13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Experimental requirements of small and large scale dynamic fracture mechanics testing Wolfram Baer BAM Federal Institute for Materials Research and Testing, Berlin, Germany Corresponding email address: wolfram.baer@bam.de Abstract The paper deals with experimental requirements of dynamic fracture mechanics tests in general with special focus on ductile cast iron materials. This is based on instructions for dynamic measurement techniques provided by the standards ASTM E 1820 and BS 7448-3 which were compared and rated. The low blow multiple specimen technique using a drop tower was investigated for crack resistance curve determination on SE(B)25x25 specimens. It was shown that no detrimental effects should be expected from the second impact of the rebounded striker. Strain gage instrumentations for direct force measurement according to the standards provided different results. The ASTM position should be preferred. Important conclusions were drawn concerning hidden friction in drop towers from a comparison of electro-optically measured displacement and calculations from force-time record. Full blow large scale tests on SE(B)140x280 specimens were performed using a servo-hydraulic test system to determine dynamic fracture toughness values. Different strain gage instrumentations were compared with respect to their force measurement capability. For displacement measurement only a non-contact electrooptical camera technique was applicable. An appropriate method of instrumentation was identified whose results show good agreement with numerical simulations of the tests. It can be concluded that the recommendations of the test standards provide different results. Therefore, they must not simply be transferred to the own specific test requirements. It is regarded absolutely essential for dynamic fracture mechanics tests in general that all measuring techniques are being validated in advance. Keywords dynamic fracture mechanics testing, ductile cast iron, measurement 1. Introduction Design and safety assessment of components subjected to high strain rate or impact (dynamic) loading require adequate material data. Therefore, the focus of this paper is on the experimental determination of dynamic fracture mechanics material data. The problem of how to measure basic quantities like force and displacement correctly and with sufficient precision in dynamic fracture mechanics tests has still been an experimental challenge, although such tests have been performed for at least 40 years. Advice given on that by test standards such as BS 7448-3 [1] or ASTM E 1820 [2] is fairly limited. The ISO standard 26843 [3] on the determination of dynamic fracture toughness using precracked Charpy specimens (PCVN) is still being drafted while comparable contents has recently been included in the very new annex 17 of [2]. A major lesson to be learned from experience is that the first step of experimental dynamic fracture mechanics investigations should always be to establish an appropriate test method. This seems to be trivial only in the first instance. In fact, the material behavior mainly governs the test techniques which are applicable. Many materials - such as the ductile cast iron (DCI) investigated here - substantially change their deformation, damage and fracture behavior from ductile to brittle by increasing loading rate, decreasing temperature and/or increasing stress triaxiality. Unfortunately, this does not happen suddenly so that the corresponding measuring techniques have to be adapted with deliberation. As will be shown below, transferability of measuring techniques from one lab to another, from small to large scale tests or vice versa and of more or less common advice from standards to the own very special task cannot simply be taken for granted. In contrary, it is of vital importance to validate the basic measured quantities independently before using the data for further analyses and
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