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Mechanical Testing.pptx

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Ajith Damodaran

The document discusses various mechanical property tests used to characterize materials including tensile tests, hardness tests, and impact tests. It provides details on how these tests are conducted and the types of properties that can be determined from the test results, such as strength, stiffness, ductility, and toughness. Both destructive and non-destructive methods are covered. Specific tests discussed in detail include tensile testing, Brinell hardness testing, Rockwell hardness testing, Charpy/Izod impact testing, and wear testing.

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Unit II
Mechanical properties are relevant for engineering applications. Mechanical Property Test Strength Tensile / Compresssion / Shear Stiffness Slope of Stress-vs-Strain curve Hardness Rockwell / Brinell / Vickers / Shore Toughness Impact: Charpy / Izod
Destructive and Non-Destructive Testing Destructive Testing requires destroying the specimen in order to measure the property. Often requires a specially prepared specimen. (e.g. Tensile test). Destructive testing is called mechanical testing. Non-Destructive Testing (NDT)measures attributes of the specimen without damaging it. Does not normally need a prepared specimen. Typically used to find flaws inside a part. Ultrasonic Weld Inspection Tensile Test specimens
Tensile Test Hardness Tests Impact Tests Creep Fatigue Wear Other Mechanical Tests
Tensile Test The tensile test pulls a test-piece until it breaks. Both force and extension are continuously measured. The specimen has thicker ends for attaching by grippers/collet/thread/s houlder.
What is Tensile tester? A typical tensile testing machine consists of a load cell, crosshead, extensometer, specimen grips, electronics and a drive system. It is controlled by testing software used to define machine and safety settings and store test parameters specified by testing standards such as ASTM and ISO. The amount of force applied to the machine and the elongation of the specimen are recorded throughout the test. Measuring the force required to stretch or elongate a material to the point of permanent deformation or break helps designers and manufacturers predict how materials will perform when implemented for their intended purpose.
Industry standards Industry standards such as ASTM E8 for metals, ASTM D638 for plastics, ASTM D412 for elastomers, and many more
Force vs extension diagram https://www.mtu.edu/materials/k 12/experiments/tensile/
Tensile test of Polymer
Information that can be determined from the Stress/Strain curve 1. Ductility 2. Elongation 3. Engineering strain 4. Strength 5. UTS 6. YS 7. Stiffness 1. Hardness? Not directly, but correlates with strength. E.g. High strength
Stiffness vs Density Stiff materials tend to be heavy. Composites have fairly high stiffness but almost as light as polymers.
Stress grade on bolts Determines its mechanical properties, strength, and suitability for specific applications. The stress grade is often represented by a set of numbers or alphanumeric characters that provide information about the bolt's tensile strength and other properties. 4.6 = 400 Mpa and 60% YS 8.8 = 800 Mpa and 80% YS 10.9 = 1000 Mpa and 90% YS
Force increased beyond ultimate stress of turbine blades, causing catastrophic failure.
Tensile strength
Test data Analysis Aluminium karabiner. 28kN closed, 10kN open, 8kN sideways
Ductility Ductility: this is the ability of a material to deform without breaking. The opposite to ductile is Brittle. (Like glass) Ductility allows forming processes (like pressing, wire drawing) Measured as percent elongation: How far it has stretched compared to the original length.
Ductility Plasticity: Permanent deformation Ductility: Tensile plasticity Malleability: Compressive plasticity
Assignemnt 1 Select a suitable material for the following requirement: House construction Furniture Electrical control panel Justify your answer with the prominent material properties.
Hardness test Resistance to indentation/abrasion Resistance to penetration
Brinell 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, The Brinell hardness number (H) is found from: H = load P area of curved surface of the impression
Brinell Hardness Number (HB) Figure: The relationship between ball diameter D, depth of impression h and dimensions of the test-piece in the Brinell test
Example In testing a piece of steel, we can use a 10 mm ball in conjunction with a 3000 kgf load, a 5 mm ball with a 750 kgf load or 1 mm ball with a 30 kgf load.
