Chapter 4 Flashcards
Define stiff
Small strain for a large stress.
Higher stiffness = higher YM
Define Hard
Resists indentation on impact
Define Elastic
Returns to outstretched form when stresses are removed
Define Plastic
Undergoes permanent deformation under a large stress rather than cracking
Define Strength
A measure of how much a material can resit being deformed by a force without breaking
Strong material requires a large stress is needed to break it or deform it
Define Brittle
Breaks suddenly as cracks travel through it; little or no plastic deformation
Define Toughness
Toughness is a measure of the energy a material can absorb before it breaks
A tough material undergoes considerable plastic deformation before breaking
Describe ceramics
Hard, Brittle, Stiff
Describe metals
Pure metals = soft
Metals that can be easily shaped = malleable
Those that can be drawn into wires = ductile
Alloys = usually harder
Describe polymers
Glass polymers - similar properties to glass, brittle
Semi-crystalline polymers - Tough
Give the equation for Hooke’s Law
Force F (N) = spring constant k (N/m) * extension x (m) F=kx
Describe the relationship between F and x in Hooke’s law
They are proportional
k = constant of proportionality
A large spring constant (k) means that…
Difficult to stretch
What affects the value of k?
Material, length, cross-sectional area (of the wire)
Spring constant is a value for a specimen not a material!!
Spring constant is a value for a specimen not a material!!
Hooke’s law can be applied to both extension and [ ]
Compression
Describe a wire performing elastically
Will return to its original length when the load is removed
What happens when a wire exceeds the elastic limit?
The wire deforms plastically. It will not return to its original length once the load is removed
Describe a extension (x) by force (y) graph
it’s linear for nearly all of the elastic region, curving very slightly near the elastic limit. The plastic region of the graph is non-linear. The wire fractures at the fracture point.
What is the fracture stress?
The stress at which a material breaks.
What is the yield stress?
This is the stress at which a material begins to deform plastically and becomes permanently deformed. Is the stress at which a large amount of plastic deformation takes place with constant or reduced load
Equation for stress:
Stress σ (Nm⁻²) = force F (N) / cross-sectional area A (m²)
What is strain?
The fractional increase in length
Equation for strain:
Strain ε = extension x / original length L
Why do tough materials have rounded edges, unlike the sharp, jagged edges like brittle materials when fracturing?
Tough materials undergo considerable plastic deformation before fracture
What does it mean to ‘neck’?
This means that part of it becomes narrower than the rest
What does Young’s modulus give a measure of?
The stiffness of a material rather than a particular specimen
Equation for Young’s modulus:
E = stress/strain
Unit for Y. Modulus:
Nm⁻² / Pa
When is a material in tension?
When a force is acting in a direction to stretch the material. The force = tensile force
What force compresses materials?
compressive force
What do material selection charts allow?
Quick comparisons between different classes of materials
Look at pg87 for material selection charts
Look at pg87 for material selection charts
Describe tension
Stretching materials creates tension
Forces of tension act along the same line as the forces stretching the material but in the opposite direction at each end of the material
Describe forces in springs in relation to Hooke’s Law
The extension of a spring is proportional o the force applied. If the force is compressive the spring is squashed and the extension is negative
Hookes’ law stop working when the…
Load is great enough
A material will show elastic deformation up to its [ ], and [ ] beyond it
Elastic limit
Plastic deformation
Describe a material stretching elastically in relation to the atoms
1) When the material is put under tension, the atoms of the material are pulled apart from one another
2) Atoms can move slightly relative to their equilibrium positions, without changing position in the material
3) Once the load is removed, the atoms return to their equilibrium distance apart
Describe a material stretching plastically in relation to the atoms
1) Some atoms in the material move position relative to one another
2) When the load is removed, the atoms don’t return to their original positions (equilibrium)
A stress causes a [ ]
Strain
If the forces stretch the material, they’re [ ]
Tensile
If the forces squash the material, they are [ ]
Compressive
A stress big enough to break a material is called the [ ]
Fracture stress
Describe stress in relation to atoms
1) The effect of stress is to start to pull the atoms apart from one another
2) Eventually the stress becomes so great that the atoms separate completely, and the material fractures
3) Ultimate tensile stress - max stress that the material can withstand before breaking
What is elastic strain energy?
The energy stored in a stretched material
When a material is stretched or compressed, [ ] in deforming the material
work is done
On a force against extension graph, what represents the work done?
The area under the graph
Work done on an elastic material in stretching is equal to the [ ] in the material as [ ]
Energy stored
Elastic strain energy
Work done =
1/2Fx
Elastic strain energy =
E = 1/2Fx
= 1/2kx²
Stress and strain are proportional to each other until the [ ]
Limit of proportionality
Describe what the gradient and area of a stress (y) and strain (x) graph mean
Gradient = Young's Modulus Area = Elastic strain energy per unit volume
Define ductile
You can change the shape of ductile materials by drawing them out into wires/other shapes. They keep their strength while they are deformed like this
