Short Notes: Design for Static and Dynamic Loading | Short Notes for Mechanical Engineering PDF Download

 Page 1


Machine parts fail when the stresses induced by external forces exceed their 
strength. The external loads cause internal stresses in the elements and the 
component size depends on the stresses developed.
Loading may be due to:
• The energy transmitted by a machine element.
• Dead weight.
• Inertial forces.
• Thermal loading.
• Frictional forces.
Load may be classified as static or dynamic 
Static load •
Design for Static and Dynamic Loading
• A static load is a mechanical force applied slowly to an assembly or 
object.Load does not change in magnitude and direction and normally 
increases gradually to a steady value
• Tests of static load are useful in determining the maximum allowable loads 
on engineering structures, such as bridges, and they can also be useful in 
discovering the mechanical properties of materials.
• This force is often applied to engineering structures on which peoples' safety 
depends on because engineers need to know the maximum force a structure 
can support before it will collapse.
• Any force applied steadily without moving an object is considered a static 
load, and the knowledge of how much loading a structure can handle is useful 
for setting safety margins for the structure.
Page 2


Machine parts fail when the stresses induced by external forces exceed their 
strength. The external loads cause internal stresses in the elements and the 
component size depends on the stresses developed.
Loading may be due to:
• The energy transmitted by a machine element.
• Dead weight.
• Inertial forces.
• Thermal loading.
• Frictional forces.
Load may be classified as static or dynamic 
Static load •
Design for Static and Dynamic Loading
• A static load is a mechanical force applied slowly to an assembly or 
object.Load does not change in magnitude and direction and normally 
increases gradually to a steady value
• Tests of static load are useful in determining the maximum allowable loads 
on engineering structures, such as bridges, and they can also be useful in 
discovering the mechanical properties of materials.
• This force is often applied to engineering structures on which peoples' safety 
depends on because engineers need to know the maximum force a structure 
can support before it will collapse.
• Any force applied steadily without moving an object is considered a static 
load, and the knowledge of how much loading a structure can handle is useful 
for setting safety margins for the structure.
• Limiting the loading to one half of a structure’s maximum will give a factor of 
safety of two.
• An elevator is an example where static loading occurs. When ten people stand 
in an elevator waiting for the doors to close, they are exerting a load on it that 
is static because the people and the elevator are not moving relative to each 
other. The stresses within the elevator have time to reach equilibrium under 
such conditions.
• An elevator must be tested to establish a maximum weight limit with an 
acceptable margin of safety.
T im e
Static Loading 
Dynamic load
• A dynamic load, results when loading conditions change with time. The load 
may change in magnitude, for example, traffic of varying weight passing a 
bridge.
• Cyclic load and impact load are types of dynamic loads
• The load may change in direction, for example, load on piston rod of a double 
acting cylinder. Vibration and shock are types of dynamic loading.
• When people move around in an elevator, they are creating a dynamic load, 
and the stresses at a point on the elevator may vary considerably.
Dynamic Loading
Design for Static Loading
Failure Criteria
Excessive stresses may result in yielding or fracture of a machine element making 
it unable to perform its desired function. •
• 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 the failure criterion for ductile materials.
• In the brittle material, very small plastic deformation occurs and fracture 
takes place once the stress developed reaches Ultimate Tensile Strength.
Page 3


Machine parts fail when the stresses induced by external forces exceed their 
strength. The external loads cause internal stresses in the elements and the 
component size depends on the stresses developed.
Loading may be due to:
• The energy transmitted by a machine element.
• Dead weight.
• Inertial forces.
• Thermal loading.
• Frictional forces.
Load may be classified as static or dynamic 
Static load •
Design for Static and Dynamic Loading
• A static load is a mechanical force applied slowly to an assembly or 
object.Load does not change in magnitude and direction and normally 
increases gradually to a steady value
• Tests of static load are useful in determining the maximum allowable loads 
on engineering structures, such as bridges, and they can also be useful in 
discovering the mechanical properties of materials.
• This force is often applied to engineering structures on which peoples' safety 
depends on because engineers need to know the maximum force a structure 
can support before it will collapse.
• Any force applied steadily without moving an object is considered a static 
load, and the knowledge of how much loading a structure can handle is useful 
for setting safety margins for the structure.
• Limiting the loading to one half of a structure’s maximum will give a factor of 
safety of two.
• An elevator is an example where static loading occurs. When ten people stand 
in an elevator waiting for the doors to close, they are exerting a load on it that 
is static because the people and the elevator are not moving relative to each 
other. The stresses within the elevator have time to reach equilibrium under 
such conditions.
• An elevator must be tested to establish a maximum weight limit with an 
acceptable margin of safety.
T im e
Static Loading 
Dynamic load
• A dynamic load, results when loading conditions change with time. The load 
may change in magnitude, for example, traffic of varying weight passing a 
bridge.
• Cyclic load and impact load are types of dynamic loads
• The load may change in direction, for example, load on piston rod of a double 
acting cylinder. Vibration and shock are types of dynamic loading.
• When people move around in an elevator, they are creating a dynamic load, 
and the stresses at a point on the elevator may vary considerably.
Dynamic Loading
Design for Static Loading
Failure Criteria
Excessive stresses may result in yielding or fracture of a machine element making 
it unable to perform its desired function. •
• 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 the failure criterion for ductile materials.
• In the 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 the failure criterion for 
brittle materials.
Factor of Safety & Allowable Stresses
• Value of factor of safety depends upon
° Effect of failure (level of severity, cost & danger involved),
° Type of load (static or dynamic),
° Accuracy in load calculations,
° Material selected (ductile, brittle, homogeneity), 
o Desired reliability,
° Service conditions (normal, corrosive, temperature level),
° Manufacturing quality (variation in desired dimensions, quality) and cost 
etc
Allowable stress=
[tr] = S /fos for dnctiLe materials 
= S^/ fos for brittle materials
where, Sy is yield strength, Su t is ultimate tensile strength and fos is factor of 
safety.
Design for Simple Stresses
• The maximum stress developed in a member should not exceed the allowable 
value as obtained from the material strength considering the certain value of 
factor of safety i.e any stress ‘a ' should always be s [a ].
• Limiting values of dimensions desired can be calculated by equating a and
[a].
• Equation a = [a ] is called design equation and it is used for simple stresses. 
Direct Tensile & Compressive Stress
p . . . p
Tensile stress is given bv. < j = _ < rffl Compressive stress is given by. < j = _ < rff 1
t a ~ L 1 ' c A ~ L cj
where P is external load, A is cross-sectional area of the component
and [a] and [oc ] are allowable tensile and compressive stress of the material.
Direct Shear Stress
T = P/A < [t]
where P is external load, A is cross-sectional area of the component and [ t ] is 
allowable shear stress 
Bending Stress
c r b = My/I < [it ]
where M is the Applied bending moment, I is Moment of inertia of the cross-section 
about the neutral axis and y is the distance of the fibre from the neutral axis 
Torsional Shear Stress
t = Tr/J < [t].
Page 4


