New testing facility at IVW allows investigating the fatigue crack propagation behavior in fiber reinforced composites under media influence

A high resistance to fatigue crack propagation is most important for many fibre reinforced polymer (FRP) composites. Because of the comparatively low fracture toughness, especially of thermosetting polymer matrices, repetitive, i.e. cyclically occurring loads can lead to sudden and catastrophic failure events of entire structure when critical damage is reached. To counteract this behavior, prevent crack propagation and increase the resistance to fatigue crack propagation of FRPs, different approaches are applied from a materials perspective. This can e.g. be the modification of the polymer matrix with second phase constituents, such as nanoparticles or in situ phase separated particulate structures [1], [2]. Exemplarily, Figure 1 shows a typical behavior of a fatigue crack in a polymer matrix composite, i.e. a steadily increasing crack propagation velocity (da/dN) as a function of the stress intensity factor range ΔK. By tailoring the epoxy matrix morphology (in this case a block copolymer toughening agent), the necessary stress intensity factor range strongly increases. Even more, the crack propagation velocity diminishes, i.e. the slope of the Paris-line m is reduced. Hence, higher loads are required to initiate and propagate the crack and the crack propagates slower [3].

To investigate the fatigue crack propagation behavior of polymer matrix composites and fiber-reinforced composites thereof, IVW acquired a new electrodynamic testing machine: the E1000 by Instron GmbH. The machine is especially made for dynamic measurements at low loads which are required for brittle polymer composite matrices. Even more, the device can be set up in a way enabling the determination of such properties even under influence of e.g. alkaline media (cf. Figure 2 and 3). The E1000 is capable of testing frequencies up to 100 Hz, while dynamically reaching loads of up to 1000N.

The machine offers the possibility to be operated vertically as well as in a horizontal position (Figure 2). Hence, this enables new important possibilities to design experiments, such as e.g. testing bio-based basalt fiber reinforced composites in highly alkaline environments (see BFKcraft, funded by BMWK, funding number: 03ET1653D, „Entwicklung eines energieeffizienten kostengünstigen Verstärkungssystems und Herstellungsprozesses für basaltfaserverstärkte Kunststoffe (BFK) zur statistischen Gebäudesanierung als Betonpflaster“) [4]. Future scenarios for investigating fatigue crack propagation phenomena comprise the effect of hydrogen on the materials behaviour, as it is done within the infrastructural development project (TPC-H2-Storage) for thermoplastic fiber-reinforced pressure vessels for hydrogen storage and transport [5]. Even more, it is the goal to investigate the interlaminar fatigue crack propagation of fiber-reinforced composites in the near future.

[1]        A. Klingler, A. Bajpai, und B. Wetzel, „The effect of block copolymer and core-shell rubber hybrid toughening on morphology and fracture of epoxy-based fibre reinforced composites“, Engineering Fracture Mechanics, Bd. 203, S. 81–101, 2018, doi: 10.1016/j.engfracmech.2018.06.044.

[2]        A. Klingler, M. Gilberg, B. Wetzel, U. Breuer, und J.-K. Krüger, „Temperature-rate induced polymerization and phase separation of block copolymer toughened polymer composites“, Composites Science and Technology, S. 109329, Feb. 2022, doi: 10.1016/j.compscitech.2022.109329.

[3]        A. Klingler und B. Wetzel, „Fatigue crack propagation in triblock copolymer toughened epoxy nanocomposites“, Polymer Engineering & Science, Bd. 57, Nr. 6, S. 579–587, 2017, doi: 10.1002/pen.24558.

[4]        S. Grzesiak, M. Pahn, A. Klingler, E. I. Akpan, M. Schultz-Cornelius, und B. Wetzel, „Mechanical and Thermal Properties of Basalt Fibre Reinforced Polymer Lamellas for Renovation of Concrete Structures“, Polymers, Bd. 14, Nr. 4, S. 790, Feb. 2022, doi: 10.3390/polym14040790.

[5]        Leibniz-Institut für Verbundwerkstoffe, „TPC-H2-Storage – Wasserstofftechnologien“, 31. Mai 2022. www.ivw.uni-kl.de/de/forschung-entwicklung/tpc-h2-storage