Sketch two S-N curves on the same axis, one for a metal with an endurance limit and one for a metal without an endurance limit. Mark any points of interest on the curves.

To sketch the S-N curves for a metal with an endurance limit and a metal without an endurance limit, you need to understand what the S-N curve represents and how it differs for these two cases.

The S-N curve, also known as the stress-life curve, is a graphical representation that shows the relationship between the stress applied to a material and the number of cycles it can withstand before failure. This curve is commonly used to assess the fatigue strength or fatigue life of a material.

1. S-N Curve for a Metal with an Endurance Limit:

When a material has an endurance limit, it means that below a certain stress level, it can resist an infinite number of cycles without failing. This endurance limit is also known as the fatigue limit. For materials with an endurance limit, the S-N curve exhibits a horizontal line after a certain stress level.

The sketch of this S-N curve will look like an inclined line at the beginning, and then it would change to a horizontal line once it reaches the fatigue limit. Marking points of interest on this curve would include the stress at the endurance limit, typically denoted as S_endurance.

2. S-N Curve for a Metal without an Endurance Limit:

Unlike materials with an endurance limit, some materials do not have a stress level below which they can resist an infinite number of cycles without failure. For these materials, the S-N curve does not exhibit a horizontal line; it continuously decreases as stress increases.

The sketch of this S-N curve will be a continuously decreasing line without any flat portion. Marking points of interest on this curve would include the stress at various cycle numbers, such as the stress at 10^3 cycles, denoted as S_10^3, the stress at 10^4 cycles, denoted as S_10^4, and so on.

By understanding the concept of an endurance limit and the behavior of materials with and without it, you can sketch the S-N curves accordingly and mark the points of interest on each curve.