PSI - Issue 57

Matthias Hecht et al. / Procedia Structural Integrity 57 (2024) 581–588 Matthias Hecht et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction In electromobility, the importance of lightweight design is growing rapidly, especially when it comes to enhancing the range of electric vehicles. To address this need, there is ongoing progress in the development of steel based lightweight design concepts, primarily focused on automotive body structures. Among various joining techniques, adhesive bonding emerges as an ideal choice, thanks to its ability to facilitate efficient material usage through laminar load transfer. Moreover, adhesive bonding offers advantageous characteristics in terms of crash behavior, damping, and electrochemical isolation. Bonded joints in most applications experience a complex in-service loading, characterized by a non-proportional and multiaxial stress state of variable amplitude. Currently, there is no generally accepted and reliable method for evaluating these joints under such complex load cases. As a result, bonded joints are likely to be oversized in current practice, which means that their potential for lightweight construction is not fully exploited. To ensure that evaluation concepts also provide reliable results for such complex loading cases, experimental results must be used to validate these concepts. To build a solid experimental basis for evaluating assessment approaches, this paper presents fatigue test results under load control on component-like bowl specimens under complex load cases of variable amplitude. These specimens consist of a deep-drawn bowl bonded to a sheet metal. Four loading cases were investigated: each with a different single load, as well as a combination of these two without and with the phase shift of φ = 90°. Typical SN curves are presented for specimens similar to construction elements in order to better estimate slopes of adhesively bonded sheet specimens and to illustrate the influence of a phase shift under multiaxial loading. In practice the stiffness degradation is a frequently used failure parameter. Adhesively bonded joints show often a more complex stiffness degradation compared to other joints, for example, welded components. Therefore, in addition to the SN curves, the cyclic stiffness degradation behavior is presented in this paper as well. 1.1. Variable amplitude loading In order to characterize such tests under variable amplitude loading, several parameters need to be defined, including, the sequence length s and the sequence shape [1]. Out of the minimum ̅ min and the maximum of stress ̅ max in the entire sequence. The load ratio ̅ = ̅ min ̅ max (1) and the amplitude ̅ = ̅ min − ̅ max 2 (2) can be calculated. For the fatigue test, it is essential to exercise caution while selecting the load sequence, ensuring that it is repeated a minimum of five to ten times [2]. 1.2. Research on bowl specimens The bowl specimen was developed and patented [3] as part of the dissertation work referenced as [4]. As a complex-shaped deep-drawing specimen, it served in several projects for method validation, since it was developed to mimic, for example, a structure within a vehicle body. Several investigations using this specimen as validation sample were made over the last decade to a hybrid joint of adhesive bonding and riveting [5], to estimate the cyclic stiffness behavior [6] or to a multi-material joint with Carbon Fiber Reinforced Plastic [7]. In the context of the test results discussed in this paper, three completed research projects are relevant as they used the same adhesive and similar steel sheets [8 – 10]: ▪ In [8] and [9] the bowl specimen was subjected to uniaxial loading, similar to the purely vertical loading