Stijn Donders

Reliability-based design optimization of composite and steel aerospace structures

Although the aerospace production process is much better controlled than the process in other industries, it remains true that very small manufacturing tolerances exist in the geometrical parameters (flange thicknesses, hole diameters, …) as well as in material properties. In the current design process, the effect of this manufacturing variability on the structural durability and safety cannot be accurately assessed and is hence compensated for by applying safety factors. This is not an ideal situation, as it may lead to slightly overdesigned structures. A much more promising approach is to include probabilistic models of design variables into the mechanical simulation process. Then, with a new methodology based on reliability analysis, engineers can obtain a better understanding of the actual effect of the manufacturing tolerances and of variability in material properties. Based on the analysis results, the robustness and reliability of the design can be assessed and improved if needed. In this paper, the above-mentioned probabilistic approach is demonstrated on two aerospace applications: a composite wing and a slat track structure. The material properties of the composite wing have been characterized with statistical distributions and their effect has been assessed on the static performance. For the slat track, measurements of different geometrical properties have been collected during the manufacturing process and their variability has been characterized probabilistically with statistical models. Then, a reliability analysis has been carried out using morphing technology and fatigue life predictions with an industrial-sized FE model of the slat track to assess the reliability of the structure in terms of fatigue life. The outcome of the analyses consists of a probabilistic model of the structural performance (e.g. fatigue life for the slat track), given the variability in the geometrical and material parameters.

Integration of Probabilistic Methodology in the Aerospace Design Optimization Process

** The need of improvements in engineering designs especially with composite materials is nowadays a major request of the aerospace industry . Deterministic approaches are unable to take into account all the variabilities that characterize composites properties without leading to oversized structures. This paper intends to give a description of the most commonly used methods for reliability -bas ed design optimization using the reliability index and the performance measure approaches to point out the advantages of the application of these methods in the design process. For this purpose, a Finite Element model has been created for an industrial com posite wing structure and realistic variability has been assigned to material properties and static load conditions. A range of test cases with gradually increasing complexity has then been defined, that allows the assessment of reported methods in terms o f accuracy, computation time and applicability in conjunction with Finite Element models.

Concept Modelling of Vehicle Joints and Beam-Like Structures through Dynamic FE-Based Methods

This paper presents dynamic methodologies able to obtain concept models of automotive beams and joints, which compare favourably with the existing literature methods, in terms of accuracy, easiness of implementation, and computational loads. For the concept beams, the proposed method is based on a dynamic finite element (FE) approach, which estimates the stiffness characteristics of equivalent 1D beam elements using the natural frequencies, computed by a modal analysis of the detailed 3D FE model of the structure. Concept beams are then connected to each other by a concept joint, which is obtained through a dynamic reduction technique that makes use of its vibration normal modes. The joint reduction is improved through the application of a new interface beam-to-joint element, able to interpolate accurately the nodal displacements of the outer contour of the section, to obtain displacements and rotations of the central connection node. The proposed approach is validated through an application case that is typical in vehicle body engineering: the analysis of a structure formed by three spot-welded thin-walled beams, connected by a joint.

A wave-based sub-structuring approach for fast vehicle body optimisation

Conventional sub-structuring methods involve the coupling of substructure models through the common Degrees of Freedom (DOF). A wave-based sub-structuring approach is being developed, using a set of basis functions that describe the dynamic behaviour at the coupling interfaces. The interface DOFs are expanded in terms of these basis functions, typically reducing the size of the interface description and hence resulting in faster assembly predictions. Obtaining suitable basis functions is a critical step in the wave-based sub-structuring procedure. The procedure is first demonstrated on a two-plate assembly structure. The industrial applicability is then demonstrated on three real test cases from the automotive industry.

Multi-Disciplinary Optimization of an Active Suspension System in the Vehicle Concept Design Stage

The automotive industry represents a significant part of the economic activity, in Europe and globally. Common drivers are the improvement of customer satisfaction (performance, personalization, safety, comfort, brand values,) and the adherence to increasingly strict environmental and safety regulations, while at the same time reducing design and manufacturing costs and reducing the time to market. The product evolution is dominated by pushing the envelope on these conflicting demands.

Structural Reliability Analysis of a Car Front Cradle with Multiple Design Criteria

This paper aims to estimate the structural reliability of a car front cradle’s FE model. Traditional methods, such as FORM and Monte Carlo, and Adaptive Response Surface methods are assessed for their ability to accurately predict the system-level failure probability in a reasonable amount of time.

