Today, NTSB said, possibly “metal fatigue” caused the engine of the B777 jet of United Airlines Flight 328 to break apart, spewing parts on suburban Denver on saturday. Was LCF (Low cycle Fatigue) involved in this case ?
👉On 13th Feb 2018, a similar case had happened. One of the ‘United Airlines’ B777-222 aircrafts suffered fan blades fracture, that induced the loss of inlet duct & both the LH & RH Fan cowls of No#2 PW4077 engine.
👉The metallurgical examination revealed that the No.11 fan blade separated transversely from the root section had undergone a low cycle fatigue (LCF) fracture of the hollow core of fan blade.
👉LCF had originated about 6.7-inches outboard from the blade root bottom on the interior wall of cavity of the hollow blade (Check attached pictures).
👉 What is LCF ?
LCF is a type of fatigue caused by large plastic strains under a low number of load cycles before failure occurs.
👉LCF fracture occurs due to high stresses greater than the material yield strength due to mechanical or thermal loading.
👉Crack due to LCF is usually initiated in the small areas having stress/strain concentration.
👉The lifetime of the component in LCF can be predicted by the plastic strain amplitude or stress amplitude.
Source: NTSB
Author – Tanmay Palei
Sr. Aircraft Structure Engineer