PSI - Issue 19

646 2

Fabian Becker et al. / Procedia Structural Integrity 19 (2019) 645–654 F. Becker et al. / Structural Integrity Procedia 00 (2019) 000–000

Fig. 1. FRP longitudinal leaf spring with clamping group and load case

fibers are candidates for the fiber material. As matrix material, epoxy resin as well as polyurethan resins are possible. Transverse and longitudinal leaf springs are already used in series production of automobiles (Wood (2014)). Longitudinal leaf springs allow for a simple and robust design of a vehicle suspension system. The axle is connected via a clamping group to the leafspring. The leafsprings’ ends are respectively connected to the chassis frame through a y-axis rotational-free rubber joint on the one hand side and through a connecting link on the other hand side, namely a shackle. This part enables free-rotation about the y-axis and at the same time the longitudinal length compensation of the leaf spring. (Figure 1). The load case of bending leads to a well defined stress distribution in the leaf spring, allowing a design with FRP, where the fibers are oriented in main stress direction. However, in the clamping section, additional transverse and shear stresses act on the leaf spring, leading to a complex stress state. Material testing for leaf springs is done taking into account the boundary conditions close to the real load case in three point bending experiments under pure bending stress state. Fatigue testing and modelling for multi axial stress states is frequently done on thin walled laminates for di ff erent combinations of stresses acting in the lamina plane (Gude et al. (2006), Fawaz et al. (1994), Diao et al. (1999), Petermann et al. (2001)). Static failure models exist for complex stress states with combined shear stress, transverse stress and fiber parallel stress for inter fiber failure and fiber parallel compres sive failure (Pinho et al. (2004), Puck et al. (1998)). However, none of the models or the experimental approaches are appropriate for the determination of the fatigue life of a FRP leaf spring. In the following, an experimental approach will be presented that aims to reproduce the complex stress state which occurs in the clamping region of a leaf spring on specimen level.

Nomenclature

h

Specimen thickness

w Specimen width 1,2,3Local coordinate axes f Testing frequency

F max Maximum force during a cycle F min Minimum force during a cycle k dyn Dynamic sti ff ness R Strain ratio

u max Maximum displacement during a cycle u min Minimum displacement during a cycle