Double shear test is used to determine the shear strength (ultimate shear stress) of the mild steel specimen. Universal testing machine (UTM) is used for performing double shear test. There are two types of UTM namely: screw type and hydraulic type. The latter is easier to operate.
Shear force is the load that cause two contiguous parts of the body to slide relative to each other in a direction parallel to their plane of contact. Shear Strength is defined as the maximum load typically applied normal to a fastener’s axis that can be supported prior to fracture. Double shear is load applied in one plane that would result in the fastener being cut into three pieces, while single shear would result in two fastener pieces.
- Universal Testing Machine (UTM)
- Shear Tool Assembly
- Venire Calipers.
Theory and Principle
Shear strength of the material is the ultimate shear stress (Tmax ) attained by the specimen, which under double shear given by,
Tmax= F/2A Equation 1
F: Maximum load at which the specimen breaks
A: cross-sectional area of the specimen.
The load range to which the machine is to be set for the test is selected bases on the expected maximum load F to be applied on the specimen. This is calculated from the yield stress fy and the factor of safety (F.S), as follows:
Permissible shear stress T for mild steel is,
T =0.45fy Equation 2
Tmax= (F.S) 0.45fy Equation 3
F= 0.9 (F.S)fyA Equation 4
- Determine the diameter of the given rod with the help of Vernier caliper. Measure the diameter of the specimen at three sections.
- Calculate the maximum load expected to be applied on the specimen using equation (2) and select the load range to be used.
- Set the UTM for the selected load range.
- Set the correct set or disc to assemble the shear attachment with the right set of disc in it. Insert the specimen in to the disc so that it projects equally on either side, Fig. 3.
- Place the entire bear assembly with the specimen in it centrally over the baring plate on the lower table.
- Bring the lower cross- head close to the top surface of the assembly.
- Float the lower table and set the load pointer to zero.
- Apply the load gradually until the specimen breaks, Fig. 4.
- Note the ultimate load applied on the specimen.
- Finally, Compute the shear strength of the steel specimen.
The observation taken are tabulated in Table.
Using equation (1) the ultimate shear stress is= F/2A=___________
Ultimate shear stress of the material found to be N/mn^2