ABSTRACT of the Doctoral Thesis

Development, Experimental and Numerical Investigations of a Novel Metal to Composite Joint Technology

by Stephan Ucsnik (2013)

The importance of fibre reinforced polymers increases continuously in present and future primary structures in automotive and aeronautic industries. Fibre reinforced polymers are alternative materials for future light weight design due to excellent weight-specific properties. In this regard, particular focus is put on joining technologies to enable optimum lightweight designs in terms of mechanical properties and material utilization.

The present thesis contains methodical research of a new integrative joining technology for the connection of carbon fibre reinforced polymers with metals which are used in lightweight design applications.

Small metal elevations, so called pins, with 3-dimensional textures are the key feature of the integrative joining technology. These pins are produced through a metal surface modification, the "cold metal transfer" process, an arc-welding process by FRONIUS International. This process allows to modify originally smooth metal surfaces with pins. Continuous fibre textiles are draped onto arrays of 3-dimensional textured pins, matrix resin is infiltrated and finally the composite is cured. In this way, fibres get fixed which leads to a good form-fit at little fibre undulation and fibre destruction. So to speak, a "fibre-friendly" joint.

It is the objective to investigate the use of small metal features for the integrative joining technology. Research shall provide a basic understanding of the load transfer mechanisms of the novel integrative joining technology. Furthermore, the influence of the metal features on the potential of the hybrid joining technology shall be determined.

Design, manufacture routines, test setups, and preliminary tensile shear results are preĀ­sented and cornpared for the selection of one best joint test geometry for investigations of the novel integrative multi-material joint under tensile shear load. Advantages and limitations of the manufacture routines are cormpared together with results of destructive tests.

Parameter studies are carried out on pin reinforced hybrid joint specimens in order to determine the significant load transfer behaviour and major influence parameters. The reinforcement of hybrid joints with undercut (ballhead) pins results in a major improvement of the load transfer behaviour as well as on the joining properties of the novel joining technology. Comparisons with state-of-the-art joint technologies emphasize the potential of the hybrid joining technology. These comparisons prove that the pin reinforced joining technology is the best alternative for adhesive bonded and single riveted hybrid joints.

Cylinder pin reinforced metal to composite joints are numerically analysed by means of the finite element method to determine and investigate internal processes in the pin-reinforced metal to composite joint during loading. The results reveal insights into the damage processes of the joint during loading. Results underline the possibility for a theoretical increase of ultimate joint strength for cylinder pin reinforced joints with an intact adhesive bonding in the touching interfaces.

To sum up, the novel, pin reinforced joining technology leads to lighter joints between metal and composite parts at increased joint properties compared to state-of-the-art joining technologies.

revised 130904 (hjb)