Fatigue Critical Structures Research Articles

Joint parameter-input estimation for virtual sensing on an offshore platform using output-only measurements

This paper presents a recursive Bayesian inference framework for joint parameter-input identification, and virtual sensing for strain time history prediction of an offshore platform using sparse output-only measurements. The studied offshore platform, known as FINO3, is in the North Sea and is instrumented with a variety of sensors, including accelerometers and strain gauges. Offshore platforms are fatigue critical structures due to harsh marine environmental conditions and continuous cyclic wind and wave loads. Therefore, continuous monitoring of strain time histories at hotspot locations of offshore structures is important for reducing maintenance cost and avoiding unexpected failures. A windowed unscented Kalman filter (UKF) is employed to estimate an uncertain modeling parameter (foundation stiffness) and unknown input load time histories using output-only acceleration and strain measurements. The input loads are divided into overlapping windows, and windowed inputs and model parameters are combined as an augmented state vector in the UKF framework. Then strain time histories at critical locations are estimated through a virtual sensing strategy using the estimated input loads and model parameter. A traditional modal expansion approach combined with model updating is also implemented for the purpose of verification and comparison. The proposed method is first demonstrated through a numerical study using a finite element model of FINO3, where accurate model parameter and input estimations are obtained. Then the approach is further investigated using the actual measurements on FINO3. More accurate strain predictions are provided by the UKF than the modal expansion approach, which recommends the proposed UKF method for fatigue monitoring and input estimation.

Concrete compressive strain behavior and magnitudes under uniaxial fatigue loading

Recent advances in concrete technology and construction have promoted an increasing utilization of concrete in fatigue-critical structures, such as wind turbine towers. While research into concrete fatigue is not a new topic and has been going on for several decades, a large majority of this research has primarily been focused on the determination of concrete fatigue capacity. However, understanding and advancing concrete fatigue behavior (e.g., fatigue stress vs. strain response) has received limited attention, even though this is also crucial, especially considering the inherently nonlinear behavior of concrete. To address this knowledge gap and further advance our understanding of concrete fatigue behavior, an experimental investigation consisting of uniaxial compressive fatigue testing of concrete cylinder specimens until failure was undertaken, and axial strains were recorded throughout the duration of fatigue loading. The behavior and evolution of concrete axial strains, including the rate of accumulation of strains, through the fatigue loading process were extensively analyzed and are reported. Additionally, the influence of key parameters, such as peak stress levels and number of cycles to failure, on peak strain evolution were investigated and are described. Magnitudes of peak axial strain at the end of the stable strain accumulation phase, as well as at failure, are also reported, and recommendations are made for estimating strain magnitudes at these critical junctures when conducting fatigue analysis and design. The results will help advance the understanding of concrete axial strain behavior under compressive fatigue loading. The results also provide guidelines for estimating the magnitudes of concrete strains under fatigue loads, which in turn will be beneficial to designers, as it allows them to also consider structural performance during the analysis and design of fatigue critical concrete structures.

A digital twin feasibility study (Part I): Non-deterministic predictions of fatigue life in aluminum alloy 7075-T651 using a microstructure-based multi-scale model

The Digital Twin (DT) concept has the potential to revolutionize the way systems and their components are designed, managed, maintained, and operated across a vast number of fields from engineering to healthcare. The focus of this work is the implementation of DT for the health management of fatigue critical structures. This paper is the first part of a two-part series. The second part focuses on the use of in-situ diagnostics to reduce uncertainty in fatigue life predictions once a life-limiting fatigue crack has reached the point where it is observable. In contrast, the focus of this work is to form a non-deterministic prediction of fatigue life prior to the accumulation of observable damage, a condition that often dominates the majority of a component or system’s lifetime. This predictive capability is enabled by a DT framework that simulates crack growth from initiation to failure using microstructure-based, multi-scale modeling. Two models are used in this work, one that incorporates microstructure explicitly and one that uses more traditional linear elastic fracture mechanics. The models are joined by a transition crack length between the micro- and macroscales, which facilitates both the coupling of fatigue life predictions and the transference of uncertainty across scales. The ability to predict fatigue life under uncertainty and across scales is demonstrated in the context of a geometrically complex, metallic test specimen subjected to uniaxial fatigue loading. Conclusions are drawn regarding the feasibility of the approach as a DT-based health management method.

