Design for Static Loading
Load may be classified as :
• Static load is the load which does not change in
magnitude or direction and gradually increases to a
steady value e.g. dead weight of machine elements.
• Dynamic load is the load which changes in
magnitude or direction or both with respect to time.
Example : Impact Load, Suddenly Applied Load
Determination of appropriate loads acting on a
machine member is a critical and challenging task
Failure Criteria
A machine element is said to have failed
when it ceases to perform its intended
function. It may happen if:
• Its stress crosses the acceptable limit
OR
• deflection crosses the acceptable limit.
Failure Criteria
For Ductile Materials: When stress developed in a
ductile material reaches the yield strength, it starts
yielding and excessive plastic deformation occurs,
therefore Yield strength is taken as failure criterion for
ductile materials.
For Brittle material: In brittle material, very small
plastic deformation occurs and fracture takes place
once the stress developed reaches Ultimate Tensile
Strength. Therefore Ultimate Tensile Strength is
considered as failure criterion for brittle materials.
Factor of Safety & Allowable
Stresses
The factor of safety is a measure of reserve strength
provided to take care of any unexpected or
unpredicted conditions that may arise due to
uncertainties in the properties of the material,
magnitude & direction of the load and operating
conditions.
Design for Simple Stresses
• The maximum stress developed in a member
should not exceed the allowable value as
obtained from the material strength
considering certain value of factor of safety
• Induced stress [ σ ] ≤ σ design
• Limiting values of dimensions desired can be
calculated by equating σ and σ design
Direct Tensile & Compressive
Stress
When the fibers of the component tend to
elongate under the external load, stress
developed in the component is called tensile
stress. On the other hand, when the fibers
tend to shorten under the external load, stress
developed in the component is called
compressive stress.
σt = P/A ≤ σ design
σc = P/A ≤ σ design
Direct Shear Stress
• When the external load acting on the
component tends to slide the adjacent planes
with respect to each other, the resulting
stresses on these planes are called direct
shear stresses.
ζ = P/A ≤ ζdesign
Bending Stress
• When a machine member is subjected to
bending moment, tensile stress develops on
one side of the neutral axis and compressive
stress on the other.
M/I = σ/y
σinduced = My/I
σinduced ≤ σ design
Torsional Shear Stress
Stress induced in a machine member to resist
the action of twist is called torsional shear
stress
T/Ip =ζ/r
ζ= Tr/Ip
ζinduced ≤ ζdesign
Bearing or Crushing Stress
Crushing or Bearing pressure is defined as the
compressive stress developed at the surface of
contact between two interacting members
• The local compression that exists at the
surface of contact between two members that
are in not relative motion is called crushing
stress.
σc
=load/ projected area
=P/dt
Bearing Pressure
The local compression that exists at the
surface of contact between two members that
are in relative motion is called bearing
pressure.
