Ellen Kraus Sep 9, 2016:

What is the difference between hairline cracks and hair-like cracks. or is there any at all ?. Hairline cracks I found termed as overuse or injury cracks, such as occur in the wake of repetitive physical stress. Hairline fractures, also called as stress fractures, are now slowly becoming an infamous dilemma in the area of sports medicine. Caused by application of repetitive strain and excessive training, hairline fractures are minute cracks on the bones, which can become severe if not immediately treated. Basically, this injury is caused by the depressed response of the bone with the ground-reaction forces that are applied to the bones during activities like jumping, running, or walking. Of Course they may also develop in Building material such as concrete for instance,
whereas the term "hair-like cracks" is used if one wants to emphasize the tiny size of cracks, cracks of the thickness of a hair.