Chapter Three Flashcards
1
Q
What is the definition of strength?
A
The ability of a material to resist forces.
2
Q
What is the definition of stress?
A
Force per unit cross-sectional area of material and usually is expressed in pounds per square inch (psi) or megapascals (MPa)
3
Q
What is the equation for stress?
A
Stress=F/A
F is the applied force
A is the cross-sectional area
4
Q
A force exerted on a small area always causes what?
A
More stress
5
Q
What are the different types of stresses?
A
Tensile
compressive
shear
flexural
6
Q
What are tensile stresses?
A
They act to pull apart an object or cause it to be in tension.
7
Q
How do tensile stress occur, in regards to the direction of force?
A
They occur parallel to the line of force, but perpendicular to the area in questions.
8
Q
What is an example of an object being in tension?
A
If an object is pulled at both ends
9
Q
What are compressive stresses?
A
They are stresses that act to squeeze or compress objects.
10
Q
How do compressive stresses occur, in regards to the direction of force?
A
They occur parallel to the line of force and perpendicular to the corss-sectional area.
11
Q
What is an example of a compressive stress?
A
A blacksmith shapes metal by hitting the material with a hammer to squeeze or compress the metal into the desired shape.
12
Q
What are shear stresses?
A
Stresses that act to scissor or shear the object, causing the planes of the material to slide over each other.
13
Q
How do shear stresses occur, in regards to the direction of force?
A
They occur parallel to the applied force.
14
Q
What is an example of shear forces?
A
Two blocks are bonded to each other. If they are pulled in opposite direction they blocks will eventually be slide apart.
15
Q
What are flexural stresses?
A
Stress that is a combination of tension and compression stresses.
16
Q
What is an example of flexural stresses?
A
A beam that is loaded transversely. It will sag, and the top fibers will be compressed while the bottom side is in maximum tension.
17
Q
What is yield stress?
A
Also known as yield point
The point at which the material begins to maintain a deformational change due to the load and therefore the internal stresses under which it has been exposed.
18
Q
What is ultimate stress?
A
The stress at which a material ruptures.
19
Q
Which has the lower ultimate stress? Ultimate shear stresses or ultimate tensile stresses?
A
Ultimate sheer stresses.
So less force must be applied for tensile stress before the material ruptures.
20
Q
What is strain?
A
If a material lengthens or shortens in response to stress, it is said to experience strain.
21
Q
What is the equation for strain?
A
Strain= Change in Length/(-Original length)
22
Q
What is the most common way to determine the mechanical properties of materials?
A
The Tension Test
23
Q
The shape and magnitude of the stress-strain curve of a metal depends on what?
A
the composition, Heat treatment, History of plastic deformation
Strain rate
Temperature
State of stress imposed during testing
24
Q
What are the parameters used to describe the stress-strain curve of a metal?
A
Tensile strength Yield strength Yield poitn percent elongation reduction in area
25
Which of the parameters are strength parameters?
Tensile strength and yield strength
26
Which of the parameters are ductility?
Percent elongation and reduction area
27
What is Hooke's Law?
Stress (psi)=Strain(Young's Modulus)
28
In a stress strain curve, what is the area that is a straight line called?
The elastic region.
29
What is the equation used for a routine tension test based on Hooke's law?
```
Change in L=F(Lo)/AE
L=Length
F=Force
Lo=Original length
A=Area
E=Modulus of elasticity
```
30
What is plastic range?
the range beyond the elastic range, where the material plays plastically. The material has a set or permanent deformation when externally applied loads are removed.
31
What is the equation for the elastic strain?
Elastic strain=Total strain under load - plastic strain
32
What is yield point?
The point at which a marked increase in strain occurs without a corresponding increase in stress.
33
What does the horizontal line in the stress-strain curve indicate?
The yield stress corresponding to the yield point.
34
What two areas does the yield point separate in the stress-strain curve?
Separates the elastic and plastic portions of the curve
35
How is the tensile strength of a material obtained?
Dividing the maximum tensile force reached during the test by the original cross-sectional area of the test specimen.
36
What area of the stress strain curve indicates toughness of the material?
The area under the curve to the point of maximum stress
37
What is toughness of a material?
Ability of a material to withstand shock loads before rupturing.
38
What is ductility?
The ability of a material to sustain large permanent deformations in tension, as drawing a rod into a wire.
39
What is the difference between ductility and toughness?
Ductility deals only with the ability to deform, whereas toughness considers both the ability to deform and the stress developed during the deformation.
40
What plays important roles in the mechanical properties of objects?
Centroid and center of gravity of objects
41
What has to be true of two objects to make the center of gravity and centroid the same?
If the density is uniform in each object.
42
What is the centroid of an area?
