Chapter 4 Flashcards by Rachel Allan

Define stiff

Small strain for a large stress.

Higher stiffness = higher YM

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Define Hard

Resists indentation on impact

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Define Elastic

Returns to outstretched form when stresses are removed

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Define Plastic

Undergoes permanent deformation under a large stress rather than cracking

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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

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Define Brittle

Breaks suddenly as cracks travel through it; little or no plastic deformation

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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

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Describe ceramics

Hard, Brittle, Stiff

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Describe metals

Pure metals = soft
Metals that can be easily shaped = malleable
Those that can be drawn into wires = ductile
Alloys = usually harder

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Describe polymers

Glass polymers - similar properties to glass, brittle

Semi-crystalline polymers - Tough

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Give the equation for Hooke’s Law

Force F (N) = spring constant k (N/m) * extension x (m)
F=kx

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Describe the relationship between F and x in Hooke’s law

They are proportional

k = constant of proportionality

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A large spring constant (k) means that…

Difficult to stretch

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What affects the value of k?

Material, length, cross-sectional area (of the wire)

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Spring constant is a value for a specimen not a material!!

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Hooke’s law can be applied to both extension and [ ]

Compression

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Describe a wire performing elastically

Will return to its original length when the load is removed

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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

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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.

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What is the fracture stress?

The stress at which a material breaks.

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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

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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

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