Product design in today’s environment requires careful attention to all aspects of the design, including such issues as sustainability, recyclability and durability of the product. Durability of ground vehicles can be particularly challenging because of the way the vehicle is used and the roads on which the vehicle is driven and the product design can change significantly during both the design phase and the product lifecycle. Testing the product is the most common way to verify durability, but testing is also expensive and time-consuming. Virtual durability testing methods in today’s computing environment offer rapid, repeatable predictions of the loads a vehicle will experience and the resulting life that the product will have when put into service.
Simulating, analyzing and predicting durability and fatigue are aspects of a complex process made up of many different pieces and can be approached using a variety of methods. The reality is that simulating the overall durability process is typically defined by an organization’s access to the necessary data and simulation tools. This article will focus on the various components of durability analysis, with respect to vehicles that travel our roads, and how the challenge of vehicle durability is being addressed.
Vehicle Durability Factors
A modern passenger vehicle is expected to last at least 100,000 miles in the U.S. market and may see loads repeated 1 million times or more during its life. An extreme load, such as driving through a deep pothole with the brakes locked, will be repeated only 10 or 20 times during the life of the vehicle. A typical field-loads measurement group will instrument and measure customer-owned vehicles going through standard-use events. This data will be analyzed using multiple methods to break it into small customer-use samples that will be extrapolated so they represent the entire population of customers. The engineering team decides the percentile customer to which they wish to design or test, and targets for different subsystems will be developed using the data.
The proving grounds team will use the customer data to develop correlated events to repeat the customer-use cases in a compressed schedule at the proving grounds. International teams will correlate schedules at various proving grounds so they have the option of testing at any location. The proving grounds schedule will add events, such as car washes and tire changes, to reproduce the customer experience and also remove non-damaging drive time from the schedule to significantly compress the time required to test the vehicle.
Damage plots (Figure 1) are commonly used to track the damage on a vehicle. The damage plot shown in the chart below compares the damage in two proving ground schedules for busses. The damage shows that the bus will see different loads when run at the two different schedules. Typically a bus will be developed to pass both schedules.
Early in a vehicle program the weight of the vehicle is set and the resulting loads are predicted. Early load predictions are either historically based or estimated with g-loads. Loads also can be predicted using road surfaces, tire models and early design multi-body dynamic models. During the vehicle development cycle, load predictions must be constantly updated to respond to design changes, consumer-use changes, and other program-related decisions. When high loads are found, it is desirable to use the load prediction process to reduce the loads. There is a significant amount of variability in any load, both measured and predicted, because of the effects of the operator.
The tire patch is the only means by which forces are transferred from the road to the vehicle. This patch supports the vehicle, provides directional control and provides acceleration and braking to the vehicle.
Modeling the physics of a tire is typically done by a combination of a complex tire model (such as the Pacejka model, the Cosin Ftire model, or the RMODK model) combined with actual vehicle testing to validate and tune the model.
Simple tire models are made up of a series of non-linear equations that are used as “lookup” values during simulations early in the design phase, in combination with road surface data. More complex models incorporate both the physics of the tire belt and sidewall construction, an enveloping tire-to-road surface model and the volumetric change of the captured volume of the tire to simulate the tire forces. As a vehicle program evolves, the tire construction may change and either increase or decrease loads going into the vehicle.
Finite Element Models
Finite element models are employed to understand the stresses in a component using a discretized mesh that accurately represents the geometry and material properties of the component. A set of loads is then required to excite the model. There are many load cases that can be applied to a component to represent worst-case loading for different events, such as maximum acceleration and maximum braking. Observing the stress location, the level of the stress and the resulting deflection of the component during the load event gives the engineer information that can be used to reduce stress by modifying the part shape, using a different material, or reducing cost by taking material out of the component. This model provides valuable insight into the component performance that is not available from lab or proving grounds tests.
The concept of fatigue was originally developed by August Wohler while studying train axles in the 1860s. The understanding of fatigue mechanisms in materials and the process for analyzing them have improved immensely since then. While modern fatigue testing and methods for predicting material fatigue are performed using precision test equipment, most fatigue predictions today still rely on measured data.
A typical fatigue curve for steel is shown below. The fatigue data is plotted versus the number of reversals. There are several things the user needs to keep in mind when using these curves:
1) The fatigue curve represents the number of cycles required to develop a 2 mm-long crack in the material. The component may still maintain load with a 2 mm crack in it. The 2 mm figure is an arbitrary criterion but is representative of a crack that can be detected with common commercial fatigue inspection methods and a detailed inspection.
