Critical Load Cases Research Articles

An analytical solution to “pillowing effect” for composite fuselages

Abstract “Pillowing effect”, introduced by inner cabin air pressure on stringer-frame stiffened fuselage cylinder, is a critical loading case for civil aircraft fuselage. The “pillowing effect” causes out-of-plane bending and shear to fuselage panels, which is a rather huge load for such membrane structures. The fuselage’s longitudinal load and bending load interact nonlinearly with each other in a so called “beam-column” formation, invalidating the superposition principle for linear elasticity. An analytical solution to the axisymmetric “beam-column” model is given in this study to account for the “pillowing effect” of orthotropic composite fuselages. This formula can be employed to perform stress checks against “pillowing effect” during fuselage design.

Failure intervention of transmission line tower primary legs using bi-angled cruciform retrofitting

Retrofitting has been suggested by researchers as an intervention measure against failure of transmission towers which needs to be done with minimal disruption. Retrofitting of the primary leg members of transmission line towers by bi-angled cruciform arrangement has been shown in literature to be effective. However, the influence of stresses and deformations existing in the leg member prior to retrofitting on the capacity of the final retrofitted member is not known. Therefore, this study assesses the effectiveness of bi-angled cruciform sections under preload conditions through experiment followed by a numerical study. The level of preload in the primary leg member with respect to its theoretical capacity, slenderness ratio and width-thickness ratio of the angle section were the parameters. The results suggest that bi-angled cruciform retrofit is more effective at lower slenderness and width-thickness ratios and the reduction in capacity of retrofitted member is more noticeable till 50 % preload after which the changes are not significant. While the former two parameters lead to lower load share in the retrofitting member, higher preloads however lead to more equal load share. The application of this retrofitting pattern is demonstrated numerically with a full-scale tower model simulated under code specified load cases. Critical load cases leading to tower failure before design loads were identified and the retrofitting was incorporated into the model for these load cases. The analysis showed that the final loads attained in these load cases improved by upto 70 % thus confirming the potential of this retrofitting technique in preventing tower failure.

Mast Design Performance - Influence of the Non-Linear Elastic Behaviour of Carbon Fibres on the Accuracy of Fluid/Structure Interaction Models for Estimating Mast Bending

_ The masts of racing yachts are generally made of carbon fibre reinforced plastics (CFRP) and are subject to two main loads: dock tuning and the forces exerted by the sails. Both result in compression and bending of the mast. The mast design must ensure that for any given sailing configuration, the mast deflection is matched to the sail design to maximise the boat’s performance. Fluid/structure interaction models are generally employed by sail designers who use a stick model for the structural definition of the mast. The elastic properties of the CFRP plies involved in mast lamination are assumed to be constant in this latter model. However, stiffness increases under tension and decreases under compression (up to 30% at failure depending on the fibre type, standard modulus, intermediate modulus, etc) due to some reversible changes in the microstructure of the carbon fibres. As a result, the actual curvature of the mast can be very different from that calculated with the stick model, leading to a significant error in the actual shape of the sail during sailing. In this work, we use a series of three critical loading cases corresponding to those of an IMOCA mast. The influence of the elastic non-linearity of the CFRP plies on the performance of the racing yacht mast is quantified. It is concluded clearly that this elastic non-linear behaviour of carbon fibres is to be accounted for by sails designers to optimise performance. Such an influence is expected to be even more pronounced on spreader masts such as those used for America’s Cup. Keywords Yacht design; IMOCA mast; Carbon fibre composites; Non-linear elasticity

Optimizing Structural Integrity of Fighter Aircraft Wing Stations: a Finite Element Analysis Approach

