Design of Accelerated Fatigue Tests for Flame Free Refrigeration Fittings

Refrigerant leakage from failed braze joints is a multi-billion dollar problem for the global HVAC&R industry. Leaks are typically caused due to mechanical fatigue from extreme pressure cycling, temperature cycling including exposure to freeze/thaw cycles, or vibrational wear induced from rotating electrical machinery. Three tests to accelerate mechanical fatigue were devised to simulate real world extreme conditions to determine possible failure modes of refrigerant components and joining technologies. The first test is a combined thermal/pressure shock test designed to simulate abrupt temperature and pressure changes due to start/stop cycles and frost/defrost mode changes. Field failures of brazed joints have been detected due to water being trapped in tight spaces and expanding during freezing, causing high stress on brazed joints. The second test is a vibration test, designed to simulate vibrational loads induced from rotating components in the system. The third and final series of testing is a freeze/thaw cycling profile which simulates ice buildup and defrost observed during heat pump operation. The test article is a flame-free tube fitting designed to work with refrigerants. Six different fitting sizes designed to connect tubes between 6.35 mm and 28.5 mm were subjected to the three tests described above.