A394 MATERIALS ANALYSIS Problem 1: The ALOHA Incident 6th Presentation Approved by: Ms. Vicky Wu Module Chair: Dr. May Sim Copyright by Republic Polytechnic, Singapore Copyright © © 2016 by Republic Polytechnic, Singapore OFFICIAL (CLOSED) \ NON-SENSITIVE The Aloha Incident Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE The Problem You were asked to: • investigate the reasons behind for the plane fuselage to be torn off during the incident. • suggest some means to prevent such occurrence based on the technology in materials science. Copyright © by by Republic Polytechnic, Singapore Copyright © 2016 Republic Polytechnic, Singapore Official (Closed), Non-Sensitive What do you recognize? • An explosive decompression in a 19-year-old ALOHA airline Flight 243 resulted in the structural failure. • Investigation was conducted to find out the cause. • Specimens were collected from the accident site. • Photograph was taken of build-up of voluminous corrosion products inside the lap joints and scanning electron micrograph (SEM) of the lap joint provided clearer examination of the crack lines. • Eddy current and fatigue tests were conducted on the lap joint sample of the airplane. • The ALOHA event has a significant effect in aviation safety and security regulations afterwards. Copyright © by Republic Polytechnic, Singapore The Approach • What is the failure? • What could be the causes of the failure? • What are the steps taken to perform failure analysis? • How was the ALOHA incident investigation conducted? • What is root cause of failure for ALOHA incident? Copyright © by Republic Polytechnic, Singapore What is the Failure? Failure • Inability of a component, machine or process to function as intended. • Types of failure includes distortion, fracture, corrosion and wear. Failure Analysis • Examination of a failed component and of the failed situation in order to determine the causes of the failure. Copyright © by Republic Polytechnic, Singapore What is the Failure? Purpose of failure analysis • Purpose of failure analysis is to determine root causes of the failure. • Once root cause is discovered, the corrective action to prevent future occurrences is implemented. • Hence, significant improvements to the quality of products and systems could be made through this. What is the failure in the ALOHA incident? Structural failure of the ALOHA airline flight 243 Copyright © by Republic Polytechnic, Singapore What could be the causes of the failure? Fishbone diagram can be used as a cause and effect diagram to illustrates the main causes leading to an effect. In the ALOHA flight incident, some possible causes are: Copyright © by Republic Polytechnic, Singapore OFFICIAL (CLOSED) \ NON-SENSITIVE What are the steps taken to perform failure analysis? Failure Analysis Procedure Step 1: Background Review • Understand the role of failed component and its environment • Or review the original design, manufacturing, assembly and operating procedures Step 2: Catalogue Evidence • Conduct visual examination (macroscopic examination) • Catalogue and record physical evidence Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE What are the steps taken to perform failure analysis? Failure Analysis Procedure Step 3: Examine and Test Evidence • Detailed examination to determine cause of failure. • Tests: • Microscopic examination (eg SEM) • Non destructive testing NDT (eg Ultrasonic, Radiography) • Destructive Testing (eg fatigue tests, chemical analysis) • Metallographic examination (microstructure) • Mechanical Testing (strength/hardness) Copyright © by Republic Official (Closed), Polytechnic, Non-Sensitive Singapore OFFICIAL (CLOSED) \ NON-SENSITIVE What are the steps taken to perform failure analysis? Failure Analysis Procedure Step 4: Analyze all Data • Decide on the likely causes, failure mechanism or failure sequences Step 5: Conclusions & Recommendations • Explain why failure occurred and recommend how to prevent future failures. Copyright © by Republic Official (Closed), Polytechnic, Non-Sensitive Singapore OFFICIAL (CLOSED) \ NON-SENSITIVE What are the steps taken to perform failure analysis? Types of Examination • Macroscopic examination (Figure A): Unaided eye examination. Able to look out for evidence of corrosion and surface disbonded area. • Microscopic examination (Figure B): Examination was conducted at higher magnification (higher than 50 times). Able to provide microstructure feature of failure surface such as fine crack lines. Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE What are the steps taken to perform failure analysis? Types of Testing • Destructive test: Involves sectioning of a component for analysis in a laboratory setting. After performing this test the component will ultimately be destroyed. Example: tensile test, fatigue test, impact test etc. • Non destructive test: Test method that allows certain physical properties/structural integrity to be examined without taking the samples completely out of service. Examples: ultrasonic, liquid penetrant, x-ray imaging, eddy current etc. Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE How was the ALOHA Incident investigation conducted? Step 1: Background Review • Aircraft was operated for many years by flying between Hawaiian islands and was exposed to sea mist and salty air. • Information gained during interview: On levelling at 24000 feet, a small section of the roof ruptured, resulting an explosive decompression. The wind was stronger than a hurricane raced through the cabin. Flying debris badly injured a number of passengers and the first five rows were exposed to the open sky. Airport emergency personnel quickly prepared for emergency landing at Kahului Airport and it landed safely. Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE How was the ALOHA Incident investigation conducted? Step 2: Catalogue Evidence • Aircraft maintenance record was retrieved. • Visual inspection : major portion of the upper crown skin was separated. The decompression tore off an 18.5 feet section of the front cabin roof. • Some pieces of structure from the damaged area were recovered for further analysis including the S-4R lab joint sample. • Corrosion and disbonded surfaces were noted in the S-4R lap joint. Visual inspection Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE How was the ALOHA Incident investigation conducted? Step 3: Examine and Test Evidence • Non-destructive test: eddy current and scanning electron microscope (SEM). • Destructive tests: fatigue tests including coupon test and Quonset hut test. Eddy current Scanning electron microscope (SEM) Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive Fatigue test OFFICIAL (CLOSED) \ NON-SENSITIVE How was the ALOHA Incident investigation conducted? Step 4: Analyze All Data • Eddy current revealed some fatigue crack lines larger than 0.08inch in the lab joint. • Scanning electron microscope (SEM) revealed the presence of many more cracks within the eddy current detectable range. • In the fatigue testing, the durability of the lap joint appeared to be the governing factor producing multiple site of fatigue cracking. • The salt and moisture in the environment and undetected corrosion contribute to further disbonding of the lap joint, leading to crack initiation. Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE How was the ALOHA Incident investigation conducted? Step 5: Conclusion and Recommendations • Failure of ALOHA airlines maintenance program to detect the presence of significant disbanding of lap joint due to fatigue and consequently aggravation. • This had led to multiple site fatigue cracking of the lap joint adjacent to some rivet holes. • The nearly 90000 flight cycles exceeded the lifetime of validity of 34000 flight cycles under the new rule after this incident. • Recommendations: all Boeing 737 with over 50000 cycles are required to have their lap joints reinforced with external sheet metal patches. Environment factor must be taken into consideration for corrosion and fatigue tests. Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE What is the root cause of failure of ALOHA incident? - Corrosion The resulting National Transportation Safety Board investigation report attributed the ALOHA incident to the failure of the operators maintenance program to detect corrosion damage and hence it accelerated fatigue failure. Fundamental of corrosion: • Corrosion involves chemical changes involving flow of electrical charge rom corroding metal to environment: Oxidation: Lose electrons (anode) Reduction: gain electrons (cathode) Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive OFFICIAL (CLOSED) \ NON-SENSITIVE What is the root cause of failure of ALOHA incident? - Corrosion • Rusting (formation of iron oxides) is a well-known example of electrochemical corrosion • For rusting to occur, oxygen in the air and water have to be present. Iron is oxidized by oxygen and water to form hydrated iron(III) oxide, Fe2O3 . H2O (rust). • Oxidation: Fe(s) Fe2+(aq) + 2e • Reduction: O2 (g) + 2H2O(l) + 4e- 4OH-(aq) (neutral to basic condition) 2H+ (aq) + 2e- H2 (g) (acidic condition) Copyright © by Republic Polytechnic, Singapore Official (Closed), Non-Sensitive What is the root cause of failure of ALOHA incident? - Corrosion Factors Affecting Rate of Corrosion Humidity & temperature of the environment Presence of continuous wetness on the metal surface Concentration of corrosive agents eg salts Others: Presence of defects, types of protective film & heat treatment of the metal Copyright © by Republic Polytechnic, Singapore OFFICIAL (CLOSED) \ NON-SENSITIVE What Have You Learnt Today? • Define what is failure. • Describe the purpose of failure analysis and failure prevention. • Describe what happens in each step of the failure analysis procedure: • Background review • Catalogue evidence • Examine and test evidence • Analyze all data • Conclusions and Recommendations • Apply the failure analysis process to evaluate a failure. • Recognize rusting as a form of corrosion and the fundamentals of corrosion. 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