Experimental analysis of mechanical joints strength by means of energy dissipation

Designing complex structures with the demand for weight reduction leads directly to a multi-material concept. This mixture has to be joined securely and welding, mechanical joining and the usage of adhesives are commonly used for that purpose. Sometimes also a mix of at least two materials is useful to combine the individual advantages. The challenge is the non-destructive testing of these connections because destructive testing requires a lot of preparation and expensive testing equipment. The authors show a testing method by measuring and analysing the energy dissipation in mechanical joints. Known methods are radiography, thermography and ultrasound testing. Unfortunately, the usage of these methods is difficult and often not usable in fibre-reinforced-plastics. The presented approach measures the propagation of the elastic strain wave through the joint. A defined impact strain is detected with by strain-gauges whereby the transmitter is located on one side of the joint and the receiver on the other, respectively. Because of different mechanisms, energy dissipates by passing the joint areas. Main reasons are damping caused by friction and material specific damping. Insufficient performed joints lead to an effect especially in the friction damping. By the measurement of the different strains and the resulting energy loss a statement to the connection quality is given. The possible defect during the execution of the joint can be identified by the energy loss and strain vs. time curve. After the description of the method, the authors present the results of energy dissipation measurements at a bolted assembly with different locking torques. By the adjustable tightening torques for the screw connections easily a variation of the contact pressure can be applied and analysed afterwards. The outlook will give a statement for the usability for other mechanical joints and fibre-reinforced-plastics.