Modern fighter aircraft are equipped with multiple stations on the fuselage and under the wings to accommodate various external stores, both jettisonable and non-jettisonable. Each configuration undergoes airworthiness certification, including structural analysis of individual stations within the carriage flight envelope. This study focuses on the structural analysis of a fighter aircraft wing station within this specified envelope. To perform this analysis, the wing station is extracted from the comprehensive global wing model, creating a sub-model with equivalent stiffness properties. Utilizing ANSYS Workbench®, Finite Element Analysis (FEA) is conducted for critical load cases to determine the Factor of Safety (FoS). The initial analysis reveals that the wing station has an FoS of 1.2 under the maximum design load. Prestressed modal and buckling analyses indicate a 10% increase in stiffness due to stress-stiffening effects. To further enhance load-carrying capacity, parametric design changes are introduced. Increasing the bolt diameter from 8 mm to 10 mm raises the FoS to 1.33, resulting in an 8% increase in the maximum load-carrying capacity of the wing station. This comprehensive approach, employing FEA, ensures the wing’s structural integrity under static load conditions within the carriage envelope. The study's findings support the wing station's enhanced performance and contribute to safer and more efficient aircraft operations.

Critical design load case fatigue and ultimate failure simulation for a 10-m H-type vertical-axis wind turbine

While previous studies investigating critical vertical-axis wind turbine (VAWT) design load cases have focused on large and relatively flexible Darrieus designs, the bulk of current commercial products seeking certification fall in the relatively small, stiff, H-type configuration. Understanding the critical design load case impacts for both fatigue and ultimate failure for this size and type of VAWT is imperative for certification and to help break the cycle of historical VAWT failures. A reevaluation of each of the design load cases specified in IEC 61400-1 using the Offshore Wind ENergy Simulator (OWENS) validated aero-servo-elastic software is conducted for both fatigue and ultimate failure contributions. Several design load cases previously thought negligible may have high enough fatigue damage rates for H-VAWTs to warrant more careful consideration; these cases include parked, extreme wind shear, and direction change with gust. Additionally, full operation stop-start-stop cycles, which historically have not been a part of the standards, may contribute fatigue damage similar to other normal design load cases. In light of these potentially critical conditions, and the sizes of many of the current H-VAWT designs falling in the IEC 61400-2 small wind turbine standard, the standard may need to be expanded to enable design success of certified H-VAWT systems.

Fatigue-resistant design of carbon/epoxy composites based on a failure tensor polynomial model by particle swarm optimization-sequential quadratic programming algorithm

This article introduces a design procedure to find the optimum fiber orientations of carbon/epoxy composite laminates for fatigue life advancement. The approach incorporates a fatigue failure tensor polynomial model and employs a hybrid algorithm, combining particle swarm optimization and sequential quadratic programming. Firstly, material properties of quasi-static and fatigue of the carbon/epoxy composites, fabricated by the vacuum-assisted resin transfer molding method, were determined to be used in the model. Various design problems involving two optimization scenarios were then solved using the hybrid algorithm. The algorithm’s performance was also evaluated by specific test problems, confirming its speed and robustness. The optimally fiber-oriented carbon/epoxy composite laminates having maximum fatigue lives were obtained for many critical in-plane cyclic loading cases. To validate the proposed design procedure, two optimum designs were experimentally verified under uniaxial loading conditions. The results indicated a good correlation between the estimated fatigue life of the optimally designed laminates and experimental data. This methodology offers a promising approach for the design of carbon/epoxy composite laminates with superior fatigue strength, particularly significant in specific industrial applications.

Heliostat Consortium: Gap Analysis on State of the Art in Wind Load Design

Abstract Wind loads are a major driver of heliostat cost. Standardized methods and tools are needed for a more detailed understanding of the static and dynamic loads of a heliostat design. This will enable cost reduction of wind-dependent heliostats to avoid unnecessarily conservative heliostat designs and increase field efficiency and reliability to reduce the risk of component failures due to high-wind events. Gaps related to wind load include lack of site characterization for wind measurements, insufficient critical load cases for heliostat design, insufficient understanding of turbulence impacts on heliostat tracking error, lack of knowledge on wind load under various heliostat array configurations, and underexplored heliostat field wind-load reduction and operating strategies. Recommended pathway forward is to develop wind load and site characterization guidelines for heliostat design and develop heliostat field wind-load models with optical performance impacts.

