Damage accumulation in a press-fit contact under mechanical loadingWednesday (26.09.2018) 11:00 - 11:15 S1/01 - A02 Part of:
Press-fit contacts are a vital part of mechatronic devices in which large currents occur. The contacts consist of a pin, which is pressed into a shaft in a printed circuit board. An impermeable cold welding zone consisting of a dissolution phase of Cu and-Zn develops between the shaft in the board and the pin due to contact forces. This welding zone provides the electric contact between the board and the pin. Any mechanical damage in this zone increases the resistivity of the device with ensuing deterioration of the electric performance of the mechatronic device. Mechanical characterization of contact zone is vital for the life assessment of the press-fit contact. However, classical mechanical characterization method involving sizable specimens cannot be applied, as the diameter of the pin amounts to about 1 mm, and the material properties of the cold-welded zone depend strongly on the contact pressure. Therefore, a characterization method was developed which relies strongly on coupling numerical modelling and quantitative analysis using far-field optical microscopy and digital image correlation. On the numerical side, the first task consists in simulating the press-fit process yielding the contact pressure and the exact size of the cold welded zone. All simulations were performed either with an idealized symmetrical pin-shaft configuration or with realistic geometric boundary conditions based on high-resolution computer tomography. The contact zone was added to this could in terms of a cohesive zone. Mechanical characterization is thus reduced to determining the material parameters of the cohesive zone model. On the experimental side, flat dog-bone specimens were cut from the board material. A press-fit contact was placed in the center of each specimen with the pin perpendicular to the specimen axis. The specimens were then placed in a testing machine, and the movement of the pin ends with respect to each other and with respect to the shaft was observed using a high resolution long rang optical microscope. An ensuing analysis using digital image correlation allowed quantifying the displacement field as a function of the applied load. Thus, the onset of mechanical damage could determined very well as the instant at which the displacement of a given pin tip did not follow the shaft movement anymore. This definition proved to be a very reliable damage sensor by which the damage accumulation process could be monitored precisely.