Plastic Deformations of Members with a Single Plane of Symmetry

In plastic deformation, the neutral axis does not coincide with the centroid of the cross-section. Instead, it divides the member into two equal areas. The neutral axis position is determined by analyzing the resultant compressive forces above it and tensile forces below it. These resultants pass through the centroids of the two equal areas created by the neutral axis.

Stresses are uniformly distributed above and below the neutral axis, with negative stress (compression) above and positive stress (tension) below. The magnitude of stress remains constant as -σy and +σy respectively. This uniform distribution creates equal and opposite resultant forces that form a couple equivalent to the applied bending moment.

The neutral axis divides the cross-section into two equal areas. The compressive and tensile resultant forces pass through the centroids of these two equal areas. This equal-area division is fundamental to understanding force equilibrium during plastic bending and determining the neutral axis position in members with a single plane of symmetry.

The plastic moment Mp is expressed as half the product of the cross-sectional area, the magnitude of yield stress, and the distance d between the centroids of the two equal areas created by the neutral axis. This formula directly relates the material's yield stress and geometric properties to the maximum moment the section can withstand before significant plastic deformation occurs.

During plastic bending, compressive forces act above the neutral axis and tensile forces act below it. These resultant forces have equal magnitudes but opposite directions and are separated by distance d. This configuration creates a couple that is equivalent to the applied bending moment, maintaining rotational equilibrium in the member.

The distance d between the centroids of the compressive and tensile force resultants is critical for calculating the plastic moment. This distance depends on how the neutral axis divides the cross-section into equal areas. Understanding this distance is essential for predicting the maximum moment capacity and assessing structural performance under bending loads.

The neutral axis does not align with the centroid when a member undergoes plastic deformation. This misalignment occurs because the neutral axis is determined by force equilibrium rather than geometric properties alone. The neutral axis divides the cross-section into equal areas, whereas the centroid is based on geometric distribution, making their positions distinct in plastic bending scenarios.