The point of application of the resultant of a uniformly distributed force acting on the area.
43
What is the moment of inertia of finite area about an axis in the plane of the area?
The summation of the moments of inertia about the same axis of all elements of the area constained in the finite area.
44
What is the definition of the moment of inertia?
The product of the area and the square of the distance between the area and the given axis.
45
What is the parallel axis theorem?
It states that the moment of inertia with respect to any axis is equal to the moment of inertia with respect to a parallel axis through the centroid added to the product of the area and the square of the distance between the two axes.
46
What is the equation for the parallel axis theorem?
Moment of inertia about x-axis= Moment of inertia about centroid+Area(Distance^2)
47
What is the neutral surface?
The area of a beam where the fibers do not undergo any extension or compression and thus are not subject to any tensile or compressive stress.
48
What is the neutral axis?
The intersection of the neutral surface with any cross-section of the beam perpendicular to its longitudinal axis.
49
What is the equation for normal stress of a beam?
Normal stress=Ty/I
T=Torque
y=Distance from the neutral axis of the beam
I=Moment of intertia of the cross-sectional area about the neutral or centroidal axis in inches.
50
What are the three stress of a beam?
Bending, flexure, fiber stresses
51
What is the equation for maximum torque of cantilevered beams?
Tmax=FL
52
What is the equation for torque of freely supported beams
Tmax=FL/4
53
How do you calculate bending torque with multiple forces acting on a beam?
Sum of the torques caused by all the external forces acting on the beam.
54
Where do most device failures occur?
The corresponding point of maximum torque (bending moment)
55
What is the equation of stress in a cantilevered beam?
Bending stress=F(Lc)/I
56
What is the equation of bending stress in a freely supported beam?
Bending stress=F(Lc)/4I
57
Excessive deflection will do what?
Will disturb alignment or prevent successful operation.
58
What is the equation for deflection y, anywhere along the length?
y(x)=(Fx^3/-6EI)+(FxL^2/2EI)-(FL^3/3EI)
59
What is the equation for maximum deflection of the cnatilevered beam (ymax)? Where does it occur?
ymax=-(FL^3/3EI)
| Occurs at the free end when x is 0
60
What is the equation for deflection of the freely supported beam with the midspan loaded?
y(x)=(Fx^3/12EI)-(FxL^2/16EI)
61
What is the equation of maximum deflection of the freely supported beam? Where does it occur?
ymax=FL^3/-48EI
| It occurs at the midspan when X=L/2
62
What is the definition of a metal?
A chemical element that is lustrous, hard malleable, heavy, ductile, and tenacious and usually is a good conductor of heat and electricity.
63
What are the most widely used metallic elements?
```
Iron
Copper
Lead
Zinc
aluminum
Tin
Nickel
Magnesium
```
64
What is the definition of an alloy?
A combination of elements that exhibits the properties of a metal.
65
Orthotics and prosthetics typically contain alloys of what?
Aluminum, carbon steels, particularly stainless steel.
| Titanium also is used
66
What is the most important characteristic of all metals?
Their Crystallinity
67
What is a crystalline substance?
One in which the atoms are arranged in definite and repeating order in a three-dimensional pattern.
68
What is the regular arrangement of atoms called in a crystalline substance?
A space lattice
69
What are the three types of space lattices?
Cubic system
Tetragonal system
hexagonal system
70
What is the cubic system?
Three contiguous edges of equal length and at right angles.
| Simple lattice, body-centered lattice, and face-centered lattice
71
What is the tetragonal system?
Three contigous edges, two of equal length, all at right angles.
Simple lattice, and body-centered lattice
72
What is the hexagonal system?
Three parallel sets of equal length horizontal axes at 120 degrees and a vertical axis
Close-packed hexagonal.
73
How is the orderly state of crystallinity described?
Balanced, unstrained, or annealed.
74
What are examples of metals that normally exist in only the one form of face-centered cubic?
Ca, Ni, Cu, Ag, Au, Pb, Al
75
What are examples of metals that normally exist in only the one form of body-centered cubic?
Li, Na, K, V, W
76
What are examples of metals that normally exist in only the one form of Face-centered tetragonal?
In
77
What are examples of metals that normally exist in only the one form of Close-packed hexagonal?
Be, Mg, Zn, Cd
78
What metal is an example of a metal that can exist in more than one lattice form?
Iron
79
What are the different temperatures and lattice structures that Iron can exist at?
Body-centered cubic: Below 1663 Degrees F
Face-centered cubic: 1663 degrees to 2557 degrees F
Body-centered cubic: 2557 degrees to 2795 degrees F
80
A metal in the liquid state is not what?
Crystalline, and the atoms move freely among one another without regard to interspatial distance.