2) The fatigue test specimen is typically a hand-selected specimen with a finely ground exterior surface. The finely ground exterior surface represents the upper limit of fatigue life. Forming processes, such as hot or cold rolling, will reduce the fatigue life significantly. These effects should be taken into account during any analysis performed on a part. The factors are part engineering and part the art of predicting fatigue.
Many structures are designed so the stress level never is high enough to develop a fatigue crack. Many structures and products do not see enough stress to develop a crack. In the automotive industry, structures used in internal combustion engines require fatigue analysis because of the number of cycles the engine runs, which can easily exceed 10 million cycles.
The finite element model can be used in conjunction with optimization software, such as Altair OptiStruct to develop the lightest and strongest component that achieves the stress targets required to meet the program objectives. A single load case can be optimized or a set of load cases can be applied to the model so the component will meet multiple load-case criteria. A wide number of methods and techniques can be used to arrive at an optimized part, and Altair engineers doing product design will typically explore the optimization space broadly by running different optimization algorithms and understanding the effects of adding or subtracting material from the design to see the robustness of the design. Virtual optimization of components is generally faster and more efficient and arrives at a lighter design than manual optimization.
Durability Director is Altair Engineering’s solution to manage the entire durability-prediction process. Durability Director is an open solution for durability that leverages different portions of the HyperWorks suite and has a flexible architecture enabling the employment of additional tools, such as the products within the HyperWorks Partner Alliance and additional third-party products. While Durability Director supports three different process flows, the typical process requires measured loads, a completed multi-body model, a finite element model, fatigue properties and a defined schedule to define the number of occurrences in the test schedule. This is not a trivial set of requirements.
Measured loads typically come from a proving ground or a laboratory test schedule. The loads can be forces or can be displacements, velocities or accelerations applied to a multi-body model to load the vehicle or component of interest. Data is typically acquired at high frequency (512 data points per second) for the entire test schedule. Individual routes that are repeated in a test schedule can be combined mathematically to represent the entire test schedule.
The duty cycle represents the number of times a vehicle drives down discreet routes. Duty cycles are defined as part of a test schedule and are created so the damage created at the proving grounds also represents the damage seen by the customer during the life of the vehicle.
The finite element model needs to be completely developed and populated with all of the material properties. Locations to apply loads at boundary conditions need to be defined in the model, and a check run of both a simple stress model and a normal modes run should be performed to validate the model.
Fatigue properties of the material of interest are required to predict the fatigue life. Surface conditions also need to be defined on the model either through the Durability Director interface or in the finite element model. Durability Director supports strain-life and stress-life methods for predicting fatigue life.
The solvers used to perform the simulation can be part of the HyperWorks suite, Alliance Partner products or third-party applications. A multi-body solver, finite element, and a fatigue solver are required to complete the fatigue life prediction process. The open architecture of HyperWorks enables multiple solvers to be supported, allowing a common process to exist while supporting an individual company’s choice of solvers. This is a unique feature of the HyperWorks suite.
Durability Director provides a streamlined method for re-submitting the complete simulation process. This capability enables the study and analysis of various changes and adjustments on a vehicle’s durability. A new finite element model, representing a new design or a competitive design, can be selected and a new life prediction can be run using the fingerprinting mode. Also, the entire durability schedule can be modified, and changes can be made in components in the multi-body (such as bushing rates or stabilizer bar diameters), and the entire process can be rerun to reflect design changes or just to understand the variation of the part component to different content in the vehicle build (such as a sport suspension).
Compute Cluster or Cloud Use
The Durability Director is compute-cluster enabled using compute-load management interfaces within HyperWorks. This feature allows for easy submission of jobs that may have high memory requirements. In addition, it provides easy management of a set of runs that simulates entire proving ground schedules which can easily include 60 different road schedules, and can require the simulation of over an hour of real-time durability-schedule driving simulation.
Vehicle Durability Solved
The complete vehicle durability problem can be analyzed using Durability Director within the HyperWorks suite. Durability director employs the HyperWorks tools to manage all of the different stages of durability. These include:
- Multi-body solution: To predict early loads and chassis loads employing Ftire and a road surface, or measured loads from wheel force transducer systems.
- Finite element pre-processing and solver solution: To create finite element models and predict stress or fatigue-life of a component.
- Post-processing solution: To visualize and streamline automation of results analysis for multiple CAE solvers.
- Open architecture: To enables fast and easy export of stress and load results to fatigue codes for advanced fatigue life prediction.
- Process automation solution: To automate the durability prediction process and makes it simple to perform multiple iterations of the durability process to fine-tune a model.
There are many challenges involved in ensuring that a vehicle can withstand the expected loading requirements over its lifetime. HyperWorks and Durability Director represent a comprehensive and streamlined solution that guides organizations through the complex process of predicting fatigue and improving vehicle design to meet today’s durability targets.