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Materials Flashcards

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1
Q

What are forces that produce extension known as

A

Tensile forces

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2
Q

What are forces that compress an object known as

A

Compressive forces

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3
Q

What does a helical spring undergo when tensile forces are exerted

A

Tensile deformation

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4
Q

What does a helical spring undergo when compressive forces are exerted

A

Compressive deformation

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5
Q

Describe a force extension graph

A

Straight line from the origin to elastic limit of the spring - in this linear region, spring undergoes elastic deformation - it will return to its original length when the force is removed

Beyond this point - it will go plastic deformation - permanent structural changes occur and the spring won’t go back to its original length when the spring is removed

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6
Q

What is Hooke’s Law

A

Extension of the print is direct proportional to the force applied as long as the elastic limit of the spring is not exceeded

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7
Q

Hookes Law

A

F = kx

K = force constant (Nm^-1)

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8
Q

What is a force constant

A

Measure of stiffness of a spring

Larger force constant means it’s more difficult to extnemt

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9
Q

Where can Hookes Law also be applied to

A

Wires under tension

Concrete columns under compression

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10
Q

What happens to work done when a material has gone though plastic deformation and when it hasn’t

A

Some of the work done has gone to moving the atoms into new positions - is not recoverable

When it hasn’t - work done can be fully recovered

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11
Q

Work done by a force in extending the spring equation

A

Change in work done = force x change in extension

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12
Q

What is the area under a force extension graph

A

Work done

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13
Q

What is work done on a spring transferred into and is it recoverable

A

EPE

Yes because if the elastic behaviour of the spring

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14
Q

Equation for EPE

A

E = 1/2kx^2

For a spring obeying Hooke’s Law substitute into 1/2Fx (Area under graph)

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15
Q

What is the hysteresis loop

A

The loop formed by loading and unloading curves for objects that don’t obey Hooke’s Law

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16
Q

When is more work done - stretching a rubber band or decreasing it

A

Stretching a rubber band

17
Q

What is the area inside the hysteresis loop

A

Thermal energy released when a material is loaded then unloaded

18
Q

What is tensile strsss

A

Force applied per unit cross sectional area of the wire

19
Q

Tensile stress formula

A

T.S = Force / cross sectional area

(Sigma is sign for T.S)

20
Q

Tensile strain formula

A

Fractional change in the original length of the wire

21
Q

Tensile strain formula

A

Extension / original length

T.S is represented with epsilon

22
Q

What is a ductile material

A

A material that can easily be drawn into a wire or hammered into thin sheets

23
Q

Describe a stress strain graph for a metal wire

A

Stress is directly to strain from origin to the limit of proportionality - obeys Hooke’s law in this region

Small curve to E - represents the Elastic
limit - some materials obey Hooke’s law to this limit - elastic deformation occurs beyond this limit

Curves up to Y1 - upper yield point
Curves down to Y1 - Lower yield point

Yield point is where the material extends rapidly - yield points may be absent in some materials - typical for mild steel

Y2 to UTS - Ultimate Tensile Strength - maximum stress a material can go under before breaking

24
Q

What do strong materials have

A

High UTS

25
Q

What is Young’s Modulus

A

Ratio of stress to strain for material within the limit of proportionality

26
Q

Youngs Modulus Formula

A

YM = Tensile Stress / Tensile Strain

Nm^-2 or Pa

Fl/Ax

27
Q

Determine Youngs Modulus

A

Measure diameter of a wire with a micrometer

Calculate cross sectional area with this measurement

Repeat and average measurements from several places along the wire for more accurate reading

Tensile force acting on wire can be calculated with F=mg (m is hanging Mass)

After applying each additional mass - extension calculated is (extended length - original length) - repeat these for 6 different masses

Plot a stress and strain graph after calculating them for each load

Youngs Modulus can be calculated up from the linear section of the graph

28
Q

Stress Strain graph for brittle materials

A

Elastic behaviour up to its breaking point with plastic deformation (just straight lines both ways)

29
Q

What are polymeric materials

A

Materials with long molecular chains

30
Q

Stress strain graph for polymers

A

Dependent on material

Some show elastic behaviour and others show plastic behaviour

31
Q

What is force directly proportionally to and when

A

Extension

For a spring obeying Hooke’s Law

32
Q

What is the gradient of a force-extension grant

A

The force constant

33
Q

Where does Hooke’s Law apply to besides springs

A

Wires under tension and concrete columns under compression

Any object that can be elastically squashed or extended

34
Q

Loading curve (force-extension graphs) for a metal wire

A

Follows Hooke’s Law until the elastic limit of the wire

Unloading graph will be identical for forces less than the elastic limit

Beyond the elastic limit it is parallel to the loading graph but not identical to it - wire is permanently extended after the force is removed - longer than it was at the start

35
Q

Loading curves (force-extension graph) for a rubber band

A

Do not obey Hooke’s Law - will return to its original length once Hooke’s Law is removed

Loafing and unloading curves are different - a loop is formed between them

Thermal energy released is area inside the hysteresis loop

36
Q

Loading curves (force-extension graph) for polythene

A

Do not obeys Hooke’s Law - suffer plastic deformation under a relatively little force

37
Q

What is the breaking strength of a material

A

The stress value at the point of fracture

Physics (19 decks)

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