81
What is the freezing point?
The temperature at which atoms begin to arrange themselves in a regular geometric pattern (lattice).
82
If a space lattice is permitted to grow freely without interference what is produced?
A single crystal is produced that has an external shape typical of the system in which it crystallizes.
83
As a liquid metal cools what is formed?
Crystallization centers form at random throughout the liquid.
84
As crystallization continues from a liquid metal what happens?
More crystallization centers are produced and develop with space lattices of random orientation. During the last stages of formation, the different crystal centers meet, but there are places at the surface of intersections where development of other space lattices is impossible. This accounts for the irregular appearance of crystals in a piece of metal that is polished and etched.
85
During the growth process of crystals , the development of external features, may be prevented due to what?
The interference from the growth of other centers.
86
What is grain?
Each unit with a crystal center. Essentially a single crystal.
87
The term crystal usually refers to what?
A group of space lattices of the same orientation that show symmetry by the development of regular faces.
88
The size of the grain is dependent on what?
The temperature from which the metal is cast, the cooling rate, and the nature of the metal.
In general, slow cooling leads to coarse grain and rapid cooling to fine grain metals.
89
Which lattice structure is more ductile or wrokable?
Cubic patterns.
90
Which lattice structure is more brittle or more rigid?
Hexagonal and more complex patterns.
91
What is the elastic limit?
The force required to bring the first permanent displacement.
92
What are slip planes?
The specified planes where permanent displacement, or slip occurs in the lattice.
93
What is the criterion of plasticity?
The ability of a crystal to slip in this manner without separation.
94
Practically all metals are what?
Plastic to a certain degree.
95
During plastic deformations, the lattice undergoes what?
Distortion, thus becoming highly stressed and hardened.
96
What are the characteristic of plans that will most likely undergo slip?
The planes that have the greatest population of atoms and, likewise, the greatest separation of atoms on each side of the planes under consideration.
97
Sliding movements tend to take place at what angle to the direction of the applied load? Why?
45 degrees
| Because much higher stresses are required to pull atoms directly apart or to push them straight together.
98
The mechanical properties of metals depends on what?
Their lattice structures.
99
Which lattice structure is most ductile throughout a wide range of temperatures?
Face-centered cubic.
100
Which lattice structure is hardened by cold working, and plastic deformation takes place most easily on planes parallel to the base of the lattice?
Close-packed hexagonal lattice.
101
Plasticity depends on what?
The ability to shape and contour aluminum and stainless steel to match body contours
102
Elasticity depends on what?
The safe and economical use as load-bearing members.
103
What is the elastic limit?
The maximum stress at which the body behaves elastically.
104
What is the proportional limit?
The stress at which strain ceases to be proportional to applied stress.
It is practically equal to the elastic limit.
105
What is plasticity?
The term used to express a metal's ability to be deformed beyond the range of elasticity without fracture, resulting in permanent change in shape.
106
What is the ratio of plastic-to-elastic deformation in metals?
It is high around 100:1 or 1000:1.
107
What are things that can intensify the stresses to an area and cause breaking?
```
Imperfections
Flaws in the regularity of the crystal lattice
Microcracks within a grain,
shrinkage
voids
nonmetallic inclusions
rough surfaces
notices of all kinds
```
108
Notches act not only as stress raisers but what?
Also as stress complicators, because they frequently induce stress in many directions.
109
Iron as a pure metal does not posses what?
Sufficient strength or harness to be useful for many applications.
110
What is carbon steel?
Iron with added carbon.
111
What is toughness?
Ability to withstand shock forces
112
What is hardness/
Resistance to penetration and abrasion
113
What is ductility?
Ability to undergo permanent changes of shape without rupturing
114
What is corrosion resistance?
Resistance to chemical attack of a metal under the influence of a moist atmosphere.
115
The addition of elements to carbon steel can what?
Increase elasticity and tensile strength as well as improve surface finish and machinability.
116
Nickel steels are characterized by what?
Improved toughness, simplified heat treating, less distortion in quenching, and improved corrosion resistance.
117
Nickel chromium steels are characterized by what?
Increased depth hardenability and improved abrasion resistance.
118
Molybdenum steels are characterized by what?
The greatest hardenability, with manganse and chromium, increased high-temperature strength, and increased corrosion resistance.
119
Chromium steels are characterized by what?
Increased hardening effect.
120
Vanadium steels are characterized by what?
increased refinement of the internal structure of the alloy, making them suitable for spring steels and construction steels.
121
Silicone manganese steels are characterized by what?
Increased strength and hardness.
122
Chromium molybdenum steels are characterized by what?
Excellent hardenability and satisfactory ductility.
123
Chromium nickel steels are characterized by what?
Good hardenability and satisfactory ductility.
