Introduction to Mechanical Properties

• Engineering Materials
• Further Education (Key Stage 5)

Introduction to Mechanical Properties

The Basics

What do you think when you hear the word, 'Mechanical'?

Definition

According to Cambridge Dictionary: 

"Operated by a machine, or connected with machines or their parts." 

Based on the Definition on the previous slide, give some examples of machines that have mechanical action associated to them.

Forces acting on a body:

When a force acts upon the body it can either change the shape or the motion of that body.

**NO BODY IS PERFECTLY RIGID WHEN A FORCE IS APPLIED TO IT - THE MATERIAL HAS BECOME STRESSED**

For example a spanner when tightening a nut.

What are the forces that produce these changes?

1. Tension
2. Torsion
3. Compression
4. Shear

Definitions:

Tension: Is a force that tends to stretch a material.

Compression: Is a force that tends to squeeze or crush a

material

Shear: Is a force that tends to slide one face of the

material over an adjacent face

Torsion:  The force that causes twisting, or the state of being twisted

Example Applications

Ask two students to label the sections that are in:

• Tension
• Compression

Example Applications cont'd

Further applications

Further Applications

What part of the following diagram is in:

• Tension
• Compression
• Shear

Worked Examples

Tension & Compression

As noted earlier, materials are stresses when in tension. Stress can be defined as: the ratio of the applied force F to cross-sectional area A of the material. The symbol used for tensile and

compressive stress is σ (Greek letter sigma). The unit of

stress is the pascal*, Pa, where 1 Pa = 1 N/m2. 

Hence > σ = F/A = Pa

A rectangular bar having a cross-sectional area of 75 mm2 has a tensile force of
15 kN applied to it. Determine the stress in the bar.

A square-sectioned support of side 12 mm is
loaded with a compressive force of 10 kN.
Determine the compressive stress in the support.

A bolt having a diameter of 5 mm is loaded so that the shear stress in it is 120 MPa.
Determine the value of the shear force on the
bolt.

Shear 

For a shear force the shear stress is equal to Force / Area, where the cross-sectional area A is that which is parallel to the direction of the shear force. The symbol used for shear stress is

the Greek letter tau, τ.

A bolt having a diameter of 5 mm is loaded so that the shear stress in it is 120 MPa.
Determine the value of the shear force on the
bolt

A rectangular bar having a cross-sectional area of 105 mm2 has a tensile force of
25 kN applied to it. Determine the stress in the bar.

Recall two new things that you have learnt from todays lesson.

How did you find todays lesson?

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