PSI - Issue 42

K. Koch et al. / Procedia Structural Integrity 42 (2022) 506–512

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

loading rates, inertia effects can occur in the specimen and in the components of the test setup. This often leads to an incomplete energy conversion in the test (Henschel (2018)). Dynamic fracture toughness tests can be carried out with an instrumented pendulum impact test machine (ISO (2015)). Also, the application of a drop weight test machine or a split Hopkinson pressure bar (Kolsky bar) is suitable depending on the loading rate (Henschel and Krüger (2016, 2015b)). Nomenclature crack length 0 initial notch or crack length ∆ stable crack N

net specimen thickness 0 initial impact energy integral cd critical J integral under dynamic loading stress intensity factor pl area under force versus displacement curve 0 impact velocity specimen width 0 pendulum deflection angle Poisson ratio

The causes for an incomplete energy conversion in tests with a pendulum impact test machine have already been investigated extensively. Energy losses arise from small plastic deformations in the Hertzian contact zone between the tup, the specimen and the support (Ireland, 1974). Due to their stiffness or compliance, a part of the energy is also stored as elastic deformation in the tup and the support (Henschel and Krüger (2015a), Manahan and Stonesifer (2000)). This leads to an overestimation of the energy which is absorbed by the specimen. Due to the dissipation of the elastic deformation energy, vibrations of the machine occur. This effect is more significant at high impact velocity (Magnus et al. (2016)). However, energy losses due to vibrations are negligible for long test durations (Server (1978)). In case of ductile material testing and high impact velocities, the pendulum also rotates due to inertia effects (Henschel and Krüger (2015a)). A reduction of the vibrations can be achieved, for example, by using machine components with high stiffness and a smooth bearing of the pendulum. An experimental measurement of the inertia effects and the amounts of the dissipated energy is possible using laser measurements of displacement and velocity (Lorriot (2000), Tronskar et al. (2002)). The vibrations of the individual components can be identified based on their natural frequency (Böhme and Kalthoff (1982)). The aim of this article is to investigate the energy conversion in dynamic fracture toughness tests with an instrumented pendulum impact test machine using different specimen materials. In particular, energy losses at low impact energies should be identified. In addition to the strength of the material, the stiffness of the specimens and the impact velocity of the tup was also considered. 2. Materials and methods For the present investigations high-strength quenched and tempered steel 42CrMo4 and nodular cast iron EN-GJS 400-18 was used. The 42CrMo4 steel was processed in a steel casting simulator (Dudczig et al. (2014)). The machined specimens were hardened (austenitizing at 840 °C in a vacuum, 20 min, quenching in a stream of He, 15 bar) and tempered (450 °C, 1 h, N2 atmosphere). Tab. 1 gives an overview of the mechanical properties of the investigated materials. Compared to cast steel, the cast iron material has a significantly lower strength. This can be explained by the ferritic matrix and the presence of strength-reducing nodular graphite. By varying the strength, the material-related effect on the impact pulse in dynamic fracture toughness tests was investigated.