Every car owner expects their vehicle to be reliable.
Unlike other performance criteria like dynamics, comfort or acoustics that can be directly experienced by the end-user, vehicle durability issues are perceived to occur only after long usage and a heavy odometer reading. This implies that creating a car brand known for long-lasting vehicles is a very hard and lengthy process. Conversely, the brand’s image for durability can quickly be destroyed by a few recalls.
Increasing a vehicle structure’s durability performance by material reinforcement comes at the cost of increased weight (and fuel consumption), which leads to higher carbon dioxide (CO2) emissions and more expensive usage. Other design updates improving durability performance also affect other vehicle performance attributes. Therefore, designing a durable car is a multi-attribute optimization process. Weight and durability performance of structural components must be balanced, while being as cost effective as possible, and not negatively impact other performance attributes.
Using accurate and efficient simulation in the early stage of the vehicle design process to predict and optimize vehicle durability is crucial because that is when there is a wide range of design freedom at affordable cost.
The earlier the better: creating models before prototypes exist and optimizing performance on these models is much cheaper and faster than applying fixes on the existing, physical vehicle, in particular when it is already released on the market.
The Simcenter™ software portfolio includes all the solutions to cover the standard durability engineering process in a vehicle development scenario (figure 1). It all starts with an understanding of the loads that products will undergo during their anticipated lifetime. Strength and durability-specific characteristics are analyzed and a target is derived. Such data typically serves as input for the virtual and physical product validation and optimization process. The last two steps in the durability development process involve optimization and validation of the physical prototype before testing the final product on the road.
Simcenter™ 3D software is a comprehensive, fully integrated 3D CAE solution with connections to design, 1D simulation, test and data management. It contains a central platform used to model, perform and evaluate multidiscipline simulations, thanks to the integration of fast and accurate solvers powering structural, acoustics, durability, flow, thermal, motion, materials, electromagnetic as well as optimization and multiphysics simulation.
Figure 1: Durability engineering process in a vehicle development scenario.
Automotive durability simulation challenges
To efficiently perform early durability simulations, CAE engineers face well-identified challenges and need access to various data and use a range of tools:
• First, accurate road load data must be available. Durability loads are mostly coming from the road, transferred through the tires, the wheel’s spindles, and then to the chassis and all of the vehicle’s structural components. The main challenge is to have accurate load data to account for both critical scenarios (like paved roads, potholes and misuses) and more usual ones (smooth driving on highways), while the vehicle prototype does not exist yet. They can either come from measurements done on previous vehicles, or from simulations done on digital roads.
• Then, the CAE engineer will need tools for load data prediction at the component or subsystem level as an input to durability simulations and optimization.
• Access to up to date computer-aided design (CAD) data and an accelerated CAD-to-CAE process are necessary. In general, designers are not working in the same department as CAE analysts. They typically update the designs of their components daily to answer update requests from the manufacturing department or from another CAE analyst. Durability results typically come rather late, when the design has already been updated several times, so that less design freedom remains (as parts of the design may have been frozen for other attributes, and/or tooling has been started such that some changes are no longer allowed from the manufacturing perspective).
• Standard fatigue simulation methods can be different from one company to another and can change with time. They must be implemented in the durability solution and easily accessible. Moreover, due to the increasing complexity of vehicle assemblies and technologies, the variety of materials types, connections strategies, manufacturing processes, and old and well-known durability methods are not always enough. New advanced fatigue methods are necessary.
• Finally, fatigue-specific postprocessing tools must be available for CAE specialists to thoroughly analyze, visualize and interpret the results, and derive design updates. This drastically benefits the added value of the engineers on the final product.
Durability 3D CAE models preparation
Access to up-to-date CAD data and acceleration of the CAD-to-CAE process
In a typical CAE process, designers share CAD files with CAE analysts in a shared folder or an e-mail. The CAE analyst needs time to integrate the CAD in the CAE model, prepare and run the simulation and postprocess the results.
Figure 2: Typical 3D CAE durability process, from road load data to strength and durability assessment.
In the meantime, the designer updates the CAD several times based on feedback from other departments (analysts, manufacturing, program management, etc.).
The results of the CAE analysis quickly become obsolete. The entire CAD-to-CAE process must be accelerated to avoid this situation.
This is why Siemens Digital Industries Software proposes, as part of its Simcenter 3D portfolio:
• A strong CAD integration, giving full access to CAD items, from a Siemens platform (NX™ software) or third-party ones
• Teamcenter for Simulation as a simulation-data management solution that automatically associates the most up-to-date CAD part to CAE analyses, and then link CAE results to CAD parts
• Automation and customization capabilities with NX Open to accelerate the CAD-to-CAE process and adapt it to customer needs
• Efficient and accurate durability analyses and postprocessing
Road load data prediction
In a “real-life” vehicle durability scenario, complex loads act on components and subsystems. These loads can only be measured when prototypes are produced. Therefore, a comprehensive digital twin is necessary to get the correct loads on the full system. This can be used to transfer the loads to the components.
