then be determined from the stress-strain curve as described below (ASM International, 2004). Elastic deformation versus plastic deformation When load is applied to a material, the material may undergo elastic deformation or plastic deformation. Elastic deformation occurs at small stresses when the material specimen is able to regain its initial dimensions upon the removal of the stresses. Subjecting a material to stress causes the bonds that hold atoms together to stretch. Once the stress is removed, the bonds relax enabling the material to regain its initial shape. The stress-strain curve in the elastic deformation part is linear. The gradient of this curve presents an important mechanical property of materials known as Young’s Modulus, Modulus of Elasticity or Elastic Modulus, which is used widely in design and selection of materials (ASM International, 2004). At higher stresses, atomic planes slide over each other such that upon removal of the stress, the material is unable to regain its original shape. The resulting deformation is known as plastic deformation. The end of elastic deformation and the beginning of plastic deformation help in determining another important mechanical property, Yield Strength. It is defined as the point at which plastic deformation begins. Applying stresses beyond yield strength will cause permanent/plastic deformation of the material. For some materials especially ductile materials, the transition from elastic to plastic deformation is not very clear on the stress-strain curve. In such materials, the yield strength is determined as offset yield strength (for example 0.2% offset yield strength). A line parallel to the elastic deformation line is drawn but offset 2% along the strain line. The intersection of this line with the stress-strain curve gives the offset yield strength. For other materials especially linear polymers and low carbon steels, two yield strengths are shown: upper yield strength and lower yield strength as shown in appendix 4. Some engineers will use the upper value while others use the lower value. Other engineers may prefer to get an average between the upper and lower yield strength (ASM International, 2004). The other mechanical property that is derived from the stress-strain curve is the ultimate strength or the tensile strength. This property is extremely used in design. Tensile strength is defined as maximum engineering stress a material can accommodate. It is worth noting however that in some materials especially low carbon steels, the upper yield strength may be higher than the tensile strength. Upon reaching this stress, the material breaks, which corresponds to the point at which necking occurs for ductile materials. Necking is the localisation of deformation and the material breaks at the point of necking. Less brittle materials such as high carbon steels do not neck or rather they fracture before they undergo necking. However, for very brittle materials such as glass and ceramics at room temperature, necking does not occur but the material breaks without warning. In other words, such materials do not yield. As such, such materials have tensile strength but they do not have yield strengths (ASM International, 2004). The other important mechanical property of material is ductility. This is calculated from the stress-strain curve as the percent elongation, which is the ratio of the extension to the original length expressed as a percentage. Ductility is also calculated as percentage reduction in area of the specimen, which is the ratio of reduction in cross-sectional area to the original cross-sectional area expressed in percentage. Ductile materials will show high values of percent elongation and reduction in cross-sectional area while less ductile materials will have lower values. In short, ductility measures the extent to which a material undergoes plastic deformation upon the application of tensile force. Hardness Tests The other set of destructive tests is hardness tests. These tests are done to determine the hardness property of materials. Hardness is defined as the property of a material that enables it to resist permanent indentation (Kakani & Kakani, 2004). Hardness also refers to resistance to cutting, scratching and abrasion. There are different types of hardness tests. They include Rockwell hardness test, micro-hardness test, Vickers hardness test, Rockwell superficial hardness test, Scleroscope test, Moh’s hardness test and Brinell hardness test among others (Polzin, 2011). Therefore, unlike the previously described mechanical properties of materials, hardness values will depend on the method and procedure used to test for hardness. Moh’s hardness scale has been used for centuries now. The scale consists of 10 materials ranked with the order of hardness. In the scale, diamond is ranked highest in hardness (number 10) while talc is ranked lowest (1) (Polzin, 2011). Each material will scratch all those materials ranking lower. These standards materials are then used to test the hardness of other materials. The test involves determining which of these standard materials will scratch or be scratched by the test material. This way, the test material can be given a hardness value. However, this method does not produce quantitative values. More reliable test methods include Rockwell hardness test, Vickers hardness test, Rockwell superficial