72 terms

Testing Materials

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definition of stress
force divided by cross sectional area
unit of stress
pascal (Pa)

Nm -2
what are the 3 types of stress
1. Tension (stretching)
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
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
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
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
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
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
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