124
What is the effect of combining three alloys with iron?
It produces a material superior in specific characteristic performance to the sum of each alloy used separately.
125
What are stainless steels?
Steel alloys containing a large amount of chromium (>3.99%)
126
What are the three general categories of stainless steel?
Austenitic
martinsitic
ferritic
127
What is cold working?
Plastic deformation of a metal at temperatures that substantially increase its strength ad hardness.
128
How does aluminum have an advantage over steel?
Lighter in weight but also easier to work with than steel.
129
What is a disadvantage of aluminum over steel?
The aluminum device is more subject to fatigue and failure. It is also not as strong.
130
Are all aluminums weaker than all steels?
No
131
How can aluminum be stronger than steel?
By adding certain alloy elements, proper heat treatment, or cold working, some alumiums can be increased in strength to an ultimate stress tolerance of 90,000 psi, which is above the strength of some steels.
However aluminum is still more subject to fatigue failure than the steel.
132
What are the purposes that heat treatment can accomplish?
Increase or decrease hardness and tensile strength, relieve internal stresses because of hot or cold working, improve machinability, and increase toughness.
133
If steel is heated above its critical temperature range, it undergoes what?
Definite internal changes, which have time to reverse themselves during the cooling period. If the steel is cooled more rapidly than the internal changes can reverse, the structure of the steel is modified and its mechanical characteristics are altered.
134
How can you tell if a metal is austenite?
The carbon can longer remain chemically combined with the iron, so the carbides are broken up, and the carbon goes into solution in the iron, which is called solid solution.
135
What is normalizing the steel?
The steel returns to its original or normal internal structure.
136
What is the benefit and problem of normalizing steel?
It has higher strength and hardness but less ductility.
137
What is annealing steel?
The steel is heated to a temperature above the critical range and then is slowly cooled in the furnace.
138
What is the benefit of annealing steel?
It relieves internal stresses and lowers the yield point to obtain maximum ductility.
139
What is tempering steel ?
The steel is heated again to a point below the critical range and then cooled at a controlled rate. The higher the temperature during the tempering process, the lower the strength and hardness and the higher the ductility.
140
What is the main cause of breakage?
Fatique stresses, which are the result of repeated application of small loads rather than the application of a large load.
141
How can you lessen fatigue stress?
Have the person stop walking and rest, so the brace/metal has time alleviate the fatigue stresses.
142
Stress in braces is concentrated where?
Nicks, notches, drilled holes, sharply bent corners or name stampings.
143
What are recommendations to help minimize the points of stress concentration?
Remove nicks and scratches from the material by polishing.
Cap the checkered jaws of the cise before clamping the work into the vise.
Ensure that contouring instruments have smooth, curved surfaces.
Plan all cuts and bends to ensure smooth transitions and minimal changes.
Do not shape the orthosis stirrups with a metal hammer.
Avoid abrupt changes in cross section, such as the joint area.
144
Many orthotic parts act in the same manner has what?
Beams
145
When the direction of maximum stress is not known the material should be distributed in the form of what?
A ring. The ring shape resists binding equally well in all directions.
146
Why is riveting an advantage for combining material compared to welding, brazing, and soldering?
The other methods depend on the addition of heat to the material, and if the material dependson heat treatment for strength prior, the additional heat may alter the desired mechanical properties that have been achieved.
147
What is the preferred material of rivet for fastening aluminum?
Aluminum
148
What happens if dissimilar metals are in contact with each other in a moist atmosphere?
Galvanic corrosion takes place which lowers service life.
149
Why would stainless steel, hot-dipped aluminized or cadmium plated steel rivets be used?
If the larger holes required for aluminum rivets would weaken the material.
150
What should the rivet size be in regards to the material?
The diameter of the rivet should not be less than the thickness of the thickest part through which the rivet is driven, but not greater than three times the thickness of the thinnest part.
151
What should the spacing be with the rivet?
The minimum spacing between rivets is three times the nominal rivet diameter. As a general rule, the maximum distance should not be greater than two to four times the thickness of the thickest member.
152
What should the edge distance of the rivet be?
From the center of the hole to the end of the member should be at least twice the diameter of the rivet.
153
Rivets sets should be what? To allow what? The bucking tool should have sufficient what?
Rivet sets should have smooth, polished surfaces to allow the metal to flow readily during the forming operation. The bucking tool should have suffieint mass.
154
The length of the rivet necessary for proper forming of a head depends on what?
The total thickness of metal through which the rivet is driven, the clearance between the rivet and the rivet hole, and the form of the head.
155
When should a strong rivet no t be used?
In a weak plate because the plate may become distorted.
156
What are plastics?
Synthetic materials made from raw chemical materials called monomers.
