MIL-HDBK-1530B(USAF)
5.1.2.1.1 Damage tolerance. The damage tolerance design guidance is provided in JSSG-2006 and should be applied to the principal structural elements and mission-essential structure. Damage tolerance designs are categorized into two general concepts:
a. fail-safe concepts where unstable crack propagation is locally contained through the use of multiple load paths or crack arrest structures in multiple-load-path structures, and
b. slow crack growth concepts where flaws or defects are not allowed to attain the size required for unstable, rapid propagation in single-load-path structures.
Either design concept should assume the presence of undetected flaws or damage, and should have a described residual strength-level both during and at the end, of a prescribed period of unrepaired service usage. The initial damage size assumptions, damage growth limits, residual strength requirements, and the minimum periods of unrepaired service usage depend on the type of structure and the appropriate inspectability level.
5.1.2.1.2 Durability. The durability design guidelines are provided in JSSG-2006. The airframe should be designed such that the economic life is greater by the desired margin than the design service goal when subjected to the design service loads/environment spectrum. The design service goal and typical design usage requirements will be described by the U.S. Air Force in the contract specifications for each new air vehicle. The design objective is to minimize cracking or other structural or material degradation which could result in excessive maintenance problems or functional problems such as fuel leakage, loss of control effectiveness, or loss of cabin pressure.
5.1.2.1.3 Corrosion control and prevention. Corrosion control and prevention guidelines are provided in JSSG-2006, AF Policy Directive 21-1, and AF Instruction 21-105. The goals are to control the maintenance cost burden associated with corrosion and ensure that it does not cause a safety-of-flight/structural integrity problem. These goals are attainable if corrosion control and prevention are addressed early in the design. Materials and processes, finishes, coatings, and films which have been proven in service or by comparative testing in the laboratory should be the basis for choices to meet the goals. Corrosion prevention should also be a primary
consideration in the development and implementation of the durability and damage tolerance control process and the fleet management process.
5.1.2.2 Battle damage criteria. Where applicable, specific battle damage criteria will be provided by the U.S. Air Force. These criteria will include the threat, flight conditions, and load-carrying capability and
duration after damage is imposed, etc. The structure should be designed to these criteria and to other criteria as described in JSSG-2006.
5.1.2.3 Repairability. Repairability must be designed into the air vehicle from the beginning and must be a design influence throughout the design process. Repairability is required to support production, maintain the fleet, and maximize operational readiness by repairing battle damage. High- or moderate-maintenance items and items subject to wear must be repairable. The structure should be designed to these criteria as described in
JSSG-2006.
5.1.3 Durability and damage tolerance control. The System Program Office (SPO) and the contractor should prepare durability and damage tolerance control processes and conduct the resulting programs in accordance with this handbook and JSSG-2006. These processes should identify and define all the tasks necessary to ensure compliance with the damage tolerance requirements as described in 5.1.2.1.1 and JSSG-2006, and the durability requirements as described in 5.1.2.1.2 and JSSG-2006. The disciplines of fracture mechanics, fatigue, materials selection and processes, environmental protection, corrosion prevention and control, design, manufacturing, quality control, and nondestructive inspection are involved in damage tolerance and durability control. The corrosion prevention and control process should also use the guidelines in JSSG-2006. These processes should include the requirement to perform durability and damage-tolerance design concepts, material, weight, performance, cost trade studies during the early design phases to obtain low weight, and cost-effective designs which comply with the requirements of Durability, Damage tolerance, and Durability and damage tolerance control (3.11, 3.12, and 3.13; respectively) in JSSG-2006.
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