Compression, tension, torsion, bending, and shear are different loading modes classified by how forces act on a structural member and how the member tends to deform. Compression shortens, tension lengthens, torsion twists, bending curves, and shear causes adjacent sections to slide relative to one another.
The Mechanism
An applied load creates internal forces and stress within a material. The type of loading depends on the direction and position of the applied forces relative to the structural member.
| Loading mode | Force action and deformation | Structural example |
|---|---|---|
| Compression | Forces act towards each other along the member, causing it to shorten or crush. Long, slender members may buckle. | A column supporting a roof |
| Tension | Forces pull away from each other, causing the member to stretch or elongate. | A cable in a suspension bridge |
| Torsion | A turning moment acts around the member's longitudinal axis, causing it to twist. This produces internal shear stress. | A screwdriver shaft transmitting torque |
| Bending | A transverse load causes the member to curve. One side experiences tension while the opposite side experiences compression. | A shelf carrying books |
| Shear | Opposing, parallel forces cause adjacent layers or cross-sections to slide past each other. | A bolt connecting two loaded plates |
In a bent beam, material above and below the neutral axis experiences opposite stresses. The neutral axis itself has zero longitudinal tensile or compressive stress, although shear stress may still be present.
A common misconception is that bending produces only one type of stress. In fact, bending normally creates both tensile and compressive stress across the beam's cross-section. Similarly, torsion describes the twisting loading mode, while the principal internal stress it produces is shear stress.
IB Exam Technique
For A3.2 questions, identify each loading mode from the direction of the arrows and state the resulting deformation. When explaining bending, explicitly mention tension on one side, compression on the other, and the neutral axis between them. Do not define the forces only by examples; link each force to its direction and structural effect.
