Multiple Site Fatigue Damage Research Articles

Lamb Wave–Based Structural Health Monitoring for Aluminum Bolted Joints with Multiple-Site Fatigue Damage

Lamb Wave–Based Structural Health Monitoring for Aluminum Bolted Joints with Multiple-Site Fatigue Damage

Probabilistic Model of Multiple-Site Fatigue Damage of Riveting in Airframes

The probabilistic model of multiple-site damage of riveting in airframes is based on the statistical description of kinetics of the three fatigue fracture factors: time to crack initiation near the rivet hole, dimensional nonuniformity of flaws, and coalescence of counterpropagating cracks from neighboring holes. The probabilistic measure of these factors is used to determine time-dependent performance characteristics of a rivet joint: no-failure operation function and lifetime distribution function. The limiting state of the structure comes if at least one of the ligaments between the holes is fractured.

Advanced damage tolerance and risk analysis methodologies and tools for aircraft structures containing multiple-site and multiple-element fatigue damages

This article presents the results obtained from the development of advanced damage tolerance analysis (DTA) and risk analysis methodologies and tools for aircraft structures, including build-up structures containing multiple-site fatigue damage (MSD) and multiple-element damage, to support the Canadian Forces aircraft structural life cycle management. The DTA methods developed include new closed-form solutions and generic finite element (FE) based tools to calculate the stress intensity factors and the β-solutions for build-up structural configurations. To facilitate the risk assessment, an in-house crack growth analysis program, CanGROW, was developed to simultaneously grow multiple cracks. Guidelines were established to calculate the residual strength of structures with MSD and multiple-element damage using global and/or local FE models, considering load redistribution among adjacent components. For the MSD risk analysis, an efficient Monte Carlo simulation technique was developed to determine the crack size distributions at different inspection intervals, which were then used in National Research Council Canada’s risk analysis code probabilistic damage tolerance analysis to calculate the single flight hour probability of failure. Case studies on critical locations of the CC-130 centre wing structure are presented to demonstrate the capability of the developed methods and tools.

COMPARISON BETWEEN TWO AND THREE PARAMETERS OF FATIGUE CRACK GROWTH MODELS IN AIRFRAME STRUCTURES

Comparison of two and three parameters models of fatigue crack growth is discussed. For calculation of failure probability, the Monte Carlo (MC) method is used. The number of MC trials was 60. Multiple site fatigue damage specimens corroded to 5% to 6% average thickness loss and non‐corroded specimens were used in the fatigue tests. As initial data, the result of these fatigue tests of corroded and non‐corroded specimens are used. It is shown that at a small number of inspections there is a significant difference between probabilities of failure (fatigue crack is not discovered before specified life). But if the number of inspections is large enough, than the difference between considered models is negligible for both corroded and non‐corroded specimens.

Fuzzy Probabilistic Assessment of Aging Aircraft Structures Subjected to Multiple Site Fatigue Damage

Abstract A strategy is developed for fuzzy probabilistic assessment of the fatigue resistance of aging aircraft structures due to multiple site fatigue damage (MSD). The residual strength of an aircraft structure may be significantly reduced by the existence of fatigue damage at multiple locations. Depending on the level of subjectivity and degree of knowledge, MSD-related parameters may be represented as either purely random variables or fuzzy random variables. The membership functions of probabilistic characteristics of fuzzy random variables, namely mean values and standard deviations, are developed. Mechanistic and probabilistic models used to evaluate multi-site fatigue damage are also presented. A probabilistic solution strategy, employing the first order reliability method (FORM), is combined with the response surface-based fuzzy modeling approach to develop possibility distributions of the probabilistic response quantities (namely reliability indices and failure probabilities) for components subjected to multiple site fatigue damage. Instead of providing the traditional single valued, purely probabilistic measure for reliability, the present formulation proves its merit in its ability to combine experimental data with expert knowledge to provide confidence bounds on the structural integrity of aging aircraft. Moreover, the predicted bounds are dependent on the level of knowledge regarding the fuzzy input parameters, with a higher degree of knowledge resulting in more narrow bounds. An example problem is used to demonstrate the advantages of the proposed methodology.

Corrosion Risk Assessment of Aircraft Structures

Abstract This paper presents a comprehensive summary of IAR/NRC research on risk assessment of aging aircraft structures in the presence of corrosion and fatigue. Extensive test data, especially for fuselage splices containing corrosion and multiple site fatigue damage (MSD) and coupons cut from service exposed aircraft lap joints, have been generated under previous projects. Based on these test data, an empirical stochastic crack growth model was first developed to analyze the probabilistic fatigue characteristics of the splices. Risk analysis was then performed on the fuselage splices using the computer codes PRISM (Bombardier Aerospace, Inc.) and PROF (United States Air Force). Holistic life assessment methodology (HLAM), which aims to quantify structural integrity in the entire life cycle by addressing the interaction effects of corrosion and fatigue, has advanced considerably during the past five years. Risk analysis based on HLAM was carried out on the fuselage splices. The analytical results, which were obtained at different stages, are compared with the test results. It is shown that corrosion in lap joints, even at the levels less than the typical maintenance limit (i.e., 10 % material loss), could significantly increase the risk level of structural failure.

Development of a Fuzzy Probabilistic Methodology for Multiple-Site Fatigue Damage

A strategy for fuzzy-probabilistic assessment of the impact of multiple-site fatigue damage (MSD) on the fatigue resistance of aging structures is developed. The residual strength of a structure may be significantly reduced by the existence of fatigue damage at multiple locations. Depending on the level of knowledge with which they are known, MSD-related parameters may be represented as either purely random variables or fuzzy random variables. The membership functions of the probabilistic characteristics of fuzzy random variables, namely, mean values and standard deviations are developed. Mechanistic and probabilistic models used to evaluate multi-site fatigue damage are also presented. A probabilistic solution strategy, employing a first-order reliability method, is combined with a response-surface-based fuzzy modeling approach to construct the possibility distributions of the probabilistic safety indicators (namely, reliability indices and failure probabilities) for components subjected to multiple-site fatigue damage. Instead of providing the traditional single-valued, purely probabilistic measure for reliability, the present formulation proves its merit in its ability to combine experimental data with expert knowledge to provide confidence bounds on the structural integrity of aging structures. Moreover, the predicted bounds are dependent on the level of knowledge regarding the fuzzy input parameters, with a greater knowledge producing more narrow bounds. An example problem is used to demonstrate the advantages of the proposed methodology.

Interaction between a major crack and small crack damage in aircraft sheet material

A study is made of the effect of small crack damage on the fracture tolerance of an elastic-plastic sheet material to a major crack. The study is motivated by concern for the influence of multiple-site fatigue damage in lap joints on the tolerance of aging aircraft fuselages to major cracks. Flat sheet geometries are analysed, both unreinforced and reinforced. Several analysis approaches are explored and assessed, including linear elastic fracture mechanics (LEFM), a modification of LEFM employing a damage-reduced fracture toughness, and a modification of the Dugdale model which makes use of a damage-reduced yield stress of the sheet material. An important feature of the interaction between a major crack and small crack damage is the fact that the plastic zone of the major crack engulfs at least several damage sites in geometries typical of most lap joint designs. The damage-reduced yield strength of the lap joint emerges as being central to understanding the role of damage, and simple formulas are given which indicate how damage erodes tolerance in the presence of a major crack.