A digital twin feasibility study (Part II): Non-deterministic predictions of fatigue life using in-situ diagnostics and prognostics

The Digital Twin (DT) concept has the potential to revolutionize the way systems and their components are designed, managed, maintained, and operated across a vast number of fields from engineering to healthcare. The focus of this work is the implementation of DT for the health management of fatigue critical structures. This paper is the second part of a two-part series. The first of the series demonstrated the use of multi-scale, initiation-to-failure crack growth modeling to form non-deterministic predictions of fatigue life. In this second part, a general method for reducing uncertainty in fatigue life predictions is presented that couples in-situ diagnostics and prognostics in a probabilistic framework. Monte Carlo methods and high-fidelity finite element models are used to (i) generate probabilistic estimates of crack state throughout the life of a geometrically-complex test specimen and (ii) predict fatigue life with decreasing uncertainty as more of these diagnoses are obtained. The ability to predict accurately and in the presence of uncertainty is demonstrated, suggesting that the proposed DT method is feasible for fatigue life prognosis and should be pursued further with a focus on increasing application realism.

Assessment of failure toughening mechanisms in continuous glass fiber thermoplastic laminates subjected to cyclic loading

Tensile fatigue behaviour of glass fiber/polyamide composites, including unidirectional ([0]8, [90]8) and cross-ply ([02/902]s, [04/904]s and [904/04]s) laminates, was studied and compared to that of similar glass fiber/epoxy composites. The fatigue resistance of cross-ply glass/polyamide was greater than that of glass/epoxy while also exhibiting lower stiffness reduction. To explain this key observation, residual stiffness and residual strength fatigue tests were performed on cross-ply laminates, while optical microscopy was used to measure ply crack density during the different stages of cycling. Testing of the cross-ply laminates at lower peak stresses of 50% of the ultimate tensile strength (i.e., high cycle fatigue regime) revealed partial cracks that did not propagate completely through the width and thickness of plies due to high matrix toughness and other observed toughening mechanisms such as matrix bridging. A micromechanical finite element model with explicit ply cracks was also used to predict laminate stiffness degradation corresponding to observed ply crack densities, revealing that stiffness degradation was overpredicted when cracks were assumed to span the entire specimen width. Additional finite element simulations with partial cracks showed notably less stiffness reduction. These observations suggest glass/polyamide is inherently more damage tolerant than glass/epoxy and may be a suitable replacement for fatigue critical structures.

Influence of drilling and abrasive water jet induced damage on the performance of carbon fabric/epoxy plates with holes

The influence of conventional drilling (CD) and abrasive water jet (AWJ) cutting on the fatigue performance of carbon fabric/epoxy plates was investigated. Observations of machined hole regions revealed more severe damage on the CD hole surfaces with larger damage regions penetrating into the material. For both material systems investigated, the surface roughness for AWJ holes was in fact higher in comparison to CD holes. Nonetheless, static properties, short-term fatigue properties and endurance limits of the plates were not affected by the machining technique. Observed damage, stiffness and temperature evolutions during short-duration cyclic tests showed no significant changes in fatigue response for CD and AWJ plates, suggesting that surface roughness is not a good indicator of hole quality. However, long-duration cyclic tests revealed that CD plates exhibited greater stiffness degradation and advanced damage after a cyclic threshold, beyond which machining-induced delamination cracks began to propagate. This was not observed for the AWJ plates, where instead cyclic loading-induced delamination cracks initiated and propagated much later. These key findings are critical for assessing the unique performance characteristics of fabric/epoxy plates with holes. Since these materials are increasingly used for primary fatigue-critical structures, the presented outcomes are vital for designing corresponding structure joining methods.

