Fatigue Load Cases Research Articles

Optimisation of bonded and hybrid step lap joints of thick carbon fibre/epoxy panels used in aircraft structures

The use of composite materials is widely spread across the Aerospace sector. Scarf and step lap joints are amongst the most commonly used methods for flush joint repairs; they restore original stiffness and nearly original strength. Prior investigations conducted by the authors have shown that bonded and hybrid step lap joints have superior static strength capabilities than bolted joints. Hybrid step lap joints have shown the highest fatigue resistance than its bolted and bonded counterparts. This investigation aims at optimising the previous step lap joint design containing five steps with a 90 mm long overlap length in order to improve joint efficiency. Four new step lap joint configurations are considered based on a parametric study conducted using Abaqus; two of which contain seven steps and a 130 mm long overlap (bonded and hybrid) whilst the other two contain six steps and a 105 mm long overlap (bonded and hybrid) with a tapered outer step region. Experimental tests right through to numerical modelling using three-dimensional (3D) finite element (FE) models have been investigated. Adhesive non-linear material properties, fastener contacts and friction forces were all included in the 3D FE models. The Multicontinuum Theory (MCT) is used to simulate the progressive failure process and determine the stress states in the four configurations. Overall the FE models are able to accurately predict the bonded and hybrid joint strengths and highlights the importance of not only optimising the number of steps and their lengths but also the individual step heights. This is a parameter which has often been overlooked by fellow researchers. Experimental results showed that all four step lap joint configurations have significant improvement under static and fatigue load cases; the greatest improvement is seen in the two bonded step lap joint configurations. Overall the hybrid joint configuration consistently outperformed the bonded joint configurations. The presence of fasteners supress crack propagation, minimise peak peeling stresses at the ends of the overlap and increase the durability of the joint. This enables early crack detection which can assist composite joint repair certification.

VBA fatigue analysis program for metallic structural components preliminary design

The main objective of this paper is to describe the creation and use of a fatigue analysis program written in VBA, designed for the preliminary sizing of metallic structural components used in aerospace applications, subjected to one or multiple fatigue loading cases. The VBA programing language was chosen because of its direct control over the most common spreadsheet computational program, Microsoft Excel. Metal fatigue analysis is an important type of analyses for modern structures. Fatigue failure accounts for around 80-90 percent of common structural failures, and therefore, a quick and reliable analysis is necessary so as to evaluate the structure’s bearing capacity to fatigue load. Due to the nature of the fatigue load and the importance of the structural component, such an analysis can be very time consuming, starting from the finite element model preparation and going through the actual analysis; thus, there is a need for a tool that can evaluate the stress data from the numerical simulation and give reliable information about the behavior of the structural component.

Lateral vibration control of monopile supported offshore wind turbines with toroidal tuned liquid column dampers

This study investigates the lateral vibration control efficiency of toroidal tuned liquid column dampers (TTLCDs) for monopile supported offshore wind turbines. The TTLCD has the advantage that it can be tuned to match the natural frequencies of wind turbines both in fore-aft and side–side directions simultaneously. A general mathematical modeling of the TTLCD inside the nacelle is derived and an interface between the simulation platforms MATLAB/Simulink and FAST is established. A TTLCD design procedure is provided for monopile wind turbines. Numerical calculations are carried out for different wind velocities, wave-wind misalignment angles and working conditions considering fatigue and ultimate load cases. A parametric analysis is performed on the flow resistance coefficient of TTLCDs. The results demonstrate that TTLCDs can effectively control lateral vibrations and improve the structural performance of monopile wind turbines against wind and wave loads.

A comparison of dynamic inflow models for the blade element momentum method

Abstract. With the increase in rotor sizes, the implementation of innovative pitch control strategies, and the first floating solutions entering the market, the importance of unsteady aerodynamic phenomena in the operation of modern offshore wind turbines has increased significantly. Including aerodynamic unsteadiness in blade element momentum (BEM) methods used to simulate wind turbine design envelopes requires specific sub-models. One of them is the dynamic inflow model, which attempts to reproduce the effects of the unsteady wake evolution on the rotor plane induction. Although several models have been proposed, the lack of a consistent and comprehensive comparison makes their relative performance in the simulation of large rotors still uncertain. More importantly, different dynamic inflow model predictions have never been compared for a standard fatigue load case, and thus it is not clear what their impact on the design loads estimated with BEM is. The present study contributes to filling these gaps by implementing all the main dynamic inflow models in a single solver and comparing their relative performance on a 220 m diameter offshore rotor design. Results are compared for simple prescribed blade pitch time histories in uniform inflow conditions first, verifying the predictions against a high-fidelity free-vortex-wake model and showing the benefit of new two-constant models. Then the effect of shed vorticity is investigated in detail, revealing its major contribution to the observed differences between BEM and free-vortex results. Finally, the simulation of a standard fatigue load case prescribing the same blade pitch and rotor speed time histories reveals that including a dynamic inflow model in BEM tends to increase the fatigue load predictions compared to a quasi-steady BEM approach, while the relative differences among the models are limited.

