Mechanical Properties

Properties of Materials Characteristics Used to Identify a Material Chemical Properties – how a material interacts with other materials Physical Properties – characteristics the material possesses in and of itself Mechanical Properties – material’s response to force or stress Thermal Properties – material’s response to heat Properties of Materials 100 - S

Properties of Materials Mechanical Properties Color, Density, Size, Magnetism, Melting and Boiling points, Crystal Structure, Luster, Viscosity Workability, Brittleness, Hardness, Elasticity, Plasticity, Toughness, Strength Conductivity, Specific Heat, Thermal Expansion Burning Reaction to Acids/Water Corrosion/OxidationReduction Chemical Properties Physical Properties Properties of Materials Mechanical Properties of materials Mechanical Properties Thermal Properties

Mechanical Properties How materials respond to force or stress force – a push or pull stress – force causing a deformation or distortion (force per unit area) http://www.artie.com/summer/arg_diving_board_url.jpg

Stresses and Forces Tension: the pulling force stretches materials Compression: a pushing force squashes materials Torsion: a twisting force Shear: opposing forces Testing a box of paperclips is a lot less expensive than testing a car body for strength. http://www.legoeducation.com/sharedimages/content/Large/L_cancrusher.gif www.global-b2b-network.com/direct/dbimage/502

Mechanical Properties workability malleability – can be flattened ductility – can be drawn into wire (stretched), bent, or extruded brittleness breaks instead of deforming when stress is applied hardness resistance to denting or scratching Malleability - (show students a rolled penny) Ductility - (show drawn wire) Show Play doh fun factory for extrusion http://www.chem-pics.co.uk/animations.htm

Mechanical Properties elasticity ability to return to original shape after being deformed by stress plasticity retains new shape after being deformed by stress toughness ability to absorb energy strength resistance to distortion by stress or force Show happy/sad balls blogs.msdn.com/.../images/610897/original.aspx

Tensile Test http://www.youtube.com/watch?v=W5A8gU37wGg&feature=related

Metal Property Changes http://www.youtube.com/watch?v=gPKkgmDoEoU http://www.youtube.com/watch?v=3jSz11lz8MA http://www.youtube.com/watch?v=fzHMnaRinJ8 Work hardening, strain hardening, or cold work is the strengthening of a material by increasing the material's dislocation density. In metallic crystals, irreversible deformation is usually carried out on a microscopic scale by defects called dislocations,which are created by fluctuations in local stress fields within the material culminating in a lattice rearrangement as the dislocations propagate through the lattice. At normal temperatures the dislocations are not annihilated by annealing. Instead, the dislocations accumulate, interact with one another, and serve as pinning points or obstacles that significantly impede their motion. This leads to an increase in the yield strength of the material and a subsequent decrease in ductility. Work hardening is a consequence of plastic deformation, or a permanent deformity to a material Increase in the number of dislocations is a quantification of work hardening. Plastic deformation occurs as a consequence of work being done on a material; energy is added to the material. In addition, the energy is almost always applied fast enough and in large enough magnitude to not only move existing dislocations, but also to produce a great number of new dislocations by jarring or working the material sufficiently enough.

At the molecular level… Crystal Defects and Imperfections What does ‘defect’ mean? Any piece of metal is made up of a large number of "crystal grains", which are regions of regularity. At the grain boundaries atoms have become misaligned. Explain that the following activities have to do with creating defects in metals. http://originoflife.net/information/graphics/dislocation.png http://www.chemguide.co.uk/atoms/structures/metals.html#top

Types of Crystal Defects Point defects The simplest point defects are as follows: Vacancy – missing atom at a certain crystal lattice position; Interstitial impurity atom – extra impurity atom in an interstitial position; Self-interstitial atom – extra atom in an interstitial position; Substitution impurity atom – impurity atom, substituting an atom in crystal lattice; Frenkel defect – extra self-interstitial atom, responsible for the vacancy nearby. www.substech.com/dokuwiki/lib/exe/fetch.php?w...

Crystal Line Dislocation Defects Atoms get jammed Work hardening If you have a pure piece of metal, you can control the size of the grains by heat treatment or by working the metal. Heating a metal tends to shake the atoms into a more regular arrangement - decreasing the number of grain boundaries, and so making the metal softer. Banging the metal around when it is cold tends to produce lots of small grains. Cold working therefore makes a metal harder. To restore its workability, you would need to reheat it. media-2.web.britannica.com/.../72/2372-004.gif

Crystal Interfacial Defects Interfacial defects occur wherever the crystalline structure of the material is not continuous across a plane. Examples are surfaces, grain boundaries, and interfaces between different layers of materials . www.cartage.org.lb/.../Crystalline/Image486.gif

Copper Wire Bending Bend the wire as shown: Use a black marker to draw a line along the wire’s middle section Twist the wire as many times as you can – you are work-hardening the wire. Count the number of turns. Holding the wire with pliers, heat the twisted area. Let it cool) and try twist it again. You have annealed the wire. Note your results. Copper Wire Activity 78-S Bending and hammering increases the dislocations in the metal, making it less workable. Heat allowed atoms to move and re-grow crystals. Dislocations were eliminated (decreased) -softer, more workable metal.

Heat-Treating Steel Lab Work Hardening - to strengthen a material by reshaping it while the part is cold. Forging - shape or form metal by beating or hammering it Annealing - heat to red hot, air cool - metal is heated and cooled so that crystal can reform. - softens metal by relieving stress Quenching/Hardening - heat to red hot, quench in cool water - rapid cooling of metal locks atoms into place in an unstable crystal structure - strengthens metal but brittle Tempering - heat to red hot, quench, re-heat to blue, air cool - heating material so atoms re-orient themselves - removes brittleness but keeps strength Review the terms on the slide and do the Lab - Heat-Treating Steel 83-S MAST Module 90-S to 99-S http://en.wikipedia.org/wiki/Image:Bobby_pin.jpg

Bobby Pin and Paper Clip Steel is an alloy consisting mostly of iron, with a carbon content between 0.2 and 2.04% by weight, depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten.[1] Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and form of their presence in the steel (solute elements, precipitated phase) controls qualities such as the hardness, ductility and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but is also more brittle. Types of steel Carbon steel (≤2.1% carbon; low alloy) Stainless steel (steel with chromium) HSLA steel (high strength low alloy) Tool steel (very hard) Other iron-based materials Cast iron (>2.1% carbon) Wrought iron (contains slag) Ductile iron Bobby pins and paper clips are processed in much the same way but contain different amounts of carbon. Bobby pins and paper clips are formed from cold worked steel wire. The paper clip, containing little carbon, is mostly pure Fe with some Fe3C particles. The bobby pin has more carbon and thus contains a larger amount of Fe3C which makes it much harder and stronger.