72 terms

definition of stress

force divided by cross sectional area

unit of stress

pascal (Pa)

Nm -2

Nm -2

what are the 3 types of stress

1. Tension (stretching)

2. Compression

3. Torsion (twisting)

2. Compression

3. Torsion (twisting)

definition of toughness

energy required to create a new surface area (J/m2)

When some materials are stretched past their elastic limit, they don't immediately undergo plastic deformation before breaking - they absorb alot of energy

When some materials are stretched past their elastic limit, they don't immediately undergo plastic deformation before breaking - they absorb alot of energy

what 3 things makes a material tough

1. Needs high amounts of energy to break it

2. Undergoes lots of plastic deformation before breaking

3. Resists crack propagation

2. Undergoes lots of plastic deformation before breaking

3. Resists crack propagation

what is the opposite to tough

brittle

unit of toughness

J/m2

definition of malleable

can be hammered or pressed into shape

definition of ductile

can easily be drawn into a wire

definition of strength

maximum stress a material can withstand without failure

units for strength

Pa

what are the 2 types of failure

1. yielding

2. breaking/fracturing

2. breaking/fracturing

definition of hardness

resistive to dents and scratches

unit of hardness

Pa

definition of strong

large stress is needed to cause failure

When springs stretch the extension is proportional to

the force

What is force?

any interaction which tends to change the motion of an object

What is pressure?

Force per unit area. It is usually more convenient to use pressure rather than force to describe the influences upon fluid behaviour.

Force is measured in

Newtons (N)

Composite materials combine

the properties of more than one material

Compressive forces tend to

squash an object

Tensile forces tend to

stretch an object

Brittle materials snap

easily

Strain in relation to length is

the change in original length / the original length

Elasticity is the word to describe a material which

returns to its original shape when the load which has been deforming it is removed

definition of plastic

won't regain original size and shape when force is removed

Hooke's law

up to a maximum (known as the limit of proportionality) the extension of a wire (or spring) is proportional to the applied load

The elastic limit is

the maximum load which a body can experience and still regain its original size and shape once the load has been removed

equation for hooke's law

F=kX

what does K represent

spring constant

how do you find K from a graph

gradient of the graph of F/X

What does a large spring constant say about a material

a specimen is hard to stretch

what does the value of K depend on

1. Cross sectional area

2. length

3. material of wire

2. length

3. material of wire

what happens to the amount of energy as a specimen is stretched

more energy is stored

why is the energy stored in an elastically stretched spring not simply force x displacement (Fx)

Force grows steadily larger as the spring is stretched

what is the equation for energy stored

1/2kx2

what does the area under the graph of Fx represent

energy stored

what does doubling the length of a wire do to the extension

also double it

what is the difference between fracture stress and yield stress

1. fracture stress is the stress at which a material breaks

2. yield stress is the stress at which a material begins to deform plastically

2. yield stress is the stress at which a material begins to deform plastically

Strength is related to

the maximum force which can be applied to a material without it breaking

Yield point

if the stress is increased beyond the elastic limit a point is reached at which there is a marked increase in expansion - the material will suddenly start to stretch without any extra load

what is yield strength

the level of stress at which a material will deform permanently

Breaking stress is the

the ultimate tensile strength- the maximum stress which can be applied to a material

Ceramic materials such as brick and concrete are

strong in compression but weak in tension.

Stiffness relates to

the resistance which a material offers to having its size and or shape changed - how hard it is to deform a material

The strength of a material is represented by

it's breaking stress or yield stress.

The stiffness of a material is represented by

it's Young modulus.

The Young modulus is found from

the initial gradient of a stress-strain graph.

why is the stress/strain graph of a brittle material linear for all its length

does undergo alot of plastic deformation before breaking - breaks straight away after elastic limit is met

why does a tough material have rounded edges after it breaks

a tough material will undergo alot of plastic deformation before breaking so the actual breaking will occur over a longer time

Materials are elastic up to the elastic limit then

they either fracture or show plastic deformation.

Stress is equal to

Force/Area

Strain is equal to

extension/original length

Tensile stress and compressive stress are

the force per unit area acting at right angles to a surface. Tensile strain is, the change of length per unit length. Strain is a ratio of two lengths and therefore has no unit.

The (engineering) breaking stress of a material =

F / A where F is the force needed to break the material by stretching it and A is the initial area of cross section of the material.

The actual stress in the material at the breaking point

will usually be larger since the area of cross section will be somewhat reduced.

The Young modulus E of a material =

tensile stress / tensile strain provided the limit of elasticity of the material is not exceeded.

what does UTS stand for and what does it mean

ultimate tensile strength is the level at which a material will fracture

young modulus equation

fl / Ax

In a stress-strain graph for a metal strain is

proportional to stress up to a limit. This is the initial straight section of the graph. In this part of the graph the ratio stress / strain is constant and equal to the Young modulus of the material. Here the material behaves elastically.

The elastic limit is the point beyond which

a material does not regain its initial shape when the tension is removed. It is also called the yield point.

When a material is stretched beyond its elastic limit and is stretched beyond the yield point

it behaves plastically suffering permanent deformation. The yield stress is the stress at the yield point. As the tension is increased beyond the yield point, the strain increases and a neck forms. Further stretching causes the stress to concentrate at the neck until it breaks.

The (engineering) breaking stress is equal to

F / A where F is the force needed to break the material by stretching it and A is the initial area of cross section of the material.

The breaking stress is also called

the tensile strength of the material.

equation of % uncertainty

% uncertainty = uncertainty in reading / actual reading

TIMES 100

TIMES 100

what is accuracy

how close a measurement result is to the true value- the closer it is, the more accurate it is

what is precision

hoe close repeated measurements are to each other

equation of % uncertainty in gradient

absolute uncertainty / gradient of best fit line x 100

when is a material under tension

when a force is acting in a direction to stretch the material

what is the force therefore described as

a tension force

what is density

mass/ volume

what do material selection charts allow

quick comparisons of materials to be made between different classes of materials