Metal Bending Assessment - Metals Lecture

Metal (definition)

a chemical element that is lustrous, hard, malleable, heavy, ductile, tenacious, and usually a good conductor of heat and electricity

Alloy (definition)

combination of elements which exhibits the properties of a metal

• alloying used to improve properties such as strength, ductility, hardness, wear resistances and corrosion resistance

All metals are _____.

crystalline

Metals crystallize into a regular arrangement of atoms called________.

space lattices

Cubic System (space lattice)

three contiguous edges of equal length and at right angles

Tetragonal (space lattice)

three contiguous edges, two of equal length, all at right angles

Hexagonal (space lattice)

three parallel sets of equal length horizontal aces at 120 degrees and a vertical axis

In molten form, metals are _________

non-crystalline

the temperature at which atoms begin to crystallize is called __________

its freezing point

Grain

• each unit is called a grain

• each grain is essentially a single crystal

size of grain depends on the temperature from which the metal is cast, the cooling rate and nature of the metal.

slow cooling - large grain

fast cooling - small grain

Slip planes (below elastic limit)

• applied force distorts crystal lattice

• remove force, crystal goes back to shape

Slip planes (above elastic limit)

• force causes permanent displacement or slip

• occurs in lattice on certain specified plane called slip planes (crystal slips without separation, plasticity)

• lattice undergoes distortion, becomes highly stressed and hardened

• slip planes tend to greatest concentration of atoms and be 45 degrees from applied force

• slip is not confined to one set of planes during plastic deformation

Mechanical Properties of Metals

• dependent on lattice structure

  • in general, cubic lattices are more ductile or workable materials

  • FCC - ductile through a wide temp range

  • CPH - good hardening by cold working

• ability to shape and contour AL and stainless steel depend on plasticity

• elasticity governs safe and use as load bearing members

Plasticity

Metals ability to be deformed beyond the range of elasticity without fracture resulting in permanent change in shape

• for metals, plastic to elastic ratio is high

• 100:1

Permanent set as result of slip

further force means that the slip doesn’t happen along original slip plan indefinitely, it gets increased resistance to further motion therefore the slip now occurs along new parallel slip plane

as slip shifts from one slip plane to another progressively higher forces required to cause slip, metal has become work hardened

Actual strength of metals are a ______ of their theoretical strength

fraction

metal strength can be compromised by imperfections like. . .

• flaws in regularity of crystal lattice

• micro cracks in grains

• shrinkage voids

• nonmetallic impurities

• rough surfaces

• notches

Notches are great _______

weakeners

• stress raisers and stress complicators, induce stress in many directions

• deeper the notch and the sharper its root, the more of a stress problem it is

• beware of notches from contouring instruments, grain boundaries

Types of steel and aluminum used in O and P applications

• general designations can be misleading

• terms don’t accurately describe the material

Carbon Steel

• iron as pure as metal - too low strength and hardness for many applications

• iron with added carbon

• hardness and strength of carbon steel are directly proportional to amount of added carbon

• manganese and small amounts of sulfur and phosphorous are also present in carbon steel

Alloy steel

• adding other elements to carbon steel to achieve certain properties gives an alloy steel

• general characteristics used to describe alloy steel properties

  • toughness - ability to withstand shock force

  • hardness - resistance to penetration an abrasion

  • ductility - ability to undergo permanent changes in shape without rupturing

  • corrosion resistance - the resistance to chemical attack of a metal under the influence of a moist atmosphere

Characteristics of Specific Alloys

• nickel steels - increased toughness, easier heat treating, decreased distortion in quenching, increased corrosion resistance

• nickel chromium steels - increased depth hardenability and abrasion resistance

• molybendenum steels - greatest hardenability, increased high temp strength and increased corrosion resistance

• chromium steel - increased hardening affect (can decrease carbon % and get a steel with high strength and satisfactory ductility)

Steel and Aluminum Alloys

• vanadium steel - increaased refinement of internal structure of the alloy

• silicon manganese steels - increased strength and hardness

Double and triple alloys have some characteristics of each

• chromium molybendenum steels - excellent hardenability and satisfactory ductility

