Design of Mechanical Elements

6Design for Fatigue Strength

6.1 Introduction

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

When designing or verifying a mechanical element for dynamic strength, similar to static strength, we first analyze the physical connection between the mechanical element and its environment, and replace it 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. These diagrams show each internal force component as a function of the location in the mechanical element and, thus, we can determine which section of the mechanical element experiences 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.

The key difference between designing a mechanical element for static and dynamic strength is that in the former case, all stress components remain constant as a function of time, whereas in the latter case, we must account for stress components that vary with time.

6.1.1 Types of Dynamic Loads

Two types of dynamic loads exist: an alternating and a fluctuating load, which result ...