Efficient Planning: In-Service Inspections of Fatigued Structures under Parametric Uncertainty

Efficient Planning: In-Service Inspections of Fatigued Structures under Parametric Uncertainty

N. A. Nechval (Baltic International Academy, Latvia) and K. N. Nechval (Transport and Telecommunication Institute, Latvia)
DOI: 10.4018/978-1-5225-1639-2.ch016
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Abstract

The main aim of this chapter is to present more accurate stochastic fatigue models for solving the fatigue reliability problems, which are attractively simple and easy to apply in practice for situations where it is difficult to quantify the costs associated with inspections and undetected cracks. From an engineering standpoint the fatigue life of a structure consists of two periods: (i) crack initiation period, which starts with the first load cycle and ends when a technically detectable crack is presented, and (ii) crack propagation period, which starts with a technically detectable crack and ends when the remaining cross section can no longer withstand the loads applied and fails statically. Periodic inspections of aircraft, which are common practice in order to maintain their reliability above a desired minimum level, are based on the conditional reliability of the fatigued structure. During the period of crack initiation, when the parameters of the underlying lifetime distributions are not assumed to be known, for efficient in-service inspection planning (with decreasing intervals as alternative to constant intervals often used in practice for convenience in operation), the pivotal quantity averaging (PQA) approach is offered. During the period of crack propagation (when the damage tolerance situation is used), the approach, based on an innovative crack growth equation, to efficient in-service inspection planning (with decreasing intervals between sequential inspections) is proposed to construct more accurate reliability-based inspection strategy in this case. To illustrate the suggested approaches, numerical examples are given.
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Introduction

Fatigue in engineering structures involves crack initiation and growth processes. Current life-prediction approaches for fatigue-sensitive aircraft structure components address both crack initiation and propagation lives. Typically, the components are designed to have a minimum low-cycle fatigue (LCF) crack-initiation life exceeding the total specified service life. The current life-prediction systems may be improved by development of better crack-initiation life models and probabilistic treatment of the variability of crack-initiation life. One of the limitations of current crack-initiation life models is that they are incapable of predicting the crack size at fatigue-crack initiation. Consequently, an initial crack size must be assumed in the prediction of crack-growth life.

In spite of decades of investigation, fatigue response of materials is yet to be fully understood. While most industrial failures involve fatigue, the assessment of the fatigue reliability of industrial components being subjected to various dynamic loading situations is one of the most difficult engineering problems. The traditional analytical method of engineering fracture mechanics (EFM) usually assumes that crack size, stress level, material property and crack growth rate, etc. are all deterministic values which will lead to conservative or very conservative outcomes. According to many experimental results and field data, even in well-controlled laboratory conditions, crack growth results usually show a considerable statistical variability.

In this chapter, sequential inspections are obtained to ensure that the conditional fatigue reliability is at the required level. Sequential inspections are important, especially during the set-up and installation stage. Frequent inspection leads to a high cost and infrequent inspection will lead to low fatigue reliability of the system upon demand. Although the cost might be an issue in this type of analysis, the focus here is to meet the fatigue reliability requirement with an appropriate time to next inspection. The sequential inspection procedure and decision making procedure studied in this chapter allows an appropriate level of fatigue reliability to be reached with minimum cost as well. Furthermore, we do not assume the distribution of system lifetime to be completely known which is usually the case. The information from the inspection can be used to determine the parameters of the underlying system lifetime distribution. Hence, such a combined estimation and decision-making analysis is important and useful in practice. The models are to allow both the understanding of physical phenomena and the practical application in engineering design with maintaining a clear physical interpretation of the stochastic crack growth.

Fatigue is one of the most important problems of aircraft arising from their nature as multiple-component structures, subjected to random dynamic loads. The analysis of fatigue crack growth is one of the most important tasks in the design and life prediction of aircraft fatigue-sensitive structures (for instance, wing, fuselage) and their components (for instance, aileron or balancing flap as part of the wing panel, stringer, etc.). From an engineering standpoint the fatigue life of a structure (or component) consists of two periods (this concept is shown schematically in Figure 1):

Figure 1.

Schematic fatigue crack growth curve (Crack initiation period (A-B); Crack propagation period (B-C))

  • 1.

    Crack initiation period, which starts with the first load cycle and ends when a technically detectable crack is present, and

  • 2.

    Crack propagation period, which starts with a technically detectable crack and ends when the remaining cross section can no longer withstand the loads applied and fails statically.

Periodic inspections of aircraft are common practice in order to maintain their reliability above a desired minimum level.

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