• chromium nickel steels - good hardenability and satisfactory ductility

Stainless Steels

• steel alloys with >3.99% chromium

• AISI 3 digit grading system for types of stainless steels

  • austenitic - most common, cannot be heat treated, cold working increased properties, high nickel and chromium content, rapidly work harden (use sharp tools), highest corrosion resistance

  • martinistic - can be head treated to improve properties

  • ferritic - non hardenable by heat treatment and only slightly by cold working

SAE Number

• 4 digits

• 1st digit - type of steel

• 2nd digit - approximately % alloying element

• 3rd and 4th digits - % of carbon in 1/100’s of a percent

Comparison of Steel and Aluminum

Size, weight, and strength comparisons

• aluminum 1/3 modulus of steel

• aluminum ~ 1/3 weight of steel

• AL more subject to fatigue failure than steel

Strengthening Aluminum and Steel

• AL can be stronger than some steels (proper alloying, heat treatement or cold working)

• still more subject to fatigue

Heat Treatment Purposes

• increase or decrease hardness or tensile strength

• relieve internal stresses due to hot or cold working

• improve machinability

• increase toughness

Techniques for steel

• steel heated above critical range undergoes definite internal changes

• if steel is slowly cooled from this elevated temp, internal changes will have to reverse themselves

• if steel is cooled more rapidly, structure will be modified and mechanical characteristics altered

Heat treat cycle

• normalized steel - returns to its original or normal internal structure

  • will have increased strength and hardness but decreased ductility than same piece annealed

• annealed steel - relieve internal stresses and lower yield point for max ductility

Strengthening Aluminum and Steel

• tempering - often follows quenching. Heat treated steel can be too brittle and hard. Tempering can make it softer more ductile and tougher

• aluminum alloy - large increased in both yield and ultimate strengths when heat treated

• steel alloy - effects of hot and cold working and heat treating

• carbon steel - the difference between a carbon steel and a high carbon steel that has been heat treated and tempered

Strengthening Aluminum

• tempering aluminum alloys is the major determinant of its strength, hardness, ductility, and other properties

  • some are tempered with heat treating, and others by cold working

  • temper designations are added to the four identifying digits

Preventing Failure

fatigue stress - fluctuating stressed lower than the ultimate stress of the material, but will cause after a number of cycles

this is called fatigue failure

Comparison of Fatigue in Aluminum and Steel

• steel curve levels of at about 50% of its original stress

• aluminum does not level off, more subject to fatigue failure

• adding alloying elements can increase yield and ultimate stress but doesn’t change fatigue strength much

• heat treating also doesnt’t change fatigue strength a lot

Minimizing Stress Concentraion

following practice helOvp minimize stress concentrations in orthoses:

• remove nicks and scratches by polishing

• cap vice jaws that are checkered before clamping your workpiece

• make sure contouring instruments have smooth curved surfaces

• do not shape orthotic stirrups, uprights with a metal hammer

• avoid abrupt changes in cross section

Overall what is the best way to minimize stress concentration due to bending?

avoid sharp bends

Orthotic bars are designed to be more _____________ (with knee locked) than M-L direction.

rigid in flexion

Titanium

titanium has strength comparable to steel while having a density of 56% steel but 60% greater than aluminum

• double the strength of aluminum

• better strength than steel, steel has better fatigue resistance

• good biological compatibility

• very resistant to corrosion

• more expensive

Metals are highly __________ and crystallize into ______________.

crystalline, space lattices

Alloying steel is done to improve properties such as _____, __________, ________, ________________

toughness, hardenability, ductility, corrosion resistance

_______ is the major determinant of strength, hardness, and ductility for AL alloys.

Tempering

Aluminum is _____ density of steel, while steel has _______ the modulus of AL

1/3, 3x

Steel has a better _______ _________ than AL.

fatigue resistance

The best way to minimize stress concentrations in orthotic fabrication are:

• prevent nicks, notches, large drilled holes, sharp contours