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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 July 18, 2015 1 Outline of the presentation Features of ductile/brittle materials Destructive testing & explanations Fundamental mechanical properties Stress-strain relation for different engineering materials Examples July 18, 2015 2 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 = Increasein lengt h 100 Originallengt h 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 July 18, 2015 3 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 July 18, 2015 4 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 Grey cast iron (example) A specified amount of stress applied to produce desired strain 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 July 18, 2015 6 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 i. Yield strength deformation) (elastic to plastic ii. Ultimate strength (maximum stress that can withstand) iii.Breaking strength (strength upto the rupture) July 18, 2015 8 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 Yes, you could use AFM tip as a nanoindenter July 18, 2015 Hardness Measurement Methods • • • • • Rockwell hardness test Brinell hardness Vickers Knoop hardness Shore 9 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. July 18, 2015 10 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 July 18, 2015 11 The _______ type hardness test leaves the least amount of damage on the metals surface. A: Rockwell B: Brinell C: Scleroscope D: Microhardness 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 Affected by the rate of loading, temperature variation in heat treatment, alloy content Try to hit it sharply and impact strength suddenly -(as with a hammer) Impact Strength The ability of a material to withstand shock loading July 18, 2015 13 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. July 18, 2015 14 July 18, 2015 15 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) July 18, 2015 16 Will you be embarrassed by reviving “Who you are??????????” You are the message (based on several consequences) July 18, 2015 17 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 July 18, 2015 Surface roughness/finishing thermal treatment Residual stresses Strain concentrations 18 Creep property of a material by virtue of which it deforms continuously under a steady load Adopts this kind of relationship slow plastic deformation (slip) of material occurs at high temperatures. Iron, nickel, copper and their alloys exhibited this property at elevated temperature. Undergo a timeBut zin, tin, lead and their alloys shows dependent creep at room temperature. 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. Logarithmic Creep (low temp) Recovery Creep (high temp) Diffusion Creep (very high temperatures) July 18, 2015 3) Tertiary creep shows a rapid increase in the creep rate due to effectively reduced cross-sectional area of the specimen 20 Factors affecting Creep Heat Treatment Alloying Grain size Dislocations Slips Grain boundaries Atomic diffusion Types of stress applied July 18, 2015 21 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 July 18, 2015 22 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 July 18, 2015 23 Different stages of Fracture July 18, 2015 24 Equation governing fracture mechanisms s = 2E e 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 July 18, 2015 25 The Ductile – Brittle Transition Surface energy increases as temperature decreases. The yield stress curve shows the strong temperature dependence 26 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 July 18, 2015 27