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Difficulty: MediumFasteners and Joints

A mechanic is constructing a heavy-duty tow linkage. Two thick steel plates are overlapped to form a lap joint and are secured together using three high-strength bolts. When a tractor pulls a heavy load using this linkage, the plates are pulled in opposite directions, parallel to their touching surfaces. Which of the following best describes the primary mechanical stress exerted on the bolts?

  1. Shear stress, because the opposing lateral forces attempt to slice through the cross-section of the bolts.Answer
  2. B
    Tensile stress, because the pulling force from the tractor attempts to stretch the bolts lengthwise.
  3. C
    Compressive stress, because the plates squeeze the body of the bolts as the load is applied.
  4. D
    Torsional stress, because the pulling action causes the bolts to twist violently inside their mounting holes.

Answer

Shear stress, because the opposing lateral forces attempt to slice through the cross-section of the bolts.
When overlapping plates in a lap joint are pulled in opposite directions (parallel to their mating surfaces), the fasteners holding them together must resist a lateral force. This force tries to cut the fasteners in half straight across their diameter. In mechanical engineering, a force that attempts to slice through a cross-section of material is known as shear stress.

Step-by-Step Solution

1
Analyze the direction of the applied forces relative to the fasteners.
The force pulls the steel plates in opposite directions, parallel to their touching surfaces. This means the force acts perpendicular to the length (axis) of the bolts.
Identifying the orientation of force vectors relative to the fastener is critical for determining the type of stress.
2
Evaluate the physical effect of this perpendicular force on the bolts.
Because the top plate pushes one way and the bottom plate pushes the other way at the exact point where they meet, it creates a cutting or slicing action across the middle of the bolts.
Visualizing how the material deforms or fails under load defines the mechanical nature of the stress.
3
Match the physical action to the correct mechanical terminology.
A force that attempts to cut or slice a material parallel to its cross-section is formally known as shear stress.
Standard engineering terminology must be applied to correctly classify joint loads.

Key Concept

Fastener Joint Loading: Shear vs. Tension
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Topics

Fasteners and Joints

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