Chapter 13- Solid Materials Flashcards
Elastic deformation
Will return to its original position dimensions when deforming force is removed
Plastic deformation
Plastic material will remain deformed
What kind of diagram can be used for material properties
Spider diagram
Hardness
Surface phenomenon
More difficult to scratch the surface
Which scale to measure hardness
MOHs scale
Grades 10 minerals from softest rated at 1 to hardest rating of 10
Stiffness
Deformation resisting potential
Toughness
Ability to absorb energy from impacts and shocks without breaking
Brittleness
Will shatter or crack when subjected to dynamic shocks or impacts
No or little plastic deformation before breaking
Strength
Can withstand large forces without breaking. Will depend on size and its breaking stress
Malleability
Can be hammered into thin sheets
Ductility
Can be drawn into wires
Hooke’s law
States that up to a given load, the extension of a spring is directly proportional to the force applied to the spring and is given by
F=kx
x(extension)
k(stiffness- spring constant)
Type of graph for an extension
Force- extension graph
Line to curve
Limit of proportionality
Point a which if force is increased material won’t return back to original dimensions once deforming force is removed
Elastic region of extension
Area in which loading and reloading are reversible
Arrows drawn on graph to illustrate this cycle
Atoms are held together by
Bonds
Elastic limit
Beyond this point wire ceases to be elastic
Has passed point of reversibility
Has undergone permanent deformation
Yield point
As load increase the wire yields and will not contract at all when the load is removed
Beyond this the material is plastic
Look at graph
Point A: limit of proportionally Point B: elastic limit Point C: yield point Lines 1) elastic region Lines 2) strange effect is noticed if load is removed and wire is reloaded- wire regains springiness and has same stiffness as before
Natural rubber
Stretches v easily at first
Then becomes v stiff and difficult to stretch as it approaches it’s breaking point
Is a polymer
Polymer
Long chains of atoms that are normally tangled in a disordered fashion
Small forces needed to untangle these molecules so long extension is produced for small loads
When chains are fully extended, additional forces needed to stretch the bonds between the atoms, so much smaller extensions are produced for a given load; the rubber becomes stiffer
Stretching forces are called
TENSILE forces
Force- compression graphs
Placed on force sensor
Then lever is twisted so screw clamps object
Displacement sensor calculates data and this is shown on display unit
Elastic strain energy
Ability of deformed material to do work as it regains its original dimensions
Elastic strain work eq
W= average F X distance moved in direction of force
Work done calculation on a graph
The area under a force- extension graph
(Elastic strain energy)
Work done= 1/2 x F x X
= 1/2 x KX x X
= 1/2 x K x X^2
= 1/2Kx^2
When a stress is applied to a material what is the effect?
STRAIN
Stress eq
Force/ cross sectional area
Symbol (o-)
Unit= Pa
Strain eq
Extension/ original length
Symbol (E) curly
Unit= no unit
Young modulus def
A property of materials that undergo tensile or compressive stress
Young modulus eq
Stress/ strain
Unit= Pa
Why are stress- strain graphs useful
Because they will be the same for any sample of a given material and the gradient of the proportional region equals the Young modulus of the material
Stress- strain graph
Look at graph :
0-A: hookes law region Young mod is gradient of this section B: elastic limit C: yield stress D: max stress (ultimate tensile strength UTS) E: breaking point
Energy density
The work done in stretching a specimen (the strain energy stored) per unit volume of the sample
=1/2 stress x strain
Area under stress strain graph
Hysteresis
Area under loading curve
Area beneath the unloading curve
Difference is known as
Shaded area is
Represents the work done per unit volume ON the band as it stretches
Represents the work done per unit volume BY the band as it relaxes
Hysteresis
Called a hysteresis loop
Hysteresis loop
Represents the energy per unit volume transferred to internal energy during the load- unload cycle
(For stress-strain graph)
For force- extension
= represents total energy transferred to internal energy for each cycle