Machine parts fail when the stresses induced by external forces exceed their 
strength. The external loads cause internal stresses in the elements and the 
component size depends on the stresses developed.
Loading may be due to:
• The energy transmitted by a machine element.
• Dead weight.
• Inertial forces.
• Thermal loading.
• Frictional forces.
Load may be classified as static or dynamic 
Static load •
Design for Static and Dynamic Loading
• A static load is a mechanical force applied slowly to an assembly or 
object.Load does not change in magnitude and direction and normally 
increases gradually to a steady value
• Tests of static load are useful in determining the maximum allowable loads 
on engineering structures, such as bridges, and they can also be useful in 
discovering the mechanical properties of materials.
• This force is often applied to engineering structures on which peoples' safety 
depends on because engineers need to know the maximum force a structure 
can support before it will collapse.
• Any force applied steadily without moving an object is considered a static 
load, and the knowledge of how much loading a structure can handle is useful 
for setting safety margins for the structure.
• Limiting the loading to one half of a structure’s maximum will give a factor of 
safety of two.
• An elevator is an example where static loading occurs. When ten people stand 
in an elevator waiting for the doors to close, they are exerting a load on it that 
is static because the people and the elevator are not moving relative to each 
other. The stresses within the elevator have time to reach equilibrium under 
such conditions.
• An elevator must be tested to establish a maximum weight limit with an 
acceptable margin of safety.
T im e
Static Loading 
Dynamic load
• A dynamic load, results when loading conditions change with time. The load 
may change in magnitude, for example, traffic of varying weight passing a 
bridge.
• Cyclic load and impact load are types of dynamic loads
• The load may change in direction, for example, load on piston rod of a double 
acting cylinder. Vibration and shock are types of dynamic loading.
• When people move around in an elevator, they are creating a dynamic load, 
and the stresses at a point on the elevator may vary considerably.
Dynamic Loading
Design for Static Loading
Failure Criteria
Excessive stresses may result in yielding or fracture of a machine element making 
it unable to perform its desired function. •
• 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 the failure criterion for ductile materials.
• In the 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 the failure criterion for 
brittle materials.
Factor of Safety & Allowable Stresses
• Value of factor of safety depends upon
° Effect of failure (level of severity, cost & danger involved),
° Type of load (static or dynamic),
° Accuracy in load calculations,
° Material selected (ductile, brittle, homogeneity), 
o Desired reliability,
° Service conditions (normal, corrosive, temperature level),
° Manufacturing quality (variation in desired dimensions, quality) and cost 
etc
Allowable stress=
[tr] = S /fos for dnctiLe materials 
= S^/ fos for brittle materials
where, Sy is yield strength, Su t is ultimate tensile strength and fos is factor of 
safety.
Design for Simple Stresses
• The maximum stress developed in a member should not exceed the allowable 
value as obtained from the material strength considering the certain value of 
factor of safety i.e any stress ‘a ' should always be s [a ].
• Limiting values of dimensions desired can be calculated by equating a and
[a].
• Equation a = [a ] is called design equation and it is used for simple stresses. 
Direct Tensile & Compressive Stress
p . . . p
Tensile stress is given bv. < j = _ < rffl Compressive stress is given by. < j = _ < rff 1
t a ~ L 1 ' c A ~ L cj
where P is external load, A is cross-sectional area of the component
and [a] and [oc ] are allowable tensile and compressive stress of the material.
Direct Shear Stress
T = P/A < [t]
where P is external load, A is cross-sectional area of the component and [ t ] is 
allowable shear stress 
Bending Stress
c r b = My/I < [it ]
where M is the Applied bending moment, I is Moment of inertia of the cross-section 
about the neutral axis and y is the distance of the fibre from the neutral axis 
Torsional Shear Stress
t = Tr/J < [t].
where T is applied torque, r is a radial distance of the fibre from the axis of rotatioi] 
and J is Polar moment of inertia of the shaft about the axis of rotation
Bearing or Crushing Stress
load P
Verona = projected = J~t ~ ^
where d is the diameter of the rivet and t is the thickness of the plate.