# SAB2223 Mechanics of Materials and Structures

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SAB2223 Mechanics of Materials and Structures
TOPIC 1 STRESS AND STRAIN
Lecturer: Dr. Shek Poi Ngian

TOPIC 1 STRESS AND STRAIN

Introduction
MECHANICS : Physical science that covers the problems of bodies at rest or in motion due to the forces acting on the bodies.
Can be divided into two areas: 1. Statics ‐ Study of bodies at rest or with uniform
velocity. 2. Dynamics ‐ Study of bodies in motion.

Introduction
• Mechanics of materials is a study of the relationship between the external loads on a body and the intensity of the internal loads within the body/material (if on structures – mechanics of structures).
• This subject also involves the deformations and stability of a body when subjected to external forces.
Tension – Elongation
Compression – Shortening
Torsion – Twisting
Bending Moment ‐ Rotation

Equilibrium of a Deformable Body
External Forces 1. Surface Forces
- caused by direct contact of other body’s surface
2. Body Forces - other body exerts a force without contact

Equilibrium of a Deformable Body
Reactions • Surface forces developed at the supports/points
of contact between bodies.

Equilibrium of a Deformable Body

Equations of Equilibrium

• Equilibrium of a body requires a balance of forces and a balance of moments

F  0

MO  0

• For a body with x, y, z coordinate system with

origin O,  Fx  0 ,  Fy  0 ,  Fz  0

Mx 0,My 0,Mz 0

• Best way to account for these forces is to draw the body’s free-body diagram (FBD).

Equilibrium of a Deformable Body
Internal Resultant Loadings • Objective of FBD is to determine the resultant force and
moment acting within a body. • In general, there are 4 different types of resultant
loadings: a) Normal force, N b) Shear force, V c) Torsional moment or torque, T d) Bending moment, M

Types of Forces and Stress
NORMAL FORCE ‐ The force that acts perpendicular to a certain surface and passes through the symmetrical axis of the body.

T (Tension)

T

C

C

(Compression)

Types of Forces and Stress
Normal Stress – The intensity of force, or force per unit area.
P
A

Tensile Stress (+ ve)

Compressive Stress (- ve)