Volvo Car Corporation uses nCode DesignLife to predict fatigue life performance

Fatigue life prediction at Volvo Cars

When a new car is developed, all components and assemblies have to be tested for fatigue life, first virtually and later through physical testing, to be approved for production. At Volvo Cars, the CAE analysis teams work closely with testing and design teams to keep required development iterations to a minimum.

“The most complex fatigue analysis we have is the body rig test,” says Sällström. “To physically test the body structure a complete car is taken to the test track and driven a few laps to measure accelerations and wheel forces. The car is then brought to the test rig. The loads applied in the rig are iterated until they give the same accelerations and forces as on the test track. The load cycles are then repeated to correspond to a specified number of laps on the test track.”

At the beginning of a new car development project, the durability CAE analysis team receives CAD data along with other information about the components from the design teams. With this input, the CAE engineers create a multi-body simulation model of the car, which is then analyzed on a virtual test track. The results of the multi-body analysis include time history loads at the body attachment points, which are used for the fatigue life prediction with nCode DesignLife.
During a car development project, Volvo Cars has a number of CAD model freezes, each of which is used by the CAE team to create CAE models. A dynamic finite element (FE) analysis of the car body structure is performed to find the eigenfrequencies and eigenmodes of the structure. The FE-model usually consists of around 5,000,000 elements and includes 5,000 spot welds, as well as arc welds, laser welds, weld nuts, and other connections. Body trim elements such as the radiator, seats, doors, fuel tank, etc., are also included. The eigenfrequencies and eigenmodes are needed together with the time history loads to conduct the fatigue life prediction with nCode DesignLife.

If the fatigue analysis indicates a place where a crack might occur, the CAE engineers give this input to the design teams. Together, design and CAE engineers discuss how to improve the body design by changing the geometry or just a radius, adding or moving spot welds, or using a different material or material thickness. If the fatigue analysis indicates very long fatigue life, this information can be used to reduce the weight of the car. nCode DesignLife offers sophisticated spot weld and seam weld analysis capabilities, which are widely used by the fatigue engineers at Volvo Cars, together with the strain life and stress life modules.

Depending on whether a new car is created on an entirely new car platform, or if an existing platform is used, the engineers might simulate hundreds of runs of the body rig during the full car development project.

This visual shows the correlation between CAE and physical testing. A fatigue crack is predicted in CAE using nCode DesignLife (left). The same fatigue crack is found in the physical correlation test (right).

With increased analyzing capability, the number of components that are evaluated for fatigue life by the CAE team has dramatically increased. As a result of CAE tools such as nCode DesignLife, many more components can be analyzed virtually in the same time frame, offering a more complete picture and an overall better quality of the car. When a physical car is finally built, the design is much more mature than in the past.

Sällström sums up the advantages of this virtual approach, saying, “With nCode DesignLife and today’s development process, we have our answers much faster than in the past, and they are more accurate. We get better designs faster, and can reduce the overall development time for the car. In addition, nCode DesignLife enables the repeatability of the process, which is very important when identifying critical areas.”

In the future, Volvo Cars’ CAE durability team expects to need even fewer physical tests, and to use those only for final validation of the CAE analyses, reducing overall lead time even more. One challenge will be finding the right balance between the speed and accuracy of the analyses, depending on how much detail is needed in each development phase. And, as the automotive industry continues its quest for lightweight cars with long fatigue life, new materials and new types of joints will provide new challenges for the CAE team. Increasing efficiency in modeling, so that the same model can be used for different analyses, will be a key to meeting these goals, and Volvo Cars’ CAE team is on this road to success.