Class 1 Single load path Class 2 Class 3 Single load path— Multiple load path damage arrest capability Redundant load path Fig. 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. | CREEP AND STRESS RUPTURE 437 Fig. Structural arrangements. After Ref. 74. Class 3 Multiple load path Redundant load path 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 subsequent 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 established on the basis of crack growth information assuming a specified initial flaw size and a detectable 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 engineering system. CREEP AND STRESS .
