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TRANSCRIPT
Introduction
The mechanical behavior of a material reflects the relationship between its response or deformation to an applied load or force. Therefore, The importance of determine materials characteristics is to make sure that any resulting deformation will not be excessive and fracture will not occur and to design the member from which it is made, where many materials, when in service, are subjected to forces or loads; examples include the aluminum alloy from which an airplane wing is constructed and the steel in an automobile axle. Important mechanical properties are strength, hardness, ductility, and stiffness.
The main factors to be considered include: The nature of the applied load and duration as well as the environmental conditions.
Mechanical properties are of concern to a variety of parties (e.g., producers and consumers of materials, research organizations, government agencies) that have differing interests. Consequently, it is imperative that there be some consistency in the manner in which tests are conducted, and in the interpretation of their results. This consistency is accomplished by using standardized testing techniques. Establishment
and publication of these standards are often coordinated by professional
societies. In the United States the most active organization is the American Society for Testing and Materials (ASTM). Its Annual Book of ASTM Standards comprises numerous volumes, which are issued and updated yearly
Concepts of Stress and Strain
To understand and describe how materials deform (elongate, compress, twist) or break as a function of applied load, time, temperature, and other conditions we need first to discuss standard test methods and standard language for mechanical properties of materials.
If a load is static or changes relatively slowly with time and is applied uniformly over a cross section or surface of a member, the mechanical behavior may be ascertained by a simple stress–strain test; these are most commonly conducted for metals at room temperature. There are three principal ways in which a load may be applied: namely, tension, compression, and shear (Figures 24a, b, c). In engineering practice many loads are torsional rather than pure shear; this type of loading is illustrated in Figure 24d.
Figure 24. Types of loading (a) Schematic illustration of how a tensile load produces an elongation and positive linear strain. Dashed lines represent the shape before deformation; solid lines, after deformation. (b) Schematic illustration of how a compressive load produces contraction and a negative linear strain. (c) Schematic representation of shear strain γ, where γ = tan θ. (d) Schematic representation of torsional deformation (i.e., angle of twist ) produced by an applied torque T.
Therefore:
Stress is ratio of force applied on the system to the per unit area of the body.
Strain is relative change in shape or size of an object due to externally applied forces, it is dimensionless (has no units).
Tension and Compression
Nearly all materials can be tested in tension include metals, plastics, woods, polymers and textiles. Materials with high compressive strength values have relatively low tensile strength, such as brick and aerospace composites. These are not generally tested in tension as their applications do not normally require them to withstand tensile loads.
Tension tests
Purpose of tensile testing:
Generally a tensile test is designed to be run until the sample fails or breaks under the load. The values that may be measured from this type of test like tensile strength, ultimate strength, elongation, modulus of elasticity, yield strength, Poisson’s ratio, and strain hardening. The measurements taken during the test reveals the characteristics of a material while it is under a tensile load.
Tensile specimen
A tensile specimens is a standardized sample cross-section. It has two shoulders and a gage (section) in between. The shoulders are large so they can be readily gripped, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area. Specimen of a standard shape (dog-bone) and dimensions (prepared according to ASTM.
Figure 25. standard tensile specimen with circular cross section.
Tensile Test Machines
Tensile test machines are universal testing machines specially configured to evaluate tensile strength of specimens. Each tensile test machine is configured to your testing needs by our application engineers with the correct controller, grips. Due to our modular machine design, your tensile tester can also be equipped to perform other applications such as compression, cyclic, shear, flexure, bend, peel, and tear just by adding fixtures.
Figure 26. Schematic representation of the apparatus used to conduct tensile stress–strain tests. The specimen is elongated by the moving crosshead; load cell and extensometer measure, respectively, the magnitude of the applied load and the elongation.
Procedure of Tensile Test
1- Care is to be taken to ensure that the specimens did not have any notching or cracks from manufacturing or any surface defects that would adversely affect the tensile tests.
