Ceiling lamps hang from their caps, and the cap is held to the glass by cement. Under a sustained torque, many cements creep: they deform slowly without failing, so the lamp rotates a few degrees over months and eventually the contacts no longer align with the holder. Nothing has broken, which is why the failure is hard to diagnose.
A cement can have adequate immediate shear strength and still creep badly, because creep depends on the glass transition temperature of the cured cement relative to the service temperature. A cement that is fine for an indoor fitting at 25 °C can creep in a recessed downlight where the cap sits at 90 °C. Testing at room temperature only, which is what most incoming inspections do, cannot detect this.
Apply a defined torque at the maximum rated cap temperature, hold it for a defined period, and measure the angular displacement. The result is a creep rate rather than a pass or fail, and it should be compared between cement batches rather than against a single absolute threshold. A batch that creeps twice as fast as the previous one is a problem even if both are within a loose limit.
Cement mix ratio, cure time and cure temperature are the three variables. All three drift when a line is pushed for output, and all three are invisible in the finished lamp until the creep test or the customer complaint arrives.
