Briefly explain the difference between fatigue striations and beachmarks in terms of (a) size and (b) origin.

Fatigue striations are microscopic parallel lines observed on the fracture surface of a material that has failed due to fatigue loading. They indicate the incremental progress of the crack front under cyclic loading. Beachmarks, also known as clamshell marks or arrest lines, are macroscopic lines found on the fracture surface of a material that has failed due to fatigue loading. Beachmarks are visible to the naked eye and indicate interruptions or changes in the load applied to the material.

(a) Size: Fatigue striations are much smaller than beachmarks. Striations are typically on the order of micrometers (\(\mu\)m), while beachmarks can be several millimeters (mm) or more in size. Striations can only be seen using a microscope, whereas beachmarks are visible to the naked eye.

(b) Origin: The origins of fatigue striations and beachmarks are related to different aspects of fatigue loading. Fatigue striations form due to the incremental nature of crack growth under cyclic loading. Each striation represents one cycle of loading; thus, the distance between striations can provide information about the rate of crack growth under fatigue loading. On the other hand, beachmarks form due to interruptions or changes in the load applied during fatigue loading. The formation of beachmarks occurs when the fatigue loading process is suspended for a period and then restarted, causing a change in the stress intensity or environmental conditions, like temperature. These changes in conditions lead to the formation of macroscopic lines that are visible to the naked eye. In summary, fatigue striations and beachmarks are different in terms of size and origin. Striations are microscopic lines that form under cyclic loading due to the incremental nature of crack growth, while beachmarks are large, visible lines that form due to interruptions or changes in fatigue loading.