Loading conditions impact on the compression fatigue behavior of filled styrene butadiene rubber

Abstract Fatigue failure, commonly encountered in rubber materials, is a critical issue. In this study, the compression fatigue tests of filled styrene-butadiene rubber (SBR) under different loading conditions were performed, applying cylindrical specimens. A stress–strain curve and modulus drop curves were generated by nine fatigue loading cases, covering different R ratios in the range of 0 < R < 1. The temperature variation in the process of compression fatigue was explored. Three different approaches were applied to investigate the fatigue life of the SBR (it is used twice hence abbreviation should be used) vulcanizates. These methods were validated in assessing the fatigue failure of the specimens. According to the experimental fatigue life, a fatigue life prediction model based on strain amplitude as the damage parameter was established. The results demonstrated that both R ratio and strain amplitude could affect the fatigue life. For all the loading cases, the fatigue life generally increases with the increase of R ratio. Under the compression loading condition, the narrower range of strain and the lower mean strain are beneficial to the fatigue resistance of rubbers, which also indicates a larger pre-load provides much higher fatigue resistance. During the fatigue loading, the temperature rises rapidly until it reaches a peak value, then drops slightly, and finally reaches a plateau.

Flexible riser tensile armour stress assessment in the bend stiffener region

The flexible riser within the bend stiffener in the top connection is a structurally vulnerable region for extreme and fatigue loads as it is subjected to large dynamic tensile forces and curvatures. This paper presents a nonlinear finite element model (FEM) for the flexible riser with bend stiffener taking into account interlayer friction and riser-bend stiffener contact interaction subjected to combined axisymmetric and bending loads. A case study with typical extreme and fatigue load cases is carried out to assess the curvature variation and bend stiffener contact pressure influence in the riser tensile armours stresses. For the extreme load case, detailed finite element results such as contact pressures and friction, normal and transverse bending stresses are compared with a uniform curvature FEM and available analytical models. A fatigue load case is also performed, where the stress amplitudes and cumulative fatigue damages for the internal tensile armour wires are compared with a uniform curvature FEM and analytical models. From the case study results under extreme loading, it can be observed that the model with bend stiffener interaction predicts larger stresses in comparison to the uniform curvature FEM and analytical models. Under typical fatigue loading conditions, significant differences in the cumulative damage are observed between the models, highlighting the importance of an accurate modeling for lifetime integrity assessment in a region of highly variable riser curvature.

Fatigue failure simulation of a double-lap composites-metal bolted joint structure

The fatigue damage behavior and the failure characteristics of the T300/QY8911 laminate and 7050-T7451 aluminum single-bolt bolted joints were investigated by test and progressive damage analysis (PDA) in this article. The manufactured composites-to-metal bolted joints (CMBJs) with different thickness metal plates and lap-patterns have been tested in static and fatigue load case and the failure characteristics are compared. A modified Lemaitre fatigue damage model and a phenomenological composite fatigue damage model were proposed and applied simultaneously to capture the damage evolution in the laminate and the metal parts. The competition of the fatigue damage of the laminate and the metal parts in affecting the joint stiffness and the coupling of the damage accumulation were studied by PDA, and the results indicates that: the failing sequence of the CMBJs constructive parts may change between the static load case and the fatigue load case; the fatigue damage of the metal parts has little effect on the joint stiffness of the CMBJs, nevertheless, there is certain coupling of the laminates fatigue damage and metal fatigue damage in the CMBJs, the harden or soften of the metal parts by fatigue hardening or crack progression would promote or slow down the damage accumulation in laminate.