157
How is polyethylene formed/
A monomer (one chemical unit) such as ethylene is reacted with other monomer molecules into long chains of repeating ethylene units.
158
Polymers consist of what?
Atoms of carbon in combination with other elements.
159
As molecules are made longer and become heavier, the polymer wax becomes what?
Harder
| Tougher
160
At approximately, what, the material is tough enough to become a plastic?
C100H202
| Continueing to add CH2 groups to the chain increases the strength and toughness even more.
161
The CH2 chain length of monomers determines what?
It determines many properties of plastic, it also affects its processing characteristics.
162
What are the major effects of increasing chain length?
```
Greater touchess
Creep resistance
stress-crack resistance
melt temperature
melt viscosity
processing difficulty
Crystallinity
```
163
What happens with a crystallized area of plastic? How about a noncrystallized, or amorphous, area?
It's stiffer and stronger
| It's tougher and more flexible.
164
What are other effects of increased crystallinity in a polyethylene?
resistance to creep, heat, stress cracking, and increased shrinkage after forming.
165
In general, what are the characteristics of crystalline polymers?
They have higher forming temperatures and melt viscosities.
They have a sharp melting point, meaning they don't soften gradually, but remain hard until a given quanitity of heat is absorbed, then they rapidly change into a low viscosity liquid.
Reinforcement of crystalline polymers with fiber of glass or other materials improves their load-bearing capabilities significantly.
166
What are the characteristics of amorphous polymers?
They soften gradually as they are heated, but do not flow as easily as crystalline materials. Reinforcing fibers do not significantly improve the strength of amorphous materials at higher temperatures.
167
What are examples of amorphous thermoplastics?
acrylonitrile-butadiene-styrene (ABS) polystyrene, polycarbonate, polysulfone, and polyethermide.
168
What are examples of crystalline plastics?
Polyethylene, polypropylene, and polyethertherketone
169
What is a copolymer?
A polymer made from two different monomers.
170
What is a terpolymer?
A polymer made from three different monomers.
171
When is the term homopolymer used?
When a polymer family includes copolymers, then homopolymer is used to identify the single monomer type.
172
How are the properties of a copolymer determined?
The percentage of monomer A to monomer B, the properties of each, and how they are arranged along the chain.
173
What are block copolymers?
Copolymers that have large areas of polymerized monomer A alternating with large areas of polymerized monomer B.
174
How are block copolymers different to alternating copolymers?
It has stronger crystalline areas and tougher amorphous areas.
175
What is a graft copolymer?
It is made by attaching side groups of monomer B to a main chain of monomer A.
176
Do copolymers always have different properties from those of a homopolymer made from either monomer?
Yes.
177
Are thermoset plastics made the same way as thermoplastics?
NO.
178
How are thermoset plastics made?
```
The polymerization (curing) of thermoset plastics is done in two stages, partly by the material supplies and partly by the molder.
First you polymerize by reacting phenol with formaldehyde under heat and pressure. The reaction is stopped at the point at which mostly linear chains have formed. The chains will contain unreacted portions that are capable of flowing under heat and pressure. The final stage is completed in the modling press, which the partially reacted phenolic liquefied under pressure, producing a cross-linking reaction between molecular chains.
```
179
What is a typical thermoset plastic?
Phenolic
180
Rigid thermosets have what type of chains and what links?
Have short chains with many cross-links.
181
Flexible thermosets have what type of chains and what links?
Long chains with fewer cross-links.
182
After a thermoset has been heated and molded, is it reversible?
No. It has a permanent structure.
183
What are examples of addition-cured thermoset plastics?
epoxy
| polyester
184
In general what is the benefit of thermosets compared to thermoplastics?
They resist higher temperatures and provide greater dimensional stability than do most thermoplastics.
185
Most thermoset composites are based on what materials?
Polyester and epoxy resins
186
Which of the two material of thermosets is the most dominant?
Polyester
187
How can thermoset composites be molded?
By any process used for thermosetting resins. They can be cured at room temperature and atmospheric pressure.
188
Thermoset composites balance what?
Low cost and ease of handling along with good mechanical properties and dimensional stability.
189
What are epoxies?
Low-molecular-weight, syruplike liquids that are cured with hardeners to cross-link thermoset structures that are hard and tough.
190
What material is often used for reinforcing thermoset composite material?
Glass fiber with a tensile strength of thermosetting resins.
191
What are other reinforcements used in thermoset composite material?
Carbon, Born, aramid (kevlar).
192
What are the different forms that glass fiber is available as?
Roving (continuous strand)
Chopped strand
Chopped-strand mat
Milled fivers
193
Which of the glass fiber forms provide greatest strength?
Longer fibers
194
Which of the glass fiber forms provides set in tension and are the strongest?
