Summary of "10 Ensayo de Corte Directo"

Summary of “10 Ensayo de Corte Directo” (Direct Shear Test)

This video explains the concept and procedure of the direct shear test, comparing it with tension and compression tests, and demonstrating how to perform the test on an aluminum specimen.

Key Concepts and Techniques

Normal Stress vs. Shear Stress:
- Normal stress (tensile or compressive) acts perpendicular (90°) to the fracture area.
- Shear stress acts parallel to the fracture area.
Shear Stress Examples:
- Riveted joints where rivets may fail by shear.
- Crane hooks experiencing shear at specific points.
- Punching holes in plates, where the punch cuts the plate by shear.
Shear Stress Calculation:
- Shear stress, (\tau = \frac{\text{Load}}{\text{Shear Area}}).
- Shear area depends on the specimen geometry (e.g., for circular cross-section: (A = \pi d^2 / 4)).
- In double shear, the shear area is doubled.
Direct Shear Test Setup:
- Specimen embedded in a fixture.
- Load applied via a punch until fracture.
- The test measures the maximum load before failure.
- The maximum shear stress is calculated by dividing this load by the shear area.
Complexities in Direct Shear:
- The specimen may experience combined stresses (bending, tension, shear), making it impossible to obtain pure shear stress-strain curves.
- Only the maximum shear stress (at failure) is reliably obtained.
Comparison with Pure Shear:
- Pure shear stress can be obtained in torsion tests, where stress-strain curves and shear modulus can be measured.
- This video focuses only on direct shear and maximum shear stress.

Step-by-Step Procedure for Direct Shear Test

Prepare the specimen:
- Use an aluminum specimen with diameter approximately 9.45 mm.
Mount the specimen:
- Place the specimen in the shear fixture.
- Tighten nuts to secure the specimen.
- Use a percussion tool to embed the specimen properly.
Set up the testing machine:
- Attach the shear test fixture to the machine.
- Center the specimen and fixture properly.
- Set the machine parameters (e.g., test speed at 2 mm/min).
Conduct the test:
- Apply load gradually until the specimen fractures or the load drops to 70% of the maximum.
- The machine automatically stops the test at this point.
Record data:
- Note the maximum load at fracture (e.g., 3250 kgf).
- Remove and inspect the fractured specimen.
Calculate results:
- Calculate the shear area: [ A = \frac{\pi d^2}{4} ]
- Convert load to Newtons if necessary.
- Calculate maximum shear stress: [ \tau = \frac{\text{max load}}{\text{shear area}} ]
- Example: For aluminum, maximum shear stress calculated was approximately 455 MPa.
Optional:
- Repeat test for other materials (steel, copper, brass, cast iron) for comparison.
- Use different fixture attachments for various specimen diameters.

Additional Notes

The test focuses on maximum shear stress, not on elastic or plastic behavior or shear strain.
Future sessions planned include torsion tests (pure shear), but the current machine is not operational for torsion.
Teachers are encouraged to use data from various materials to enrich learning.