Chapter 6, Materials Flashcards
What is elastic deformation?
When a spring or material is being stretched such that when the force is removed it will return to its original shape.
What is plastic deformation?
When a spring or material is being stretched beyond the elastic limit such that when the force applied is removed it will not return to its original shape.
What is the elastic limit?
The point which if a material is stretched beyond it will not return to its original shape (when it will be plastically deformed).
State Hooke’s law.
The extension of a spring is directly proportional to the force applied (F = kx or force = constant * extension).
When is Hooke’s law true?
Before the elastic limit of a spring has been exceeded.
What is the name given to k in the equation F = kx and what is it’s units?
k is the force constant measured in Nm^-1.
How do you get the elastic potential energy of a extended spring?
E=Fx/2 (energy = a half * force * extension) or the area under a force-extension graph. Using Hooke’s law also gives the alternative formula E = (kx^2)/2.
What’s a loading/unloading curve?
The curve drawn on an force-extension graph from applying then removing the force.
What are the differences between the loading/unloading curves of a metal wire, rubber band and polythene?
A metal wire has a straight loading curve until its elastic limit where it bends, the unloading curve is parallel to the loading curve meaning if the elastic limit has been reached it will not return to the same point. A rubber band has a sideways s shaped curve where the loading / unloading curves are different but return to the same origin meaning energy is lost to heat, rubber is elastic meaning it does not obey Hooke’s law. The loading curve of polythene is similar to the rubber band but the unloading curve is a straight line under the loading curve.
Define stress, strain and young modulus.
Stress (σ) is the force applied per unit corss sectional area of the wire or σ = F/A. Strain (ε) is the ratio between the extension and original length of the wire or ε = x/L. The young modulus (E) is a constant for each material and is stress / strain (σ/ε).
Describe the stress-strain graph of a ductile material and mention any key points.
The first part of the curve is linear where Hooke’s law applies until the “limit of proportionality”, soon after a material hits it’s elastic limit giving a negative quadratic shape, the material extends rapidly between two “yield points” (one at the top of the quadratic and one at the bottom right), the curve appears to bounce off the second yield point until it hits it’s “ultimate tensile strength where the material is under its maximum stress, the curve then goes back down until the material breaks.