Test standards Brinell test methods are defined in the following standards- ASTM E10 ISO 6506 JIS Z 2243
Limitations Not suitable for very hard materials
The Vickers Diamond hardness test This test uses a square-based diamond pyramid as the indentor. One great advantage of this is that all the impressions will be the same square shape, regardless of how big an indentation force is used
Vickers hardness number HD = load P area of curved surface of the impression 諮 = 1.8554 2 F is the applied load in kg, and D is the mean diagonal length in mm.
Advantages The hardness values for very hard materials (above an index of 500) are likely to be more accurate than the corresponding Brinell numbers A diamond does not deform under high pressure to the same extent as does a steel ball, and so the result will be less uncertain. The impression made by the diamond is generally much smaller than that produced by the Brinell indentor, a smoother surface finish is required on the test-piece. Surface damage is negligible, making the Vickers test more suitable for testing finished components.
Rockwell hardness Test It is particularly useful for rapid routine testing of finished material, since the hardness number is indicated on a dial, and no subsequent measurement of the diameter is involved.
Rockwell hardness
Rockwell hardness number The Rockwell hardness number is given by: Rockwell hardness = E - h where E is a constant determined by the form of the indentor; for a diamond cone indentor E is 100, for a steel ball 130 Standards (BS 891: Rockwell Hardness Test; BS 4175: Rockwell Superficial Hardness Test).
Principle of Rockwell hardenss Rockwell hardness test measures the permanent depth of indentation on the material by applying a fixed load using an indenter. The smaller the indentation value, the harder is the material. The Rockwell hardness test follows the principle of the differential-depth method. Here, the indenter makes a residual depth called the indent and it is measured. The total test force is applied in two stages to eliminate errors caused due to the roughness of the surface and measurement.
Rockwell hardness The Rockwell machine is very rapid in action, and can be used by relatively unskilled operators. Since the size of the impression is also very small, it is particularly useful for the routine testing of stock
Questions What are the metals on which Rockwell hardness test can be conducted? The Rockwell hardness test can be used to determine the hardness of metals like aluminum, thin steel, lead, iron, titanium, copper alloys, and cemented carbides.
How to determine Rockwell hardness number as per ASTM E18? As per ASTM E18, the indenters can either be diamond spheroconical or tungsten carbide balls. If a diamond spheroconical indenter is used, then Rockwell Hardness Number = 100 - (h/0.002) If a ball indenter is used, then Rockwell Hardness Number = 130 - (h/0.002) Where 'h' is the indentation depth in mm.
Knoop Hardness test The Knoop hardness test method is one of the microhardness tests tests for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. The Knoop and Vickers techniques are called micro- indentation-testing methods based on indenter size. Both are well suited for measuring the hardness of small, selected specimen regions; furthermore, the Knoop technique is used for testing brittle materials such as ceramics.
The Shore scleroscope (Dynamic test) This is a small portable instrument which can be used for testing the hardness of large components such as rolls, drop-forgings, dies, castings and gears. The scleroscope embodies a small diamond-tipped 'tup', or hammer, of mass approximately 2.5 g, which is released so that it falls from a standard height of about 250 mm inside a graduated glass tube placed on the test surface. The height of rebound is taken as the hardness index
Shore Durometer Used for rubber and plastic materials
Wear Resistance Wear characteristics refer to the behavior and properties of a material or surface when it comes into contact with another surface or abrasive medium, resulting in the removal or deformation of material from the contacting surfaces. Wear is a natural and that occurs in various mechanical systems, such as machinery, tools, automotive components, and industrial equipment. Understanding wear characteristics is crucial for designing materials, coatings, and lubrication systems that can minimize wear and extend the lifespan of components.
Wear test- Hip joint simulator
Impact Test These tests are used to indicate the toughness of a material, and particularly its capacity for resisting mechanical shock. Brittleness, resulting from a variety of causes, is often not revealed during a tensile test. The test specimen struck by a fast moving hammer and the energy that is absorbed in fracturing the test piece is measured.
