Lect 6 - Design Philosophy PDF

ME2511 Aircraft Structures Design Philosophy ME2511 – Revision 1.0  Designing Philosophy MM2514 AIRCRAFT STRUCTURES & REPAIR  Designing Philosophy Learning Objectives PART 1: Safe life Design Define the...

ME2511 Aircraft Structures Design Philosophy ME2511 – Revision 1.0  Designing Philosophy MM2514 AIRCRAFT STRUCTURES & REPAIR  Designing Philosophy Learning Objectives PART 1: Safe life Design Define the Fail Safe Design philosophy Damage Tolerance for Design Part 2: Application for the 3 design philosophies MM2514 AIRCRAFT STRUCTURES & REPAIR  Aircraft Design Philosophies Designing Philosophy / Principle: Design Philosophies are used to ensure that an aircraft structure will remain SAFE for operations over extended period of time, while being subjected to repeatedly applied loads. These Philosophies includes both ‘Safe Life’, ‘Fail Safe’ and ‘Damage Tolerance’. Design philosophy applied in the design of aircraft structural components changes over time with new design experiences and new material applications MM2514 AIRCRAFT STRUCTURES & REPAIR  Safe-Life Design What is Safe-Life Design? This term relates to a design method whereby the ‘Safe Fatigue life’ of a structure or individual member is calculated. It is assumed that cracking would develop at areas deemed particularly susceptible to extreme fatigue Those parts are designed under the assumption that cracks will not develop into fatal flaws before being detected during the next maintenance MM2514 AIRCRAFT STRUCTURES & REPAIR  Safe-Life Design What are the terms of units for a Safe-Life component? The safe fatigue life is the operational period, usually expressed in terms of flying hours or number of load applications during which the possibility of fatigue failure of the part concerned, due to the normal loads likely to be experienced in service, is estimated to be extremely remote. This includes the growth of cracks slow enough to permit detection at normal inspection intervals. In practical the term of units are:  TSO – Time since Overhaul  TSN – Time Since New  CSO – Cycle Since Overhaul  CSN – Cycle Since New  Flying Hour MM2514 AIRCRAFT STRUCTURES & REPAIR  Safe-Life Design How to determine the life-span of a component? Components were put under destructive test until failure to determine the average lifespan (Lf) In order to calculate the safe-life of a member or structure, the designer must estimate the magnitude and frequency of application of the loads that it will encounter in the course of its normal operation. This is derived from results obtained over a period of years on other types of aircraft fitted with load measuring and counting devices. MM2514 AIRCRAFT STRUCTURES & REPAIR  Safe-Life Design Maintaining a Safe-Life Component A specimen of the structure or member in question is then subjected to similar loads before and length of time it can endure these loads before failing is measured. The result is divided by a ‘ Life Factor’ of between 3.6 and 6 (decided by OEM) and this gives the ‘Safe Life’ of the part. A ‘Lifed’ member must be removed from service before the expiry of its life period. A failure of one of the safe life structural elements could leads to the complete failure of that structure and possibly to significant consequences for the aircraft. Who monitor the life-span of a Safe-life component and what method do they used to monitor the life? MM2514 AIRCRAFT STRUCTURES & REPAIR  Fail-Safe Design What is Fail-Safe Design Philosophy? This term relates to a structure that is designed such that after the failure in operation to a part of the primary structure, there is sufficient strength and stiffness in the remaining primary structure to permit continued operation of the aircraft for a limited period at a lower but acceptable level of safety. MM2514 AIRCRAFT STRUCTURES & REPAIR  Fail-Safe Design What are the 2 designs for Fail-Safe philosophy? The Fail-Safe philosophy prevents against fatigue failure of the aircraft structure and failure due to any other causal factor. It is fundamental to a safe design and its features includes: 1a) Alternate load path construction, wherein one member normally takes all the load with another unloaded member standing by ready to take over in the event of failure or 1b) Multiple load path construction where an applied load is resisted by more than one member such that should one member fail, its share of the load is distributed to the remaining members. MM2514 AIRCRAFT STRUCTURES & REPAIR  Fail-Safe Design MM2514 AIRCRAFT STRUCTURES & REPAIR  Fail-Safe Design Can you describe what type of Fail- Safe design is this? MM2514 AIRCRAFT STRUCTURES & REPAIR  Fail-Safe Design What are the design limitations and features for Fail-Safe design?  The material thickness used must be such as to keep the stress levels low, to reduce the chance of fatigue failure, and to provide the required strength and stiffness in the remaining structure in the event of a failure.  The material used must have good resistance to crack propagation (i.e. relatively slow increase in length of crack that has occurred)  The structural design should include features that tend to limit the maximum length of a crack which has developed.  The structure must be designed such that it facilitates regular inspection. MM2514 AIRCRAFT STRUCTURES & REPAIR  Fail-Safe Design How do I set the Fail-Safe design limits?  For Fail-Safe structure, the occurrence of corrosion and fatigue cracks are more of concern, with regards to maintenance cost as well.  To ensure that these cracks are detected before they reach fail-safe damage limits, interval inspections are established. The inspection intervals are based on past service experience or/and crack growth historical data collected from analysis and/or tests.  When detected before the damage size reaches fail-safe limits, the member that exhibits the crack will be repaired or replaced.  For civil aircraft, inspection frequency and procedures are worked out jointly between the manufacturer, customer and certifying agency. MM2514 AIRCRAFT STRUCTURES & REPAIR  Fail-Safe Design What does FAR states about Fail-Safe design? Reference FAR 25.571, the fail-safe requirements states that: “It must be shown by analysis, or tests, or both that catastrophic failure or excessive structural deformation that could adversely affect the flight characteristics of the airplane are not probable after fatigue failure or obvious partial failure of a single principal structural element” MM2514 AIRCRAFT STRUCTURES & REPAIR  Damage-Tolerance Design History of Damage-Tolerance Design? Philosophy formally endorsed by the FAA in 1978 Consider the existence of minor flaws in manufactured components, study and anticipate their growth rate Establish appropriate inspection programs to arrest these flaws before they pose detrimental effects on the aircraft airworthiness What is Damage-Tolerance Design? Damage tolerance of an aircraft structure is the ability of the structure to sustain operating loads in the presence of flaws or damages e.g. fatigue, corrosion, cracks etc until flaws/damages are detected through inspections or malfunctioning of components for repairs MM2514 AIRCRAFT STRUCTURES & REPAIR  Damage-Tolerance Design ‘Damage Tolerance Requirement’ ….. assumed damages and these includes: Any single member completely severed. Extensive structure severed between boundaries of effective crack barriers. Major structural member attached directly to skin fractured together with the skin between adjacent crack barriers. (Above cases demonstrated by analysis and/or tests that damage will propagated slowly under normal operational loads so that detection and repair can be carried out) All adhesive bonded lap or butt joints will contain sufficient rivets to carry ultimate design loads without benefits of adhesive. All fail-safe joints and skin splices will be designed to have sufficient shear lag to distribute loads from failed sections. MM2514 AIRCRAFT STRUCTURES & REPAIR  Any Questions??? http://www.youtube.com/watch?v=GJy-DX-1c04 C-Check MM2514 AIRCRAFT STRUCTURES & REPAIR