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30079_18c - Class 1 Single load path Class 2 Class 3 Single...

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Class 1 Class 2 Class 3 Single load path Single load path— Multiple load path damage arrest capability Redundant load path Fig. 18.61 Structural arrangements. (After Ref. 74.) 2 structures, including pressurized cabins and pressure vessels, relatively large amounts of damage may be contained by providing tear straps or stiffeners. There is usually a high probability of damage detection for a class 2 structure because of fuel or pressure leakage, that is, "leak-before-break" design is characteristic of class 2 structures. Class 3 structures are usually designed to provide a specified percentage of the original strength, that is, a specified residual strength, during and subse- quent to the failure of one element. This is often called "failsafe" type of structure. However, the preexisting flaw concept requires that all members, including every member of a multiple load path structure, be assumed to contain flaws. It is usual to assume a smaller initial flaw size for class 3 structures because it is appropriate to take a larger risk of operating with cracks if multiple load paths are available. The development of inspection procedures is an important part of any fracture control program. Appropriate inspection procedures must be established for each structural element, and regions within elements may be classified with respect to required NDI sensitivity. Inspection intervals are estab- lished on the basis of crack growth information assuming a specified initial flaw size and a "detect- able" flaw size that depends on the NDI procedure. Inspection intervals are established to ensure that an undetected flaw will not grow to critical size before the next inspection, with a comfortable margin of safety. The intervals are usually picked so that two inspections will occur before any crack will reach critical size. A good fracture-control program should encompass and interact with design, materials selection, fabrication, inspection, and operational phases in the development of any high-performance engi- neering system. 18.6 CREEP AND STRESS RUPTURE Creep in its simplest form is the progressive accumulation of plastic strain in a specimen or machine part under stress at elevated temperature over a period of time. Creep failure occurs when the ac- cumulated creep strain results in a deformation of the machine part that exceeds the design limits. Creep rupture is an extension of the creep process to the limiting condition where the stressed member actually separates into two parts. Stress rupture is a term used interchangeably by many with creep rupture; however, others reserve the term stress rupture for the rupture termination of a creep process in which steady-state creep is never reached, and use the term creep rupture for the rupture termination of a creep process in which a period of steady-state creep has persisted. Figure 18.62 illustrates these differences. The interaction of creep and stress rupture with cyclic stressing and the fatigue process has not yet been clearly understood but is of great importance
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