Fatigue performance of Friction Stir Welded titanium structural joints

The Friction Stir Welding process for producing corner and T-joints out of 6mm Ti–6Al–4V was developed in this effort using previous work on butt weld joints as a starting point. A limited number of corner joints were also subjected to a bending fatigue test to preliminarily assess the applicability of the process in producing fatigue critical structures. These results were also compared to predictions made by applying stress concentration factors to previously generated uniaxial butt joint test data. While additional testing is still required to obtain a higher degree of confidence in the conclusions of this study, it was found that the performance of these corner joints in fatigue could be compared to butt joint data when a geometrically based stress concentration factor is applied. Furthermore, these welded joints possessed equivalent fatigue performance relative to identical test specimens machined from wrought product forms, both bar and extrusion. Thus, from the perspective of fatigue design, this study has shown that Friction Stir Welding is able to produce structures with the same performance as currently made from wrought materials.

Inflow Parameter Effects on Wind Turbine Tower Cyclic Loading

Wind turbine towers are fatigue-critical structures subjected to a variety of atmospheric wind regimes at onshore installations. At a commercial wind farm in Southwestern Ontario, the structural steel supporting tower of a multi-megawatt horizontal-axis turbine has been instrumented with a fiber Bragg grating strain gauge array. A nearby meteorological mast allows wind conditions to be classified on the basis of horizontal turbulence intensity and vertical wind shear. Data has been collected over a measurement campaign spanning multiple months, and a sample of the tower's cyclic loading history has been compiled. Comparisons are drawn between accumulated hourly loading cycle spectra for operation across different upwind inflow parameters.

A Modified Compliance Method for Automatically Measuring Fatigue Crack Growth <i>In Situ</i> during Spectrum Fatigue Testing

The fatigue critical structures of military aircraft are generally subjected to variable amplitude flight spectrum loading. Maintaining aircraft structural integrity to ensure safe operation of the fleet is critically dependent on accurate analysis and reliable prediction of fatigue crack growth in those structures under service operating conditions. To achieve this goal, laboratory experimental methods that can accurately measure and monitor fatigue crack growth under variable amplitude loading are required. This can be challenging as no test standard exists to guide the process of fatigue crack growth measurement under variable amplitude loading conditions to ensure the accuracy of the test results. This challenge was addressed by developing a modified compliance method as described in this paper. The results presented employed a modified compliance method complemented with a travelling microscope technique and marker band loads. The modified compliance method developed is able to measure in-situ, fatigue crack growth of standard compact-tension specimens under a fighter flight spectrum loading. The marker band loads and microscope readings were used to assist the post-test validation using quantitative fractography. The results from this study have demonstrated that the modified compliance method can produce consistent and accurate fatigue crack growth data under variable amplitude loading conditions.

Hard Landing이 항공기 구조물에 미치는 영향성 연구

Aircraft MLG and wing structures have been recognized as fatigue critical structures and exposed to the risk of fatigue crack initiation and propagation. Furthermore, these structures are frequently subjected to serious dynamic loading condition during a Hard Landing which may lead to their failure. Especially, structural integrity of MLG and wing components is decreased as the flight time increased because of the fatigue damage accumulated on the aircraft. In this study, the effects of Hard Landing on the MLG and wing components of aging aircraft were evaluated by using numerical approach. To achieve the aim, a finite element model has been developed and simulations were conducted by varying the landing conditions. As a result, it was revealed that the high stress concentration phenomenon was occurred at the lower Side Brace of MLG. Thereby, the intensified inspection for the lower Side Brace should be considered to prevent unexpected aircraft mishap.

Assessment of Durability and Damage Tolerance of β-Annealed Ti-6Al-4V

-annealed Ti-6Al-4V ELI alloy has a special chemical composition and manufacturing process, intended to optimise its fatigue and fracture properties. This alloy is used in primary fatigue-critical structures of advanced military aircraft, but little has been generally published about its properties, particularly the fatigue and crack growth behaviour under service loading. A test programme has been set up to assess the fatigue durability and damage tolerance properties of the alloy, in the context of the two most relevant analysis methods. The first is strain – life analysis, used to estimate safe lives (durability). The second is fatigue crack growth analysis, used to assess the damage tolerance behaviour. This paper presents preliminary results of the test programme.

