UNIT 1 - Solids Under Stress Flashcards
What is the limit of proportionality
the point at which the forces on an object are so intense that hookes law is no longer obeyed
What is elastic limit
the maximum stress that can be applied to an object without deformation
What is permanent extension?
There is a permanent, irreversible deformation
What is elastic behavior
When the extension disappears when the forces is removed.
What is plastic behaviour
When the extension decreases only slightly when the stress is removed. Doesn’t return to original size and shape
What is spring constant
The force per unit extension
What is stress and units
The force per unit cross-sectional area when equal opposing forces act on a body
Pa or Nm⁻ ²
Define strain
The extension per unit length due to an applied stress
Define Hooke’s law
The tension in a spring or wire is proportional to its extension from its natural length, provided the extension is not too great
What is hooks law equation
F = kx
Force = spring constant x extension (from springs natural length)
How to calculate ktotal (spring constant total)
K= f/x
For parallel
Ktotal = k1 + k2
For series
1/Ktotal = 1/k1 + 1/k2
What happens to k if springs are put in series
X is doubled so k is halved
What happens to k if springs are put in parallel
X is halved so k is doubled
Unit for young modulus
Pascals
Pa
Explain the term young modulus
Ratio of stress/strain
Elastic def
A material that regains its shape after stresses are removed
Ductile definition
Can easily be stretched or drawn into a wire
Malleable
Can be hammered or pressed into a shape without breaking or cracking
Brittle definition
Can snap without any yield
Stiff definition
Small strains for large stressed, not stretchy or bendy (large young modulus)
Plastic definition
Undergoes permanent deformation under large stress rather than cracking
Strong definition
A large stress is needed to break it
Hard definition
Resists indentation on impact
What condition needed for young modulus to be calculated
When the work done is within the limit of proportionality
What are the 3 broad categories of solids
Crystalline
Amorphous
Polymeric
Explain what is crystalline solid and give 4 examples
Atoms are arranged into a crystal lattice which consists of regularly repeating unit cells. Crystalline solids exhibit long range order and symmetry.
Eg. Diamond, metals, graphite, salt
Explain amorphous solids and 3 examples
Atoms have no long range order or symmetry. Considered to be super cooled liquid, atoms are randomly arranged with no overall pattern. More brittle and small elasticity.
Eg. Glass, Ice, Ceramics
Explain polymeric solids and give 3 examples
Comprised of long molecular chains, usually containing carbon. Chains are randomly tangled up.
Eg. Rubber, cellulose, polyethylene
What are the 4 parts on the line of a stress strain graph
Limit of proportionality- up until the stress and strain stop being proportional. (Young modulus)
Elastic limit - material can move up to elastic limit, and still return to original shape
Yield point - when u have such a stress u have a very large strain
Ultimate tensile strength (UTS) - the point where failure occurs
