Light Fixture ESC Failures

Ceiling-mounted light fixtures failed in a metalworking facility that performs laser cutting, welding, machining, and metal stamping. The light fixtures had an expected service life of ten years but started falling from the ceiling after less than a year of service. Eight of the 72 lights in service at the metalworking facility failed. Thousands of light fixtures had been sold, but the only reported failures were at this metalworking facility. The light fixtures were purportedly made from one of three materials: Cycloy, Bayblend, or Noryl. Cycoloy and Bayblend are blends of polycarbonate and acrylonitrile butadiene styrene (PC/ABS). Noryl is a blend of polyphenylene ether and polystyrene (PPE/PS).

The failed light fixtures had broken tabs on the mounting plates that secure it to the ceiling. When the tabs fractured, the light fixtures fell. Proof-load testing demonstrated that the tabs could withstand 100-pound loads. The tabs folded over but did not fracture.

Fractographic examination of the broken tabs revealed predominantly smooth, brittle fracture surfaces with multiple crack origins. Ductile fracture morphology was observed at the final fast overload region of the broken tabs. Laboratory-induced fractures exhibited features of ductile fracture throughout the fracture surface. These two observations indicate that the base material was ductile.

FTIR analysis identified that the primary resin of the mounting plate material was polycarbonate. FTIR also detected a residue on the exterior surface of the failed samples that was consistent with hydrocarbon oil, a known ESC agent for PC materials.

The failure mechanism was determined to be environmental stress cracking (ESC). The three prerequisites for ESC were met: (1) a susceptible polymer (polycarbonate); (2) a material-specific aggressive chemical agent (hydrocarbon oil from gases released during metalworking); and (3) tensile stress (support of the light fixture and/or pinching of the tabs).