Joining of steel/CFRT-hybrid parts via non-rotational symmetric cold formed pin structures

Hybrid parts, consisting of a steel and continuous fiber reinforced thermoplastic (CFRT) component offer new possibilities such as the combination of good weight related mechanical properties and high temperature resistance that can exceed the potential of single material parts. This study investigates the direct pin pressing process with single cold formed, non-rotational symmetric pins into a quasi-unidirectional CFRT component. In the current state of the art, primarily rotational symmetric cross sections of the pins are investigated although non-rotational symmetric cross sections could offer potential in means of material and load appropriate joining and introduction of forces into the reinforcing fibers: Depending on the failure mode of the joint, the orientation of the non symmetric pin strucuture is expected to influence the joint characteristic. 

Consequently, in the scope of this study, three different geometries are investigated: As a baseline geometry, a cylindrical pin with a diameter of 1 mm is used, which is utilized to compare the results of the non-rotational symmetric pins. The second geometry is an oval shaped pin with an aspect ratio of 1:2 and length of the short side of 1 mm aiming at a more gently fiber displacement in the quasi unidirectional composite and also increased bending resistance of the pin structure. The third geometry is a polygon, which is based on a triangular shape with rounded corners and sides and an in-circle diameter of 1 mm aiming at an increased, load direction dependent, joint strength. Thereby, the fiber component is a custom-manufactured quasi-unidirectional CFRT glass fiber/polypropylene composite with a fiber volume content of approximately 45 %.

In a first step, the joints are characterized via imaging methods in order to create a thorough understanding of the geometries’ influence on fiber displacement mechanisms. Thereby the structures are joined with the CFRT in different orientations in relation to the fiber direction in order to create a thorough understanding about the anisotropic fiber displacement.

In a second step, the joints are characterized in quasi-static single lap shear tests in 0° and 90° to the fiber orientation in order to determine the mechanical performance and the results of the different geometry types. Besides the fiber orientation, the orientation of the non-symmetric pin structures is also varied in relation to the load direction resulting in a total of six tested combinations.