SAFE 605:
Application of Safety
Engineering Principles
Strength of Materials
Load
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A load is a force applied to a body.
In engineering, a load can be due to any one of the
following forces:
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Stationary load (dead load)
Change in velocity (inertia force)
Rotation
Friction
Bending
Twisting (Torsion)
Change in temperature
Load Units are in force per unit length
Force

Represents the action of one body on
another which may or may not change the
motion of the body.
 External
forces: Reactions and loads placed
on a structure.
 Internal forces: Forces present in a structure,
which are developed within the body of the
structure necessary to balance the external
forces.
Resultant Forces
The reduction of several forces in a force
system is referred to as “resolving the
force system.”
 The resulting single force is called the
“resultant force.”
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Moment
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A measure of the tendency of a force to cause
rotation about an axis.
The graphical representation of a moment acting
on an object is called a curl.
A curl is an arc shaped arrow drawn near and
about the axis of rotation.
Typical units are in-lbs, ft-lbs and ft-kips, N-m
Moment(M) = Force(F) times the perpendicular
distance to the axis(d).
M
=Fxd
Stress
The application of force to a body causes
deformation which results in an equal and
opposite resisting force in the material
 Stress is the ratio of applied load to the
cross-sectional area of an element
in tension and is expressed in pounds per
square inch (psi) or kg/mm2.

 Stress
= Load/Area
Strain
Strain is the measure of deformation
produced in a component due to the load
imposed.
 The measure of the deformation of the
material is dimensionless.
 Strain = New length/original length

Elasticity
Metal deformation is proportional to the
imposed loads over a range of loads.
 A material is elastic as long as the strain
disappears with the removal of the load.
 The load point which this does not happen
and the corresponding stress is referred to
as the elastic limit.

Modulus of elasticity (Young’s
Modulus)
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Since stress is proportional to load and strain is
proportional to deformation, this implies that stress is
proportional to strain.
Hooke's Law is the statement of that proportionality.
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Stress
= E X Strain
The constant, E, is the modulus of elasticity, (Young's
modulus) or the tensile modulus and is the material's
stiffness.
Young's Modulus is in terms of 106 psi or 103 kg/mm2.
If a material obeys Hooke's Law, it is elastic.
Ultimate strength
The maximum stress a material withstands
when subjected to an applied load.
 Dividing the load at failure by the original
cross sectional area determines the
value.
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Elastic limit
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The point on the stress-strain curve
beyond which the material permanently
deforms after removing the load .
Bending stress
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When bending a piece of metal, one surface of the material
stretches in tension while the opposite surface compresses.
It follows that there is a line or region of zero stress between the two
surfaces, called the neutral axis.
Yield Strength
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Point at which material exceeds the elastic limit
and will not return to its origin shape or length if
the stress is removed (permanent deformation
occurs).
This means, in effect, the maximum load that will
allow the material to return to its original shape
when the load is removed.
The value is determined by evaluating a stressstrain diagram produced during a tensile test.
Yielding
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Yielding occurs when the design stress exceeds
the material yield strength.
Safety factor is a function of design stress and
yield strength.
The following equation denotes a safety factor:
 Safety
Factor = YS / DS
 Where YS is the Yield Strength and DS is the
Design Stress
Stress-Strain Diagram
Coplanar Force Systems
Coplanar force systems are analyzed on
an X – Y coordinate system.
 The assumption is that forces may exist
upon the structure in two dimensions.
 To solve a coplanar force system, all
forces acting upon the structure are
resolved into their respective X – Y
components.
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Moment of Inertia
The Moment of Inertia (I) is a term used to
describe the capacity of a cross-section to
resist bending.
 It is a mathematical property of a section
concerned with a surface area and how
that area is distributed about the reference
axis.
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Deflection
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When a load is placed on a beam, the
formerly-straight horizontal (centroidal)
axis of the beam is deformed into a curve