The effect of decarburization on the fatigue life of overhead line hardware

Altering the microstructure in order to improve the tensile properties of bow shackles resulted in inconsistency in the fatigue performance. This raises the question whether the inconsistency in fatigue life can be attributed to microstructural changes along the profile of the shackle or to decarburization at the surface. Bow shackles forged from 080M40 (EN8) material were subjected to different heat treatments in order to alter the microstructure. The shackles were subjected to five different fatigue load cases, which represented typical loads experienced at termination points for an overhead power line with a span length of 400 m, with changes in conductor type, configuration, wind, and ice loading. Although the change in microstructure does improve both the tensile and fatigue performance, we found that the depth of the decarburization layer has a greater effect on the high cycle fatigue life of bow shackles than the non-homogeneous microstructure.

Assessment of mooring configurations for the IEA 15MW floating offshore wind turbine

To achieve cost-effective deployment of floating offshore wind farms, it is necessary to reduce mooring costs of Floating Offshore Wind Turbines (FOWTs). Beyond the cost, in terms of environmental impact, the seabed disrupted area due to mooring lines should be mitigated with care. The objective of this paper is to shed light on design parameters for cost- effective and low-footprint mooring configurations for FOWTs using coupled dynamic analyses. A design space is explored for mooring configurations with different pretension ratios, laid down length ratios and clump weight sizes. Ultimate and fatigue load cases are simulated in OpenFast to compute the floater motions, mooring line tensions and fatigue damage. With constant pretension ratio of 0.15 and adding clump weights of 40t, mooring line length, mooring footprint and peak tension can be reduced by 14%, 15% and 9% respectively, while maximum surge and fatigue damage increase by 25% and 12% respectively. This paper will serve as a basis for further work on mooring design in the EU H2020 funded project COREWIND and provide a practical reference for the mooring system design for FOWTs.

A continuum damage model for composite laminates: Part III - Fatigue

The mesoscale continuum damage model for laminated composites that has been developed for the past 10 years at AMADE research group is extended for fatigue life and residual strength predictions. The same intra-laminar damage mechanisms are considered by means of the LaRC04 failure criteria, but a new set of functions are introduced to account for the degradation of the material under fatigue loads. The cycle jump approach is adopted within the finite element code to reduce the computational cost. In the follow-up paper part IV (Llobet et al., submitted for publication), the computational model is validated by simulating the progressive failure mechanisms of open-hole and double-edge notched specimens under static, tension-tension fatigue and residual strength load cases. All the experimental data is also described in detail in the accompanying part.

Fatigue behavior of composite laminates with automated fiber placement induced defects- a review

Numerical and experimental studies have been conducted to try to quantify the influence of Automated Fiber Placement (AFP) induced manufacturing defects on the quasi-static behavior of composite parts ranging, on the building block approach, from coupons to sub-structural parts. In more recent studies, researchers started to conduct the same investigations for fatigue load cases with the purpose of determining the influence of defects on damage initiation and propagation mechanisms. This document will present a review of both experimental and numerical studies conducted so far on the influence of AFP defects on laminate response and failure characteristics during fatigue loading. In the first section, a brief introduction of the types of AFP manufacturing defects identified will be presented before focusing on publications related to the influence of the aforementioned defects on the quasi-static behavior of composite laminates. Following, in the second section, we propose to detail the numerical investigations on the fatigue behavior of composite laminates. Then, the third section will present a combined experimental and numerical investigation of the influence of the manufacturing defects presented in the first section on the fatigue behavior studied in detail in the second section. Finally, the fourth section is dedicated to a comparison of the time cost associated with numerical and experimental approach.

Identification of cumulative damage at the blade root of AB92 blade using Palmgren-Miner’s rule

This study presents the identification of cumulative damage at the blade root of AVANTIS AB92 wind turbine blade by evaluating the simulations generated output obtained from GH Bladed software. The blade length was 45.3 m and consisted of six (6) different airfoils. The blade material was made of GRP/Epoxy with a weight of 10.2 tons. At the blade root, the bolt hole circle diameter was 2.3 m with a 1.3 m distance from hub centre to rotor shaft flange. The fatigue loads at the blade connection of AB92 were performed according to Germanischer Lloyd guideline for a 2.5 MW horizontal axis wind turbine generator with wind class IEC IIA. The simulations were performed when the turbine operates during power production with and without occurrence of fault with wind condition satisfying the normal turbulence model. Design load cases such as during start-up and normal shutdown were also carried out with wind condition similar to normal wind profile model. The fatigue load cases at the blade connection were post-processed by determining the rainflow cycle counting and damage equivalent loads using GH Bladed software. Statistical estimation models such as graphical and exponential methods were also used to predict the Weibull parameters. The total fatigue damages due to bending moments and torsion at the blade connection were evaluated using the Palmgren-Miner’s rule at different expected behaviour of S-N curves. Calculation results indicate that the total fatigue damage due to torsion, flapwise and edgewise bending moments at the blade connection of AB92 when the slope parameter of S-N curve i.e., m = 3 are less than unity with values of 0.8702, 0.0354 and 0.0180, respectively.

