Materials Tutorial 3 - Stress, Strain, and the Young Modulus

Stress and Strain

Wires obey Hooke's Law just like a spring.  This is because bonds between atoms stretch just like springs:

If we stretch a wire, the amount it stretches by depends on:

• its length

• its diameter

• the material it’s made of.

If we have two of the same material and length but of different thicknesses, it is clear that the thicker wire will stretch less for a given load.  We make this a fair test by using the term tensile stress which is defined as the tension per unit area normal to that area.  The term normal means at 90o to the area.

We can also talk of the compression force per unit area, i.e. the pressure

Stress = Load (N) = F

area (m2)   A

In some text books you may see stress given the code s (sigma, a Greek letter 's'). Therefore:

You will have met the expression F/A before.  It is, of course, pressure, which implies a squashing force.  A stretching force gives an expression of the same kind.  Units are newtons per square metre (N m-2) or Pascal (Pa).

1 Pa = 1 N m-2

 You must always convert areas to square metres for this equation to work.  Remember that radii will often be given in mm or cm.  This is a common bear trap. 1 mm2 = 1 x 10-6 m2 Therefore if you get an area of 10-2 m2,  you probably have forgotten to do the conversion to square metres.

 Find the area of a wire of diameter 0.75 mm in m2.

If we have a wire of the same material and the same diameter, it doesn’t take a genius to see that the wire will stretch more for a given load if it is longer.  To take this into account, we express the extension as a ratio of the original length.  We call this the tensile strain which we define as the extension per unit length.

Strain = extension (m)

original length (m)

There are no units for strain; it’s just a number.  It can sometimes be expressed as a percentage.

You will find that the same is true for when we compress a material.

 What is the strain of a 1.5 m wire that stretches by 2 mm if a load is applied?

Stress-Strain Curves

Stress-strain graphs are really a development of force-extension graphs, simply taking into account the factors needed to ensure a fair test.  A typical stress-strain graph looks like this:

We can describe the details of the graph as:

• P is the limit of proportionality, where the linear relationship between stress and strain finishes.

• E is the elastic limit.  Below the elastic limit, the wire will return to its original shape.

• Y is the yield point, where plastic deformation begins.  A large increase in strain is seen for a small increase in stress.

• UTS is the ultimate tensile stress, the maximum stress that is applied to a wire without its snapping.  It is sometimes called the breaking stress.  Notice that beyond the UTS, the force required to snap the wire is less.

• S is the point where the wire snaps.

We can draw stress-strain graphs of materials that show other properties.

• Curve A shows a brittle material.  This material is also strong because there is little strain for a high stress.  The fracture of a brittle material is sudden and catastrophic, with little or no plastic deformation.  Brittle materials crack under tension and the stress increases around the cracks.  Cracks propagate less under compression.

• Curve B is a strong material which is not ductile.  Steel wires stretch very little, and break suddenly.  There can be a lot of elastic strain energy in a steel wire under tension and it will “whiplash” if it breaks.  The ends are razor sharp and such a failure is very dangerous indeed.

• Curve C is a ductile material

• Curve D is a plastic material.  Notice a very large strain for a small stress.  The material will not go back to its original length.

The Young Modulus

The Young Modulus is named after the British polymath Thomas Young (1773 - 1829).  (A polymath is someone who knows everything.)  The term modulus means a little measurement.   I hate to spoil a good story (again), but it was actually Leonhard Euler (1707 – 1783) who worked it out.  Young’s Modulus experiments were performed by Giordano Ricatti in 1782.  Young reported his findings in 1807.

The Young Modulus is defined as the ratio of the tensile stress and the tensile strain.

So we can write:

Young modulus = tensile stress

tensile strain

We know that:

tensile stress = force F

area     A

and that

tensile strain =  ___extension __   = Dl

original length        l

Young Modulus has the physics code E, so we can write:

which becomes:

Units for the Young Modulus are Pascals (Pa) or newtons per square metre (N m-2).

The Young Modulus describes pulling forces

We can link the Young Modulus to a stress strain graph.

The Young Modulus is the gradient of the stress-strain graph for the region that obeys Hooke’s Law.  This is why we have the stress on the vertical axis when we would expect the stress to be on the horizontal axis.

The area under the stress strain graph is the strain energy per unit volume (joules per metre3).

Strain energy per unit volume = 1/2 stress x strain.

The units arise because stress is in N m-2 and strain is m m-1 (NOTE: This unit here is not "millimetres to the minus one", but metres per metre which mean no units).

N m-2 x m m-1 = N m m-3.  N m is joules, hence J m-3

Area is the strain energy per unit volume.  So we can write this equation:

Area = ½ × stress × strain

In Physics code:

The term E' is pronounced "E-prime" or "E-dashed" and is being used as the code for the elastic strain energy per unit volume.

Al is area × length = volume

 Question 3 A wire made of a particular material is loaded with a load of 500 N.  The diameter of the wire is 1.0 mm.  The length of the wire is 2.5 m, and it stretches 8.0 mm when under load.  What is the Young Modulus of this material? Give you answer to an appropriate number of significant figures. Question 4 What is the elastic strain energy per unit volume for the wire in question 3?

Measuring Stress and Strain

We can measure the Young Modulus doing a simple experiment like this:

We need to measure the diameter of the wire using a micrometer.  A video tutorial in how to do this is in one of the links.  Then we measure the extension as we increase the load, recording the observations.  We need to convert the force into stress, and extension into strain.  From that we plot a stress-strain graph, using the gradient to calculate the Young Modulus.

You need to have a couple of slotted masses on the wire to pull it taut.  This will be your zero reference point.  Suppose you need 0.3 kg to pull the wire taut.  Then you add 0.1 kg to make the total load 0.4 kg.  The load you note in your results is 0.98 N.

A useful tip is to place a sheet of graph paper underneath.  Mark the position of the flag at your zero load, then mark the position under load.

You need to do at least three readings for each load and take an average.

The value we get for E is often rather low.  This is because the wire might suffer the following defects:

• It might not be of uniform diameter.  A smaller diameter will cause a greater stress.

• The crystal structure of the wire may be degraded by the wire drawing process.  The wire is made by pulling the metal through a small hole called a die.

There are also many uncertainties.  The greatest uncertainty comes in measuring the diameter.  You will need to use a micrometer screw gauge like the one shown below:

Source not known

You need to do at least three readings along the length of the wire that you are using and take an average.

Your tutor will show you how to use it.  There is an excellent tutorial on how to do this HERE.

This experiment is a required practical for A-level syllabuses in England.

Searle's Method

This is an alternative method that uses apparatus like this.  You may have seen it hanging from the ceiling at the back of the physics lab at your school or college.

By Ewen at English Wikibooks, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=61793896 (Adapted)

This is how such an experiment is carried out:

• There are two wires of the same material, same diameter, and the same length.  The control wire is held taught by a constant control mass which is not altered.  It acts as a reference.

• The diameter needs to be measured with a micrometer.

• The variable mass is placed on the test wire.  Initially it is the same mass as the control mass.  This is the zero point.

• The reference bar is levelled off using the spirit level, and the vernier scale is set to zero.

• Then a mass is added to variable mass.

• The masses need to be converted to weights (g = 9.8 N kg-1)

• The vernier scale is adjusted, so that the reference bar is level.

• Then the reading is taken.

The advantages of this method are:

• The wires are long, so the strain is more easily measured.

• Both wires undergo the same thermal expansion, so the results are not affected by variations in the temperature in the lab.