The durability engineers first need to define the loads that will act on the whole vehicle, thus strongly influencing its lifetime. This depends on the type of vehicle, the driver and the type of roads on which the vehicle will be used. This definition helps define a test campaign or extract existing testing data on existing vehicles. The loads on new components are then simulated and the design of these components optimized to meet multiple performance targets.
Using the comprehensive digital twin, all you need to have is the test track and the driving maneuvers to get loads on the complete vehicle, on subsystems and components. The load schedules on a test rig can be tailored and optimized to the individual subsystems and components.
There are two typical approaches for performing this road load data prediction: the hybrid approach and the digital road approach.
Hybrid approach
The hybrid test/CAE approach combines tests with simulation. It allows engineers to apply measurements taken on test tracks and transfer this data to the CAE environment in a consistent and pragmatic way. This method provides a realistic road-load prediction, even for loads that cannot be measured.
More precisely, the input is the measurement of wheel spindle forces on existing vehicles applied to the multi-body simulation model of the future vehicle. The model calculates spindle displacements for the new vehicle, from which loads on components and subsystems needed for durability CAE analyses are derived. One of the main benefits of this solution is that the model does not need to include advanced models for tires, the driver and the digital road as the loads are coming from real measurements done on predecessor vehicles.
Simcenter 3D Motion-TWR from Siemens is a unique, integrated application that captures the entire road load prediction process.
This application is not limited to full vehicles. Data from sensors used in test track measurements, including wheel force transducers, accelerations, strain gauges and dis-placements, are accepted. The loads on subsystems and components needed for durability CAE analyses can be back calculated from this type of data as well.
Figure 3: Hybrid test-CAE approach.
Digital road approach
The digital road approach is the method of choice when test data is unavailable. The road profile is measured and applied to the CAE model as a virtual road. The driving maneuvers are applied to the virtual vehicle, rolling on the virtual road.
While the hybrid approach needs measurements on a similar vehicle, the digital road approach needs some investments in the road measurements, an accurate tire model, a driver model and a good knowledge of driving maneuvers.
Definition of complex load events in the durability analysis
Simcenter 3D Specialist Durability handles a variety of complex time-based and frequency-based load events (figure 4).
Figure 4: Load events in Simcenter 3D Specialist Durability
Load events can directly use Simcenter 3D Motion results coming from the load data prediction phase described above. Stress results from Simcenter 3D Motion flexible bodies can be analyzed for durability performance with the flexible body events. Hundreds of measured or simulated load histories can be combined using superposition events, and duty cycles allow the designer to create an ordered list of events, including repetition factors.
In some cases, durability engineers need to work on a more reduced scope, for example, if the full vehicle multi-body model, or testing data and digital road data, do not exist yet. Applied loads must still be sufficiently realistic to make the durability results valuable. Applying random events is a way to account for road load data when it is not available from tests or digital road analyses. These events are best represented in frequency domain, with power spectral density (PSD) functions.
Of course, basic load events like simple block loads or transient events are also available. Transient events can be defined directly by importing the transient load history data or by writing it.
Figure 5: Digital road approach.
The comprehensive digital twin concept proposed by Simcenter 3D enables a highly accurate and efficient end-to-end 3D CAE durability process in an integrated environment.
It all starts with getting access to road load data and predicting loads at components levels thanks to a hybrid approach that combines intelligent test data processing and a direct link with multibody simulation. Another option is to directly run multibody simulation from a digital road, virtual driver and accurate tire models.
The durability CAE model is efficiently created thanks to a fully integrated and managed environment that keeps simulation models in sync with the latest design revision. Durability engineers can then select the analysis parameters they prefer, either among a very extensive library available in the standard product, or by easily implementing their own methods. They can also create templates to make durability analyses more accessible to a wider community that may include CAE generalists or design engineers.
An efficient, industry-proven durability solver is integrated in the solution, including best-in-class solutions for component fatigue and connections fatigue.
A range of advanced methodologies validated in the context of research and technology development (RTD) projects with leading academia and industrial end-users delivers innovative solutions in terms of materials engineering.
Finally, postprocessing allows you to identify critical areas, load events and situations to improve the vehicle (component) structure in terms of durability performance while reducing the cost of tests.
With Simcenter 3D Specialist Durability, the durability targets can be efficiently balanced with other attributes, like vehicle weight, noise, vibration and harshness (NVH), comfort, stiffness and safety, all before the first vehicle prototype has been created.
Credit: Siemens Digital Industries Software