Representation of wind turbine blade responses in power production load cases by linear mode shapes

AbstractThe wind turbine industry is designing large MW size turbines with very long blades, which exhibit large deflections during their operational life. These large deflections decrease the accuracy of linear models such as linear finite element and modal‐based models, in which the structure is represented by linear mode shapes. The aim of this study is to investigate the competence of the mode shapes to represent the large blade responses in normal operation load cases. For this purpose, blade deflections are projected onto the linear modal space, swept by mode shape vectors. The projection shows the contribution of each mode and the projection error. The blade deflections are calculated by a nonlinear aero‐servo‐elastic solver for power production fatigue load cases with normal turbulence. The mode shapes are calculated at the steady‐state deflected blade position computed at different wind speeds. Three reference turbine blades are used in the study to evaluate the effects of various blade design parameters such as length, stiffness, mass, and prebend. The results show that although the linear mode shapes can represent the flapwise and edgewise deflections accurately, axial and torsional deflections cannot be captured with good accuracy. The geometric nonlinear effects are more apparent in the latter directions. The results indicate that the blade deflections occur beyond the linear assumptions.

The Effect of Geometry on the Fatigue Life of Overhead Line Hardware

Bow shackle failures, over the years, raised the question whether these failures are attributed to microstructural changes along the profile of the shackle or due to the geometry of the shackle. Bow shackles forged from 080M40 (EN8) material were subjected to different heat treatments in order to alter the microstructure thereof. The shackles were 3D-scanned prior to fatigue testing, and the data points were imported into engineering simulation software (ANSYS), to build a finite element model of each shackle tested. The shackles were subjected to five different fatigue load cases, which represented typical loads experienced at termination points for an overhead line with a span length of 400 m, with changes in conductor type, configuration, wind and ice loading. The fatigue tests revealed that the improvement in fatigue performance with an increase in hardness was limited to the lower load levels. In addition, the finite element model indicated that the misalignment of the bolt holes results in unequal load distribution between the two legs, with a considerable increase in the bending stress experienced by the leg carrying the higher loading. The influence of the bow shape of the shackle was confirmed by testing straight-leg shackles also manufactured from 080M40 material, which outperformed the fatigue performance of the bow shackles. Furthermore, misalignment of the bolt holes for the straight shackles did not have the same detrimental effect compared to the bow shackle. Although the change in microstructure does influence the fatigue performance, this investigation concludes that the combined influences of the curved leg and misalignment of the bolt holes pose a greater impact on the fatigue performance of bow shackles than microstructure. Furthermore, the fatigue performance of line hardware shackles is significantly improved by changing the geometry of the shackle from a curved leg to a straight leg.

An experimental approach for characterization of fracture process zone in concrete

In this work, an attempt has been made to understand the evolution of fracture process zone in plain concrete member utilizing Digital Image Correlation (DIC) technique. Series of experiments have been performed in centre point bending under the action of monotonic and repetitive loadings and DIC technique is used during the entire test process. A systematic procedure has been developed to monitor the evolution and propagation of the fracture process zone and traction free crack. Size of fully developed FPZ has been found to be approximately equal in both static and fatigue loading cases. It has been also seen that traction free crack propagation has occurred only in medium and large size beams. DIC analysis results have been compared with the measured CMOD values through validation study and both are found to be in good agreement. An analysis has been performed for evaluating the resolution and standard uncertainty in the DIC measured displacements and strains.

