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 November 22, 2018November 22, 2018

Outline of the presentation Features of ductile/brittle materials Destructive testing & explanations Fundamental mechanical properties Stress-strain relation for different engineering materials Examples November 22, 2018November 22, 2018

Stress measures the force required to deform or break a material 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 November 22, 2018November 22, 2018

Issues of ductile material 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 November 22, 2018November 22, 2018

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

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 November 22, 2018November 22, 2018

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

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) November 22, 2018November 22, 2018

Destructive testing Yes, you could use AFM tip as a nanoindenter (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 November 22, 2018November 22, 2018

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. November 22, 2018November 22, 2018

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 November 22, 2018November 22, 2018

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

The ability of a material to withstand shock loading Destructive testing 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 Impact Strength The ability of a material to withstand shock loading November 22, 2018November 22, 2018

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. November 22, 2018November 22, 2018

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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) November 22, 2018November 22, 2018

Will you be embarrassed by reviving “Who you are??????????” You are the message (based on several consequences) November 22, 2018November 22, 2018

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 November 22, 2018November 22, 2018

Undergo a time-dependent 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

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) November 22, 2018November 22, 2018

Factors affecting Creep Dislocations Slips Grain boundaries Atomic diffusion Heat Treatment Alloying Grain size Types of stress applied November 22, 2018November 22, 2018

Types of Fracture Brittle Fracture Ductile Fracture Fatigue 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 Types of Fracture Brittle Fracture Ductile Fracture Fatigue Fracture Creep Fracture November 22, 2018November 22, 2018

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 November 22, 2018November 22, 2018

Different stages of Fracture November 22, 2018November 22, 2018

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 November 22, 2018November 22, 2018

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

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 November 22, 2018November 22, 2018