2- Before loading the specimens in the machine, the computer system connected to the machine was set up by inputting the necessary information of gauge length and width of the specimen. The computer system was then prepared to record data and output necessary load-deflection graphs.
3- The specimens were loaded into the machine, and a tensile test was performed. The data was recorded electronically
A tensile load is applied to the specimen until it fractures. During the test,
the load required to make a certain elongation on the material is recorded. A load elongation curve is plotted by an x-y recorder, so that the tensile behavior of the material can be obtained. An engineering stress-strain curve can be constructed from this load-elongation curve by making the required calculations. Then the mechanical parameters that we search for can be found by studying on this curve.
A typical engineering stress-strain diagram and the significant parameters
1- Stress = P/Ao ( Load/Initial cross-sectional area)
2- Strain = = l/lo (Elongation/Initial gage length)
3- Elastic Region: The part of the stress-strain curve up to the yielding point.
4- Elastic deformation is recoverable. In the elastic region, stress and strain are related to each other linearly.
Hooke’s Law: = E
The linearity constant E is called the elastic modulus which is specific for each type of material.
5- Plastic Region: The part of the stress-strain diagram after the yielding point.
6- Maximum point of the stress-strain diagram (σUTS), necking starts.
UTS = Pmax/Ao
7- Yield Strength is the stress level at which plastic deformation starts. The beginning of first plastic deformation is called yielding. It is an important parameter in design.
Figure 27. Typical engineering stress-strain behavior to fracture, point F. The tensile strength is indicated at point M. The circular insets represent the geometry of the deformed specimen at various points along the curve.
The Stress-Strain Curve
Ductile Material Stress-Strain Curve
In this ductile material curve, you can see a point labeled yield strength, also known as yield point. The dip in the curve at this point is an indication that the material has yielded or deformed. After the load is removed, this deformation will be permanent. Before this point, the material is elastic. Elastic materials deform but return to the original shape after the load is removed.
Brittle Material Stress-Strain Curve
In the brittle material curve, a yield strength or yield point is the same as the fracture point. The brittle material curve reveals that the material fractures or breaks instead of bending when the force is high enough.
Compression Test
A compression test is any test in which a material experiences opposing forces that push inward upon the specimen from opposite sides or is otherwise compressed, “squashed”, crushed, or flattened. The test sample is generally placed in between two plates that distribute the applied load across the entire surface area of two opposite faces of the test sample and then the plates are pushed together by a universal test machine causing the sample to flatten. A compressed sample is usually shortened in the direction of the applied forces and expands in the direction perpendicular to the force. A compression test is essentially the opposite of the more common tension test.
Purpose of Compression Tests
The goal of a compression test is to determine the behavior or response of a material while it experiences a compressive load by measuring fundamental variables, such as, strain, stress, and deformation. By testing a material in compression the compressive strength, yield strength, ultimate strength, elastic limit, and the elastic modulus among other parameters may all be determined. With the understanding of these different parameters and the values associated with a specific material it may be determined whether or not the material is suited for specific applications or if it will fail under the specified stresses.
Types of Compression Testing materials:
Typically materials subjected to compression testing have a compressive strength generally accepted to be high and a tensile strength (e.g tensile test ) that is considered to be of a lower value. Almost all materials can experience compressive forces in one way or another depending upon their application, but the most common materials are composites, concretes, wood, stone, brick, mortars, grouts, polymers, plastics, foam and metals among many others.
Machines used for compression testing are basically similar to those used for tensile testing often the same machine can be used to perform both tests.
Shape of the specimen: The shape of the machine to be used for the different materials are as follows:
(i) For metals and certain plastics: The specimen may be in the form of a cylinder
(ii) For building materials: Such as concrete or stone the shape of the specimen may be in the form of a cube.
Shape of stress stain diagram
(a) Ductile materials: For ductile material such as mild steel, the load vs compression diagram would be as follows:
(1) The ductile materials such as steel, Aluminum, and copper have stress – strain diagrams similar to ones which we have for tensile test, there would be an elastic range which is then followed by a plastic region.
