Everyone recognizes what a material crack is.  However, we distinguish between cracks and crazing, since they have a different appearance and somewhat different causes.  A crack is defined as a single separation of a surface; whereas, a crazed surface has many micro-cracks spreading randomly like a spider web.  There can be multiple cracks in a surface.  Cracks are easily visible.  Crazing may not be visible without magnification.

Causes for cracks are normally either an impact from an object or excessive stress from a constant load.  They can also occur from fatigue caused by stress cycles (e.g. thermal expansion/contraction) Since the strength of a material declines as temperature increases, cracks occur more readily when the material is hot.

The photo below shows a crack in a plastic automobile bumper.crack001.jpg

Cracks occur in spas, bathtubs, showers, sinks, and countertops regardless of the type of material.