STRAIN MEASUREMENT IN BEAM BENDING

Abstract

In this lab experiment the beam design was evaluated for a given bending load so that the best engineering design can be used to design the beam for a given load. In order to evaluate the given design of bean against a given load stress and strain produce in beam when load is applied in it was calculated. Stresses in beam were calculated using theoretical formulas and the deflection was measured using foil strain gauges. Similarly the buckling of columns was also studied experimentally for different ends conditions to evaluate the load bearing capacity of columns made of aluminum and brass.

STRAIN MEASUREMENT IN BEAM BENDING

Introduction

For any load bearing system beams are its main supporting structure which provides necessary strength to the structure. Beam provides the required strength to the structure due to strength of its material and it shape. Shape of the beam defines the second moment of inertia which when combine with the load and point of application of load gives the value of strength a beam can provide to its structure. There are many different types of beams like simply supported beam, continuous beam and over hanging beam were each type can have different types of shapes like square beam, rectangular beam or I beam. From the above mention shapes of beam I beam is the most used beam due to its greater second moment of inertia for given height and width.

In construction work application one of the most important elements used is beam. Beam is used to withstand load which is applied laterally along the length of the beam. The applied load can be a point load which is applied at one single point or it can be distributed load. The uniformly distributed load expands all over the length of beam and acts with same magnitude at all points on the beam. Due to bending load the beam can be deflected downwards under the applied load [1]. This deflection must be measured and analyzed to evaluate the effectiveness of beam towards its load bearing capacity. The deflection in beams can be measured using strain measurement and this is done by using strain sensors [2]. The examples of such strain gauges are foil gauges, vibrating wire gauges, micro electromechanical strain gauges and optical sensors etc. in this experiment the foil gauges were used because they are easy to use and are more economical and produce accurate results.

Experimental apparatus 

Wooden plank

Foil gauges (attached to wooden plank by a glue with good strain transference)

Wheatstone bridge circuit

Oscilloscope or multi-meter

Loads

CALCULATIONS

The following dimensions of beam are given

Load applied; W = 100 kN/m2

Beam length; L = 100 cm = 1 m

Beam width; w = 3.5 cm

Beam Breadth, h = 14.3 cm 

From beam dimensions the second moment of area, ‘I’ can be calculated as 

Second moment of area

I =(wh^3)/12 

Putting in values

I =(0.035* (0.143)^3)/12   = 8.53 x 〖10〗^(-6)  m^4

The beam is assumed to be simply supported and the load is applied at the middle point

For a maximum load of 100 kg 

W = 100* 9.81 = 981 N

For simply supported beam the maximum bending moment will be at the centre of beam where the deflection will be the maximum but the shear force will be the minimum. 

The maximum moment would be 

M =WL/4  =(981*1)/4  = 245.25 Nm 

The maximum bending stress can be calculated as 

Bending stress 

σ =My/I

Where ‘y’ is the distance from neutral axis to the edge of beam and this is equal to half of beam breadth (height) 

so y =14.3/2   = 7.15 cm

Bending stress

 σ =(245.25* 0.0715)/(8.53 * 〖10〗^(-6)  )

σ = 2.05* 〖10〗^6  Pa = 2.05 MPa

For three point test, the flexural strength can be calculated using the following formula 

σ =3LF/(2wh^2 )

Putting in values

σ =(3*1*981)/[2*0.035*(0.143)^2 ] 

σ = 2.05* 〖10〗^6  Pa = 2.05 MPa

This is exactly same as the maximum bending stress in the beam

For uniform distributed load

For this particular case when the load is 100 k N/m2 which is uniformly distributed and the beam length is 2.6 m. Let’s assume the width of beam is 10 cm and height is 20 cm. Then the load on the beam will be 

100 kN/m2* 0.1 m = 10 k N/m

Therefore 

W =10 k N/m

This is uniformly distributed load and in this case the second moment of area would be

Second moment of area

I =(wh^3)/12

Putting in values

Second moment of area 

I =(0.1* (0.2)^3)/12  = 6.67 x 〖10〗^(-5)  m^4

The distance from neutral axis to the beam edge

y =20/2  = 10 cm = 0.1 m

The maximum bending stress can be calculated as 

Bending stress 

σ =My/I

The maximum moment for uniformly distributed load would be 

M =(WL^2)/4  =(10*1000* 2.62)/4   = 16900 Nm

Now

Bending stress

σ =(16900 x* 0.1)/ 6.67* 〖10〗^(-5)

σ = 25.33*〖10〗^6  Pa = 25.33 MPa

This is the maximum stress the beam can take before failure. With a factor of safety of 3 the ultimate stress would be 25.33 x 3 = 76 MPa

W	I	M	Y	stresses	strain
196	0.00000853	49	0.0715	411145.96	2.74097E-05
245	0.00000853	61	0.0715	513932.44	3.42622E-05
491	0.00000853	123	0.0715	1027864.9	6.85243E-05
736	0.00000853	184	0.0715	1541797.3	0.000102786
981	0.00000853	245	0.0715	2055729.8	0.000137049

Discussion

A square shape simply supported beam was used during this experiment and material used for the beam was wood. As per instruction for experiment load was applied on the beam and stresses were calculated using formulas for simply supported beam. Strain was recorded using the strain gauge and Wheatstone bridge. Five different loads were applied on beam and stresses and strain for each of the load were calculated.  Graphs for stress and strain was made against the force applied. According to the graph one which is between stress and force (load) the relation between stress and load is linear and directly proportional to each other. This means that increase in load increase the stress produce in beam and reduction in load reduces the stress produce. This change in stress is equal to change in load as the graph is linear. According to the graph two which is between strain and force (load) the relation between strain and load is linear and directly proportional to each other. This means that increase in load increase the strain produce in beam and reduction in load reduces the s strain produce. This change in strain is equal to change in load as the graph is linear.