Lecture 2 - Mechanical Properties of Materials Flashcards
What do extensive properties depend on?
the amount of a material
What are two examples of extensive properties?
mass and volume
What do intensive properties not depend on?
the amount of a material
What are two examples of strength
stress and strain
What is stress?
force applied to deform a structure
What is the formula for stress?
sigma = F/A
What are the units for stress?
pascal (MPa) or PSI
What are the 3 kinds of stresses?
compression, tension, shear
How is compression created?
when force is applied inward on an object
How is tension created?
when forces are pulled away from the object
How is shearing created?
when a parallel force is applied to the area
What does strain result in?
deformation of a material from a force
What kind of direction of deformation can a strain have?
perpendicular or parallel
What is strain?
change in the length of materal/resting length of material
What is the formula for strain?
e = deltaL/L
What is a stress-strain curve?
a graph relating stress to strain
What are stress-strain curves useful for?
- examining how materials change with age
- examining how materials react to different forces
- examining how materials react to everyday stress
What happens during the toe region?
initial un-crimping of fibers
What are the 4 regions of stress-strain curves?
toe region, elastic region, plastic region, failure point
What happens during the elastic region?
material returns to original length when load is removed
What happens during the plastic region?
structure does not return to original length when load is removed
What happens during failure point?
fracture or breaking point
What is Young’s modulus also referred to?
modulus of elasticity (material’s stiffness)
What is Young’s modulus?
the slope of the straight line (elastic region) of a stress-strain curve
A material with a high Young’s modulus undergoes….
less strain under a given load
A material with a low Young’s modulus undergoes…
more strain under a given load (stiffer)
What kind of material is ductile?
it deforms plastically before failure
What kind of material is brittle?
fails before plastic deformation
What kind of relationship is brittle failure?
stress-strain
What happens at ultimate tensile strength?
the material breaks
What is ductile failure?
when material yields with continued increase in the applied load
What does Poisson’s Ratio state (2)?
- when a material is stretched in one direction it tends to get thinner in the other two directions
- when a material is compressed in one direction it tends to get thicker in the other two directions
What is Poisson’s Ratio?
the ratio between a material’s transverse and longitudinal strain when it is stretched
What does the negative sign do in Poisson’s Ratio?
it keeps the Poisson’s ratio positive for objects in which stretching longitudinally leads to a decrease in the lateral dimension
Most materials have Poisson ratio between…
0 and 0.5
What is material fracture?
when a material is compressed in one direction it tends to get thicker in the other two directions
When does fatigue fracture happen?
when a material is loaded and unloaded repeatedly, and the maximum loads are below the ultimate tensile strength(UTS)*
What is fatigue limit?
the stress below which the material will never fail in fatigue
What are stress rates?
psi/second or MPa/second
What are strain rates?
pure frequency, per second, inch/inch/second, mm/mm/second
What is sensitive to strain rate?
soft tissues
What is stress relaxation?
the reduction of stress within a material over time as the material is subject to constant deformation
What is creep?
the continued deformation of material over time as the material is subjected to a constant load
What do bones have a similar fashion to?
beams
What is a neutral axis?
the location where a beam experiences zeros tress
How are the stresses on each side of the neutral axis defined as?
compressive and tensile
What is applied instead of a bending movement?
a torque or twisting force
What does torsion generate?
shear stresses that are distributed over the entire structure
What is buckling?
fracture mode of a structure
Short and wide column =
compression under a load
As a column gets taller and thinner,
it becomes less stable
What happens to a perfectly centered force on a column?
it will continue to compress
What will cause the column to bow out?
an off-centre force
