BOA optimizes exhaust flex-joints to market faster and reduces design risk

BOA specializes in the engineering and production of flex-joints for exhaust emission systems that decouple the engine motion from the exhaust structure. Exhaust flexjoints need to be custom engineered for each application in order to balance their static offset and dynamic motion, and ensure they meet fatigue life requirements for any automotive, commercial vehicle and off-road application. The data required for this custom design process is captured during road load data acquisition (RLDA) at a proving ground. In the past, several steps were required to analyze the data and it was necessary to manage every single step of this complex process using different software products which slowed the decision-making process. BOA has streamlined its engineering process by implementing nCode GlyphWorks. GlyphWorks automates most of the test data analysis process and eliminates the need to convert file formats, thereby reducing the possibility of having to redo the analysis due to input errors. With some of the data analysis now completed on-site, soon after RLDA, preliminary engineering decisions can be made on the spot and the RLDA can be concluded with confidence that additional data will not need to be collected later.

BOA Group is a leading system developer and supplier of flexible element technology, bellows, hoses and decoupling devices worldwide. Its customers include almost all European and North American, and many Asian manufacturers of passenger cars and commercial vehicles. BOA pioneered the multi-ply bellows design which absorbs thermal expansion and vibration in engine and compressor piping systems. The multi-ply bellows is manufactured from a laminated tube that consists of thin gaugestainless steel plies. This tubular body is formed into corrugations by a hydroforming process that delivers close tolerances. The use of thin gauge material combined with a large number of corrugations per unit length reduces deflection forces acting on and increases the flexibility of the bellows

Importance of physical testing

BOA creates custom mechanical bellows designs to meet the requirements of specific automotive, on-road and off-road applications for exhaust, on-engine and intercooler systems. The company uses finite element analysis software to simulate the ability of proposed bellows designs to decouple the engine motion from the rest of the system. The company then builds a prototype of the proposed design that the customer installs on a vehicle for testing at the proving grounds, capturing data from accelerometers, strain gauges and displacement sensors.

The primary purpose of physical testing for BOA is to validate the critical tradeoff between static manufacturing/assembly offset and dynamic range. The exhaust flex joint has a certain amount of flexibility, or dynamic range, which is needed to accommodate the relative motion of the engine and the exhaust system. However, the initial variation in alignment between the engine and exhaust system, called the static offset, takes up a certain amount of that dynamic range. Physical testing is needed to determine the dynamic range required by the vehicle’s operating envelope to ensure it is within the flex hose’s dynamic range.

Physical testing is also performed to determine the fatigue life of the flex joint on this particular vehicle. The required life of the flex joint might be 150,000 miles, yet there is rarely time to drive the prototype for this full distance. In any case, the life of the flex joint is determined not by how many miles the vehicle is driven but by the number and magnitude of severe loads it receives, such as when the vehicle goes over a pothole. The amount of damage produced by these events is estimated by driving a prototype with the flex joint installed over potholes, Belgian blocks, or other hazards designed to generate severe loads on the flex joint. The loads from each event are recorded by the sensors.

The damaging events are converted to stresses and strains. The vehicle OEM determines a duty cycle that defines how many of each of these damaging events the vehicle is expected to see during its warranty life. The test data is compressed and accelerated to generate a damage profile that corresponds to the OEM’s duty cycle and is used as input for a fatigue analysis that predicts the life of the flex joint.

Optimizing the design

BOA engineers take advantage of GlyphWorks’ capability to utilize test data to perform designed experiments to evaluate fatigue life as a function of design parameters such as the diameter of the flex hose. This approach is performed with DesignLife and can be used to improve the existing design without having to run additional RLDA. BOA engineers have also written scripts that evaluate each potential design combination for manufacturability so only designs that can actually be built are evaluated. 

Engineers have developed a custom process within GlyphWorks to generate accelerated drive files for a 6 DOF simulation used to test the part without having to install it in a prototype vehicle and run it on the proving ground. This approach can be used after RLDA is performed on a vehicle to evaluate other flex hose designs without having to re-run the RLDA.

High frequency data acquisition is also performed using a microphone inside the passenger cabin to evaluate the impact of the flex joint on noise/vibration/harshness (NVH). BOA uses GlyphWorks to process the data and perform order analysis, which involves matching the accelerometer data to the speed of the motor and makes the data much easier to interpret.

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