Tensile Testing of Materials

Title

Tensile Testing of Metals

Objective

·         To determine the tensile strength of metal

Introduction

Tensile testing is an important test for getting information about materials and their properties. Some of the properties are ultimate tensile strength, maximum elongation, yield strength, Young’s modulus and Poisson’s Ratio. The results can be used when choosing suitable materials for production and construction.

The test is done by applying an increasing axial force to a standard tensile specimen until failure. The specimen has two large ‘shoulders’ for gripping and a thinner middle section for deformation and fracture. The dimensions differ in various standardised tests. During the test the tensile force and the extension are recorded. The information is used to calculate the stress and strain. First the sample would undergo elastic deformation until the yield point. After that the plastic deformation takes place until failure.

Theory

In the test the elongation is measured against the tensile force. This is used to calculate the strain.

                         

The force is used to calculate the engineering stress.

·         ∆L – Elongation

·         L₀ – Initial gauge length

·         F – Tensile force

·         A₀ – Initial cross section

The following image shows the stress-strain curve and the condition of the specimen at significant moments.

Figure 2: Stress-Strain curve of ductile material (Shah, 2011)

Apparatus/Instrument

Medium Carbon steel specimen                                 Area Reduction Gauge

Vernier Calliper                                                          Elongation Gauge

Tensile testing machine

Computer

     

Procedure

·         First specimen’s gage length and diameter of the cross section were measured using the Vernier caliper.

·         Specimen was placed between the jaws of the tensile testing machine and by rotating a wheel by manually applied the load to the specimen.

·         The display in the tensile machine indicates the load and the elongation of the specimen with load.

·         Display of the tensile testing machine was videoed to read the exact load in the maximum elongation.

·         Load was applied until the specimen was broke into two pieces. o Two parts of the specimen was placed on the elongation gauge and calculate the elongation.

·         A piece of a specimen was placed in the area reduction gauge and measures the reduction of the cross-section area.

Observation

 

                                      

Calculations

Load	Elongation	Stress	Strain
0	0	0	0
70	0	4.71	0
1400	0.2	94.21	0.009
5500	0.8	370.12	0.038
9860	1.3	663.53	0.063
10440	1.5	702.56	0.073
10500	1.6	706.59	0.078
10100	2.0	679.68	0.097
9670	2.5	650.74	0.122
9120	2.8	613.73	0.136
8210	3.2	552.49	0.156
6900	3.5	464.33	0.170
6600	3.7	444.14	0.182

Discussion

Tensile stress is the resistant force that build inside the material which against the force tending to tear it. In this test applied force and elongation was measured to identify the special properties of the material. All the materials have characteristic points and different regions that have different properties.

When consider the   stress and strain of a specific object, at low stresses it acts in the elastic region. When given stress is released the object comes to the initial length and this property in this region is called elastic deformation or called liner deformation.  The size of the elastic region is differing from material to material. In the elastic deformation connection between stress and strain is liner.

Yield strength is the end of the elastic region and also it’s the beginning of the plastic region. So the transformation from the elastic behavior to plastic behavior is called yield. After the yield point deformation will be permanent and non-reversible and when the load is removed some amount of extension will remain. 

Under an applied load, change in the shape/structure of the material called deformation. Elastic deformation involves stretching the bonds, but it does not involve in slipping the atoms and pass each atom or break bonds. Slip occurs in the directions of atoms are most strongly packed because least amount of energy is required to slip is in that direction.

           

Ultimate tensile strength is the maximum stress that material can bear before it fails. After the necking/UTS begins the non-uniform plastic deformation. Here the plastic deformation is not spread through the gauge of the specimen, but it is occurring in one local region. Finally at the fracture stress material fails.

                                                         

                              

 Fig- 01 Stress strain curve of Steel specimen                                                              Fig-02 Stress strain curve of Aluminum specimen

                                

Fig-03 Stress strain curve of Glass                                                                            Fig-03 Stress strain curve of Rubber  

Above 4 graphs of stress vs. strain shows the four different curves those four different materials give. When we consider steel and aluminum there are some considerable differences in the two curves.   The difference in strain values between yield point and ultimate point is the plastic strain and it is quite large for Aluminum. Hence it can be beaten to thin plates and sheets and wires and all that. Steel has small plastic region compare to Aluminum so it can’t be beaten into thin sheets like Aluminum if we do so it will break, but steel has more elastic region stress with low strain than Aluminum , it mean steel can bear more energy than Al and that give the idea that steel is more stronger.

           

As we can see generally glass and rubber are very much differ from other two materials, but when analyze the graphs of rubber and glass. Glass is a rigid material and it has no characteristic points or regions. As in the Fig-03 glass has a very small elongation before fracture, but rubber has huge elongation before break. Rubber has moderate tensile strength. For rubber elastic region is large and plastic region is not very much clear and it is not well defined.  Rubbers cis or trance molecular arrangement is the cause to this behavior.

                                                                                                        

            In engineering material selection is a very important thing. Tensile test is a very useful test to select a material for any engineering project. By doing this test engineers/scientists can have some technical and scientifically important data before their work. Also the users can pre identify if these materials are complete their requirements or not.  In legal processes this tensile test is a good evidence to prove facts about materials.

Conclusion

As any engineer we should have the knowledge about how the materials act in various situations. In this lab practical it gives us good experience and idea about the properties of the materials. We learned about why different materials has different properties and how those properties comes to those materials by analyzing the atomic and molecular structures and mainly the stress vs. strain graph. 

References

·         En.wikipedia.org. (2018). Tensile testing. [online] Available at: https://en.wikipedia.org/wiki/Tensile_testing [Accessed 26 Aug. 2018].

·         Practicalmaintenance.net. (2018). Practical Maintenance » Blog Archive » Plastic Deformation and Fracture. [online] Available at: http://practicalmaintenance.net/?p=1135 [Accessed 26 Aug. 2018].

·         Jove.com. (2018). Stress-Strain Characteristics of Aluminum | Protocol. [online] Available at: https://www.jove.com/science-education/10362/stress-strain-characteristics-of-aluminum [Accessed 26 Aug. 2018].

·         En.wikipedia.org. (2018). Ultimate tensile strength. [online] Available at: https://en.wikipedia.org/wiki/Ultimate_tensile_strength [Accessed 26 Aug. 2018].