Flexural or Bending Test Lab Report

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OBJECTIVE

To determine the value of flexural strength (), maximum flexural strain () and flexural

modulus (Ef) of materials.

INTRODUCTION

This mechanical testing method measures the behaviour of materials subjected to simple

bending loads. Like tensile modulus, flexural modulus (stiffness) is calculated from the slope

of the bending load vs. deflection curve. Flexural testing involves the bending of a material,

rather than pushing or pulling, to determine the relationship between bending stress and

deflection. Flexural testing is commonly used on brittle materials such as ceramics, stone,

masonry and glasses. It can also be used to examine the behaviour of materials which are

intended to bend during their useful life, such as wire insulation and other elastomericproducts

The three point bending flexural test provides values for the modulus of elasticity in

bending , flexural stress , flexural strain and the flexural stress-strain response of the

material. The main advantage of a three point flexural test is the ease of the specimen

preparation and testing. However, this method has also some disadvantages: the results of the

testing method are sensitive to specimen and loading geometry and strain rate.

Flexural stress () can be calculated on any point on the load deflection curve by using

following equation 1.

- (1)

where;

: flexural stress (MPa)

p : the load at a given point on the load-deflection curve (N)

L : the length of the support span (mm)

b : width of the specimen (mm)

d : thickness of the specimen (mm)
http://en.wikipedia.org/wiki/Flexurehttp://en.wikipedia.org/wiki/Elastic_modulushttp://en.wikipedia.org/wiki/Flexural_stresshttp://en.wikipedia.org/wiki/Flexural_stresshttp://en.wikipedia.org/wiki/Elastic_modulushttp://en.wikipedia.org/wiki/Flexure


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Flexural strength () is the maximum capability of a material to resist the plastic

deformation. Equation 2 is used to calculate the value of flexural strength ().

- (2)

where;

: flexural strength (MPa)

Y : yield point which the load does not increase with an increase in strain (N)




Flexural strain () is nominal fractional change in the length of an element of the outer

surface of the specimen at middle of span, where the maximum strain occurs. Equation 3 is

used to calculate the value of flexural strain ().

- (3)

where;

: flexural strain

D : maximum deflection of the centre of the beam (mm)



Modulus of Elasticity (MOE) is the ratio, within the elastic limit, of stress to correspondingstrain. Equation 4 is used to calculate the value of Modulus of Elasticity (MOE).

- (4)

where;

: modulus of elasticity (MPa)m : slope of the tangent to the initial straight-line portion of the load deflection curve

(N/mm)L : the length of the support span (mm)




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SPECIMEN AND EQUIPMENTS

1. Instron Series 8500 (5kN)

2. Vernier caliper

3. Test jig

4. Loading block5. Flexural specimens

PROCEDURES

1) The thickness and width of the beam are measured.2) The loading block is gripped and test jig in the upper and lower gripping head,

respectively.

3) The specimen is located so that the upper surface is to the side and centered in loadingassembly.

4) The machine is operated until the loading block was bought into contact with the uppersurfaces of the specimen. Full contact between the load (and supporting) surfaces and thespecimen is ensured to secure.

5) The required parameters are set on the control panel.6) The load recorder is adjusted on the front panel controller to zero, to read load applied.7) Start button is pressed to start the flexural test.8) The specimen is observed, as the load was gradually applied.9) The maximum load is recorded and loading is continued until complete failure.

Results

1. Show all the measurements of beams.

Beam length L

[mm]

Beam width

Beam thickness

Beam working

length

Aluminium 150.04 24.92 2.06 70

Steel 150.00 24.94 1.96 70


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2. Plot the loaddeflection graph for the tested specimen.Aluminium

Graph 1: loaddefl ection of alumini um

Steel

Graph 2: loaddeflection of steel


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DISCUSSIONS

1. Discuss on the shape of obtained loaddeflection graph.

The flexure test method measures behaviour of materials subjected to simple beam

loading. This is calculated at the surface of the specimen on the convex or tension side.

Flexural modulus is calculated from the slope of the stress vs. deflection curve. If the curve

has no linear region, a secant line is fitted to the curve to determine slope.

According to the aluminium graph:-

C

B D

A

From 0 to 300 (N) A to B the aluminium was elastic deformation

From 300 to 510 (N) B to C the aluminium was plastic deformation

From 500 to 450 (N) C to D the aluminium was strain hardening or it can call necking

At point D = 450 (N) D the aluminium was fracture
http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=130http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=130


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According to Steel Graph

C

D

B

A

From 0 to 250 (N) A to B the steel was elastic deformation

From 250 to 400 (N) B to C the steel was plastic deformation

From 400 to 380 (N) C to D the steel was strain hardening or it can call necking

At point D = 380 (N) D the steel was fracture

2. What is the percentage error (%) between experiment results with the theory? Why?

For aluminium the percentage was at 65 % to 70 %

For steel the percentage was at 60 % to 75 %

It is because the graph it makes from the parallel line and the theory we use the calculations

3. What is the critical application of the experiment in industry?

Flexural testing is predominately used in industries where materials are subject to

some form of bending force. The construction industry is a typical example in that the mostcommon test for structural steels, concrete beams, timber joists, GRC panels, ceramic tiles etc

is flexural testing.

Flexural testing is also widely used to evaluate materials that can be difficult to test in

tensile mode. This technique requires specialised fixtures and precision displacement

measurement coupled with advanced flexural testing software. Test metric offer a

comprehensive range of 3 and 4 point bend fixtures, displacement systems and dedicated

software to suit all applicable materials.


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C o n c l u s i o n s

The system functions by using metal bending bars of varying thickness and stiffness to

deform the test specimen. The force applied is measured by use of a built-in calibration and

calculation system. Due to the large margin of error from the measured and

calculated results, the experimental results are not acceptable for practical

application. At maximum deflection, the percentage of error of the experimental

result for aluminium is 65% - 70%. One cause for this error occurs because the

equations used are ac cur ate in sma ll def le ct i ons and loa ds eas i l y han dle d by

the mate r ia l tes t ed . The perc en tag e er ro r fo r s te e l i s 60 % to 75 %. Also ,

Hooke's law is only valid for a portion of the elastic range for some materials, including

aluminium.

R e f e r e n c e s

Gilbert, J. A and C. L. Carmen. "Chapter 8Flexure Test." MAE/CE 370Mechanics of Materials Laboratory Manual. June 2000

Dowling, N.E., Mechanical behaviour of materials: Engineering methods fordeformation, fracture and fatigue, 2nd edition, 1999, Prentice Hall, ISBN-0-13-010989-4.

Hibbleler, R.C.,Mechanics of Materials, SI second edition, 2005, Prentice Hall, ISBN0-13-186-638-9.


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Attachment

Documents

Flexural or Bending Test Lab Report