EDP4130 Design and Technologies Resource

Teacher background information:

Tensile strength measures the force required to pull or stretch a material to the point where it breaks. The term tensile refers to being under tension or having a stretching force applied. To make a rope that could be used to tow a car, it is useful to use material with high tensile strength so that it does not break.

Materials with high tensile strength do not snap, tear or stretch under tension. Materials that stretch but do not snap have medium tensile strength. Materials with low tensile strength snap or tear easily under tension. Some materials have different tensile strength depending on whether they are wet or dry. For example, a wet paper towel is not as strong as a dry paper towel. Materials that contain short fibres often tear under tension. Each fibre breaks separately so the edge of the fabric becomes uneven during the tearing process. Materials which are not made of fibres may snap and make a clean break when pulled, whereas other materials may stretch. Materials that can be stretched without snapping are said to be ductile, for example, chewed chewing gum. The more ductile a material is, the higher its tensile strength.

Materials that can be hammered into sheets or pressed permanently out of shape are malleable. Malleable materials may or may not have high tensile strength. For example, plasticine is malleable but it snaps when stretched and so has low tensile strength.

Elastic materials will stretch under tension and then return to their original shape or size when the force is removed. Even elastic materials have limits, however, as to how much force they can withstand before they are stretched irreversibly and do not go back to their original shape. The more force needed for elastic materials to reach that elastic limit, the higher the tensile strength of the material.

Equipment:

Preparation:

 •      Cut a strip of newspaper 1 cm wide x 15 cm long for modelling the investigation in Lesson step 4. This measurement covers the average clothes peg, measure your pegs to check. Test the demonstration by wrapping a strip around the clothes peg and squeezing until it breaks or the peg is wide open.
  Note: Model the investigation test using materials other than those that the students will be using for their investigation.

•      Collect material samples of similar thickness for each team, for example, a 1 cm x 15 cm strip cut from paper, plastic and fabric bags, and something which is stretchy, for example, a rubber glove, to compare them with.  
I f students have brought in dispensable bags from home, ask them to cut a 1 cm x 15 cm strip to be included in their team’s investigation.

Lesson Steps:

1.      Review the class materials snapshot and ask questions, such as:
•      If you had something heavy to carry, what kind of material would you want your bag to be made of?
•      What would you select to wear if you want to perform gymnastics?
•      Why would you select to wear those clothes and not something like tight jeans?

2.      Ask students if they have ever had a material from their clothes, bags or other accessories snap, tear or stretch. Discuss the meaning of the terms ‘snap’, ‘tear’ and ‘stretch’, and add terms to the word wall (Literacy).

3 .     Explain that students will be working in collaborative learning teams to investigate the question ‘What happens to the material when pulled by the ends of a peg?’ (Differentiation).

4.      Model how to wrap the strip of newspaper around the clothes peg, how to hold it  and how to squeeze the ends of the peg until the material breaks or the peg opens to its limit.

5.      Ask students what variables could affect whether the material snaps, tears or stretches. Suggestions might include the size of the strips, the amount of force and the time taken to apply the force.

6.      Ask students what they might observe if something snaps, tears or stretches. Model how to use the magnifying glass to examine the snap, tear or stretch in the material.

7.      Ask students to think about how they will ‘Keeping the test fair’ and record it in their e journal, for example:
•      Change: the material tested
•      Observe: using a magnifying glass, if the material snaps, tears or stretches
•      Keep the same: the size of the material, the way the material is wrapped around the peg, how hard the peg is pushed to open it (the force) and how fast the peg is opened.

8.     Model how to fill out the ‘Recording results’ table using the headings: What material was used, prediction, reason for prediction, and result. Students to upload any digital photographs to e journal.

                    Note: Remind students to record both their prediction and their reasons for the prediction, before they test each material.

9.    Form teams and allocate roles. Ask Managers to collect team equipment. 

10.  Ask selected Speakers to report on what happened to each material and how their team predictions compared to their findings.

11.  Discuss the term ‘tensile strength’ and how it links to the terms ‘snap’, ‘tear’ and ‘stretch’. Add ‘tensile strength’ to the word wall and glossary (Literacy). Discuss how to rate  the materials according to their tensile strength, such as ‘high’, ‘medium’ and ‘low’  (see ‘Teacher background information’). Record in the class science journal a description of each rating and examples of findings, for example, the piece of fabric had high tensile strength because it didn’t change when pulled.

12. Ask students to record in their science journal what they have learned about the tensile strength of materials and to add new ideas to their notes about fair testing.

13. Ask students to reflect on how knowing about tensile strength can be useful in selecting materials for particular purposes, such as to make clothes and bags or ropes.

Optional: Ask students to wet the materials and repeat the investigation. Ask students to compare the results to see how moisture affects the tensile strength of materials.(Differentiation)