Chapter 12_Damage Tolerance

# Chapter 12_Damage Tolerance - CHAPTER 12 DAMAGE TOLERANCE...

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1 CHAPTER 12 DAMAGE TOLERANCE In this chapter the following topic will be covered: Safe life design Damage tolerant (Fail-Safe) design Stress intensity factors Mode I fracture, mode II fracture and mixed mode fracture Fatigue crack growth: Paris equation Aerospace structures are subjected to cyclic loadings as follows: Pressurization and depressurization of passenger cabin Acoustic loadings on rocket during the launch phase. Takeoff and landing Maneuvers Wing vibration in turbulence A structure under a cyclic load may fail after a number of cycles of load application even when the amplitude of the cyclic load is below the static failure load. This is called fatigue failure. Accordingly, structures must be designed for structural integrity against fatigue failure. Aerospace structures are designed based on the following two design philosophies: Safe-life design and damage tolerant (or fail-safe) design. Safe-life design assumes that a structural component has no initial cracks while damage tolerant design takes into account pre-existent small undetectable cracks. Structures designed with damage tolerance are lighter than those designed under the safe life approach. However, damage tolerant structures must be designed and the inspection intervals arranged such that damage such as a crack must not grow to a catastrophic size before it can be detected in the next inspection.

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2 Safe Life Design The safe-life design assumes that structural components are initially free of any cracks. The effect of fatigue damage on allowable fatigue life (in terms of flight hours) of a structural component is determined via calculation using the S-N curve and the Palmgren-Miner hypothesis for cumulative damage and extensive testing of the structural component under simulated cycling loadings. S-N Curve under Constant Stress Amplitude The simplest cyclic loading is one that varies between the maximum stress max σ and the minimum stress min as shown in figure. Simple cyclic loading The following terms are often used to describe fatigue behavior. stress range min max = Δ alternating stress (or stress amplitude) Δ = 2 1 a S mean stress ) ( 2 1 min max + = m stress ratio max min = R fatigue life = f N number of cycles to fatigue m a a S max min N Δ
3 In order to evaluate the fatigue properties of a material, tests are conducted using a cyclic load of constant amplitude superimposed over a mean stress. The results are plotted in stress amplitude ( a S ) versus cycles to failure f N . This is so-called S - N curve . A typical S-N curve is shown in the following page.

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Chapter 12_Damage Tolerance - CHAPTER 12 DAMAGE TOLERANCE...

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