Stress, Strain And Young Modulus Flashcards
What is Hooke’s Law?
The force applied will be directly proportional to the extension produced within the elastic limit (unit of proportionality)
What does Hooke’s Law look like on a graph?
- Straight line
- Through the origin
- Scatter is close
Equation for the spring constant?
F = kx, where k = force required to produce unit extension
Equation for the gradient of a spring constant on a graph
K = 🔼 Force / 🔼 extension
Cm^2 ➡️ m^2
x10^-4
mm^2 ➡️ m^2
x10^-6
cm ➡️ m
x10^-2
mm ➡️ m
x10^-3
Explain in terms of compression and tension, what happens to concrete
- Weak under tension
- Strong under compression
- Cracks will propagate upwards until failure
Explain the process of adding a steel rod to concrete
- Steel rod initially under tension
- Tension released as concrete sets
- Puts lower surface under compression making it as strong as the top
What is the area under a F=kx graph equal to?
Energy
Why does the elastic limit occur in metals?
Metals have regular atomic structure and long range order, meaning that any force transmitted will be produced equally between each bond
Why does the plastic limit occur in a material?
When a material is stretched past the elastic limit, atoms slip past each other
When does edge dislocation occur?
Edge dislocation occur when half a plane is missed out. The atoms around this point are places of weakness as they are under strain
What happens when the forces are;
(A) small
(B) large
(A) - bonds stretch reversibly so they behave elastically (Hooke’s Law)
(B) - strained bonds snap and migrate in the direction the stress is applied
When does edge dislocation stop?
The dislocation carries on until it reaches the edge of the crystal
What strengthens materials?
- having smaller grains restricts movement
- introducing foreign atoms
What is the elastic limit on a crystalline stress-strain graph?
Where no bonds are broken. If the force is removed, the material will return to its original shape
What is the yield point?
Where large extensions occur as planes of atoms slide past each other
What is the plastic region?
Where bonds are broken so will NOT return to original shape
What do ductile materials do at the end of stress-strain graph
They become narrower and extend rapidly until breaking
What features does a polymeric material have?
A polymeric material has strong covalent bonds called cross-links which form between molecules allowing the molecules not to slide past each other
What does adding impurities do to a polymeric material?
Adding impurities makes the material rigid however these impurities are scarce
Explain what happens at the start of a polymeric stress-strain graph
There are weaker bonds formed called vat der waals. These form between molecules to keep the molecules together making it harder to stretch (steep gradient)
Explain the second part of the graph
The van der waal forces break easily and rapidly which therefore untangles the molecules. This makes large extensions with little strain (low gradient)
Explain the final part of a polymeric stress-strain graph
The molecules have straightened meaning to stretch the material further, you are lengthening cross-links and lengthening strong bonds between the molecules (steeper gradient)
Definition of stress?
Stress is the force per unit cross-sectional area when equal opposing forces act on a body
Definition of strain?
Strain is the extension per unit length due to an applied stress
Definition of young modulus?
Stress per unit strain
Hooke’s Law region