hardness test and Brinell hardness test (Polzin, 2011). The Rockwell hardness test involves using a diamond cone or an indenter made of hardened steel to indent the test material. Usually, the process starts with a minor load of 10kgf until equilibrium is reached when no further indentation is achieved. At this point the indicating device is set to a datum position. The indicating device tracks the movement of the indenter. Therefore, the indicating device measures the depth through which the indenter moves into the test material. An additional force above the preliminary force is applied. This results to an increase in the indentation depth. Equilibrium will be reached when the constant load does not produce any further indentation depth. One the equilibrium is reached, the additional force is removed so that the preliminary load remains in application. This action allows the test specimen to recover to some extent so that the depth of penetration reduces. The Rockwell hardness number of the material is then determined by deducting the minor penetration resulting from the initial penetration due to the application of the preliminary load from the major penetration resulting from the addition of the major load. In other words, the Rockwell hardness number is the permanent penetration resulting from the application of the major load (Polzin, 2011). Rockwell hardness scale has been developed that categorises materials from A to V. Materials in class A have higher hardness number than materials in lower classes. This scale shows the application of materials with different Rockwell hardness numbers. For example, class A materials include case hardened steels and cemented carbides used in places requiring high hardness values such as bearings. Materials in class V are very soft materials such as plastics (Polzin, 2011). Other Tests Other tests include torsion test, shear test and compression test. Compression test is the opposite of tension test because a uni-axial compressive force is applied to the specimen. The aim of conducting the compression test is to determine the behaviour of the material under compression. The main material property determined through compression test is the bearing capacity of the material. Compression tests are usually done on concrete specimens to determine how strong the concrete mix is under compression. The specimen is subjected to compression force until it breaks. The maximum force is the used to determine bearing capacity by dividing the force with the cross-section area of the specimen (Carino, 2006). Shear tests are done to determine the behaviour of material under shear force. The primary mechanical property under consideration when conducting shear test is the shear strength. It is defined as the resistance of a material to shear failure or sliding failure (Meyer & Halle, 2011). Finally, torsion test involves subjecting a material specimen under a twisting force and determining the resistance of the material to twisting. This test is usually done for materials that engineers intend to use as shafts. A specimen of the material is fixed to a torsion testing machine that exerts controlled torsion load until the specimen fails. The angle of twist can be measured directly. The shear modulus of elasticity (G) of the material can be determined from the test results. The idea is to select a material of a given diameter and length to facilitate the transfer of the required power without twisting. The angle of twist is directly proportional to the torque and the length of the shaft while it is inversely proportional to the diameter and the shear modulus of elasticity (Singh, 2007). Therefore, the idea is to select a shaft of the right length, cross-sectional area and shear modulus of elasticity to handle the required toque without exceeding the allowable twist. Once the shear modulus of elasticity of the material is determined, then it is possible to play with the other number depending on design requirements. Factors Affecting the Mechanical Properties of Materials Material Structure Mechanical properties of materials are, in general, structure sensitive (Sharma, 2004). This implies that the mechanical properties of materials depend on the microstructure of a material. Indeed, Sharma (2004) notes that of all the mechanical properties of materials, only the modulus of elasticity is structure insensitive (does not depend on the microstructure of the material). In particular, grain size greatly affects the mechanical properties of materials. Fine grained materials are made of small grains while coarse grained materials are made of coarse grains. In general, fine grained materials have higher values of tensile strength and fatigue strength. Indeed, Khurmi and Sedha (2008) note that the material strength is inversely proportional to the square root of its grain size. This means that the bigger the grains, the less strong the material. Temperature Why do blacksmiths heat metals to high temperatures before trying to deform them to the desired shape? It will be noted that trying to deform a cold metal will result fractures. On the other hand, if a metal is heated before it is worked on, it is easy to deform it. In addition, the metal does not fracture. Heat treatment enhances ductility thereby enabling the material to be deformed easily (Kakani & Kakani, 2004). In turn, it increases workability