Objectives:

1. To study and examine the flexural properties of materials.

2. To investigate how the dimension and shape of materials affect the flexural.

3. To develop an understanding about the flexural properties of materials.

Theory:

This mechanical testing method measures the behavior 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 behavior of materials which are

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

products. The three points 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

Specimen and equipment:

1. Instron Series 8500

2. Vernier caliper

3. Test jig

4. Loading block

5. 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 centred in

loading assembly.

4) The machine is operated until the loading block was bought into contact with the

upper surfaces of the specimen. Full contact between the load (and supporting)

surfaces and the specimen 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.

Result:

1. Measurement of specimens.

Beam length,

L(mm)

Beam width,

b(mm)

Beam

thickness,

h(mm)

Beam working

length, l(mm)

Specimen 1 197 9.76 9.76 100

Specimen 2 201 24.21 3.06 100

Specimen 3 194 38.32 4.51 100

2. Graph of load-deflection for the tested specimen.

3. Calculation:

flexural strength

σf,(MPa)

maximum flexural

strain εf

Flexural modulus

Ef,(GPa)

specimen 1

(circular)

experimental 995.80768 0.11717 8498.83

theory 586.579 0.0302 19423

specimen 2

(thin-

rectangular)

experimental 637.12695 0.03667 17374.61

theory 637.390 0.00908 70197.1

specimen 3

(thick-

rectangular)

experimental 608.83954 0.05408 11258.13

theory 609.157 0.0144 42302.6

-1000

0

1000

2000

3000

4000

5 10 15 20

Load

(N

)

Deflection (mm)

Load-deflection graph

specimen 1

specimen 2

specimen 3

Calculation of specimen 1:

Flexural strength = 3Wl / 2bh²

= 3(3635.67)(0.1) / 2(0.00976)(0.00976)²

= 586.579 MPa

Maximum flexural strain = 6Dd / L²

= 6(20.01)(9.76) / 197²

= 0.0302mm

Flexural modulus = σf / εf

= 586579000 / 0.0000302

= 19423 GPa

Experimental flexural modulus = σf / εf

= 995807680 / 1.1717×10^(-4)

= 8498.83GPa

Calculation of specimen 2:

Flexural strength = 3Wl / 2bh²

= 3(962.88)(0.1) / 2(0.0242)(0.00306)²

= 637.39 MPa

Maximum flexural strain = 6Dd / L²

= 6(19.98)(3.06) / 201²

= 0.00908mm

Flexural modulus = σf / εf

= 637390000 / 0.00000908

= 70197.1 GPa

Experimental flexural modulus = σf / εf

= 637126950 / 3.667×10^(-5)

= 17374.61 GPa

Calculation of specimen 3:

Flexural strength = 3Wl / 2bh²

= 3(3163.66)(0.1) / 2(0.0383)(0.00451)²

= 609.157 MPa

Maximum flexural strain = 6Dd / L²

= 6(19.98)(4.51) / 194²

= 0.0144mm

Flexural modulus = σf / εf

= 609157000 / 0.0000144

= 42302.6 GPa

Experimental flexural modulus = σf / εf

= 608839540 / 5.408×10^(-5)

= 11258.13 GPa

Discussion:

Based on the result obtained, the load-deflection graphs for the three specimens are

almost similar overall. Specimen 1 has higher gradient compared with specimen 2 and

specimen 3. All of the graphs tend to flatten at the beginning because the load has not

applied to the specimen yet. When the load was applied on the specimens, the graphs start

to show increasing in value for both load and deflection. The deflection of the three

specimens increase directly proportional to the load applied. Once reaching the proportional

limit, the graph tends to increase with a less steep gradient until it reached the ultimate

flexural strength. Besides, the graphs differ in the numerical value due to their different

dimension and shape (thickness, width, rectangular or circular in shape).

The percentage error of flexural strength for specimen 1 is 69.77%, specimen 2 is

0.041% while specimen 3 is 0.052%. The percentage error differences are quite large

between specimen 1 with specimen 2 and specimen 3. This is due to some error that occur

during the experiment was conducted that were parallax error and systematic error. Parallax

error maybe occurred when obtaining the value of thickness, width and length of the

specimen because the scale of the vernier caliper is not parallel with observer’s eyes.

Systematic error also may be occurred along the whole experiment without consciousness.

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 most

common test for structural steels, concrete beams, timber joists, GRC panels, ceramic tiles

is flexural testing. Flexural testing is also widely used to evaluate materials that can be

difficult to test in tensile mode. This technique requires specialized 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. The flexural testing is widely used to

evaluate materials that can be difficult to test in tensile mode. So that the strength of the

material that would be used in the industry could be determined and could increase the

technology and also explore more about the uses of materials in the industry.

Conclusion:

The specimens with same working length but different dimensions and shapes give

different value of flexural strength, maximum flexural strain and flexural modulus. From

the experiment, bigger values in dimensions and circular in shape give higher value of

flexural strength, maximum flexural strain and flexural modulus. The students get more

understanding about the mechanics of materials especially in flexural properties.

References:

1. Flexural or bending test lab, retrieved from:

http://www.scribd.com/doc/145757294/Flexural-or-Bending-Test-Lab-Report

2. Harlina, Bending-tensile strength, retrieved from:

http://metalab.uniten.edu.my/~Halina/EXP3~1.pdf

3. Strength of Material Laboratory Manual, 2014, Department of Mechanical And

Manufacturing Engineering, Faculty Of Engineering UPM, Serdang.