Fretting tests on connector terminals

Fretting is a complex set of mechanical and electrical mechanics that occurs at the contact surface between two vibrating objects. It is widely present in electrical connectors close to combustion engines due to the constant exposure to vibrations. Fretting is an extensive problem in the automotive industry but little research is done. In this thesis, a connector attached to a high pressure fuel pump that is present in several cars made by Volvo Car Corporation is analysed. The goal is to investigate the design properties of the connector and evaluate if laser vibrometry is a valid method for connector vibration measurement. Fretting has previously been observed to correlate with the relative displacement between the male and female connector terminals. Accelerometers are widely used to measure acceleration and displacement of various components of the engine, but to measure directly on the terminals inside the connector is impossible. To instead use lasers to measure the relative displacement could prove to be an effective way of predicting fretting damage. The connector is analysed in a vibration laboratory where it and the pump it is connected to is mounted to a fixture. This way the vibration schedule can be designed and controlled with accuracy. To have a frame of reference when designing and analysing the results of this test, a real engine vibration test is first analysed. In this test, the connector is attached to the engine in the same way as in production cars. The engine is then run from idle to full throttle and the vibrations in the connector is measured. It is found that the measurement of the terminal relative displacement is much more accurate when using lasers instead of accelerometers. The result of using the accelerometer showed a misleading result. The relative displacement was up to five times higher and three times lower than the true displacement, measured with the lasers. A method to predict fretting damage can be developed using laser vibrometry. The thesis also present suggestions on details in the connectors that are beneficial from a fretting point of view.Electrical connectors not connecting Modern cars move to electrify more and more systems in the car. This puts higher demands on the electrical distribution systems. Vibrations originating from the engine can cause damage to components in the car, slowly leading to failure. One big contributor to this damage is referred to as fretting. Fretting is a collection of phenomena caused by microscopic vibrations and often affect electrical components. This thesis work explored how fretting can be discovered and handled before any cars break down. Everybody knows that combustion engines vibrate, creating noise inside the car. It is less known that they also generate microscopic vibrations that cause a lot of problems for the components in the car. One of these problems is called fretting and it occurs where current has to travel between two surfaces. These high frequency vibrations from the engine cause damage to the surfaces, eliminating their conductivity which ultimately results in a failed connection. Higher effectiveness of combustion engines and lower voltage in electrical distribution systems of modern cars can lead to fatal connector degradation. It might take years before a degrading connector fails, causing safety risks and forcing reparations. Fretting is a relatively common but complex problem involving material science, contact mechanics, fatigue mechanics, tribology and corrosion science. It is estimated that fretting damage in connectors contributes to up to 35% , of all electrical problems in a car. Despite fretting being a considerable problem in the automotive industry, very little research has been made. With more and more systems being electrified, involving breaks and steering, a failed electrical system could be catastrophic. For example, in 2016, a modern car had a total electrical failure which caused the death of a nearby car driver as he tried to steer clear. In this thesis, progress on how to predict fretting is made. One of the main questions is if lasers can be used to measure the vibrations inside the connector. If it is, is possible to get a more accurate measurement of the movement of the connector when compared to accelerometers? It is found that the measurements with the lasers are much more accurate. The relative displacement between the male and female part of the connector is up to five times higher and three times lower than the true displacement, measured with the lasers. By implementing laser measurement when analysing new connectors, it could be easier to identify if the connector will suffer from fretting. Further, the structure of the connector is analysed in detail. Dangerous frequencies which cause resonance in the structure were identified and evaluated. These resonance frequencies cause large displacement vibrations, which are the underlying cause of fretting. How the mass and stiffness can affect these resonance frequencies is explored. It is determined that the connector withstands fretting worse when the connector is glued in place. A connector with several parts connected with springs is more resilient to fretting. The terminals in the connector are then less affected by the large displacement of the outer parts of the connector. A 15 minute presentation of my thesis can be found here: https://youtu.be/JdrCyPyWw-I 1 J. Swingler and J. W. McBride, “The synergistic relationship of stressesin the automotive connector,” in Proc. 19th Int. Conf. Electric ContactPhenom., Nuremburg, Germany, 1998, pp. 141–145. 2 T. Gissila, Connectors and Vibrations – Damages in Different Electrical Environments. 2013