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Star shaped cracks

Star shaped cracks
Uneven heat distribution of a focusing optic will frequently cause a star shaped fracture. Common causes of uneven heat distribution includes:

• Laser beam interaction with metal spatters.
• Uneven beam shape
• Inadequate lens cooling

Lens contamination
            This type of failure initiated in a top down direction, extending a hole completely through the center of the lens. Brown residue and "rainbow" coloration on the top side, indicating the presence of petroleum products on the surface of the lens.
            The optic failure in this case is a direct result of contaminated beam delivery purge air. Oil residue on top of the lens absorbed a significant amount
of laser power, causing an uneven heat distribution (hot spot) which
led to lens explosion. It is possible that the source of this contamination is from the compressor which is pressurizing the beam delivery system.

Ref: http://www.ophiropt.com/co2-lasers-optics/focusing-lens/knowledge-center/lens-failure?r=drp