Jännitysrasituskäyrä

Jännitysrasituskäyrä, often referred to as the S-N curve or fatigue curve, is a graphical representation of a material's resistance to fatigue. It plots the stress amplitude (S) against the number of cycles (N) to failure. Typically, stress is shown on the vertical axis and the logarithm of cycles to failure is shown on the horizontal axis.

The curve illustrates that as the stress amplitude increases, the number of cycles a material can withstand before fracturing decreases. Conversely, at lower stress levels, the material can endure a greater number of cycles. For some materials, particularly ferrous metals, the curve may level off at a certain stress level, known as the fatigue limit or endurance limit. Below this stress, the material can theoretically withstand an infinite number of cycles without failing. Other materials, like aluminum alloys, do not exhibit a distinct fatigue limit and will eventually fail even at very low stress levels, although the number of cycles will be extremely high.

Understanding the jännitysrasituskäyrä is crucial in engineering design, particularly for components subjected to repeated loading. It allows engineers to select appropriate materials and design structures that can safely withstand the expected operational stresses over their intended lifespan, thereby preventing catastrophic failures due to fatigue. The shape and position of the curve are influenced by various factors including material composition, manufacturing processes, surface finish, and environmental conditions.