A Resilient Approach to a Test Rig Setup in the Qualification of a Tilt Rotor Carbon Fiber-Reinforced Polymer (CFRP) Wing

The evolution of aircraft wing development has seen significant progress since the early days of aviation, with static testing emerging as a crucial aspect for ensuring safety and reliability. This study focused specifically on the engineering phase of static testing for the Clean Sky 2 T-WING project, which is dedicated to testing the innovative composite wing of the Next-Generation Civil Tiltrotor Technology Demonstrator. During the design phase, critical load cases were identified through shear force/bending moment (SFBM) and failure mode analyses. To qualify the wing, an engineering team designed a dedicated test rig equipped with hydraulic jacks to mirror the SFBM diagrams. Adhering to specifications and geometric constraints due to several factors, the jacks aimed to minimize the errors (within 5%) in replicating the diagrams. An effective algorithm, spanning five phases, was employed to pinpoint the optimal configuration. This involved analyzing significant components, conducting least square linear optimizations, selecting solutions that met the directional constraints, analyzing the Pareto front solutions, and evaluating the external jack forces. The outcome was a test rig setup with a viable set of hydraulic jack forces, achieving precise SFBM replication on the wing with minimal jacks and overall applied forces.

Heliostat Consortium: Gap Analysis on Wind Load for Achieving a Fully Competitive Heliostat Industry

The Heliostat Consortium (HelioCon) Wind Load Subtask was initiated with the aim of bringing research work pertaining to wind load measurement, characterization, and prediction taking place across several tasks, including Advanced Manufacturing, Components and Controls, and Field Deployment. The cross-cutting wind load subtopic in the HelioCon roadmap report [1] highlighted standardized methods and tools that are needed for a more detailed understanding of the static and dynamic loads on a heliostat. This will enable cost reduction of wind-dependent heliostat components to avoid unnecessarily conservative, overly constrained designs and increase field efficiency/reliability, to reduce the risk of component failures due to high-wind events (>15 m/s). Gaps related to heliostat wind load include site characterization for wind measurements, critical load cases for heliostat design, turbulence impacts on heliostat tracking error, testing of heliostat array configurations, understanding spatial variation of maximum loads across the solar field, and heliostat field layout and operating strategies. The recommended highest-priority pathway as first steps taken by HelioCon to address these gaps are to develop site characterization guidelines for heliostat design and field load measurements.

Methodology of load effect analysis and ultimate limit state design of semi-submersible hulls of floating wind turbines: With a focus on floater column design

A main challenge in the design of hull structures of floating wind turbines (FWTs) is to properly estimate the load effects and resistance to perform the safety design check. While great efforts have been devoted to the load effects in terms of global motions, there is a lack of effective methods to calculate internal loads and lateral pressures of hulls in a fully coupled aero-hydro-servo-elastic environment, which should serve as the basis for the assessment of load effects in hulls. Moreover, very limited knowledge and experience exist in the current guidelines and research publications to deal with the detailed design of hull structures of FWTs. This study outlines an effective methodology and procedure to determine load effects for structural design of semi-submersible hulls, and demonstrates their application in a case study of column design based on the ultimate limit state (ULS) criteria. First, a novel multi-body floater model is established in a fully coupled FWT numerical system, which is capable of calculating floater internal loads. Second, the environmental contour approach is employed to initially choose load cases for ULS design, based on important wind speed conditions and criterial wave periods. Next, a screening of the critical load cases is made based on Von Mises stress analysis of columns considering global loads. Then, lateral pressure on columns is calculated, and a detailed design check is made based on combined global loads and lateral pressure as well as relevant safety factors for loads and resistance. Furthermore, the relative influence of global loads and lateral pressure on ULS performance of columns is investigated, which serves as the basis to reset the critical load cases for ULS design check. Subsequently, sensitivity analysis of ULS performance with respect to column design parameters, extreme value estimation, and shell buckling design checks, are performed to determine the plate thickness and ring stiffener distances of central and outer columns. Finally, optimized ring stiffener dimensions are estimated for different design cases based on panel buckling design criteria, and detailed design parameters for the central and outer columns are obtained. The methodology and procedure established and applied in a case study in this work, contribute to improving engineering practice and establishing improved design standards.