of the material. Heat treatment also increases plasticity thereby reducing the elastic strength of the material (Khurmi & Sedha, 2008). Indeed, heating a plastic makes it easy to deform it because it is easy to break the bonds and set up new arrangements. Heat treatment also improves hardness and shock resistance (Khurmi & Sedha, 2008). That is why it is important to note the room temperature when performing tests to determine mechanical properties of materials. These values may vary significantly due to differences in testing temperatures. Atmospheric Exposure The atmosphere contains gases and water which interact with surfaces of materials to alter the mechanical properties of materials. Most metals react with water, oxygen and other gases such as carbon dioxide and sulphur dioxide (Kakani & Kakani, 2004). The reaction may enhance or interfere with the surface of the metal. For example, most metals react with oxygen and in doing so form a film on their surface, which may increase surface hardness. Reaction with water causes rust on iron, which in turn negatively affects mechanical properties of iron such as hardness and strength (Kakani & Kakani, 2004). Treatment Heat treatment of some metals especially steel has been shown to alter the mechanical properties of the material (Kakani & Kakani, 2004). The resultant is the production of different alloys with different mechanical properties (Khurmi & Sedha, 2008). For example, hot rolled 1015 carbon steel has a tensile strength of 345 MPa compared to 385 of cold drawn 1015 carbon steel (Davis, 2005). The addition of carbon, nitrogen, manganese, molybdenum, chromium, nickel and silicon increases the mechanical properties of steel including strength and surface hardness (Davis, 2005). However, this treatment reduces the machinability property of the metal. As such, the treatment is usually as the last step of the production process. This surface treatment is required for applications where high wear is unavoidable such as in making gears and bearings. High carbon steels have less ductility values than low carbon steels (Davis, 2005). High carbon steels are used where low ductility values are required such as in shafts where they are required to transmit heavy loads without twisting (Khurmi & Sedha, 2008). Conclusion The knowledge of the mechanical properties of materials, methods of determining and evaluating the different mechanical properties of materials and the factors affecting the mechanical properties of materials is fundamental to the success of any engineer. Important mechanical properties of materials that have been explained in this essay include tensile strength, yield strength, modulus of elasticity, hardness, ductility, and toughness. These properties are determined by performing destructive tests on material specimens. Tension test is the most common destructive test, which is used to determine tensile strength, yield strength, bearing capacity, modulus of elasticity and ductility. Hardness tests are used to determine the hardness property of a material. Compression test helps engineers to understand the behaviour of the material under compression. The test is very common in civil engineering especially in determining the bearing capacity of concrete mixes. Other common tests include torsion test and shear test. Factors that affect the mechanical properties of materials include material structure, temperature, material treatment and atmospheric exposure. References ASM International. (2004). Introduction to Tensile Testing. Retrieved from https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=20&cad=rja&uact=8&ved=0CJUBEBYwE2oVChMImNWakcbLyAIVTOkUCh3bPQGd&url=http%3A%2F%2Fwww.asminternational.org%2Fdocuments%2F10192%2F3465262%2F05105G_Chapter_1.pdf%2Fe13396e8-a327-490a-a414-9bd1d2bc2bb8&usg=AFQjCNHqMQlb7T4hOn-vfqzssHBez0GcKQ&sig2=XJ5A-pfu6aofQ-lt_ZMMFA Carino, N. J. (2006). Prediction of Potential Concrete Strength of Later Ages. In J. F Lamond (Ed.). Significance of Tests and Properties of Concrete and Concrete-making Materials, Issue 169, Part 4 (141-153). West Conshohocken, PA: ASTM International. Davis, J. R. (2005). Gear Materials, Properties, and Manufacturing. Materials Park, OH: ASM International. Kakani, S. L. & Kakani, A. (2004). Material Science. New Delhi, India: New Age International (P) Limited, Publishers. Khurmi, R. S. & Sedha, R. S. (2008). Materials Science. New Delhi, India: S. Chand & Company Ltd. Meyer, L. W. & Halle, T. (2011). Shear Strength and Shear Failure, Overview of Testing and Behaviour of Ductile Metals. Mechanics of Time-Dependent Materials, 15(4), 327-340. Polzin, T. (2011). Hardness Measurement of Metals-Static Methods. In K. Herrmann (Ed.). Hardness Testing: Principles and Applications (25-66). Materials Park, OH: ASM International. Sharma, C. P. (2004). Engineering Materials: Properties and Applications of Metals and Alloys. New Delhi, India: Prentice-Hall of India Private Limited. Singh, A. K. (2007). Mechanics of Solids. New Delhi, India: Prentice-Hall of India Private Limited. Appendix Appendix 1: Tensile Test Specimen Source: ASM International (2004) Appendix 2: Universal Testing Machine Source: ASM International (2004) Appendix 3: (a) Load-Extension Curve and (b) Stress-Strain Curve Source: ASM International (2004) Appendix 4: Upper and Lower Yield Strength Source: ASM International (2004) Read More