Charpy impact test For Izod tests, the pendulum is released from the lower position, to give a striking energy of 170 J and for the Charpy test it is released from the upper position to give a striking energy of 300 J. The scale carries a set of graduations for each test
The Izod Impact Testing This test employs a standard notched test- piece which is clamped firmly in a vice. The striking energy is approximately 163 J. The test-piece is notched so that there is an initial 'crack' to initiate fracture.
Test Standards The ASTM E23 standard describes Charpy and Izod impact tests on notched bar metal specimens Izod Test Charpy Test
Ductile and brittle fracture In ductile facture, failure is preceded by a considerable amount of plastic deformation of the material. In brittle fracture no plastic deformation prior to failure
Charpy Impact Test This test is of continental origin, and differs from the Izod test in that the test-piece is supported at each end (Figure 3.12); whereas the Izod test uses a test- piece held cantilever fashion. Here the load on the pendulum can be varied so that the impact energy is either 150 J or 300 J.
Mechanical Properties summary Strength (Mpa) : Ability to endure stress the intensity of force. Strain () Elastic = Elongation (mm) / Original Length (mm) Elongation (%) Plastic = Elongation (mm) / Original Length (mm) Stiffness (Mpa) : Stress to cause strain = Stress (Mpa) / Strain () Toughness (J) : Energy to break Hardness () : Resistance to indentation / abrasion.
Ashby Charts Select chart: Young's modulus - Density Young's Modulus - Cost Strength - Density Strength - Toughness Strength - Elongation Strength - Cost Strength - Max service temperature Specific stiffness - Specific strength Electrical resistivity - Cost Recycle Fraction - Cost Energy content - Cost
CREEP When stressed over a long period of time, some metals extend very gradually and may ultimately fail at a stress well below the tensile strength of the material. This phenomenon of slow but continuous extension under a steady force is known as 'creep. Such slow extension is more prevalent at high temperatures, and for this reason the effects of creep must be taken into account in the design of steam and chemical plant, gas and steam turbines and furnace equipment
Creep test Creep tests are carried out on test-pieces which are similar in form to ordinary test-pieces. A test-piece is enclosed in a thermostatically controlled electric tube furnace which can be maintained accurately at a fixed temperature over the long period of time occupied by the test. The test-piece is statically stressed, and some form of sensitive extensometer is used to measure the extremely small extensions at suitable time All creep testing is conducted in tensile mode
Creep test set up For example, for a 0.2 per cent plain carbon steel at 400属C, the stress to rupture in 1000 hours is 295 MPa; at 500属C it has dropped to 118 MPa.
Creep Variation with temperature
Creep examples
Fatigue Fatigue is gradual crack growth caused by alternating loads. If you take a paper clip and repeatedly flex it back and forth, it does not last very long before it breaks, even though you have not applied a large stress - Metal Fatigue A380 engine
S/N curve Fatigue life, at a given alternating stress level and mean stress, is the number of cycles required to cause failure due to fatigue. Specimens are tested in a series of decreasing stress levels until no failure occurs within a selected maximum number of cycles (usually 10 million cycles). The nearly horizontal portion of the curve defines the fatigue or endurance limit for the test material.
S/N curve The curve becomes horizontal at a stress which will be endured for an infinite number of reversals. This stress is the fatigue limit or endurance limit. Some non-ferrous materials do not show a well- defined fatigue limit; that is, the S/N curve slopes gradually down to the horizontal axis
Stages in Fatigue failure 1.Initiation Stage: 2.Crack Initiation: 3.Crack Propagation: 4.Final Fracture:
Important factors that influence fatigue failure include: Stress Amplitude: The difference between the maximum and minimum stress levels in each cycle. Mean Stress: The average stress level over the cyclic loading. Number of Cycles: The total number of loading cycles the material experiences before failure. Material Properties: Different materials have varying resistance to fatigue failure. Surface Finish: Smooth surfaces generally have better fatigue performance than rough ones. Environmental Factors: Corrosive environments or elevated temperatures can accelerate fatigue failure. Loading Frequency: Faster loading rates can reduce fatigue life. Overloads: Sudden increases in stress levels can accelerate crack growth. To prevent fatigue failure, engineers use various strategies: Design Considerations: Incorporating smooth transitions, fillets, and avoiding sharp corners can reduce stress concentrations. Material Selection: Choosing materials with better fatigue resistance for specific applications. Limiting Stress Ranges: Designing to keep stress amplitudes within safe limits. Surface Treatments: Processes like shot peening or nitriding can enhance the material's fatigue resistance. Inspections: Regular inspections can detect early signs of cracks and prevent catastrophic failures.
