Where Do You Get Your Loads To Assess Component Strength And Durability?

Designing high quality, reliable products is always a prime motivation for a wide variety of Industries from home appliances to land vehicles. While it is possible to use physical testing to verify durability, it is most efficiently done by applying simulation methods integral to the development process.

Here are a couple Industrial examples for you to glance over:

1. Honda – Vehicle Strength and durability

2. MABE – Designing reliable washing machines

In the past, our colleagues have written some interesting articles such as “Simulation Driven Automotive Closures” and “Analyzing Durability”. If you try to connect the dots here, you will realize that one of the most crucial aspects for predicting durability is getting accurate component loads. The challenge is that in many situations is that these cannot be directly measured experimentally.

This article gives you an insight into how accurate component loads are generated from physical or virtual tests and the different approaches in use today for vehicle durability evaluation.

Durability of chassis frame or other components

Durability is the ability of a product or a component to withstand failure due to fatigue, corrosion, wear, and creep. In most durability situations, the predominant failure mode is fatigue. Fatigue is defined as failure due to repeated cyclic loads that are well below the failure strength of the material. Therefore, avoiding premature fatigue failure is a critical element in the creation of long lasting products. Products should be validated and optimized for the dynamic loads to which they will be exposed during their operation. There is a need for durable products across all industries; this article focuses on the evaluation of the durability of the chassis of an automobile.

Figure 1: The typical process for measuring chassis durability

In conventional vehicle development, the fatigue strength of the chassis and its components is evaluated through physical testing. Since a complete set of component load and stress measurements for components is hard to obtain, field-testing is augmented with laboratory measurement and simulation.

Through coupled multi-body dynamics (MBD), finite element analysis (FEA), and fatigue simulations, the need for physical testing, is not only minimized, but available test data is used to greater benefit. Virtual durability testing is performed in two ways:

a. Semi-analytical durability evaluation

b. Analytical durability evaluation

Semi analytical durability evaluation can be condensed into five key steps:

1. Experimental measurement: The first step is to define a duty cycle for the test product. The vehicle is instrumented with wheel force transducers to capture the time history of the loads on each of the spindles. The vehicle is driven over different road conditions on the test track for varying durations and the force-time histories are collected. A duty cycle representative of real-life usage is then synthesized from the collected force-time histories.

2. MBD: The second step in the process is to perform virtual system-level testing with the duty cycle to determine component load time-histories for the duty cycle. An MBD model of the vehicle is placed on an MBD model of a test rig. The duty cycle is fed as input to virtual actuators in the virtual test rig. These apply the excitation either to the tires or directly to the spindles of the vehicle model. An MBD simulation of the combined vehicle and test rig with a synthesized duty cycle is performed. Components of interest in the MBD system model are instrumented to obtain load time histories at all the attachments on the component.

3. FEA: A unit loads FE analysis of the component is then performed. Unit loads are applied in each of the six directions at each of the attachment points. The stress or strain state for the unit loads is generated.

4. Fatigue analysis: The material model for the component is defined. The unit results from FEA are scaled with the loading obtained from MBD and a fatigue analysis of the component is performed. The fatigue analysis can be quite detailed. For instance, the fatigue life of each of the individual welds on the chassis or frame can be calculated and fatigue life at all stress concentrations can be evaluated.

5. Post-processing: The MBD, FE, and fatigue results are visualized to understand the durability behavior of the component.

This process is graphically illustrated in Figure 2.

Figure 2: The semi-analytical process for chassis durability evaluation. Experimentally measured loads are provided as input to a virtual vehicle model

Semi-analytical approaches are useful because they can be done quickly and they reduce the need for physical testing. When physical prototypes are not available, a fully analytical approach is required. Analytical approaches avoid the use of physical prototypes and experimental measurement. The prototype is tested entirely in the virtual world. The role of MBD in this scenario is to mimic the proving ground tests.

Analytical durability evaluation can be condensed into four steps:

1. System level testing: An MBD model of the vehicle is built. This system is validated first. Then it is exercised in exactly the same way one would have tested a physical model, if it were available. The virtual vehicle, outfitted with a virtual driver and high fidelity tire models is driven over a virtual road, and component load histories at each of its attachments are generated.

2. FEA: A unit loads FE analysis of the component is then performed. Unit loads are applied in each of the six directions at each of the attachment points. The stress or strain state for the unit loads is generated. This step is the same as for semi-analytical approaches.

3. Fatigue analysis: The material model for the component is defined. The unit results from FEA are scaled with the loading obtained from MBD and a fatigue analysis of the component is performed. This step is the same as for semi-analytical approaches.

4. Post-processing: The MBD, FE, and fatigue results are visualized to understand the durability behavior of the component. This step is the same as for semi-analytical approaches.

This approach is summarized in Figure 3.

Figure 3: The analytical process for chassis durability evaluation. A virtual car is driven on a virtual proving ground. Component load histories from the simulation are used to assess their fatigue life.

Analytical durability evaluation is more compute-intensive than semi-analytical approaches. Furthermore, it is hard to validate results against real experiments since physical prototypes are not available. For these two reasons, fully analytical approaches were not very common. Today, however, with the development of new computer validation techniques and faster computer systems, this approach is gaining acceptance.

The analytical approach may be used in other scenarios as well. The durability evaluation of automobile doors is an example, as illustrated in the “Simulation-Driven Automotive Closures” article. This approach is general; it also can be applied to other products.

Summary:

Simulation methods and physical testing have become an integral parts of the development process to enable organizations to create high-quality products and significantly speed their time to market. Both virtual and physical methods have evolved tremendously in the past few decades because of advances in hardware and software technology. HyperWorks offers a comprehensive set of tools for multi-body dynamics, finite element analysis and fatigue that can be used to virtually test and improve Industrial applications.

References:

NAFEMS Publication – Why do Multi-body System Simulation?

Keshav Sundaresh

Keshav Sundaresh

Global Business Development Director at Altair
Keshav is a Global Business Development Director and has been with Altair since 2006. He’s responsible for providing corporate business development leadership and direction to Altair’s Math & Systems solution.In this role, he is responsible for driving partnerships by teaming with product development, global sales & channels, marketing and customers. Based in Altair’s world HQ in Troy, MI, Keshav closely works with the software development team. Prior to this, Keshav has held several different roles at Altair including his last role as a global business development manager responsible for Altair’s Multi-body Simulation solution. Before starting his journey with Altair, he worked as a CAE Analyst at a machine tool OEM. Keshav holds a Bachelor of Engineering degree in Mechanical Engineering from India.
Keshav Sundaresh
Keshav Sundaresh

About Keshav Sundaresh

Keshav is a Global Business Development Director and has been with Altair since 2006. He’s responsible for providing corporate business development leadership and direction to Altair’s Math & Systems solution. In this role, he is responsible for driving partnerships by teaming with product development, global sales & channels, marketing and customers. Based in Altair’s world HQ in Troy, MI, Keshav closely works with the software development team. Prior to this, Keshav has held several different roles at Altair including his last role as a global business development manager responsible for Altair’s Multi-body Simulation solution. Before starting his journey with Altair, he worked as a CAE Analyst at a machine tool OEM. Keshav holds a Bachelor of Engineering degree in Mechanical Engineering from India.