Continuous fibers.
195
When forces are applied plastics are subjected to which behaviors?
Elastic (springlike)
| Viscous (slow-flow)
196
The deformation of plastic is _________ dependent.
Time
197
Which tests are used to observe the deformation of plastic?
Creep tests
198
How is a tensile-type creep test performed?
A plastic specimen is clamped on its ends on the tensile tester, and the distance between guage marks on the specimen is measured. A steady tensile force is applied to the specimen.
199
What is the tensile stress of a material?
The magnitude of the force divided by the original cross-sectional area of the specimen.
200
What is the strain in a tensile creep test?
The increase in length between the gauge marks divided by the original length.
201
what are often the results of a tensile creep test?
The plastic immediately undergoes an elastic strain when the load is applied, followed by a period of further but retarded elastic strain and finally a period of steady viscous flow.
202
How is the elastic portion of the tensile creep test determined?
By removing the load from the specimen and observing its recovery.
203
If plastic is strained and the strain is held constant with time, what will happen?
The stress in the plastic reduces with time. Thus, the viscoelastic nature of plastics can cause not only elongation creep under constant stress, but also stress relaxation at constant strain.
204
What is necking?
A phenomenon where the loaded tensile specimen begins to elongate rapidly because of the occurrence of a local area of thinning down.
205
What is static fatigue?
Plastics weaken with age even under steady loads.
206
What has a major effect on the mechanical behavior of plastics?
Temperature.
207
Increasing the temperature tends to cause what, in plastics?
Softening of a plastic, with consequent reduction in strength.
208
Decreasing the temperature in plastics tends to cause what?
The plastic to become brittle.
209
In general plastics are much _____ than many other materials.
Softer.
210
How is hardness determined in plastics?
Indentation tests of the rockwell type.
211
How are indentation tests performed?
A steel ball under a minor load is applied to the surface of the specimen. This action slightly indents the surface and ensures good contact. The major load is applied for 15 seconds and removed. Then both indentation are measure from the minor load to the major load and given a hardness number.
212
What is a thermosetting resin?
A synthetic organic polymer that cures to a solid infusible mass by forming a three-dimensional network of covalent chemical bonds.
213
How do thermoset plastics compare to metals?
They possess corrosion resistance, lighter weight, and insulating properties, and they can be processed at lower pressures and temperatures.
214
How do thermoset plastics compare to ceramics?
Thermoset plastics offer lighter weight, more toughness, and easier processing. Ceramics offer high-temperature performance.
215
How do thermoset plastics compare to thermoplastics?
They have reduced creep and improved crack resistance. They also lead to improved machinability, low shrinkage, and improved high-temperature performance. Often lower cost fabrication prices.
216
What is a limitation of thermoset plastics?
Poor impact resistance.
217
Do thermosetting plastics become soft again after they have already been heated and hardened?
They do not become soft again.
218
Thermosetting plastics exhibit little cold flow and therefore can be bjected to what?
Continuous loads.
219
What two names can be considered chemical classifications of plastics?
Thermoplastics
| Thermosetting.
220
What are the physical classifications of plastics?
Rigid
Flexible
Elastomeric
221
What is the definition of rigid?
Relatively nondeforming under loads.
222
What is the definition of flexible?
Deforming under loads.
223
What is the definition of elastomeric?
Having high elongation.
224
How are plastic resins formed?
By condensation reaction in which two or more unlike molecules are combined to form a larger molecule, accompanied by the loss of water or a gas.
225
What is polymerization?
The stage or reaction that follows condensation. The resin formed in the condensation reaction, known as the monomer, is not suitable as a molding material but can be used as the base for lacquers. The monomer is converted into the polymer, and this usually takes place during molding or fabricating processes, when the monomer is subjected to the action of a catalyst and varying conditions of heat and pressure.
226
In general, a monomer is a _________. whereas a polymer is a ________.
Liquid
| Solid.
227
What are the characteristics of Synthetic resins?
They are usually amorphous (non-crystalline)
Have no definite melting point.
Have a definite temperature range in which they soften.
There is an increase in viscosity during the polymerization process.
228
How do synthetic resins compare when used with thermoplastic and thermosetting types of molding compounds?
In thermoplastic class the resin is fully polymerized when it is used as a molding compounds, while it is only partially polymerized when in thermosetting molding material.
229
What are laminates?
A base material impregnanted with a plastic resin that is allowed to harden under pressure.
230
Which plastic resins are used in laminated plastics/
Thermosetting resins.
231
What does the base material provide in laminating? How about the resin?
Mechanical strength
| Rigidity and dimensional stability.
232
What are some base material used in laminating?
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Nylon
Dacron
Fiberglass
boron
aramid (kevlar).