Other tests- Erichsen cupping test Ductility and suitability for deep drawing processes is tested by pressing a hardened steel ball into sheet metal. The maximum depth of penetration before rupture is the Erichsen value (in mm)
Other tests Bend tests Another test for ductility, but specific to bending or plastic fatigue. Sheet metal materials are tested for bending 180o on itself (without cracking or orange peel where grain become visible). Another test bends 90o back and forth until it fails counting the number of cycles. This is definitely plastic deformation, so not really fatigue in the engineering sense.
Compression test Ductile materials simply squash (barrel). Brittle materials often fracture at 45 degrees (due to shear stress being much lower than compressive stress). Compression is the standard test for concrete.
Online material Property resources Graphical comparison of materials properties. Ashby charts Testlopedia: Testing of plastics
Questions Steel wire of 2mm diameter can withstand 250MPa. What is the force? Sketch a load extension diagram for low carbon steel and show the following points: (a) Elastic limit (b) Yield point (c) Ultimate tensile strength A certain carbon steel has hardness of 42HRC. When hardened it is 62 HRC. Which specimen would have the greater (a) wear resistance (b) toughness (c) strength? Describe the Charpy test. Name three types of hardness test. Describe the Rockwell hardness testing machine, and how a test is done. Describe the process of a typical fatigue failure List ways to improve a components resistance to fatigue. Describe shot peening in reference to fatigue treatment. Explain creep with reference to creep curve and jet engine turbine blades.

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Mechanical Testing.pptx

  • 2. Mechanical properties are relevant for engineering applications. Mechanical Property Test Strength Tensile / Compresssion / Shear Stiffness Slope of Stress-vs-Strain curve Hardness Rockwell / Brinell / Vickers / Shore Toughness Impact: Charpy / Izod
  • 3. Destructive and Non-Destructive Testing Destructive Testing requires destroying the specimen in order to measure the property. Often requires a specially prepared specimen. (e.g. Tensile test). Destructive testing is called mechanical testing. Non-Destructive Testing (NDT)measures attributes of the specimen without damaging it. Does not normally need a prepared specimen. Typically used to find flaws inside a part. Ultrasonic Weld Inspection Tensile Test specimens
  • 4. Tensile Test Hardness Tests Impact Tests Creep Fatigue Wear Other Mechanical Tests
  • 5. Tensile Test The tensile test pulls a test-piece until it breaks. Both force and extension are continuously measured. The specimen has thicker ends for attaching by grippers/collet/thread/s houlder.
  • 6. What is Tensile tester? A typical tensile testing machine consists of a load cell, crosshead, extensometer, specimen grips, electronics and a drive system. It is controlled by testing software used to define machine and safety settings and store test parameters specified by testing standards such as ASTM and ISO. The amount of force applied to the machine and the elongation of the specimen are recorded throughout the test. Measuring the force required to stretch or elongate a material to the point of permanent deformation or break helps designers and manufacturers predict how materials will perform when implemented for their intended purpose.
  • 7. Industry standards Industry standards such as ASTM E8 for metals, ASTM D638 for plastics, ASTM D412 for elastomers, and many more
  • 8. Force vs extension diagram https://www.mtu.edu/materials/k 12/experiments/tensile/
  • 9. Tensile test of Polymer
  • 10. Information that can be determined from the Stress/Strain curve 1. Ductility 2. Elongation 3. Engineering strain 4. Strength 5. UTS 6. YS 7. Stiffness 1. Hardness? Not directly, but correlates with strength. E.g. High strength
  • 11. Stiffness vs Density Stiff materials tend to be heavy. Composites have fairly high stiffness but almost as light as polymers.
