Figure 1. Typical circular dishwasher heating element.Figure 2. Scanning electron microscope micrograph of a sheath wall fracture showing progressive damage – fatigue and corrosion.
An appliance manufacturer agreed with the Consumer Product Safety Commission in June, 2010 to recall 1.7 million dishwashers made with plastic tubs because an electrical failure in the dishwasher’s heating element could pose a serious fire hazard. A typical heating element is shown in Figure 1. We were asked by one of the manufacturer’s vendors that had supplied the heating elements to conduct a root cause failure analysis to determine how they suffered failures.
We conducted a finite element analysis, strain gage testing, metallurgical examination and performance testing, and found that the steel brackets holding the heating element in place, under certain conditions, interfered with its free thermal expansion and contraction during normal heating and cooling cycles. The resulting repeated stresses were sufficient to fatigue the thin stainless steel sheath of the heating element, causing cracks and allowing for the intrusion of water.
Once inside the sheath, the water caused swelling of the insulation material, corrosion, more cracking damage, and plugging which trapped some of the water. Figure 2 shows a magnified view of a fracture in one of the failed heating elements’ thin sheath wall. In some cases the trapped water contacted the heating element wire resulting in a steam explosion breaching the sheath, expelling the wire which in turn came in contact with the plastic dishwasher tub igniting a fire.