Stress, Strain & Tensile Strength (OCR A Level Physics)

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Stress, Strain & Tensile Strength

  • Opposite forces can deform an object
  • If the forces stretch the object, then they are tensile forces
  • Tensile forces lead to the two properties of materials known as tensile stress and tensile strain

Tensile Stress

  • Tensile stress is defined as the force exerted per unit cross-sectional area of a material

Stress equation, downloadable AS & A Level Physics revision notes
  • The ultimate tensile stress is the maximum force per original cross-sectional area a wire is able to support until it breaks
  • Stress has the units of pascals (Pa), which is the same units as pressure (also force ÷ area)

Tensile Strain

  • Strain is the extension per unit length
  • This is a deformation of a solid due to stress in the form of elongation or contraction

Strain equation, downloadable AS & A Level Physics revision notes

  • The strain is a dimensionless unit because it’s the ratio of lengths
  • Sometimes strain might be written as a percentage
    • For example, extending a 0.1 m wire by 0.005 m would produce a strain of (0.005 ÷ 0.1) × 100 = 5 %

Ultimate Tensile Strength

  • The ultimate tensile strength of a material is defined as:

The maximum amount of load or stress a material can handle until it fractures and breaks

  • The table lists some common materials and their tensile strength:

Tensile strength of various materialsTable of the tensile strength of various materials, downloadable AS & A Level Physics revision notes

Worked example

A brass wire of length 4.50 m and a radius of 0.2 mm is extended to a total length of 4.53 when a tensile force of 50 N is applied.Calculate for the brass wire:

a) The tensile stress

b) The tensile strain

Part (a)

Step 1: Write down the tensile stress equation

Tensile stress = Force ÷ Cross-sectional area

Step 2: Calculate the cross-sectional area, A of the wire

    • A wire has a circular cross-sectional area = πr2

  Area = π × (0.2 × 10-3)2 =1.2566 × 10-7 m2

Step 3: Substitute values in the tensile stress equation

Tensile stress = 50 ÷ (1.2566 × 10-7) = 397.899 × 106 Pa = 400 MPa

Part (b)

Step 1: Write down the tensile strain equation

Tensile strain = Extension ÷ Original length

Step 2: Determine the extension

    • The extension is total length – the original length

Extension = 4.53 – 4.50 = 0.03 m

Step 3: Substitute values in the tensile strain equation

Tensile strain = 0.03 ÷ 4.50 = 6.7 × 10-3

Examiner Tip

Since strain is a ratio, the extension and original length do not have to be calculated in metres. As long as they both have the same units, the strain will be correct

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.