3.4 Materials Flashcards
What do tensile forces cause?
Tensile deformation
What do compressive forces cause?
Compressive deformation
What is Hooke’s Law? (2)
For forces less than the elastic limit of a spring, the extension is directly proportional to the force applied
F = kx
What does the force-extension graph of a spring show? (2)
Straight line from origin to elastic limit - elastic deformation
Begins to curve - plastic deformation
What is elastic deformation?
The material will return to its original shape when the force is removed
What is plastic deformation?
Permanent structural changes mean that the material will not return to its original shape when the force is removed
What is the work done in a spring transferred to? (2)
Elastic potential energy
Fully recoverable due to elastic behaviour of spring
What does the area under a force-extension graph represent?
Work done
How is elastic potential energy calculated?
1/2 Fx
1/2 kx^2
What is tensile stress? (2)
Force applied per unit cross-sectional area of wire
σ = force / cross-sectional area
What is tensile strain? (2)
The fractional change in the length of the wire
ε = extension / original length
What is a ductile material?
One that can be easily drawn into a wire or hammered into thin sheets (e.g. mild steel)
What are upper and lower yield points?
The points between which the material extends rapidly
What is ultimate tensile strength?
The maximum stress a material can withstand before it breaks
What happens, for a ductile material, beyond the ultimate tensile strength? (2)
Necking - the material becomes longer and thinner at weakest point
Snaps at breaking point
What is the breaking strength of a material?
The stress value at the point of fracture
What is the Young modulus? (3)
The constant ratio of stress to strain within the limit of proportionality
Depends only on material, not shape or size
E = σ/ε
What is a hysteresis loop? (2)
The loop formed by the unloading and loading curved on a force-extension graph for a material such as rubber
More work is done stretching the rubber band so the excess energy is released as thermal energy when material is unloaded (area of loop)