Risk Analysis of Fuselage Splices Containing Multisite Damage and Corrosion

repair of the components. This technology has been increasingly used in the maintenance and management of aging aircraft e eets to improve e ight safety and reduce costs. The output of risk analysis can be used to optimize the maintenance schedule of aging aircraft while maintaining the POF under an acceptable level. To examine the performance of fatigue critical structures, probabilistic risk analysis has been applied to both military and commercial aircraft, and it is believed to be the approach of choice for the future. 1 Various probabilistic risk analysis methodologies have been developed for aging aircraft structures. Generally, there are two aspects in the probabilistic risk analysis to be noted. First is deterministic damage tolerance and durability analysis, such as the analysis of the median crack growth data, the onset pattern of initial MSD, crack linkup criterion, and failure criterion. Considerable effort has been expended on this issue using fracture

A summary of the fatigue properties of wind turbine materials

Modern wind turbines are fatigue-critical machines that are typically used to produce electrical power from the wind. The materials used to construct these machines are subjected to a unique loading spectrum that contains several orders of magnitude more cycles than other fatigue-critical structures, e.g. an aeroplane. To facilitate fatigue designs, a large database of material properties has been generated over the past several years that is specialized to materials typically used in wind turbines. This article reviews the fatigue data that have been developed especially for wind turbines. Major sections are devoted to the properties developed for metals (primarily aluminium), wood and fibreglass. Special emphasis is placed on the fibreglass discussion because this material is currently the material of choice for wind turbine blades. The article focuses on the data developed in the US, but cites European references that provide important insights. Published in 2000 by John Wiley & Sons, Ltd.

Formulation of a Stochastic Fatigue Crack Growth Model and its Numberical Analysis.

Fatigue crack growth is a major factor that must be considered in the design and life prediction of fatigue-critical structures, yet recent studies suggest that much of fatigue crack growth data exhibit a large amount of statistical variability and deterministic models do not adequately describe the crack propagation behavior. Thus, the need to use probabilistic methods to predict fatigue crack growth becomes evident. In this paper, focus is centered on the formulation of a stochastic fatigue crack growth model. With the use of the diffusion Markov process theory, a partial differential equation called the Fokker-Planck equation which satisfies the probability density of the random time to reach a given crack size is introduced. Then, the equation is numerically solved by the finite differential method, and the statistical properties of crack growth fatigue life are evaluated.

Thickness Effect on the Statistical Properties of Random Fatigue Crack Propagation Resistance.

The statistical properties of crack growth fatigue life are often needed in a reliability-based design of fatigue-critical structures. However, recent research revealed that a decrease in specimen thickness caused an increase in the variability of crack growth fatigue life. In this paper, the dependence of the statistical properties of random crack propagation resistance on specimen thickness is investigated. ConstantΔK fatigue crack growth tests are conducted on 2024-T3 aluminum alloy in four cases of specimen thickness. From the experimental data, there is no negligible influence of thickness on the statistical properties of random crack propagation resistance. Using the random crack growth model proposed in the previous paper, the thickness effect on the distribution of crack growth fatigue life is also investigated, and satisfactory agreement with the experiment is obtained.

Optimal Periodic Proof Test Based on Cost-Ef f ective and Reliability Criteria

An exploratory study for the optimization of periodic proof tests for fatigue-critical structures is presented. The optimal proof load level and the optimal number of periodic proof tests are determined by minimizing the total expected (statistical average) cost, while the constraint on the allowable level of structural reliability is satisfied. The total expected cost consists of the expected cost of proof tests, the expected cost of structures destroyed by proof tests, and the expected cost of structural failure in service. It is demonstrated by numerical examples that significant cost saving and reliability improvement for fatigue-critical structures can be achieved by the application of the optimal periodic proof test. The present study is relevant to the establishment of optimal maintenance procedures for fatigue-critical structures.