Reliability-based design of a slat track fatigue life using mesh morphing technology

Although the aerospace production process is much better controlled than in other industries, it remains true that very small manufacturing tolerances exist in the geometrical parameters such as flange thickness and hole diameters. In the current design process, the effect of this manufacturing variability on the structural durability and safety cannot be accurately assessed and is hence compensated for by applying safety factors. This is not an ideal situation, because it may lead to slightly overdesigned structures. A much more promising approach is to include probabilistic models of design variables into the mechanical simulation process. With a new methodology based on reliability analysis, engineers can obtain a better understanding of the actual effect of the manufacturing tolerances and of variability in material properties. Based on the analysis results, the robustness and reliability of the design can be assessed and improved if needed. In this paper, the aforementioned probabilistic approach is demonstrated on a slat-track structure. Measurements of different geometrical properties were collected during the manufacturing process and their variability was characterized probabilistically with statistical models. Then a reliability analysis was carried out using mesh morphing technology and fatigue life predictions with an industrial-sized finite element model of the slat track to assess the reliability of the structure in terms of fatigue life. The outcome of the analysis consists of a probabilistic model of the structural performance (e.g., fatigue life for the slat track), given the variability in the geometrical parameters. Then a reliability-based design optimization procedure was carried out to improve the design of the slat track while maintaining the same reliability of the nominal design.

Integrating Vehicle Body Concept Modelling and Flexible Multi-Body Techniques for Ride and Handling Simulations

This paper deals with the integration of a vehicle body concept modeling methodology, based on reduced models of beams, joints and panels, with flexible Multi-body (MB) representation of the chassis of a passenger car. The aim is to enable ride and handling simulations in the initial phases of the vehicle design process, where the availability of predictive Computer Aided Engineering (CAE) tools is a key factor to steer design choices such that a faster convergence of the vehicle development cycle towards improved products is achieved.The proposed approach is demonstrated on an industrial case study, involving a commercial passenger car, for which a detailed chassis and suspension model for MB simulations is developed in LMS Virtual.Lab Motion. A flexible concept model of the vehicle’s Body In White (BIW) is created as well and included in the MB model to enable fast investigations on how ride and handling performance of the full vehicle are affected by body modifications.To demonstrate the validity of the resulting concept model, a number of standard handling manoeuvres and ride excitations are simulated by using both the flexible MB model described above and a rigid MB model of the vehicle, which is derived from the same FE model. The numerical results are compared to allow assessing the influence of body flexibility on the predicted handling and ride behaviour of the vehicle.Copyright

Reliability-Based Fatigue Optimization of an Air Plane Slat Track with Respect to Manufacturing Tolerances*

Abstract Especially for the design with respect to fatigue the scattering in load conditions and durability performance has a significant influence. In the case studied in this paper we concentrate on the durability performance, and here especially on manufacturing tolerance influences. For the original design all specimen of the component have been measured, such that reliable data on the manufacturing process are available. This data now enables to estimate which tolerances are necessary for a safe durability performance but also shows that there is significant potential for reducing the weight of the component. The development from optimization methods with respect to durability to reliability based optimization methods is an important step to better and safer designs.

A Fatigue Life Reliability-based Design Optimization of a Slat Track using Mesh Morphing

Although the aerospace production process is much better controlled than the process in other industries, it remains true that very small manufacturing tolerances exist in the geometrical parameters (flange thicknesses, hole diameters,. ..). In the current design process, the effect of this manufacturing variability on the structural durability and safety cannot be accurately assessed and is hence compensated for by applying safety factors. This is not an ideal situation, as it may lead to slightly over-designed structures. A much more promising approach is to include probabilistic models of design variables into the mechanical simulation process. Then, with a new methodology based on reliability analysis, engineers can obtain a better understanding of the actual effect of the manufacturing tolerances. Based on the analysis results, the robustness and reliability of the design can be assessed and improved if needed. In this paper, the above-mentioned probabilistic approach is demonstrated on a slat track structure. Measurements of different geometrical properties have been collected during the manufacturing process and their variability has been characterized probabilistically with statistical models. Then, a reliability analysis has been carried out using morphing technology and fatigue life predictions with an industrial-sized FE model of the slat track to assess the reliability of the structure in terms of fatigue life. The outcome of the analysis consists of a probabilistic model of the fatigue life, given the variability in the geo-metrical parameters. This analysis not only provides a better insight in the effect of variability in the fatigue life prediction, but also provides sensitivity measurements of the design parameters on the final performance of the structure. These results provide guidelines to improve structural designs and manufacturing tolerances, by using a reliability-based design optimization procedure. A powerful tool is thus obtained to reduce design conservatism while maintaining and even improving structural safety.