Hardness Testing - DOC by hcj

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									                            Mechanical Properties of Materials
                            Hardness Testing Teachers’ Notes            T-HT1

Aims:

To encourage pupils to carry out first hand testing to establish characteristics and
properties of materials in order to aid selection relevant to application and product
development.

Learning Outcomes:

    o Pupils will learn how to use hardness testing apparatus to compare the
      hardness of different materials.
    o Pupils will collect and analyse data in order to measure comparative hardness
      of different materials.
    o Pupils will apply knowledge of hardness data to material selection.

Target Group:

This activity is most suitable for pupils aged 11-14 in KS3, though it could be used for
GCSE lessons, in both Science and Design and Technology: Resistant Materials.
With appropriate materials, it may also be used to support learning at AS/A2 level
(particularly for the OCR Applied Science and DT Product Design A-levels)

Curriculum Links:

7-11 (KS2): QCA SoW 3c - to recognise properties such as hardness, strength and
flexibility and compare materials in terms of these properties.

11-14 (KS3): QCA SoW 9e - make some generalizations about the properties of
metals which make them useful, eg they are hard; they can be flexible.

14-19 (KS4/KS5):


            AQA                            EDEXCEL                        OCR
GCSE Additional Applied Science   GCSE Science                GCSE 21stC Add App Science
                                                                      st
GCSE Applied Science (Double)     DT – Resistant Materials    GCSE 21 C Science A
AS/A2 DT – Product Design                                     GCSE DT – Resistant Materials
                                                              GCSE DT – Product Design
                                                              AS/A2 Applied Science
                                                              AS/A2 DT – Product Design



Suggestions for use:

This activity could be used as the main experimental activity in a lesson looking at
selection of materials that are appropriate for a task, by modelling the effect of
temperature on the hardness of a disposable plate. Alternatively, it could be one
station in a circus of activities looking at different properties of materials.

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                         Mechanical Properties of Materials
                         Hardness Testing Teachers’ Notes            T-HT1

Background Information:

Hardness is the resistance of a material to plastic deformation. This property can be
used to measure the hardness of a material by deliberately indenting the surface with
a known load and an indenter of known shape and size. The experiment /
investigation is relevant to reinforce terminology such as plastic, elastic, ductile,
brittle, hardness, toughness and could be applied in Science or DT at Key stages 2, 3
or even 4 (where higher assessment marks are not an issue).
The principle behind this home-made apparatus is that a fixed, pointed shape is
pressed into a surface by a consistent force.
Plasticine can easily be tested at five different test temperatures, including room
temperature. Using ice-water (approx. 0C) and ice-salt-water (approx. -5C) mixes
allows two values below room temperature. A water bath set at 50C allows a safe
maximum value to be used, and an intermediate temperature can be achieved by
allowing hot water to cool in a suitable container, e.g. a beaker.
Plasticine is a very poor thermal conductor, so temperature variations will be minimal
if the pupils test as soon as the plasticine leaves its heat source.
If left in water at 50C for several minutes, anomalous results may be obtained
indicating that the plasticine has been hardened.
Small indentations can reasonably be observed with a magnifying glass and metal
rule. A fruitful discussion point is whether it is best to measure the depth or the
diameter of any indentation. (If the indenter is a regular conical shape either
measurement would be appropriate, whereas for the cork/nail indenter it is necessary
to use the depth measurement).

Notes:
    Could be used as a science investigation at KS3.
    Dropping masses from greater heights and/or increasing the size of the mass
      are ways of adjusting to harder materials.
    Only materials which are softer than the indenter can be measured. The point
      of the indenter is likely to wear with time if hard materials are tested.
    The thickness of the material to be tested must always be such that neither
      energy nor indention is transferred to the substrate.
    Beware testing brittle materials, such as glass.




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                              Mechanical Properties of Materials
                              Hardness Testing Teachers’ Notes                  T-HT1

Design of Apparatus for Hardness Testing:

                                   All measurements are
                                   approximate and to fit
                      24 mm
                                              Approx. 45                        Wood / Cork

                                     20 mm                                       Nail


                                       40 mm


                                  The diagram is intended to suggest a possible construction,
                                  only. Materials and dimensions can be chosen to suit the
                                  needs of the class and available materials.
Length to suit e.g.               Any suitable and available materials can be used e.g.
  200, 300, 400
                                  Tube:           Al, Cu, plastic, cardboard
       mm
                                  Base:           Al, Fe, wood or plastic
                                  Weight:         Fe, brass, lead
                                  Indenter:       Fe, Sharpened wood (pencil in simple cases),
                                                  Nail in wood


                                   3 mm
                                                            20 mm




                                                            Weights in simple ratios that can
                                                             be dropped onto the indenter.
                                                                                    20 mm
                                                                        20 mm
                                                             20 mm
                                                                                    40 mm
                                                                        30 mm
                                                             20 mm




                      150 mm


Safety Alerts:
                        Beware testing brittle materials, such as glass.