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233
What are the different categories of laminates?
High-pressure laminate
Low-pressure laminates
Contact pressure laminates
234
What are high-pressure laminates?
Laminates that are formed from thermosetting materials under pressures ranging from 1000 to 2000psi. They are strong, lightweight, and have high-impact resistance.
235
What are examples of high-pressure laminates?
Paneling
Countertops
safety helmets.
236
What are low-pressure laminates?
Laminates that are formed from thermosetting materials under pressures ranging from 15-1000psi
237
What is an example of low-pressure laminates?
Vacuum bag molding.
| Prosthetic leg lamination.
238
What is contact pressure laminates?
Laminates that are formed under pressures as low as 0.25 to 15 psi.
239
When is contact pressure laminates used?
When each piece must be custom-made using hand pressure.
240
What are cellular plastics?
Plastics that consist of plastic resins that have been foamed or filled with bubbles of gas before the resin hardens.
Another name is foamed plastics.
241
When are sandwich constructions used?
When maximum stiffness is required for a given wieght of material.
242
How are sandwich constructions made?
By laminating a cellular core between skins of metal or thin lastic fabric laminates.
243
What can act as the cellular core in sandwich constructions/
Paper honeycombs,
balsa wood
Cellular plastics.
244
What are miscellaneous structures important to practitioners?
Glass and cotton fabrics coated with thermoplastic resins and fabrics of glass and plastic fibers woven together.
245
How are polyesters versatile?
They can be molded, cast and laminated with contact pressure sleeves over inexpensive molds of plaster, rubber, or low-melting metals.
246
Polyesters are resistant to what substances?
Gasoline
alcohol
acids
moisture
247
What materials are used in th elamination process of prosthetists and orthotists/
Polyesters and other resin.
248
Why is 100% polyester resin not used?
It is too brittle.
249
What are the two types of polyester resins tha can be used in making laminates?
Air-inhibited
| Non-Air-inhibited type.
250
What is inhibition?
The slowing down of polymerization or curing of the resin because of the presence of atmospheric oxygen.
251
What does air-inhibited resins require?
Careful control of the air seal and manual mixing to control the flexibility.
252
What are polyester laminates used for?
Sockets
Cuffs
Artificial hands.
253
When laminating the first layer, what should be done?
Lubricate the plaster of paris. A polyvinyl alcohol (PVA) bag also can be used to form the first layer.
254
What's the second part of the laminating preparation process?
The fabric is next placed in contact with the plaster form (or bag), and a thin coating of resin, sufficient to saturate the fabric, is applied to the surface. Then a dry piece of fabric is smoothed into place in a way to provide intimate contact between the two layers of fabric to force any excess resin to surface. More fabric is added till the thickness is as desired.
255
What is the third part of the laminating preparation process?
Another PVA bag is placed over the whole piece, excess resin is squeezed out, and the laminate is oven cured.
256
What is another plastic lamination method?
Fiberglass, tubular nylon, or cotton stockinette is used as a resin reinforcement. 3 or 4 layers are added. After the stockinette has been pulled over the plaster cast and tied, a snug-fitting PVA bag is pulled over the stockinette to form a tight and smooth outer surface. Polyester resin is then poured into the outer PVA sleeve . Then an airless cure is achieved, by tying the end of the PVA bag and stringing the mold. It takes about 45 minutes.
257
What are the two types of thermoplastics?
Amorphous
| Crystalline
258
What are amorphous materials?
Devoid of crystallization and have a randomly ordered molecular structure.
259
The behavior of amorphous material is similar to what?
Viscous, inelastic liquid.
260
what are the benefits of amorphous materials compared to crystalline?
They normally have a better hot strength characteristic than crystalline ones.
261
What happens when amorphous material is cooled?
It never reaches a total nonflowing solid state. So they have a tendency toward creep or movement with age.
262
What are examples of amorphous material?
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ABS
Styrene
vinyl
acrylic
cellulosics
polycarbonates.
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263
What are crystalline thermoplsatics?
They have molecules that are orderly grouped and have a tendency to align in rigid, precise, highly ordered structures such as a chainlink fence.
264
What is the advantage of crystalline thermoplastics?
They have good stiffness and low creep.
265
What are examples of crystalline thermoplastics?
nylon
polypropylene
polyethylene
acetal
266
What are the characteristics of polypropylene?
Notch sensitive
Edges must be smooth
Surface easily marred when hot
May warp or distort if removed from the mold too rapidly
267
What are the common uses of polypropylene?
All orthoses where rigidity is required.
268
What is the shrinkage of polypropylene?
1.5-2%
269
What are the characteristics of copolymer?
