Dr. Alagiriswamy A A , (M.Sc, PhD, PDF) Asst. Professor (Sr. Grade),

1. Dr. Alagiriswamy A A, (M.Sc, PhD, PDF) Asst. Professor (Sr. Grade), Dept. of Physics, SRM-University, Kattankulathur campus, Chennai MECHANICS OF MATERIALS UNIT V Lecture 2

2. Outline of the presentation • Features of ductile/brittle materials • Destructive testing & explanations • Fundamental mechanical properties • Stress-strain relation for different engineering materials • Examples

3. Ductility; the property of a metal by virtue of which it can be drawn into an elongated state before RUPTURE takes place.Percentage of elongation = • Stress measures the force required to deform or break a material • s = F/A • Strain measures the elongation for a given load • e = (L-Lo)/Lo

4. Issues of ductile material • A ductile material is one with a large Percentage of elongation before failure • Ductility increases with increasing temperature. • Easily drawn into wire • Moldable, • Easily stretchable without any breakage Materials Percentage of Elongation • Low-Carbon -37% • Medium-Carbon 30% • High-Carbon- 25%

5. Quiz time • Ductility is the ability of a metal to ________ before it breaks. A: Bend B: Stretch or elongate C: Be forged D: Be indented

6. Features of Brittle material A specified amount of stress applied to produce desired strain Grey cast iron (example) • A brittle material is one with a low % of elongation before failure • Brittleness increases with pressure • ≤ 5 % elongation Dislocations/defects/imperfections could be the probable reasons

7. Fundamental Mechanical Properties • (i)Tensile strength • (ii) Hardness • (iii) Impact strength • iv) fatigue • (v) Creep

8. Destructive testing • (i)Tensile strength (Alloy steel ; 60-80 kg/mm2) • provides ultimate strength of a material • maximum withstandable stress before breakage • just an indication of instability regime • provides the basic design information to the test of engineers • Yield strength (elastic to plastic deformation) • Ultimate strength (maximum stress that can withstand) • Breaking strength (strength upto the rupture)

9. Destructive testing • (ii) Hardness factor • Ability of a material to resist before being permanently damaged • Direct consequences of atomic forces exist on the surface • This property is not a fundamental property (like domain boundary) • Measure of macro/micro & nano-hardness factors provide the detailed analyses Hardness Measurement Methods • Rockwell hardness test • Brinell hardness • Vickers • Knoop hardness • Shore Yes, you could use AFM tip as a nanoindenter

10. Destructive testing • Brinell, Rockwell and Vickers hardness tests; • to determine hardness of metallic materials to check quality level of products, for uniformity of sample of metals, for uniformity of results of heat treatment. • Knoop Test; • relative micro hardness of a material • Rock well hardness; • a measure of depth of penetration • Shore scleroscope ; • in terms of the elasticity of • the material.

11. Vickers hardness tests Microhardness test involves using a diamond indenter to make a microindentation into the surface of the test material, the indentation is measured optically and converted to a hardness value Metalography; viewing of samples through high powerful microscopes HV = 1.854(F/D2); F is the force applied, d2 is the area of the indentation

12. The _______ type hardness test leaves the least amount of damage on the metals surface. A: Rockwell B: Brinell C: Scleroscope D: Microhardness

13. Destructive testing Affected by the rate of loading, temperature variation in heat treatment, alloy content Impact Strength The ability of a material to withstand shock loading

14. Destructive testing • (i)Fatigue • Fatigue is the name given to failure in response to alternating loads (as opposed to monotonic straining • expressed in terms of numbers of cycles to failure (S-N) • Occurs in metals and polymers but rarely in ceramics. • Also an issue for “static” parts, e.g. bridges.

16. Destructive testing • (i)Fatigue • Repeated/cyclic stress applied to a material • An important mode of a failure/disaster • Loss of strength/ductility • Increased uncertainty in service • SEM Fractograph (Aluminum alloy)

17. Will you be embarrassed by reviving “Who you are??????????” • You are the message (based on several consequences)

18. Factors affecting Fatigue • What causes fatigue? • Fatigue is different for every person. Here are some causes of fatigue: • Chemotherapy/Pain • Sleep problems/Radiation • Certain medicines/Lack of exercise • Surgery/Not drinking enough fluids • Not being able to get out of bed/Nausea • Eating problems • Surface roughness/finishing • thermal treatment • Residual stresses • Strain concentrations

19. Creep Adopts this kind of relationship • property of a material by virtue of which it deforms continuously under a steady load • slow plastic deformation (slip) of material • occurs at high temperatures. • Iron, nickel, copper and their alloys exhibited this property at elevated temperature. • But zin, tin, lead and their alloys shows creep at room temperature. Undergo a time-dependent increase in length

20. Different stages of creep • 1) Primary creep is a period of transient creep. The creep resistance of the material increases due to material deformation. Predominate at low temperature test such as in the creep of lead at RT. • 2) Secondary creep provides a nearly constant creep rate. The average value of the creep rate during this period is called the minimum creep rate. • 3) Tertiary creep shows a rapid increase in the creep rate due to effectively reduced cross-sectional area of the specimen • Logarithmic Creep (low temp) • Recovery Creep (high temp) • Diffusion Creep (very high temperatures)

21. Factors affecting Creep • Dislocations • Slips • Grain boundaries • Atomic diffusion • Heat Treatment • Alloying • Grain size • Types of stress applied

22. Fracture; a disaster occurs after the application of load, • Local separation of regions • Origin of the fracture (in two stages): • initial formation of crack and • spreading of crack • Types of Fracture • Brittle Fracture • Ductile Fracture • Fatigue Fracture • Creep Fracture

23. Fracture • Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittle • • Ductile fracture - most metals (not too cold): • Extensive plastic deformation ahead of crack • Crack is “stable”: resists further extension unless applied stress is increased • • Brittle fracture - ceramics, ice, cold metals: • Relatively little plastic deformation • Crack is “unstable”: propagates rapidly without increase in applied stress • Ductile fracture is preferred in most applications

24. Different stages of Fracture

25. Equation governing fracture mechanisms •  = • Where, • e is half of the crack length, •  is the true surface energy • E is the Young's modulus. • the stress is inversely proportional to the square root of the crack length. • Hence the tensile strength of a completely brittle material is determined by the length of the largest crack existing before loading. • For ductile materials (additional energy term p involved, because of plastic deformations

26. The Ductile – Brittle Transition • Surface energy increases as temperature decreases. • The yield stress curve shows the strong temperature dependence

27. On recalling/revisiting Make sure you understand language and concepts: • Roughness/ductility/Brittleness/hardness • Isotropy/anisotropy/orthotropy/elasticity • Resilience/endurance • Brittle fracture • Corrosion fatigue • Creep • Dislocation/slip • Ductile fracture • Ductile-to-brittle transition • Fatigue /Fatigue life • Fatigue limit/Fatigue strength