  • 12. Stress grade on bolts Determines its mechanical properties, strength, and suitability for specific applications. The stress grade is often represented by a set of numbers or alphanumeric characters that provide information about the bolt's tensile strength and other properties. 4.6 = 400 Mpa and 60% YS 8.8 = 800 Mpa and 80% YS 10.9 = 1000 Mpa and 90% YS
  • 13. Force increased beyond ultimate stress of turbine blades, causing catastrophic failure.
  • 15. Test data Analysis Aluminium karabiner. 28kN closed, 10kN open, 8kN sideways
  • 16. Ductility Ductility: this is the ability of a material to deform without breaking. The opposite to ductile is Brittle. (Like glass) Ductility allows forming processes (like pressing, wire drawing) Measured as percent elongation: How far it has stretched compared to the original length.
  • 17. Ductility Plasticity: Permanent deformation Ductility: Tensile plasticity Malleability: Compressive plasticity
  • 18. Assignemnt 1 Select a suitable material for the following requirement: House construction Furniture Electrical control panel Justify your answer with the prominent material properties.
  • 19. Hardness test Resistance to indentation/abrasion Resistance to penetration
  • 20. Brinell 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, The Brinell hardness number (H) is found from: H = load P area of curved surface of the impression
  • 21. Brinell Hardness Number (HB) Figure: The relationship between ball diameter D, depth of impression h and dimensions of the test-piece in the Brinell test
  • 22. Example In testing a piece of steel, we can use a 10 mm ball in conjunction with a 3000 kgf load, a 5 mm ball with a 750 kgf load or 1 mm ball with a 30 kgf load.
  • 23. Test standards Brinell test methods are defined in the following standards- ASTM E10 ISO 6506 JIS Z 2243
  • 24. Limitations Not suitable for very hard materials
  • 25. The Vickers Diamond hardness test This test uses a square-based diamond pyramid as the indentor. One great advantage of this is that all the impressions will be the same square shape, regardless of how big an indentation force is used
  • 26. Vickers hardness number HD = load P area of curved surface of the impression 諮 = 1.8554 2 F is the applied load in kg, and D is the mean diagonal length in mm.
  • 27. Advantages The hardness values for very hard materials (above an index of 500) are likely to be more accurate than the corresponding Brinell numbers A diamond does not deform under high pressure to the same extent as does a steel ball, and so the result will be less uncertain. The impression made by the diamond is generally much smaller than that produced by the Brinell indentor, a smoother surface finish is required on the test-piece. Surface damage is negligible, making the Vickers test more suitable for testing finished components.
  • 28. Rockwell hardness Test It is particularly useful for rapid routine testing of finished material, since the hardness number is indicated on a dial, and no subsequent measurement of the diameter is involved.
  • 30. Rockwell hardness number The Rockwell hardness number is given by: Rockwell hardness = E - h where E is a constant determined by the form of the indentor; for a diamond cone indentor E is 100, for a steel ball 130 Standards (BS 891: Rockwell Hardness Test; BS 4175: Rockwell Superficial Hardness Test).
  • 31. Principle of Rockwell hardenss Rockwell hardness test measures the permanent depth of indentation on the material by applying a fixed load using an indenter. The smaller the indentation value, the harder is the material. The Rockwell hardness test follows the principle of the differential-depth method. Here, the indenter makes a residual depth called the indent and it is measured. The total test force is applied in two stages to eliminate errors caused due to the roughness of the surface and measurement.
  • 32. Rockwell hardness The Rockwell machine is very rapid in action, and can be used by relatively unskilled operators. Since the size of the impression is also very small, it is particularly useful for the routine testing of stock
  • 33. Questions What are the metals on which Rockwell hardness test can be conducted? The Rockwell hardness test can be used to determine the hardness of metals like aluminum, thin steel, lead, iron, titanium, copper alloys, and cemented carbides.