Will cold flow (creep)
Not as rigid or birttle as polypropylene
Blanching or crazing develops at areas of high or cyclic stress
Moderatley notch sensitive (need to polish edges)
270
What are the common uses of copolymer?
All orthoses where some flexibility is required; prosthetic check sockets.
271
What is the typical shrinkage of copolymer?
1.5-2%
272
What are the characteristics of polyethylene?
Flexible and easy to vacuum form
Cold flow under pressure with sustained use.
Thinner gauges can be cut by hand
Not particularly notch sensitive (but edges should be polished)
273
What are the common uses of polyethylene?
Spinal and upper limb orthoses
| Orthoses in which greater flexibility is required.
274
What is the typical shrinkage of polyethylene?
At low density: 1.5-3%
| At high density: 3-3.5%
275
What are the characteristics of surlyn (lonomer)?
```
Transparent
Not as rigid or birttle as polypropylene
Very tough
Cold flows
May be solvent bonded
Not affected by cold
Notch sensitive
Can be worked at a wide range of temperatures.
```
276
What are the common uses of surlyn (lonomer)?
Check sockets.
| All orthoses.
277
What are the characteristis of copolyester (Durr-plex)?
Very rigid and brittle.
Difficult to judge proper working temperature.
Best results and to reduce brittleness work with a warm cast.
278
What are the common uses of copolyester (durr-plex)?
Check sockets.
279
What are the characteristics of polycarbonate?
Hydrophilic (must be hydrated 48 hours at 275 degrees F for 3/8in thick material)
Rigid at proper working temperatures.
Sensative to acetone and other solvents.
280
What are the common uses of polycarbonate?
Check sockets.
281
What are the characteristics of kydex?
Abrasion resistance
Dimensionally stable
Rigid
Can be drape formed without vacuum
282
What are the common uses of kydex?
Thoracolumbosacral orthosis (TLSO)
Body jackets
Cervical orthosis.
283
What is the cast and set temperatures?
The set temperature is the temp at which the thermoplastic sheet hardens and can be safely taken from the cast. This is also defined as the heat distortion temperature at 66 psi. The closer the cast temperature is to the set temperature, without exceeding it, the less internal stress and warping.
284
What is the lower processing limit?
The lowest temperature for the sheet before it is completely formed. Material formed at or below this limit has severely increased internal stress that can cause warpage, lower impact strength, and other poor physical properties.
285
What is normal forming temperature?
The temperature the sheet should reach for proper forming conditions under normal circumstances. The core of the sheet should be at this temperature.
286
What is the upper limit temperature?
The temperature at which the thermoplastic sheet begins to degrade or decompose. The material must stay less than this amount.
287
The least amount of internal stress in a thermoformed plastic is obtained how?
With a hot cast, a hot sheet and rapid vacuum.
288
What are the four things that should be considered when selecting a sheet of plastic?
Depth of draw
Desired finished thickness
rigidity
Shrink
289
What should be considered when heating the sheet of plastic?
Open the oven door as little as possible. It might lead to cold and hot spots throughout the sheet.
Don't set the oven for a higher temperature than the upper limit temperature, because excessive heat can be absorbed by the plastic and the surface of the material will be gin to deteriorate.
290
What is convection?
Heat transfer takes place when a material is exposed to a moving fluid of hot air that is at a higher temperature, than the object being heated.
291
How is convection done in the thermoforming industry?
By hot-air recirculating ovens.
292
How does the speed of convection compare to radiation?
Covection is much slower.
293
What is the main advantage of convection heat?
It's uniformity of heating and ability to keep the sheet surfaces from getting hotter than the oven temperature.
294
Convection is recommended when heating what?
Heavy-gauge foam shetts
Very thick, solid sheets
Sheet stocks where the thickness is difficult to control accurately
Sheets where surfaces have been planished
Surfaces might degrade easily if over heated
295
How does the speed of conduction compare to convection and radiation?
It is faster than convection and slower than radiation.
296
Most conduction heater plates are made of what material?
They are made of teflon-coated aluminum plates that are electrically heated
297
What is radiation?
The energy transmitted between two separated bodies at different temperatures by means of electromagnetic waves.
298
What is the benefit of radiation?
It is much faster and it is the most energy-efficient way of heating sheet materials.
299
What is the source of heat in radiation?
Infrared wavelength radiation elements are the usual source of heat; the specific wavelength is related to a given temperature of a specific radiant heater.
300
What is the property all types of radiation have in common?
They travel with the same velocity (the speed of light)
301
How does radiation work?
As the radiant emitter is directly exposed to the material to be processed, a high percentage of the electromagnetic waves is absorbed within the plastic sheet only if the emitter operates at the proper wavelength, and the wavelength is determined strictly by the emitter-surface temperatures.
