nvestigating Shear Strength in Mild Steel through Double Shear Testing

The double shear test serves as a valuable method to ascertain the shear strength, or ultimate shear stress, of mild steel specimens. Employing a Universal Testing Machine (UTM), this test allows us to understand how materials respond to forces that cause them to slide relative to each other. Two main types of UTMs are utilized: the screw type and the hydraulic type, with the latter being preferred for its ease of operation.

Understanding Shear Force and Strength

Shear force is the load responsible for inducing relative sliding between two adjacent parts of a material. Shear strength is defined as the maximum load applied perpendicular to a fastener’s axis that the material can endure before fracturing. In the case of double shear, the load is applied in a manner that would result in the fastener being cut into three pieces, distinguishing it from the single shear, which results in two fastener pieces.

Apparatus

To conduct the double shear test, the following apparatus is required:

• Universal Testing Machine (UTM)
• Shear Tool Assembly
• Specimen
• Vernier Calipers

Theory and Principle

The shear strength (Tmax) of the material under double shear is determined by the formula:

Tmax=F2A (Equation 1)Tmax​=2AF​ (Equation 1)

Where:

• $F$ represents the maximum load at which the specimen breaks.
• $A$ is the cross-sectional area of the specimen.

The machine’s load range is set based on the expected maximum load ($F$), calculated from the yield stress (fyfy​) and the factor of safety ($F.S$):

T=0.45fy (Equation 2)T=0.45fy​ (Equation 2)

Therefore,

Tmax=(F.S)×0.45fy (Equation 3)Tmax​=(F.S)×0.45fy​ (Equation 3)

F=0.9×(F.S)×fy×A (Equation 4)F=0.9×(F.S)×fy​×A (Equation 4)

Procedure

1. Determine Diameter:
   • Use Vernier calipers to measure the diameter of the given rod at three sections.
2. Calculate Maximum Load:
   • Calculate the maximum load using Equation (2) and select the load range.
3. Set Up UTM:
   • Set the UTM for the chosen load range.
4. Assemble Shear Attachment:
   • Assemble the shear attachment with the correct set of discs and insert the specimen.
5. Placement on Lower Table:
   • Place the entire assembly centrally over the bearing plate on the lower table.
6. Adjust Cross-Head:
   • Bring the lower cross-head close to the top surface of the assembly.
7. Zero Load Pointer:
   • Float the lower table and set the load pointer to zero.
8. Apply Load Gradually:
   • Gradually apply the load until the specimen breaks.
9. Record Observations:
   • Note the ultimate load applied on the specimen.
10. Calculate Shear Strength:

• Compute the shear strength of the steel specimen.

Observations

The observations taken during the double shear test are tabulated for analysis.

Calculations

Using Equation (1), the ultimate shear stress is calculated as $F/2A$.

Result

The ultimate shear stress of the material is found to be in units of N/m^2.