CHAPTER 1(a)

FORCES ON MATERIAL

MUHAMMAD HAFIZZUDDIN BIN MD TENI

Learning OutcomeAt the end of this lecture, student should be

able to;

Understand the concept of strength of material

Define the types of loadsDefine the types of forcesUnderstand stress and strainSolve problems related to stress and strain

What is strength of material?Subject which deals with loads, deformations

and the forces acting on the material

Important in understanding; mechanical behavior/properties of material safe design of all structures- buildings,

bridges, machines, ships, airplanes etc..Material purchasing / material testing maintenance

Types of LoadA) STATIC LOAD- Forces which is fix (not moving)- A good example of this is a person seen

below. He is holding a book but he is not moving. The force downwards is STATIC.

B) DYNAMIC LOAD- Forces which is moving / changing- A good example of a dynamic load is the

person below. He is carrying a weight of books but walking. The force is moving or DYNAMIC.

C) IMPACT LOAD- Forces which is happen immediately- Example: Car crashed the lamp pole and

smashed it very badly.

D) FATIQUE AND ALTERNATING LOAD-Forces which is happen at certain time only

Fatique Load Alternating Load

Example: Wind Wheel Example: Spring

Types of Forces i) Tensile Force (+ve)- Force that tends to stretch / lengthen a

material on which it acts.- Example: The rope is in “tension” as the two

people pull on it. This stretching puts the rope in tension.

ii) Compressive Force (-ve)- Force that tends to squeeze / crush a material- Example: The weight lifter finds that his body

is compressed by the weights he is holding above his head. 

ExerciseThe bracket holding up the hanging basket is made of steel. It has been made by heating up the steel until ‘red’ hot and then bending it at 90 degrees.

1. What is the force exerting on the bracket at point ‘A’?2. What type of force is the chain under?3. What type of force are the wires holding the plant pot under?4. What force is acting on the wall at point ‘B'?5. As more weight is added to the plant pot the bracket begins to bend too much. How could the bracket be strengthened?6. How could the bracket be fixed to the wall? Use diagrams to illustrate your answer.

Answer

iii) Shear Force- Force that tends to slide one face of the

material to another face.- Example: Simple scissors. The two handles

put force in different directions on the pin that holds the two parts together. The force applied to the pin is called shear force. 

STRESSInternal resistance which react to external

forces subjected to the material .Ratio between the applied force, F and cross-

sectional area, A of the material.The symbol is σ (sigma)Unit in N/m2 or Pa

Stress

The person in the diagram is sat on the mono-bicycle and the air filled tyre is under great pressure. The air pressure inside it pushes back against his/her weight.

STRAINWhen forces act on a body, it undergoes

some deformation.Ratio between deformation per unit length The symbol is ε (epsilon)Strain has no unit

@

For example, a 2.0" titanium bar that has been stretched to 2.2" is said to have experienced a tensile strain of 0.1, or 10%.

Exercise: Stress and Strain1. A rectangular bar having a cross-sectional area of

75mm2 has a tensile force of 15 kN applied to it. Draw the figure and determine the stress in the bar.

2. A circular bar having 25 mm diameter being applied a compressive force of 100 kN. Determine the stress produce.

3. A bar has initial 1.6m length and after subjected to load 30 kN, the length become 1.4m. Determine the strain and the percentage strain.

4. A wire length 2.5m has a percentage strain of 0.012% when loaded with a tensile force. Determine the deformation of the wire.

5. A pipe has an outside diameter of 25mm and inside diameter 15mm and length 0.4 m and it support a compressive load of 40kN. The pipe shorten by 0.5mm when the load is applied.determine:

a) compressive stressb) compressive strain