Chapter-5- Pure Axial Loading

Pure Axial Loading

A member is said to be under pure axial loading, when two equal and opposite load act along the axis of the member in such a way that its magnitude remains constant throughout the length of the member. In this, we have assume that member is a prismatic bar (i.e. dimension are same throughout the structure).

Pure Axial Load 

Under axial loading condition we found that bending moment, shear force and twisting moment are zero.

SFD & BMD Diagram

There are many method in strength of material to obtains the stress occurred on member due to application of pure axial loading, here we have considered stress calculation as per “section method” which say that load acting to any cross section is equal to algebraic sum of corresponding load either on the left hand side of the cross section or on the right hand side of the cross section of the member.

 We see in the below image that at the section x-x axial load is “P” and Shear force, bending moment and twisting moment is “0”.

 Axial load is also independent of location of section x-x which signify that axial load is constant, so we can say that member is under pure axial load. We can calculate stress and deformation caused due to axial load using below equation.

Stress and Deformation

Above equation is only applicable when it satisfied following condition.

• Member should be prismatic whose magnitude remains constant throughout its length.

Loading classification in strength of materials

Loading classification:-

Sl. No.	Type of load	Plane of Cross Section	Longitudinal Axis	Centroidal Axis
1	Axial load	Perpendicular to the plane of cross section	Along the longitudinal axis	Along the polar axis
2	ECC. Axial Load	Perpendicular to the plane of cross section	Parallel to the longitudinal axis	Parallel to the polar axis
3	Transverse Shear Load	Parallel to plane of cross section	Perpendicular to longitudinal and passing through longitudinal axis	Perpendicular to poplar axis and passes through centroid of that cross section
4	ECC. TSL	Parallel to plane of cross section	perpendicular to longitudinal but away from longitudinal axis	Perpendicular to polar axis but away from centroid axis of cross sections
5	Bending Couple	Perpendicular to the plane of cross section	Along longitudinal al axis	Acts about either horizontal or vertical to centroidal axis
6	Twisting Couple	Parallel to plane of cross section	Perpendicular to the longitudinal axis	Act about the polar axis

Chapter-4- Types of Loads

Types of Loads:-

We will see in details study of load before going into depth of strength of materials. It is very important to understand the different type of load in strength of material (s o m), then only we well be able to calculate the stress on the body.

Type of load with respect to Time.

1.Static load

    a)Dead load

    b)Gradually applied load 

2.Dynamic load

    a)Impact load

   b)Fatigue load 

Static Load

are such type of load whose magnitude and direction remains constant for the entire periods of time. Where 

Dynamic Load 

magnitude and direction are continuously changing. Those load which increases gradually with respect to time is Gradually Applied Load (G.A.P).

Impact Load

is a load which is acting for short interval of time, they are always a function of velocity. Impact load is also known as Suddenly Applied Load or Instantaneous load.

Fatigue load 

are those load whose magnitude or direction or both magnitude and direction changes with respect to time. These load are repeatedly applied load with respect to time.

Fatigue life 

of a component is defined as number of revolution that a component can undergoes before initiation of first crack. Fatigue crack can be initiated first at stress concentration region, where the surface is irregular or having some kind of defect.

Endurance Limit

is the measure of fatigue life i.e. failure stress of a material under completely reversed fatigue load condition. It is obtained from S.N curve which is drawn using fatigue test reading. It gives relation between stress and number of revolution. 

It is defined as maximum value of completely reversed bending stress that a material can withstand for an infinite no. of cycle without initiation of crack.

Based on direction of load with respect to cross section.

Normal load 

which are acting perpendicular to the plane of cross section.

Shear load 

which are acting parallel to the plane of cross section.

Axial load 

are such type of load which are acting along the longitudinal axis. 

Eccentric axial load (ECC. Axial Load) 

act parallel to the longitudinal axis.

Transverse Axial load (TSL) 

are such type of load which are acting perpendicular to the longitudinal axis. 

 Eccentric transverse axial load (ECC. TSL) 

act parallel to the longitudinal axis

  

Summery of loads:-

Sl. No.	Type of load	Plane of Cross Section	Longitudinal Axis	Centroidal Axis
1	Axial load	Perpendicular to the plane of cross section	Along the longitudinal axis	Along the polar axis
2	ECC. Axial Load	Perpendicular to the plane of cross section	Parallel to the longitudinal axis	Parallel to the polar axis
3	Transverse Shear Load	Parallel to plane of cross section	Perpendicular to longitudinal and passing through longitudinal axis	Perpendicular to poplar axis and passes through centroid of that cross section
4	ECC. TSL	Parallel to plane of cross section	perpendicular to longitudinal but away from longitudinal axis	Perpendicular to polar axis but away from centroid axis of cross sections
5	Bending Couple	Perpendicular to the plane of cross section	Along longitudinal al axis	Acts about either horizontal or vertical to centroidal axis
6	Twisting Couple	Parallel to plane of cross section	Perpendicular to the longitudinal axis	Act about the polar axis