How to Perform Tension Test on Mild Steel Plain Bars?

The aim of tension test on mild steel plain bars is to determine the behavior of mild steel under tension and find the following properties:

  1. Yield Point Stress
  2. Ultimate Stress
  3. Percentage Elongation
  4. Nominal Breaking Stress
  5. Actual Breaking Stress
  6. Percentage reduction in cross-sectional area

Figure-1


Tension Test on Mild Steel Bars


The tensile strength of mild steel is of prime importance to an engineer. Mild steel is used in many of the civil engineering structures like bridges, railways, industrial buildings, water tank, transmission towers etc.

In one or the other case mentioned before, the mild steel used is subjected to direct tension or tension due to bending in some of the components of the civil engineering structures. The determination of the shapes and size of these components require the knowledge of the permissible stress in the materials and the capacity to deform consistent with the specifications of safety and serviceability. These are obtained from stress strain curve of the material.

Apparatus for Tension Test on Mild Steel Bars

The apparatus required to conduct tension test on mild steel bars are:
  1. Universal Testing Machine (UTM)
  2. Extensometer
  3. Hacksaw
  4. Scale
  5. Screw Guage

Procedure for Tension Test on Mild Steel Bars


The important procedure involved in the tension test on mild steel bars:
  1. Clean the steel rod specimen  with sand paper. Measure the diameter of the given rod at three different places and calculate the average diameter of the rod. Find the cross-sectional area of the steel rod using this average diameter, and name it as Ao.                                                           Diameter of the specimen                                                                                                                                   Trial 1, D1 =  ? ; Trial 2, D2 = ? ; Trial 3, D3 = ? ;  Average Diameter, Do = (D1+D2+D3)/3;      Average Cross-sectional Area, Ao = (𝝅/4 ) x D.D.D.D                                                      
  2. Mark the centre of the specimen with a hack saw. Calculate the guage length Lo.                        Guage Length, Lo = 5.65 sqrt (Ao) 
  3. Make marks using hacksaw at distances Lo/2 on either side from the centre throughout the entire length of the rod.
  4. Assume an ultimate stress of 600N/mm2 and calculate the maximum load that the specimen can take. The range of load is thereby applied and that range is set on the universal testing machine.   Approximate Ultimate Load = 600 Ao; { If ultimate load is 51 kN, then the load range selected is 0 to 100kN.}
  5. Now open, the relief valve of the UTM. Secure the specimen in the tension grips of the upper crosshead. Take care to insert more than 50mm into the jaws. Close the relief valve. Start the hydraulic drive. Open the control valve slightly so that the needle will deflect and close the control valve. Adjust the load point to zero, using zero adjusting knob. This process is necessary to remove the dead weight of the lower table, upper crosshead and connecting parts from the load. 
  6. Stop the hydraulic drive. Lift the lower crosshead up and grip firmly the lower part of the specimen. Then lock the jaws and close the relief valve and start the hydraulic drive.
  7. Open the control valve slowly until the desired loading rate is obtained. The load pointer will again move up gradually until at a certain point it remains constant or attempts to kick back. Note this load point as the yield point.
  8. After the yield point, the load again increases to a certain point of maximum value, which is indicated by the dead pointer and this load is stated as ultimate load.
  9. After which, the load pointer moves backward along with the formation of a neck on the steel rod which result later to the breaking of the specimen. Now, close the inlet valve smoothly and open the outlet valve.
  10. Remove the specimen and place it together and measure the length between the two punch marks or so called guage length after elongation. Also measure the reduced diameter.


Load Calculation for Tension Test on Mild Steel Plain Bars


SL.No

Parameter

Equation

Units

1

Reduced Diameter (Dr)

 

-----mm

2

Reduced Cross-sectional Area (Ar)

 

-------mm2

3

Final length Lr

 

Lr

4

Load at yield point Py

 

kN

5

Ultimate Load Pu

 

kN

6

Breaking Load Pb

 

kN

7

Yiled Stress,σy

Pv/Ao

kN

8

Ultimate Stress

Pu/Ao

N/mm2

9

Nominal Breaking Stress

Pb/Ao

N/mm2

10

Actual Breaking Stress

Pb/Ar

N/mm2

11

Percentage Breaking Elongation

[Increase in length /Original length ] x100

 

(Lr – Lo)/Lo

%

12

Percentage Reduction of area

(Ao –Ar)/Ao

%



Final Results for Tension Test on Steel Rods


Yield strength of the material =
Ultimate strength of the material = 
Normal breaking stress of the material =
Actual breaking stress of the material =
Percentage Breaking Elongation =
Percentage Reduction of Area = 

As per the Indian standard specification, the minimum yield strength of structural steel of standard quality is 260 N/mm2. Minimum ultimate strength of structural steel of standard quality is 420 N/mm2. Minimum percentage of breaking elongation for structural steel of standard quality is 23 %.


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