Fatigue testing of GFRP materials for the application in automotive leaf springs

Due to their high specific properties, fiber reinforced plastics are ideal candidates for the substitution of steel parts in the automotive sector enabling high weight savings and increasing the efficiency of automobiles. One possible application are leaf springs made of glass fiber reinforced plastics. Due to the simple bending load case, the fiber orientation can be aligned with the main stress direction. In real life such components as leaf springs are loaded under stochastic loads originated from different driving maneuvers. To replicate those fatigue load case, a high number of component tests are required. Aim of the present work (as a result of a joint research project between the Institute for Plastics Processing and Ford Motor Company) is to develop a simplified testing program that is able to reproduce the damage and the fatigue behavior which occurs in real composite components. For this purpose, three point bending experiments on unidirectional specimens made of glass fiber reinforced epoxy pre-preg material are performed. However, since the loading of the real component introduces additional transverse stresses in the region of the clamping group, the results on specimen level are not transferable to component level. For this purpose, a special clamping group is developed on specimen level to enable a better prediction of fatigue life and the transferability of the results of the fatigue tests to component level. This results in a more realistic representation of the leaf spring stress state in the fatigue experiments. As a result of this research, a higher safety margin in the design phase of such components and less testing time is achieved.

XFEM simulation of fatigue crack growth in a welded joint of a pressure vessel with a reinforcement ring

The numerical analysis of the behaviour of a pressure vessel with a reinforcement ring subjected to both static and dynamic load is presented in this paper. This research was based on a previous analysis which involved different reinforcement ring dimensions, and their influence on the stress distribution within the pressure vessel, assuming the presence of a crack (Sedmak et al., in: International Conference on Structural Integrity and Durability, 2017). The aim was to compare the numerical results for two models, one of which was a 2D model simulated with classic FEM, whereas the other included a 3D fatigue crack, and was simulated with extended finite element method, using Morfeo software. The numerical simulation was based on a pressure vessel which is typically used as a part of hydropower plants, containing a manhole which is supported by the reinforcement ring. The analysis focused on a crack that was located in the welded joint between the reinforcement ring and the pressure vessel mantle. The results obtained by the simulations have shown the differences in the stress magnitudes and in the consequences resulting from the crack growth in both cases, depending on the type of the load, wherein the second (fatigue) load case was noticeably more extreme.

Assessment of load reduction capabilities using passive and active control methods on a 10MW-scale wind turbine

In the paper, the potential to alleviate wind turbine loads through combined implementation of different passive and active control methods is assessed. Passive control of loads is accomplished through blade designs with build in material and/or geometric bend-twist coupling (BTC). The first is materialized by introducing an offset angle on the plies of the uni-directional material over the spar caps of the blade, while the latter by sweeping the blade elastic axis with respect to the pitch axis. Active control of loads is considered through individual pitch control (IPC) or concurrent use of individual pitch and flap control (IPC+IFC). Different combinations of the abovementioned techniques are tested in the paper with the aim to obtain maximum possible load reduction levels but also confine key design parameters of the various methods within reasonable limits that by no means exceed manufacturing constraints. The performance of the different control options is assessed through aeroelastic simulations for the 10MW DTU Reference Wind Turbine (RWT). A subset of representative fatigue and ultimate design load cases (DLCs) of the IEC is simulated and load reduction levels are assessed with respect to the baseline RWT configuration with no aeroelastic control of loads.

Inflow-based flap control on a 10MW-scale wind turbine using a spinner anemometer

In the paper, an individual flap control (IFC) algorithm based on wind speed measurements obtained with a spinner anemometer is presented. The controller uses flap actuators with the aim to remove any deterministic source of load variation on blades, associated with asymmetries of the inflow. Such load variations are concentrated on multiples of the rotational frequency (p multiples) and they are mainly due to wind yaw, inclination and atmospheric boundary layer (ABL) shear. The aim of the controller is to assist operation of the conventional feed-back individual pitch controller (IPC) and thereby reduce its control duty cycle. The performance of the proposed controller is assessed through aero-elastic simulations for the 10MW DTU Reference Wind Turbine. A subset of normal operation fatigue and ultimate Design Load Cases (DLCs) of the IEC has been simulated and load reduction capabilities of the control algorithm have been compared against those of the standard individual pitch control (IPC) loop.

A model for quick load analysis for monopile-type offshore wind turbine substructures

Abstract. A model for quick load analysis, QuLA, of an offshore wind turbine substructure is presented. The aerodynamic rotor loads and damping are pre-computed for a land-based configuration. The dynamic structural response is represented by the first global fore-aft mode only and is computed in the frequency domain with phases using the equation of motion. The model is compared to the state-of-the-art aeroelastic code Flex5. Both lifetime fatigue and extreme loads are considered in the comparison. In general there is good agreement between the two models. Some deviations for the sectional forces are explained in terms of the model simplifications. The differences in the sectional moments are found to be within 10 % for the fatigue load case and 10 % for the extreme load condition.