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 Beam length,
Beam width,
thickness,
Beam working
L(mm)
b(mm)
h(mm)
length, l(mm)
197
9.76
9.76
100
201
24.21
3.06
100
194
38.32
4.51
100
Specimen 1 Specimen 2 Specimen 3
2. Graph of load-deflection for the tested specimen.
Load-deflection graph 4000
Load (N)
3000
2000
specimen 1 specimen 2
1000
specimen 3
0 5
10
-1000
15
20
Deflection (mm)
3. Calculation: flexural strength σf,(MPa)
maximum flexural
Flexural modulus
strain εf
Ef,(GPa)
specimen 1
experimental
995.80768
0.11717
8498.83
(circular)
theory
586.579
0.0302
19423
specimen 2
experimental
637.12695
0.03667
17374.61
rectangular)
theory
637.390
0.00908
70197.1
specimen 3
experimental
608.83954
0.05408
11258.13
theory
609.157
0.0144
42302.6
(thin-
(thickrectangular)
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.
