Material characterization with multiaxial stress states as well as multiaxial component testing is an important but challenging task. This is especially true for fatigue testing because at least two actors need to work together simultaneously and thereby a number of additional testing parameters need to be defined and controlled. A full documentation of the specimen’s or component’s response, both in terms of reactional forces and moments as well as in terms of strains, is important to gather as much information as possible from often more expensive specimens (compared to simple lab scale specimens). During the TPC-H2-Storage project, Leibniz-Institut für Verbundwerkstoffe (IVW) developed a testing set up that enables the characterization of components and materials under multiaxial fatigue loads, namely combined axial and torsional loads.
A particular interesting field is the multiaxial material characterization of composites due to their anisotropy. The directional properties affect not only stiffness and strength but also fatigue damage. A conventional uniaxial test can only determine these effects in a limited way because interactions between the damage in different directions are not accessible. Fracture criteria allow at least to incorporate the multiaxial stress state in addition to the applied number of cycles. However, most criteria focus on the undamaged state and predict the occurrence of cracks in a multilayered laminate. Many investigations into composite fatigue show on the other hand that cracks occur early in a composites lifetime. This suggests that a significant fraction of a laminate’s lifetime is not facilitated when crack onset is used as design criterion. A deeper insight into the damage and its anisotropic nature can help to accept a certain amount of damage and thereby capitalize on additional material capabilities. Crucial is that one type of loading leads to a specific type of directional damage which affects the properties and damage evolution in all other material orientations, too. One particularly interesting combination is, for example, the interaction of normal and shear loads. This is because under shear loading, most laminates show pronounced stiffness degradation which affects stiffness properties transverse to the fiber reinforcement more than those in longitudinal direction. A uniaxial approach with which these effects can also be investigated is the Arcan testing fixture (Figure 1i). Butterfly-shaped specimens similar to ASTM 7078 are used and can be oriented freely in the loading direction. As a result, it is possible to apply different ratios of shear and normal loading and also to switch between these loading ratios by rotating the specimen intermittently. This setup is very easy to use but has drawbacks due to damage introduced at free edges and an inhomogeneous stress state. Testing tubular specimens loaded simultaneous with axial and torsional loads can overcome these problems and is therefore the preferred method. With the latest development, both types of tests can now be performed at IVW.
In the field of component testing, combined axial and torsional loads or the simultaneous control of both loads are a similarly interesting option. One subcomponent frequently encountered in this context is the hat shaped profile. These profiles are typically made from organo sheets and may be used as stiffeners for plate-like structures. With additional injection molding, it can also be used as beam-like structure. With both axis controlled, it is possible to test these profiles in torsion with a defined amount of axial load or zero axial load. Instead of using slider mechanism to compensate for the shortening of the profile under torsion, it is possible to actively control the axial load. Thereby a known axial load is acting instead of an undefined frictional force from the slide mechanism. In addition, the newly created setup in Figure 2 incorporates a multicomponent force sensor which enables the documentation of all acting forces and moments. Transverse forces might act as a result of an offset between torsional axis and shear center. The position of this force sensor allows force and moment measurement to be mace excluding effects of friction. The assembly furthermore allows the application of up to 100 kN axial force and 8 kNm torque to specimens up to 1.2 m length. Additional optical techniques provide information on deformation and strains.
Andreas Baumann, M.Sc.
Fatique & Life Time Prediction
Phone: +49 (0) 631/2017-320