Impact analysis of compression preloaded honeycomb sandwich structures

Honeycomb cores made of phenol resin impregnated paper are widely used in sandwich structures of aircraft. Owing to the rather weak core material, this kind of sandwich is prone to a range of damages as a result of impact loading which may accidentally occur during operation. When employed in aircraft these structures have to be damage tolerant. This means barely visible damages have to be considered in the structural design process. In the standard design process for damage tolerant aircraft structures especially the effect of impacts has to be considered but is mostly investigated on unloaded sandwich components. However, aircraft structures are not completely stress free, even when the aircraft is in parking position. At least the structural weight will cause a permanent static preload. In the literature there are only few investigations dealing with the impact behaviour of preloaded honeycomb sandwich structures. In general, studies on composite structures have shown that tension preloads lead to higher structural impact resistance, while prestressing in compression reduces mechanical properties such as impact strength and stiffness of the structure. For this reason, a compression preload is considered as the more critical load case. In order to evaluate the effect of static compression preloads an experimental study on five different honeycomb sandwich configurations was conducted. The experimental investigations showed that even low preloads cause a higher structural damage area, which results in a lower residual strength of the structure. The results indicated that it is recommendable to consider structural compression preloads in the aircraft structural design process.

A Comparative Analysis of the Response-Tracking Techniques in Aerospace Dynamic Systems Using Modal Participation Factors

Mechanical structural systems are subject to multiple dynamic disturbances during service. While several possible scenarios can be examined to determine their design loading conditions, only a relatively small set of such scenarios is considered critical. Therefore, only such particular deterministic set of critical load cases is commonly employed for the structural design and optimization. Nevertheless, during the design and optimization stages, the mass and stiffness distributions of such assemblies vary, and, in consequence, their dynamic response also varies. Thus, it is important to consider the variations in the dynamic loading conditions during the design-and-optimization cycles. This paper studies the modal participation factors at length and proposes an alternative to the current point-wise treatment of the dynamic equations of motion of flexible bodies during design optimization. First, the most relevant-to-structural-dynamics definitions available in the literature are reviewed in depth. Second, the analysis of those definitions that have the potential to be adopted as point-wise constraint equations during structural optimization is extended. Finally, a proof of concept is presented to demonstrate the usability of each definition, followed by a case study in which the potential advantages of the proposed extended analysis are shown.

Gust and landing impacts as critical load cases for wings with distributed propulsion

The integration of distributed electric propulsion and energy storage systems into the wing structure significantly affects the mass distribution of the wing. Compared with conventional aircraft which are designed with engines mounted on the inner wing and integral fuel tanks, this substantially changes the overall loads on the wing structure. Assuming a retrofit scenario, this paper studies the influence of transient gust and landing impact loads on the dynamic behavior and the structural integrity of the wing of a representative conventional 19-seater commuter aircraft of CS-23 category retrofitted with distributed electric propulsion. Modal and dynamic response analyses are conducted based on a beam model idealizing the wing box of three wing designs: the reference wing with one main turboprop engine per half-wing, and two wings with integrated distributed electric propulsion. The results show for both load cases that the wing section loads obtained by dynamic response analysis exceed those predicted by equivalent quasi-static calculations. Compared with the reference wing, a significant increase of the amplitudes of the plunge and twist deformation and consequently higher section loads and stresses are observed for the retrofitted wing designs. It is concluded that both load cases are critical with regard to the structural integrity of the wing. However, the gust load case is more critical than the landing load case. These conclusions suggest that for the structural sizing of the wing, in particular for wings with distributed propulsion, and with regard to gust and landing impacts, the commonly applied quasi-static analyses should be accompanied by dynamic response analyses.

ANN-based optimization framework for the design of wind load resisting system of tall buildings

Conventional design methodology for tall buildings is a time-consuming and repetitive trial-and-error procedure with a limited probability of yielding an optimal solution that satisfies architectural, structural and serviceability requirements. Tall buildings are typically slender structures and mainly depend on lateral load resisting systems (LLRS) (e.g., shear walls, cores, and bracing systems) to withstand the lateral load of earthquakes and wind events, where a minor change in their layout, size, or shape will affect the cost tremendously. Consequently, an automated layout optimization procedure will result in a more economic and sustainable design. This paper develops a novel structure-wind optimization framework (SWOF) to find the optimal shear wall layout of tall buildings subjected to wind loads. SWOF is considered a genetic algorithmbased framework that uses an Artificial Neural Network (ANN) surrogate model to evaluate its constraints and objective function. These surrogate models rely on a training dataset prepared using the Finite Element Method (FEM), which has been created using an open application program interface (OAPI) MATLAB code. An optimization problem; is presented to show SWOF's efficiency. SWOF showed significant capabilities in recognizing load direction, critical load cases and inertia concepts without explicitly defining them through the developed code.

