# May 1, 2015 1 Dr. Alagiriswamy A A, (M.Sc, PhD, PDF) Asst. Professor (Sr. Grade), Dept. of Physics, SRM-University, Kattankulathur campus, Chennai UNIT.

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May 1, 2015 1 Dr. Alagiriswamy A A, (M.Sc, PhD, PDF) Asst. Professor (Sr. Grade), Dept. of Physics, SRM-University, Kattankulathur campus, Chennai UNIT V Lecture 2 MECHANICS OF MATERIALS

May 1, 2015 2 Features of ductile/brittle materials Destructive testing & explanations Fundamental mechanical properties Stress-strain relation for different engineering materials Examples Outline of the presentation

May 1, 2015 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  = F/A Strain measures the elongation for a given load  = (L-L o )/L o

Materials Percentage of Elongation Low-Carbon -37% Medium-Carbon 30% High-Carbon- 25% 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 May 1, 2015 4 Issues of ductile material

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

A brittle material is one with a low % of elongation before failure Brittleness increases with pressure ≤ 5 % elongation May 1, 2015 6 Features of Brittle material Grey cast iron (example) A specified amount of stress applied to produce desired strain Dislocations/defects/imperfections could be the probable reasons

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

May 1, 2015 8 (i)Tensile strength (Alloy steel ; 60-80 kg/mm 2 )  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 i. Yield strength (elastic to plastic deformation) ii. Ultimate strength (maximum stress that can withstand) iii. Breaking strength (strength upto the rupture) Destructive testing

May 1, 2015 9 (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 Rockwell hardness testRockwell hardness test Brinell hardnessBrinell hardness VickersVickers Knoop hardnessKnoop hardness ShoreShore Hardness Measurement Methods Yes, you could use AFM tip as a nanoindenter Destructive testing

May 1, 2015 10 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. Destructive testing

May 1, 2015 11 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 HV = 1.854(F/D 2 ); F is the force applied, d 2 is the area of the indentation Vickers hardness tests Metalography Metalography; viewing of samples through high powerful microscopes

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

May 1, 2015 13 Impact Strength The ability of a material to withstand shock loading Try to pull it --tensile strength Try to compress it --compressional strength Try to bend (or flex) it --flexural strength Try to twist it --torsional strength Try to hit it sharply and suddenly -- (as with a hammer) impact strength Affected by the rate of loading, temperature variation in heat treatment, alloy content Destructive testing

May 1, 2015 14 (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. Destructive testing

May 1, 2015 15

May 1, 2015 16 (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) Destructive testing

Will you be embarrassed by reviving “Who you are??????????” You are the message (based on several consequences) May 1, 2015 17

May 1, 2015 18 Factors affecting Fatigue  Surface roughness/finishing  thermal treatment  Residual stresses  Strain concentrations 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

Creep  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. Adopts this kind of relationship Undergo a time- dependent increase in length

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 May 1, 2015 20  Logarithmic Creep (low temp)  Recovery Creep (high temp)  Diffusion Creep (very high temperatures) Different stages of creep

May 1, 2015 21 Factors affecting Creep  Heat Treatment  Alloying  Grain size  Types of stress applied  Dislocations  Slips  Grain boundaries  Atomic diffusion

May 1, 2015 22 Types of Fracture  Brittle Fracture  Ductile Fracture  Fatigue Fracture  Creep Fracture 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

May 1, 2015 23 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 Fracture

May 1, 2015 24 Different stages of Fracture

May 1, 2015 25  = 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 Equation governing fracture mechanisms

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

May 1, 2015 27 On recalling/revisiting  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 Make sure you understand language and concepts:

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