(2) The ductile materials (steel, Aluminum, copper) proportional limits in compression test are very much close to those in tension.
(3) In tension test, a specimen is being stretched, necking may occur, and ultimately fracture fakes place. On the other hand when a small specimen of the ductile material is compressed, it begins to bulge on sides and becomes barrel shaped as shown in the figure above. With increasing load, the specimen is flattened out, thus offering increased resistance to further shortening ( which means that the stress – strains curve goes upward ) this effect is indicated in the diagram.
Brittle materials ( in compression test )
Brittle materials in compression typically have an initial linear region followed by a region in which the shortening increases at a higher rate than does the load. Thus, the compression stress – strain diagram has a shape that is similar to the shape of the tensile diagram.
However, brittle materials usually reach much higher ultimate stresses in compression than in tension.
For cast iron, the shape may be like this
Brittle materials in compression behave elastically up to certain load, and then fail suddenly by splitting or by craking in the way as shown in figure. The brittle fracture is performed by separation and is not accompanied by noticeable plastic deformation.
SHEAR AND TORSIONAL TESTS4
For tests performed using a pure shear force as shown in Figure 24c, the shear stress τ is computed according to
where F is the load or force imposed parallel to the upper and lower faces, each of which has an area of A0 . The shear strain γ is defined as the tangent of the strain angle θ, as indicated in the figure. The units for shear stress and strain are the same as for their tensile counterparts.
Torsion is a variation of pure shear, wherein a structural member is twisted in the manner of Figure 24d; torsional forces produce a rotational motion about the longitudinal axis of one end of the member relative to the other end. Examples of torsion are found for machine axles and drive shafts, and also for twist drills. Torsional tests are normally performed on cylindrical solid shafts or tubes. A shear stress τ is a function of the applied torque T, whereas shear strain γ is related to the angle of twist, in Figure 24d.
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The mechanical behavior of a material reflects the relationship between its response or deformation to an applied load or force. Therefore, The importance of determine materials characteristics is to make sure that any resulting deformation will not be excessive and fracture will not occur and to design the member from which it is made, where many materials, when in service, are subjected to forces or loads; examples include the aluminum alloy from which an airplane wing is constructed and the steel in an automobile axle. Important mechanical properties are strength, hardness, ductility, and stiffness.
The main factors to be considered include: The nature of the applied load and duration as well as the environmental conditions.
Mechanical properties are of concern to a variety of parties (e.g., producers and consumers of materials, research organizations, government agencies) that have differing interests. Consequently, it is imperative that there be some consistency in the manner in which tests are conducted, and in the interpretation of their results. This consistency is accomplished by using standardized testing techniques. Establishment
and publication of these standards are often coordinated by professional
societies. In the United States the most active organization is the American Society for Testing and Materials (ASTM). Its Annual Book of ASTM Standards comprises numerous volumes, which are issued and updated yearly
Concepts of Stress and Strain
To understand and describe how materials deform (elongate, compress, twist) or break as a function of applied load, time, temperature, and other conditions we need first to discuss standard test methods and standard language for mechanical properties of materials.
If a load is static or changes relatively slowly with time and is applied uniformly over a cross section or surface of a member, the mechanical behavior may be ascertained by a simple stress–strain test; these are most commonly conducted for metals at room temperature. There are three principal ways in which a load may be applied: namely, tension, compression, and shear (Figures 24a, b, c). In engineering practice many loads are torsional rather than pure shear; this type of loading is illustrated in Figure 24d.
Figure 24. Types of loading (a) Schematic illustration of how a tensile load produces an elongation and positive linear strain. Dashed lines represent the shape before deformation; solid lines, after deformation. (b) Schematic illustration of how a compressive load produces contraction and a negative linear strain. (c) Schematic representation of shear strain γ, where γ = tan θ. (d) Schematic representation of torsional deformation (i.e., angle of twist ) produced by an applied torque T.