  • 34. How to determine Rockwell hardness number as per ASTM E18? As per ASTM E18, the indenters can either be diamond spheroconical or tungsten carbide balls. If a diamond spheroconical indenter is used, then Rockwell Hardness Number = 100 - (h/0.002) If a ball indenter is used, then Rockwell Hardness Number = 130 - (h/0.002) Where 'h' is the indentation depth in mm.
  • 35. Knoop Hardness test The Knoop hardness test method is one of the microhardness tests tests for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. The Knoop and Vickers techniques are called micro- indentation-testing methods based on indenter size. Both are well suited for measuring the hardness of small, selected specimen regions; furthermore, the Knoop technique is used for testing brittle materials such as ceramics.
  • 36. The Shore scleroscope (Dynamic test) This is a small portable instrument which can be used for testing the hardness of large components such as rolls, drop-forgings, dies, castings and gears. The scleroscope embodies a small diamond-tipped 'tup', or hammer, of mass approximately 2.5 g, which is released so that it falls from a standard height of about 250 mm inside a graduated glass tube placed on the test surface. The height of rebound is taken as the hardness index
  • 37. Shore Durometer Used for rubber and plastic materials
  • 38. Wear Resistance Wear characteristics refer to the behavior and properties of a material or surface when it comes into contact with another surface or abrasive medium, resulting in the removal or deformation of material from the contacting surfaces. Wear is a natural and that occurs in various mechanical systems, such as machinery, tools, automotive components, and industrial equipment. Understanding wear characteristics is crucial for designing materials, coatings, and lubrication systems that can minimize wear and extend the lifespan of components.
  • 39. Wear test- Hip joint simulator
  • 40. Impact Test These tests are used to indicate the toughness of a material, and particularly its capacity for resisting mechanical shock. Brittleness, resulting from a variety of causes, is often not revealed during a tensile test. The test specimen struck by a fast moving hammer and the energy that is absorbed in fracturing the test piece is measured.
  • 41. Charpy impact test For Izod tests, the pendulum is released from the lower position, to give a striking energy of 170 J and for the Charpy test it is released from the upper position to give a striking energy of 300 J. The scale carries a set of graduations for each test
  • 42. The Izod Impact Testing This test employs a standard notched test- piece which is clamped firmly in a vice. The striking energy is approximately 163 J. The test-piece is notched so that there is an initial 'crack' to initiate fracture.
  • 43. Test Standards The ASTM E23 standard describes Charpy and Izod impact tests on notched bar metal specimens Izod Test Charpy Test
  • 44. Ductile and brittle fracture In ductile facture, failure is preceded by a considerable amount of plastic deformation of the material. In brittle fracture no plastic deformation prior to failure
  • 45. Charpy Impact Test This test is of continental origin, and differs from the Izod test in that the test-piece is supported at each end (Figure 3.12); whereas the Izod test uses a test- piece held cantilever fashion. Here the load on the pendulum can be varied so that the impact energy is either 150 J or 300 J.
  • 46. Mechanical Properties summary Strength (Mpa) : Ability to endure stress the intensity of force. Strain () Elastic = Elongation (mm) / Original Length (mm) Elongation (%) Plastic = Elongation (mm) / Original Length (mm) Stiffness (Mpa) : Stress to cause strain = Stress (Mpa) / Strain () Toughness (J) : Energy to break Hardness () : Resistance to indentation / abrasion.
  • 47. Ashby Charts Select chart: Young's modulus - Density Young's Modulus - Cost Strength - Density Strength - Toughness Strength - Elongation Strength - Cost Strength - Max service temperature Specific stiffness - Specific strength Electrical resistivity - Cost Recycle Fraction - Cost Energy content - Cost
  • 48. CREEP When stressed over a long period of time, some metals extend very gradually and may ultimately fail at a stress well below the tensile strength of the material. This phenomenon of slow but continuous extension under a steady force is known as 'creep. Such slow extension is more prevalent at high temperatures, and for this reason the effects of creep must be taken into account in the design of steam and chemical plant, gas and steam turbines and furnace equipment
  • 49. Creep test Creep tests are carried out on test-pieces which are similar in form to ordinary test-pieces. A test-piece is enclosed in a thermostatically controlled electric tube furnace which can be maintained accurately at a fixed temperature over the long period of time occupied by the test. The test-piece is statically stressed, and some form of sensitive extensometer is used to measure the extremely small extensions at suitable time All creep testing is conducted in tensile mode
  • 50. Creep test set up For example, for a 0.2 per cent plain carbon steel at 400属C, the stress to rupture in 1000 hours is 295 MPa; at 500属C it has dropped to 118 MPa.
