Materials

Question 1

Stress

Answer 1

F/A

Force per unit area of the plane on which loading is applied

Intensity of loading

Used for

Tension
Compression
Shear

Bending is a combination of these stresses

Question 2

Plastics are viscoelastic, meaning that

Answer 2

their physical characteristics are modified by the rate of strain applied

Question 3

Stress vs. Strain Curve, testing

Answer 3

Continuously increasing tensile force applied to standard specimen until it breaks

Question 4

Elastic Deformation

Answer 4

Behave in proportional stress/strain manner

If force discontinued, specimen will relax to original length

Deformation is temporary

Question 5

If force continues to rise, one of two events will occur

Answer 5

Specimen will fracture (brittle failure)

Specimen will permanently deform (ductile failure) and ultimately fracture

Question 6

Stiffness

Answer 6

Relationship between stress and strain

Material that is stiff demands high stress levels to produce small amounts of strain, will have a steep stress/strain slope, high Young’s modulus

Question 7

Slope of elastic region in stress/strain curve

Answer 7

Young’s Modulus

Measure of stiffness (not a measure of strength)

Question 8

Strength

Answer 8

Measure of stress at which material fails for a single cycle test

Strength is the stress at fracture point

Question 9

Yield Point

Answer 9

Point at which elastic deformation stops and plastic deformation begins

Question 10

Plastic Deformation

Answer 10

Increments of stress beyond a critical level produce increasing amounts of permanent strain

Question 11

Ductile material

Answer 11

One which undergoes plastic deformation before failure

A ductile material has a large range of plastic deformation before fracture

larger area under stress strain graph indicates it requires more energy to cause catastrophic failure

Question 12

If material were undergoing plastic deformation, and load was removed

Answer 12

• it would unload in a line parallel to the elastic portion of the graph
• the amount of permanent deformation would be the resultant strain at the point where the unloading line crosses the strain axis of the graph
• Materials that exhibit this behavior are said to be ductile

Question 13

Mechanical Work

Answer 13

o Force applied to the body multiplied by distance (or deformation) that occurs under the influence of load

o Work required to bring about fracture in a material is the area under a stress-strain graph

Question 14

Toughness

Answer 14

Material’s ability to withstand impact loading

Ductile materials generally more tough, as they deform in plastic manner before catastrophic failure (as opposed to brittle materials which just fracture at a certain stress)

Question 15

Main factors in Material Selection

Answer 15

Ultimate strength (tensile)

Yield strength (usually tensile)

Young’s Modulus

Question 16

Steel vs. Aluminum Alloy

Answer 16

Steel is stronger and stiffer

• better for heavy/vigorous individuals
• larger energy to failure
• however heavier

Aluminum is lower density (lighter)

• preferable for lower limb orthotics because of weight
• possible to use larger aluminum alloy section to compensate for lower strength, and still be lighter than steel

Question 17

Heat treatment

Answer 17

o High carbon steels made harder by cooling them rapidly under water
• Stronger, but renders them very brittle
• Unsuitable for orthotic applications

o Compromise between hard brittle and soft ductile can be achieved by different types of heat treatment

Question 18

Visco Elastic Behavior

Answer 18

o When material is strained to a particular level and held at that level, stress produced within material will gradually diminish with time
• Material’s ability to resist force diminishes the longer strain level is maintained
• At the end of the day, plastic AFO is less effective than it was at the beginning
o Difficult to make comparisons of their physical properties
o Strain rate highly variable in nature
o Ambient temperature also causes significant variations in performance of plastic materials
• Yield strength of poly propylene with temperature
• Yield strength diminishes as much as 50% in range 0-40 degrees Celsius

Question 19

Plastic Advantages over metal

Answer 19

• Molded to cast of patient’s shape
• Provides more closely fitting device that is quicker and easier to manufacture
• Some can be molded directly to patient
• Can be more cosmetic
• Often lighter

Question 20

Fatigue failure

Answer 20

o Repeated loading causes materials to fail at a stress somewhat lower than their strength

• Effect of cyclic loading is to cause materials to weaken
• Failure at consequently lower stress level
• Several factors have bearing on # of cycles of loading necessary to cause fatigue failure
• Higher level of stress, lower number of cycles to failure
• Steel will withstand millions of cycles of loading when stress levels are kept to less than half the yield point
• However when stress levels are close to the yield point, number of cycles to cause failure drops to thousands

Question 21

Stress Raiser

Answer 21

• Poor finish in the form of scratches or notches, or sudden changes of section
o All have effect of producing “stress raiser” in material
• Higher stress levels cause failure to commence in stress raised area, causes failure to spread gradually across section
• Accelerated process because as it occurs, cause general stress level to rise as the material section becomes effectively smaller (F/A)
• Smooth finish is of great importance to reduce risk of early structural failure due to fatigue
• Cut marks/notches should be smoothed out before dispensing to patient

Question 22

Brittle Materials

Answer 22

Materials that exhibit little or no yielding before failure

Question 23

Ductile Material

Answer 23

Material that can be subjected to large strains before it ruptures is called a ductile material