Issues to address. Why Mechanical Test?? Mechanical Properties. Why mechanical properties?

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1 Mechanical Properties Why mechanical properties? Folsom Dam Gate Failure, July 1995 Need to design materials that can withstand applied load e.g. materials used in building bridges that can hold up automobiles, pedestrians materials for skyscrapers materials for space exploration NASA materials for and designing MEMs and NEMs Space elevators? Issues to address Stress and strain Elastic behavior Plastic behavior Strength, ductility, resilience, toughness, hardness Mechanical behavior of different classes of materials Why Mechanical Test?? For quality control, the test should preferably be as simple, rapid and inexpensive as possible Forpredicting product performance the more relevant the test to service conditions the more satisfactory it is likely to be. For producing design data, the need is for tests which give material property data in such a form that they can be applied with confidence to a variety of configurations. For investigating failures the first difficulty is to establish what to look for and then the prime need is for a test which discriminates well. Simple tension Common States of Stress Simple shear Simple compression: Bi-axial tension: Hydrostatic compression: Standard methods are vital in ensuring reliable data True in all fields Thus ASTM & ISO If a property is to be claimed it must be backed by 1. a method and 2. statistical analysis ( means,sd) For materials science and engineering, materials development must be supported by proper results. 1

2 Original cross sectional area Engineering stress And this? You need to be aware of the relationship between stress and strain You need to understand what each of the terms relating to the mechanical properties of materials are Initial linear Non-linear What sort of behaviour is this called? Young s Modulus, E E tensile The slope of this linear part of the line is called the modulus of elasticity or Young s Modulus and is given the symbol E In this region we say the material behaves elastically E(GPa) Metals Alloys Tungsten Molybdenum Steel, Ni Tantalum Platinum Cu alloys Zinc, Ti Silver, Gold Aluminum Magnesium, Tin Graphite Composites Ceramics Semicond Polymers /fibers Diamond Si carbide Al oxide Si nitride <111> Si crystal <100> Glass-soda Concrete Graphite Polyester PET PS PC PP HDPE PTFE LDPE Carbon fibers only CFRE( fibers)* Aramid fibers only AFRE( fibers)* Glass fibers only GFRE( fibers)* GFRE* CFRE* GFRE( fibers)* CFRE( fibers)* AFRE( fibers)* Epoxy only Wood( grain) E ceramics >E metals >>E polymers Based on data in Table B2, Callister 6e. Composite data based on reinforced epoxy with 60 vol% of aligned carbon (CFRE), aramid (AFRE), or glass (GFRE) fibers. 2

3 This is the amount of the force needed to deform a material to a point where it cannot return to its original shape. Half plane of atoms inserted into lattice distortion of lattice Once past the yield permanent or plastic deformation occurs This is the stress used for design 14 Slip without dislocations requires high shear force high theoretical strength all bond in plane broken at same time Slip with dislocations dislocation glides along slip plane slip facilitated by stress in lattice less force only one bond broken and reformed during glide dislocation slips to crystal ( grain ) surface Plastic deformation and the role of dislocations 800 MPa No dislocations Cu Yield strength 80 MPa Dislocations 15 dislocation free materials have much greater strength materials contain dislocations 16 Dislocation motion occurs most readily on Close packed planes smoothest surface for slipping And close packed directions smoothest surface for slipping 3

4 (Ultimate) Tensile Strength, σ TS Maximum possible engineering stress in tension. Metals: occurs when necking starts. Ceramics: occurs when crack propagation starts. Polymers: occurs when polymer backbones are aligned and about to break. Adapted from Fig. 6.11, Callister6e. Results and Analysis Metals Stress (Mpa) Cold Rolled 1018 Steel 6061-T651 Aluminum C2600 Brass, half hard Copper Annealed 1018 Steel Some snap Some bend Strain (mm/mm) 4

5 Ductile failure: --one piece --large deformation Brittle failure: --many pieces --small deformation Figures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures(2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., Used with permission. true stress at fracture elongation at fracture (ductility) an idea of the toughness an idea of the resilience Note: these are engineering stress & strain data, there is also true stress & strain No or little plastic deformation before fracture A significant amount of plastic deformation before fracture Ductility or %Elongation Plastic tensile strain at failure: Adapted from Fig. 6.13, Callister 6e. 5

6 UTS 400 E Stress, MPa Yield 100 From the previous graphs: Ductile fracture necking Brittle fracture no necking Strain % elongation Brittle Ductile Strong, not very ductile One that undergoes considerably plastic deformation 6