Implications of range of motion requirements for the laxity of ligaments in a lumbar finite element model

Finite element models of the lumbar spine often adopt ligament properties from tensile tests without accounting for possible differences between testing and in situ initial ligament length. Such differences could result in laxities or preloads at the beginning of a simulation that would affect the ligament forces, tangent stiffness, and the posture at which they fail. In vivo and in vitro human experimental data reported laxities or preloads. However, laxities or preloads, which could also result from postural differences, are often neglected in simulation studies. This study proposes a numerical methodology to identify ranges of ligament laxities or preloads compatible with the selected tensile ligament properties, the model, and the range of motion (RoM) the model aims to simulate. The approach assumes that ligaments should remain in a safe elongation range for the complete RoM, and that each ligament should play a significant mechanical role in at least one load case. The methodology was applied to the functional spinal unit (FSU) models using the RoM from healthy subjects and ligament properties from the literature. Without laxity, some ligaments reached their elongation at failure within the RoM. Laxity ranges varied considerably (from −9.2 mm preload to 10.7 mm laxity) and flexion was the most critical load case to determine them. Their effect on the mobility response was also assessed. The effect on the mobility of a FSU was also assessed. While the proposed method cannot determine an exact laxity value, it is simple and it can be applied to any model to identify a plausible range of ligament initial length.

Stress Analysis of Fuselage Stiffened Panel with and Without A Door Cutout

Aircraft is a complex engineering structure. The safety of the structure and the weight of the structure are the two important aspects to be kept in mind while designing the aircraft structure. It is an uphill task for an aircraft structural designer to bring out a safest structure with minimum weight. The current project deals with the stress analysis of the fuselage-stiffened panel with and without a large cutout for an emergency exit door. Cabin pressurization load case, which is one of the critical load cases used for the stress analysis. When the aircraft is flying above 40000 ft. altitude pressure is applied to fuselage inside cabin to create sea level atmospheric pressure . Stress analysis of the fuselage segment will be carried out to capture the global response of the fuselage under pressurization. A skin panel with fuselage frame and stringers representing the fuselage features will be considered for the local analysis to capture global response. An emergency exit door cutout will be introduced in the stiffened panel and analyzed for stress distribution around the cutout. Stress concentration factor and variation of stress from cutout edge towards the free edge of the panel will be obtained from the stress analysis results. Key Words: Transport aircraft, Fuselage, Bulkhead, Orthogonal stiffening, Finite element method, stress analysis, emergency exit door cutout, Stress concentration factor

Influence of degree of saturation on the dynamic soil properties of a sandy soil

It is commonly assumed that the fully-saturated scenario represents the critical shear strength condition for geotechnical structure design; nevertheless, this is not the case when the dynamic shear strength of unsaturated soil is considered, because the water content affects the transfer of seismic waves through the porous medium and into a structure. It is important to better understand this behavior so the soil response during an earthquake could be established to prevent damage or failure of the structure. A series of cyclic triaxial tests with controlled matric suction was conducted to determine the principal dynamic parameters for a sandy soil for different degrees of saturation. Finally, to assess the influence of soil saturation on the dynamic soil properties of a sandy soil, a dynamic lateral load pile design was performed using Reese’s p-y curve method, and the parameter obtained from the laboratory testing were used to backcalculate the seismic coefficients used for structural design. Structural designs of laterally loaded piles considering different soil degrees of saturation demonstrated that the saturated soil condition while subjected to a dynamic condition does not represent the critical load case for seismic analysis.