Therefore:
Stress is ratio of force applied on the system to the per unit area of the body.
Strain is relative change in shape or size of an object due to externally applied forces, it is dimensionless (has no units).
Tension and Compression
Nearly all materials can be tested in tension include metals, plastics, woods, polymers and textiles. Materials with high compressive strength values have relatively low tensile strength, such as brick and aerospace composites. These are not generally tested in tension as their applications do not normally require them to withstand tensile loads.
Tension tests
Purpose of tensile testing:
Generally a tensile test is designed to be run until the sample fails or breaks under the load. The values that may be measured from this type of test like tensile strength, ultimate strength, elongation, modulus of elasticity, yield strength, Poisson’s ratio, and strain hardening. The measurements taken during the test reveals the characteristics of a material while it is under a tensile load.
Tensile specimen
A tensile specimens is a standardized sample cross-section. It has two shoulders and a gage (section) in between. The shoulders are large so they can be readily gripped, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area. Specimen of a standard shape (dog-bone) and dimensions (prepared according to ASTM.
Figure 25. standard tensile specimen with circular cross section.
Tensile Test Machines
Tensile test machines are universal testing machines specially configured to evaluate tensile strength of specimens. Each tensile test machine is configured to your testing needs by our application engineers with the correct controller, grips. Due to our modular machine design, your tensile tester can also be equipped to perform other applications such as compression, cyclic, shear, flexure, bend, peel, and tear just by adding fixtures.
Figure 26. Schematic representation of the apparatus used to conduct tensile stress–strain tests. The specimen is elongated by the moving crosshead; load cell and extensometer measure, respectively, the magnitude of the applied load and the elongation.
Procedure of Tensile Test
1- Care is to be taken to ensure that the specimens did not have any notching or cracks from manufacturing or any surface defects that would adversely affect the tensile tests.
2- Before loading the specimens in the machine, the computer system connected to the machine was set up by inputting the necessary information of gauge length and width of the specimen. The computer system was then prepared to record data and output necessary load-deflection graphs.
3- The specimens were loaded into the machine, and a tensile test was performed. The data was recorded electronically
A tensile load is applied to the specimen until it fractures. During the test,
the load required to make a certain elongation on the material is recorded. A load elongation curve is plotted by an x-y recorder, so that the tensile behavior of the material can be obtained. An engineering stress-strain curve can be constructed from this load-elongation curve by making the required calculations. Then the mechanical parameters that we search for can be found by studying on this curve.
A typical engineering stress-strain diagram and the significant parameters
1- Stress = P/Ao ( Load/Initial cross-sectional area)
2- Strain = = l/lo (Elongation/Initial gage length)
3- Elastic Region: The part of the stress-strain curve up to the yielding point.
4- Elastic deformation is recoverable. In the elastic region, stress and strain are related to each other linearly.
Hooke’s Law: = E
The linearity constant E is called the elastic modulus which is specific for each type of material.
5- Plastic Region: The part of the stress-strain diagram after the yielding point.
6- Maximum point of the stress-strain diagram (σUTS), necking starts.
UTS = Pmax/Ao
7- Yield Strength is the stress level at which plastic deformation starts. The beginning of first plastic deformation is called yielding. It is an important parameter in design.
Figure 27. Typical engineering stress-strain behavior to fracture, point F. The tensile strength is indicated at point M. The circular insets represent the geometry of the deformed specimen at various points along the curve.
The Stress-Strain Curve
Ductile Material Stress-Strain Curve
In this ductile material curve, you can see a point labeled yield strength, also known as yield point. The dip in the curve at this point is an indication that the material has yielded or deformed. After the load is removed, this deformation will be permanent. Before this point, the material is elastic. Elastic materials deform but return to the original shape after the load is removed.
