Materials Flashcards
What is density?
A measure of how compact a substance is.
What is the equation for density?
ρ = m / v
ρ = density, Pa or Nm-2
m = mass, kg
v = volume, m3
1gcm-3 = ?kgm-3
1000
Do elastic objects object Hooke’s law all the time?
No, if they extend too much Hooke’s law stops being true.
What is the limit of proportionality?
Last point where Hooke’s law applies.
What happens beyond the elastic limit?
The material won’t return to its original length once force equals zero.
What is the deformation after the elastic limit?
Plastic.
What is the deformation before the elastic limit?
Elastic.
What is the equation for elastic potential energy?
U = 1/2 k ΔL^2
U = EPE, J
k = spring constant, Nm-1
ΔL = extension, m
What is the area under a force-extension graph equal to?
Elastic potential energy stored.
Define pressure.
The force applied per unit area.
What is the equation for pressure?
F = A P
F = Force, N
A = Area, m2
P = Pressure, Nm-2 or Pa
Define stress.
The force per unit area.
What is the equation for stress?
Stress = F/A
Stress, Nm-2 or Pa
F = Force, N
A = Area, m2
Define strain.
The extension per unit length.
What is the equation for strain?
Strain = ΔL / L
ΔL = extension, m
L = original length, m
What are the units for strain?
No units.
What is the equation for Young’s Modulus?
E = stress/strain
E = Young’s Modulus, Nm-2 or Pa
If E is small what does this mean?
The material stretches easily.
If E is big what does this mean?
The material is difficult to stretch.
When does Young’s Modulus change?
When a material is heated.
When a material stretches beyond its elastic limit.
What is the equation for Young’s Modulus where stress and strain are broken up into their respective equations?
E = F L / A ΔL
What is the equation linking the spring constant and Young’s Modulus?
k = E A / L
Is k constant for a material?
No, it depends on thickness and length of the material.
On a stress-strain graph what is the gradient?
E
What is the area under a stress-strain graph?
Energy per unit volume.
Define breaking stress
Stress that breaks the material.
Define ultimate tensile stress.b
The maximum stress a material can take and recover from.
Define yield point. Where is it on a graph?
Where material continues to extend with significant plastic deformation even if the load is nearly constant.
The point before it dips, before the breaking point.
Do brittle materials deform plastically?
No, they just break.
