Materials Flashcards
Stress
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
Plastics are viscoelastic, meaning that
their physical characteristics are modified by the rate of strain applied
Stress vs. Strain Curve, testing
Continuously increasing tensile force applied to standard specimen until it breaks
Elastic Deformation
Behave in proportional stress/strain manner
If force discontinued, specimen will relax to original length
Deformation is temporary
If force continues to rise, one of two events will occur
Specimen will fracture (brittle failure)
Specimen will permanently deform (ductile failure) and ultimately fracture
Stiffness
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
Slope of elastic region in stress/strain curve
Young’s Modulus
Measure of stiffness (not a measure of strength)
Strength
Measure of stress at which material fails for a single cycle test
Strength is the stress at fracture point
Yield Point
Point at which elastic deformation stops and plastic deformation begins
Plastic Deformation
Increments of stress beyond a critical level produce increasing amounts of permanent strain
Ductile material
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
If material were undergoing plastic deformation, and load was removed
- 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
Mechanical Work
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
Toughness
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)
Main factors in Material Selection
Ultimate strength (tensile)
Yield strength (usually tensile)
Young’s Modulus
Steel vs. Aluminum Alloy
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
Heat treatment
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
Visco Elastic Behavior
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
Plastic Advantages over metal
- 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
Fatigue failure
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
Stress Raiser
• 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
Brittle Materials
Materials that exhibit little or no yielding before failure
Ductile Material
Material that can be subjected to large strains before it ruptures is called a ductile material