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                          Mechanical Properties of Materials
                          Hardness Testing Teachers’ Notes            T-HT1


Time needed:

It should be possible to test at five different temperatures within 40 minutes. Less
confident groups could be asked to carry out fewer tests and to share results.

Suggestions for adapting for other target groups:

DT investigations could include the testing of different woods (parallel, perpendicular
and end-on to the grain), different plastics, different metals and the effect of using
composites.
The indenter can be used to test different samples of hardwood and softwood to
illustrate the need to match a material to the sort of wear it might expect.




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                                Mechanical Properties of Materials
                                Hardness Testing Teachers’ Notes             T-HT1


                              Hardness Testing Lesson Outline
OBJECTIVES: By the end of this lesson, pupils should be able to:
    o use hardness testing apparatus to compare the hardness of different materials.
    o collect and analyse data in order to measure comparative hardness of different
         materials.
    o apply knowledge of hardness data to material selection.
Introduction/Starter:
      Use the Starter worksheet to introduce/revise terminology regarding properties of
   materials such as elastic, plastic, ductile, brittle, hardness, toughness. Pupils complete the
   worksheet and then carry out peer assessment. (Correct answers can be displayed using
   sheet in teachers notes)
Main Activity:
    Pupils work in small group (2-4 persons) to carry out the activity. Central supplies of water
    of the relevant temperatures should be available. One member of each group collects a
    beaker of water at each temperature. Other members make discs of plasticine (a guide
    size is 3cm diameter by 1cm thick). Discs should be placed in plastic bags (to keep dry)
    and left for 10-15 minutes to reach temperature. If time is short, Pupils could collect ready-
    prepared discs from central temperature baths.
    Whilst waiting, pupils should be encouraged to think of the variable factors involved and
    detail what steps they are taking to control them.
    Average data should be plotted on a graph. Able pupils should do this for themselves.
    Alternatively, groups could enter their results on their own or a central spreadsheet which
    will draw a graph of their results.

Plenary:
  Each group writes two important things they have learned from the experiment on a piece of
  card. These are displayed on a board and the class as a whole decides on a an overall
  conclusion. Ask the class to explain how experiments like this might be of use for a vending
  machine company. Demonstrate pouring boiling water into plastic/polystyrene cups.
Success Criteria/Assessment Opportunity:
  Pupils should successfully show that hardness decreases as temperature rises.


Homework/Further Study:


Resources:
  Plasticine, plastic bags, Water baths (ice/salt;ice/water;50C), Hardness testers, magnifiers,
  rulers, (vernier gauges)




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                                   Mechanical Properties of Materials
                                   Hardness Testing Teachers’ Notes              T-HT1

      Hardness Testing - Starter Worksheet


          PLASTIC, ELASTIC, DUCTILE, BRITTLE, HARDNESS, TOUGHNESS
The ability of a material to
stretch under the influence of
an external stress / force, but
                                     ELASTIC
to return to the original shape                    O-------------------------O
and size once the stress /
force is removed.
A property due to the plastic
deformation of materials
which allows the material to
be drawn out into a wire or          DUCTILE
beaten into shape.

A measure of a materials
resistance to deformation.
Tested by measuring the
                                    HARDNESS
extent of plastic deformation
by a standard indentation
test.
The ability of a material to
flow under sufficient stress
allowing changes in shape
without breaking. *******
deformation results in               PLASTIC
permanent shape changes
when atoms or molecules
slide past one another
The ability of a material to
absorb energy from an
impact. To illustrate,
elastomers are soft and
stretch easily. By stretching
they absorb energy from an
impact and are therefore very      TOUGHNESS
*****. By contrast, ceramics
are very hard, but are also
brittle, absorbing little energy
of an impact – these
materials are NOT *****.
A fracture with little or no
plastic deformation. The
                                     BRITTLE
opposite of ductile.


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                      Mechanical Properties of Materials
                      Hardness Testing Teachers’ Notes     T-HT1

Teaching Questions:


References:




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                        Mechanical Properties of Materials
                        Hardness Testing Teachers’ Notes            T-HT1

Copy of Student Worksheet


To investigate the hardness of plasticine at different temperatures.

Theory:
Many substances soften and become more ductile when they are heated.

Prediction:
Indentations in plasticine will increase as the temperature of the plasticine
increases, because as it softens it becomes more ductile and is, therefore, more
easily deformed.

Method:
   Place equally thick discs of plasticine in different temperature
     environments for about 15 minutes to allow the temperature of the
     plasticine to become even throughout.
   Place the base of the tester on the plasticine disc.
   Place the point of the indenter on the plasticine, through the central hole.
   Fit the tube in place
   Drop a weight from the top of the tube.
   Measure the indentation.
   Record results and repeat to obtain three values at each of five
     temperatures.

Results: (Typical temperature values are given)
                                                           Indent / mm
Environment           Temperature / C
                                              1        2        3        Average
Ice-salt-water mix    -5C
Ice-water mix         0C
Room temperature      25C
Cooling water         40C

Water bath            50C


Conclusion:
To be decided…

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