Vehicles must undergo numerous tests in order to ensure that the body, axles and brakes are actually able to withstand high amounts of stress every day.Many drivers take it for granted that the body, brakes and drive of their vehicles are all in perfect condition and able to take everything in their stride. However, a great deal of know-how and technology is needed for a vehicle and its components to be able to withstand the high stresses to which they are exposed each day. For this reason, they are all tested to their limits during the course of numerous trials.
Every form of transport, whether a train, airplane or car, is exposed to enormous stresses each day. However, even in the most difficult of conditions, the body, axles, brakes and drive of a car need to function smoothly. The characteristic that allows them to do this is engineering strength, i.e. the ability of a design or component to withstand certain loads without damage during vehicle operation. It is not only decisive for the safety of the driver but also for the safety of all other road users.
When in use, a vehicle is exposed to different types of intensive stress, including operating loads, extraordinary events and misuse. Operating loads are loads that continually arise during vehicle operation, while extraordinary events, such as driving over a deep pothole, are low-frequency events that should not impair functionality, lead to damage that might impair safety or reduce the envisaged service life of a vehicle. Vehicle mishandling on the other hand occurs, for example, when it is driven over a large obstacle and damage to it becomes inevitable. What is important for the driver is that he be able to recognize the damage immediately, e.g. a bent steering wheel. A vehicle's components and systems therefore need to be designed in such a way that damage is clearly evident when a vehicle is no longer roadworthy.
Which factors, however, influence the engineering strength of a vehicle? Besides everyday stresses, the geometry, materials and manufacturing procedures used to produce a component too naturally play an important role. Moreover, environmental factors such as temperature and humidity are also highly influential. As Prof. Dr. Rudolf C. Stauber, the departmental manager of the engineering strength and materials department at the BMW Group, explains: "Lightweight construction, optimal use of materials, quality assurance and many other factors have significantly increased the requirements for engineering strength. Moreover, the importance of the predicted service life of a vehicle has also increased considerably in recent years." Added to this is the constantly increasing number of vehicle types as well their growing complexity, coupled with shorter development times.
When developing vehicles, the ever-lighter components are increasingly being taken to their limits. Stauber adds: "The global use and selling of vehicles is a challenge too. After all, we build our vehicles not just for the German market, but also for customers across the entire world. This means that vehicles need to have the correct degree of engineering strength for all road conditions - in China, South America and Europe."
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Eager to experiment
The engineering strength development process is divided up into design and safety processes. While design loads are set in the design phase, the priority during the subsequent safety phase is the experimental testing of prototypes and development assemblies.
During the early design phase, numerical simulation plays an important role. As part of the virtual road load measurement process, a vehicle is driven over measured and digitalized real road sections, allowing maximum vehicle loads to be determined at a very early stage. The derived wheel strengths are then supplemented with measurements from the predecessor model. As soon as the designs of the first axles become available during the design phase and the rigidity of the body is determined, a special "vehicle load analysis vehicle" (FABEAN) is assembled. Fitted out with corresponding trim weights and various different engines and axles, FABEAN can simulate either a BMW 7 Series model or a MINI, and this long before the first prototypes become available. FABEAN determines the stresses that arise during a journey, and in addition verifies the calculated load forecasts. Both the component design requirements and the drive data for the engineering strength trials to be carried out on the test benches are then derived on the basis of this measuring data.
The use of numerical simulation means that data becomes available at a very early stage for forecasting whether and how long components will withstand a certain stress. As a result, there is no need for time- and cost-intensive trial series.
Particularly important for body durability are welding points. Highly stressed welding points, so-called hot-spots, need to be identified during the development process and relieved through improved design. In order to be able to quickly and reliably evaluate the approximately 5,500 welding points of a vehicle, the BMW Group uses a method that combines automatically generated welding-points models with an exact evaluation procedure. The result is high forecast accuracy and a significant increase in forecast speed.
The authorization of safety components, however, is always subject to the testing of their engineering strength in actual trials. These trials require special system and component test benches that can replicate the complex load conditions of road loads. Using special iteration software, the steering of the test benches is progressively optimized until the loads generated in the axle and body components equal those of the vehicle. An example of a complex system test bench is the multi-component test bench for vehicle bodies. As Professor Stauber explains: "Using specific steering programs, the test bench, in approximately three weeks, generates loads that correspond to a normal road load of more than 300,000 kilometers."
Only when these trials provide the evidence that all the components and systems fulfil the engineering strength requirements is it recommended that they be authorized for customer use. Final confirmation comes following total vehicle trials on various trial roads.
The BMW Group has been focusing on the area of engineering strength for the past 40 years. "Naturally a lot has changed during this time, and with cars becoming increasingly lighter and continuous technological improvements, engineering strength too has increased in importance and will remain one of the most important areas within vehicle development in the future," says Stauber.
New materials and requirements with regard to load capacity and lightweight construction, combined with quicker development times mean that engineers will continue to face new and exciting challenges. The importance of engineering strength trials for vehicle development is therefore set to increase further.
