Design of Mechanical Elements

5Design for Static Strength

5.1 Introduction

We define a static load as an external force or moment that remains constant as a function of time. This contrasts a dynamic load, which changes as a function of time.

When designing or verifying a mechanical element for static strength, we analyze the physical connection between the mechanical element and its environment and replace this connection with boundary conditions and reaction forces. Together, the external forces and the reaction forces describe the free‐body diagram of the mechanical element. From the free‐body diagram, we determine the internal forces in the mechanical element as an axial force, a shear force, a bending moment, and a torque moment diagram. The internal force diagrams display each internal force component as a function of the location in the mechanical element and, thus, they enable determining which section of the mechanical element is subject to the highest load. Subsequently, when considering the internal force diagrams together with the (local) geometry of the mechanical element, we convert internal forces to internal stresses. Figure 5.1 schematically shows that we relate the external forces acting on a mechanical element to the corresponding internal forces by means of the free‐body diagram, and subsequently to the internal stress by considering the local geometry of the mechanical element.

Ultimately, we compare the internal stress to the yield stress (ductile materials) or the ultimate tensile ...