  • 51. Creep Variation with temperature
  • 53. Fatigue Fatigue is gradual crack growth caused by alternating loads. If you take a paper clip and repeatedly flex it back and forth, it does not last very long before it breaks, even though you have not applied a large stress - Metal Fatigue A380 engine
  • 54. S/N curve Fatigue life, at a given alternating stress level and mean stress, is the number of cycles required to cause failure due to fatigue. Specimens are tested in a series of decreasing stress levels until no failure occurs within a selected maximum number of cycles (usually 10 million cycles). The nearly horizontal portion of the curve defines the fatigue or endurance limit for the test material.
  • 55. S/N curve The curve becomes horizontal at a stress which will be endured for an infinite number of reversals. This stress is the fatigue limit or endurance limit. Some non-ferrous materials do not show a well- defined fatigue limit; that is, the S/N curve slopes gradually down to the horizontal axis
  • 56. Stages in Fatigue failure 1.Initiation Stage: 2.Crack Initiation: 3.Crack Propagation: 4.Final Fracture:
  • 57. Important factors that influence fatigue failure include: Stress Amplitude: The difference between the maximum and minimum stress levels in each cycle. Mean Stress: The average stress level over the cyclic loading. Number of Cycles: The total number of loading cycles the material experiences before failure. Material Properties: Different materials have varying resistance to fatigue failure. Surface Finish: Smooth surfaces generally have better fatigue performance than rough ones. Environmental Factors: Corrosive environments or elevated temperatures can accelerate fatigue failure. Loading Frequency: Faster loading rates can reduce fatigue life. Overloads: Sudden increases in stress levels can accelerate crack growth. To prevent fatigue failure, engineers use various strategies: Design Considerations: Incorporating smooth transitions, fillets, and avoiding sharp corners can reduce stress concentrations. Material Selection: Choosing materials with better fatigue resistance for specific applications. Limiting Stress Ranges: Designing to keep stress amplitudes within safe limits. Surface Treatments: Processes like shot peening or nitriding can enhance the material's fatigue resistance. Inspections: Regular inspections can detect early signs of cracks and prevent catastrophic failures.
  • 58. Other tests- Erichsen cupping test Ductility and suitability for deep drawing processes is tested by pressing a hardened steel ball into sheet metal. The maximum depth of penetration before rupture is the Erichsen value (in mm)
  • 59. Other tests Bend tests Another test for ductility, but specific to bending or plastic fatigue. Sheet metal materials are tested for bending 180o on itself (without cracking or orange peel where grain become visible). Another test bends 90o back and forth until it fails counting the number of cycles. This is definitely plastic deformation, so not really fatigue in the engineering sense.
  • 60. Compression test Ductile materials simply squash (barrel). Brittle materials often fracture at 45 degrees (due to shear stress being much lower than compressive stress). Compression is the standard test for concrete.
  • 61. Online material Property resources Graphical comparison of materials properties. Ashby charts Testlopedia: Testing of plastics
  • 62. Questions Steel wire of 2mm diameter can withstand 250MPa. What is the force? Sketch a load extension diagram for low carbon steel and show the following points: (a) Elastic limit (b) Yield point (c) Ultimate tensile strength A certain carbon steel has hardness of 42HRC. When hardened it is 62 HRC. Which specimen would have the greater (a) wear resistance (b) toughness (c) strength? Describe the Charpy test. Name three types of hardness test. Describe the Rockwell hardness testing machine, and how a test is done. Describe the process of a typical fatigue failure List ways to improve a components resistance to fatigue. Describe shot peening in reference to fatigue treatment. Explain creep with reference to creep curve and jet engine turbine blades.