Brittle Material Stress-Strain Curve
In the brittle material curve, a yield strength or yield point is the same as the fracture point. The brittle material curve reveals that the material fractures or breaks instead of bending when the force is high enough.
Compression Test
A compression test is any test in which a material experiences opposing forces that push inward upon the specimen from opposite sides or is otherwise compressed, “squashed”, crushed, or flattened. The test sample is generally placed in between two plates that distribute the applied load across the entire surface area of two opposite faces of the test sample and then the plates are pushed together by a universal test machine causing the sample to flatten. A compressed sample is usually shortened in the direction of the applied forces and expands in the direction perpendicular to the force. A compression test is essentially the opposite of the more common tension test.
Purpose of Compression Tests
The goal of a compression test is to determine the behavior or response of a material while it experiences a compressive load by measuring fundamental variables, such as, strain, stress, and deformation. By testing a material in compression the compressive strength, yield strength, ultimate strength, elastic limit, and the elastic modulus among other parameters may all be determined. With the understanding of these different parameters and the values associated with a specific material it may be determined whether or not the material is suited for specific applications or if it will fail under the specified stresses.
Types of Compression Testing materials:
Typically materials subjected to compression testing have a compressive strength generally accepted to be high and a tensile strength (e.g tensile test ) that is considered to be of a lower value. Almost all materials can experience compressive forces in one way or another depending upon their application, but the most common materials are composites, concretes, wood, stone, brick, mortars, grouts, polymers, plastics, foam and metals among many others.
Machines used for compression testing are basically similar to those used for tensile testing often the same machine can be used to perform both tests.
Shape of the specimen: The shape of the machine to be used for the different materials are as follows:
(i) For metals and certain plastics: The specimen may be in the form of a cylinder
(ii) For building materials: Such as concrete or stone the shape of the specimen may be in the form of a cube.
Shape of stress stain diagram
(a) Ductile materials: For ductile material such as mild steel, the load vs compression diagram would be as follows:
(1) The ductile materials such as steel, Aluminum, and copper have stress – strain diagrams similar to ones which we have for tensile test, there would be an elastic range which is then followed by a plastic region.
(2) The ductile materials (steel, Aluminum, copper) proportional limits in compression test are very much close to those in tension.
(3) In tension test, a specimen is being stretched, necking may occur, and ultimately fracture fakes place. On the other hand when a small specimen of the ductile material is compressed, it begins to bulge on sides and becomes barrel shaped as shown in the figure above. With increasing load, the specimen is flattened out, thus offering increased resistance to further shortening ( which means that the stress – strains curve goes upward ) this effect is indicated in the diagram.
Brittle materials ( in compression test )
Brittle materials in compression typically have an initial linear region followed by a region in which the shortening increases at a higher rate than does the load. Thus, the compression stress – strain diagram has a shape that is similar to the shape of the tensile diagram.
However, brittle materials usually reach much higher ultimate stresses in compression than in tension.
For cast iron, the shape may be like this
Brittle materials in compression behave elastically up to certain load, and then fail suddenly by splitting or by craking in the way as shown in figure. The brittle fracture is performed by separation and is not accompanied by noticeable plastic deformation.
SHEAR AND TORSIONAL TESTS4
For tests performed using a pure shear force as shown in Figure 24c, the shear stress τ is computed according to
where F is the load or force imposed parallel to the upper and lower faces, each of which has an area of A0 . The shear strain γ is defined as the tangent of the strain angle θ, as indicated in the figure. The units for shear stress and strain are the same as for their tensile counterparts.
Torsion is a variation of pure shear, wherein a structural member is twisted in the manner of Figure 24d; torsional forces produce a rotational motion about the longitudinal axis of one end of the member relative to the other end. Examples of torsion are found for machine axles and drive shafts, and also for twist drills. Torsional tests are normally performed on cylindrical solid shafts or tubes. A shear stress τ is a function of the applied torque T, whereas shear strain γ is related to the angle of twist, in Figure 24d.
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