Approach for machine learning based design of experiments for occupant simulation

The complexity of crash scenarios in the context of vehicle safety is steadily increasing. This is especially the case on the way to mixed traffic challenges with non-automated and automated driving vehicles. The number of simulations required to design a robust restraint system is thus also increasing. The vast range of possible scenarios here is causing a huge parameter space. Simultaneously biofidelic simulation models are resulting in very high computational costs and therefore the number of simulations should be limited to a feasible operational range. In this study, a machine-learning based design of experiments algorithm is developed, which specifically addresses the issues when designing a safety system with a limited number of simulation samples taking diversity of the occupant and accident scenario into account. In contrast to an optimization task, where the aim is to meet a target function, our job has been to find the critical load case combinations to make sure that these are addressed and not missed. A combination of a space-filling approach and a metamodel has been established to find the critical scenarios in order to improve the system for those cases. It focuses specifically on the areas that are difficult to predict by the metamodel. The developed method was applied to iteratively generate a simulation matrix of a total of 208 simulations with a generic interior model and a detailed FE human body model. Kinematic and strain-based injury metrics were used as simulation output. These were used to train the metamodels after each iteration and derive the simulation matrix for the next iteration. In this paper we present a method that allows the training of a robust metamodel for the prediction of injury criteria, considering both varying load cases and varying restraint system parameters for individual anthropometries and seating postures. Based on that, restraint systems or metamodels can be optimized to achieve the best overall performance for a huge variety of possible scenarios with a specific focus on critical scenarios.

Contribution for the roadmap of hydraulic short circuit implementation: Case of Grand-Maison pumped storage power plant

The objectives of the 2050 energy policy based on the decarbonization of the electric power networks generate drastic changes for grid balancing with a massive integration of non-dispatchable Renewable Energy Sources. Hydroelectric power plants already significantly support electricity power system flexibility with innovative solutions such as variable speed units, fast frequency control, fast generating to pumping modes transition, high ramping rate, inertia emulation, etc.For pumped storage power plants (PSP), a quick solution to increase the flexibility without large investment is to operate the power plant in hydraulic short circuit (HSC) mode. This technological solution is simple to implement, but requires an in-depth study of various technical aspects among which the hydraulic transients of the new operating modes is of high importance from the installation’s safety perspective. In the framework of XFLEX HYDRO H2020 European research project, the exploitation of this solution is under implementation at Grand-Maison PSP. Located in the French Alps, Grand-Maison PSP is equipped with 8 reversible multi-stage Francis pump-turbines and 4 Pelton turbines, for a total installed capacity of 1800MW, thus being the largest PSP in Europe and one of the major PSP in the world. The waterway includes a headrace tunnel, a headrace surge tank, 3 parallel penstocks feeding the 12 units operated under a maximum gross head of 955mWC.In this paper, after a description of the general HSC considerations, the 1D model of the Grand Maison PSP and the related validation are presented. Finally, the most critical load cases in HSC operation are described to identify the potential hydraulic transient issues, such as extreme water levels in the upstream surge tank, maximum static pressure along the pressure shaft and minimum static pressure along the tunnels. The analysis performed for Grand Maison PSP is a contribution to the roadmap for the implementation of HSC operation in pumped storage power plant and will be made available as a public deliverable of the XFLEX HYDRO H2020 European research project.

Study of wind loads on asphalt shingles using full-scale experimentation

Asphalt shingles constitute more than 80% of the roofing materials in current residential housing in the United States. Many post-disaster surveys have reported the failure of these roofing elements below the design level wind event. Research to realistically model the aerodynamics of asphalt shingles at full-scale is limited, resulting in knowledge gaps in peak wind loads and effects of permeability. In this study, the aerodynamics and wind resistance of asphalt shingles are studied by using a monoslope roof tested at Florida International University's Wall of Wind Experimental Facility. Results from both aerodynamic and failure assessment test protocols showed that shingles near the upper corners of the roof were subjected to high suctions due to cornering winds. This presents a critical loading case for which asphalt shingle roofing systems are not typically tested using current standard tests. Moreover, the extent of permeability underneath the shingles was observed to vary with the spatial location of shingles and wind direction. Finally, the comparison of area-averaged peak Cp values obtained from this study with GCp plots in ASCE 7-16 Standard showed that these provisions, which do not consider the effects of permeable roofing elements (e.g., shingles), can underestimate wind loads on asphalt shingles.