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

Fatigue striations and beachmarks are microscopic features found on fracture surfaces of materials that have failed due to fatigue. Fatigue occurs when a material is subjected to cyclic loads or stresses, ultimately leading to crack initiation and growth. Fatigue striations and beachmarks provide important information about the history of crack growth in a material.

Fatigue striations are fine, parallel lines that appear on the fracture surface of a material subjected to fatigue. The space between two consecutive fatigue striations represents the crack growth within one cycle of the applied stress. Fatigue striations are microscopic in nature and usually require high magnification (such as an electron microscope) to be observed. Beachmarks, also known as clamshell marks or arrest lines, are macroscopic (visible to the naked eye) concentric or semi-circular markings on the fracture surface of a failed material. Beachmarks indicate periods of stress where the crack growth was arrested temporarily. The distance between the concentric circles indicates the extent of crack growth during that period.

Fatigue striations originate from the continuous cyclic loading of the material. The presence of fatigue striations on the fracture surface indicates that the crack has grown incrementally in small, closely spaced steps. Fatigue striations are commonly observed in ductile materials and can be used to determine the frequency and amplitude of the applied cyclic stress. The direction of the fatigue crack growth can also be determined from the spacing between the striations. In contrast, beachmarks form due to intermittent periods of crack growth, where the crack temporarily arrests during the fatigue process. These periods of arrest may be caused by variations in the applied stress (e.g. changes in load magnitude, duration, or frequency), material properties, or environmental conditions (e.g. temperature, humidity). The presence of beachmarks can give insight into the history of the crack growth and help identify factors that may have contributed to the material's failure. In summary, fatigue striations and beachmarks differ in terms of size and origin. Fatigue striations are microscopic and result from the continuous, incremental crack growth due to cyclic loading, while beachmarks are macroscopic and form due to intermittent periods of crack growth arrest.