4 TESTING MATERIALSProperties of materials

o Classify and explore mechanical and electrical properties of materials

o Explain the meaning of the term intensive property, with examples

o Appreciate the need for log scales for representation of properties

Classifying materials

• Look at the vast range of solid materials used in objects around you.

• What general classes of material do they fall into?

• Can you describe the internal structure of materials in these classes?

Investigating breaking stress

• Measure the breaking force and breaking stress for a material

• Use “plot and look” to identify outliers

Starter

• How can we compare how strong/stiff different metals are when the wire samples we have are of different diameters for different metals?

• Similarly, how can you compare how elastic/stretchy different metals are when the wire samples we have are of different lengths for different metals?

Force extension graphs and Young’s modulus

• Record and interpret a load-extension graph for copper

• Make an accurate and precise measurement of the Young’s Modulus of a material, estimating the uncertainties involved

StarterA wire of Material A fractures under a load of100 N, while a wire of Material B fracturesunder a load of 10 N.

Q1. What, if anything, can you conclude about the strengths of the materials A and B?

Q2. What further information would you need in order to make a valid comparison between materials A and B?

Stress, strain and Young’s Modulus

• Define and then calculate these from experimental measurements

Olympus Mons imaged by Mars Reconnaissance Orbiter, September 2009

Measuring Young’s Modulus

• Make an accurate and precise measurement of the Young’s Modulus of a material, estimating the uncertainties involved

Stress strain curves

• Describe and explain the stress-strain curves for metals, ceramics and polymers

Note: Elastic limit and yield point are often very close to eachother

STRONG

HARD

TOUGH

STIFF

These are microscope images of the fracture cross sections of rods of two different materials.

A

Which one has undergone ductile fracture, and which one brittle fracture? Explain your answer.

AA

B

Electrical properties of materials

• Investigate the relationship between material dimensions and resistance

• Derive equations linking resistance, conductance, resistivity and conductivity

Measuring resistivity• Make an accurate and precise measurement of

the resistivity of a material, estimating the uncertainties involved

Starter: Write down the equation for working out the resistance R of a wire of length L and area A. Compare with the equation of a straight line. What should you measure and plot in order to determine the resistivity of the metal?

Making measurements

• Review key considerations relating to experimental measurement

Uncertainty and Error How do they apply in this experiment?

Random uncertainties• Identify sources of

measurement uncertainty and quantify them.

• Which contributes the most to the uncertainty in the final answer?

Systematic errors• Identify limitations in

the design of the experiment and explain how they could either be eliminated or reduced in effect.

Accuracy and precision

• Accuracy: how close your measurement is to the accepted value

• Precision: what range of values does your measurement span?

Accepted value = 110

a. 108 +/- 4b. 125 +/- 2c. 111 +/- 9d. 85 +/- 40