PSI - Issue 3

Wolfram Baer et al. / Procedia Structural Integrity 3 (2017) 25–32 Author name / Structural Integrity Procedia 00 (2017) 000–000 Author name / Structural Integrity Procedia 00 (2017) 000–000

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achieved crack extension. The investigations covered the analysis of limit loads of the specimen with respect to the measured force. The measured stiffness of the specimen was assessed and signals of crack sensors were analyzed. Furthermore, an analysis of the mechanical behavior of the loading system and the specimen by optical observation was performed. Corresponding results are discussed in the paper. It had finally been proven that additional crack extension in the specimen due to bouncing strikes of the hammer is not to be expected under the given conditions of test setup, material and loading. It can be seen as a major experimental advantage that the striker does not have to be catched after the low blow test. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of IGF Ex-Co. 1. Introduction Fracture mechanics based component design requires appropriate fracture mechanics toughness data with respect to both, loading rate as well as test temperature. Taking high-rate loading into account such as with accidental scenarios, different standards such as ASTM E 1820 (2013) or BS 7448-3 (2005) provide some information on dynamic fracture mechanics testing. Nevertheless, the designations differ and BS 7448-3 does not address the determination of R-curves at all. The new ISO 26843 (2015) standard issued in 2015 is dedicated to fracture toughness tests at impact loading rates but exclusively limited to 10 mm thick pre-cracked Charpy type test pieces. Therefore, it is at the discretion of the test laboratory to make sure that the own material specific test method used for dynamic R-curve determination is designed, performed and validated properly. Within this procedure, 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 experimental 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 to establish material characteristics. The results presented in this paper are part of investigations which were performed at BAM to establish and validate an experimental setup and an analysis procedure for the determination of dynamic R curves on ductile cast iron materials at -40 °C using a drop tower test system and the multiple specimen approach. Basic aspects of the experimental setup have already been reported in Baer (2012). This reference covers the development and validation of a strain gage based force measurement method on the specimen. Solutions for non-contact displacement measurement and numerical determination of the displacement by double integration of the force-time record have been discussed as well. The focus of the present paper is on the validation of the low blow multiple specimen technique using the BAM drop tower test system. 2. The BAM drop tower test facility The BAM drop tower test facility, Fig. 1, was designed and optimized to perform fracture mechanics low blow tests on bend type specimens of ductile cast iron materials (DCI). Single edge bend specimens SE(B)25 (length 138 mm, width 25 mm, thickness 25 mm, a 0 / W = 0.5) are the preferred specimen size. Nevertheless, the test setup allows for variable adaptation to other specimen sizes. The mass of the hammer and its height of fall may be varied (max. 20 kg, max. 3 m, max. initial speed of the hammer approximately 7.7 ms -1 ). Stress intensity rates in the linear elastic range of approximately 1∙10 5 to 2∙10 5 MPa√ms -1 are typical with DCI low blow tests. The loading fixture complies with ISO 12135 (2002) and is designed to minimize friction by allowing the rollers to move outwards during loading. Fig. 2 exposes a detail of the hammer weight design which turned out to be essential for the analysis of the recorded test signals. Fig. 2(a) shows the hammer weight before and Fig. 2(b) after design optimization. The taller new design ensures that the susceptibility of the loading device to energy losses due to friction during the fall is maximally reduced. The beneficial effect of the optimized design is due to the mass of the hammer being positioned much closer to the gravity center axis of the hammer sledge than before thus preventing the risk of canting during the fall. If the energy losses are proven to be negligible, then the option exists to calculate the load line displacement by double integration of the force-time record instead of using expensive non-contact displacement measuring equipment. For details of the validation procedure see Baer (2012). achieved crack extension. The investigations covered the analysis of limit loads of the specimen with respect to the measured forc . The measured tiffness of th spec men was assess d and gnals of crack ensors were analyzed. Furtherm re, an analysis of the m chanical behavior of the loading system and the specimen by ptic l observation was performed. Cor espondi g result are discussed n the p per. It had finally been proven at add tio al crack exten ion in the specimen due to bounci g strikes of th hamm r is not to b expected under the given conditions f test setup, material and loading. It can e see a a maj r experi ental advantage that the striker does not have t be catched after the low blow test. © 2017 The Authors. Published by Elsevi r B.V. Peer-review under espons bility of the Scientific Committee of IGF Ex-Co. Keywords: Dynamic fracture mechanics; R curve; low blow; drop tower; ductile cast iron 1. Introduction Fracture mechanics based component design requires appropriate fracture mechanics toughness data with respect to both, loading rate as well as test temperatu . Taking high-rate loading into ac ount such as with accidental scenarios, different st ndards such as ASTM E 1820 (2013) o BS 7448-3 (2005) provide some informat on on dy mic fractu mechanics testing. Nevertheless, the designations differ and BS 7448-3 d es not address the eter ination of R-curves at all. The new ISO 26843 (2015) standard issued in 2015 is edicated to fractur toughness tests at impact lo ding rates but exclusively limited to 10 mm thick pre-cracked Charpy type est pieces. Therefore, it i the discretio of he test laboratory to ake sure that the own material specific test m thod us d for dynam c R-curve det rmination is designed, perform d and validated properly. Within this pr cedure, transfer bil ty of measuring techniques from one lab to anoth r, from small o large scale tests or vice versa and of more or less c m on advice from standards to the own very special experimental task cannot simply b t ke f r granted. In contrary, it is of vital importance to validate the basic me sured quantities indepe dently befor using the data for further analyse and to establish material ch rac eristics. The results presented in his p per are part of investigations which were performed at BAM to establish and validate an expe ime tal setup and an analysis procedure for the determination of dynamic R curves on ductile cast iron materials at -40 °C using rop tower test system and t multiple specimen approach. Basic aspects of the experim nt se up have already been reported in Baer (2012). This refer nce ov rs the development nd validation of a strain gage b sed force m asurement method on the specim n. Solution for non-contact displaceme t me surement nd numerical determination of th displacement by double integration f the force-time record have b en discussed as well. The focus of the pres nt paper is on the validation of the low bl w multipl specimen t chnique u ing the BAM drop tower test syst m. 2. The BAM drop tower test facility The BAM drop tower test facility, Fig. 1, was designed and optimized to perform fracture mechanics low blow tests on bend ty e specimens of ductile cast iron materials (DCI). Single edge bend specimens SE(B)25 (length 138 mm, wi th 25 mm, thickness 25 mm, 0 / W = 0.5) a e the preferred specim n size. N vertheless, the test setup allows for variable adap ation to other specimen sizes. The mass of th hammer and its height of fall may be varied (max. 20 kg, max. 3 m, max. initial speed of the hammer approxima ely 7.7 s -1 ). Stre s intensity r tes in th linear elastic ran e of approximately 1∙10 5 to 2∙10 5 MPa√ s -1 are typic l with DCI low blow st . The loading f xtu e complies with ISO 12135 (2002) and is designed to minimize friction by allowing the rollers to move outwards during load ng. Fig. 2 exposes a detail of th hammer weight d sign which turned out o be essential for the analysi of the recorded test signals. Fig. 2( ) shows t e hammer weight before and Fig. 2(b) after de ign optimization. The taller n w design en ures that the susceptibility of the loadin device to energy losses due to fricti n during the fall is maximally reduced. The beneficial effec of the optimize design is due t the mass of he hammer being positioned much closer to th grav ty center axis of the hammer sledge than before thu preventing th risk of canting during the fall. If the energy losses are proven to be negligible, then the option exists to calculate the load line displacement by doubl integration of the force-tim record instead f using expensive non-cont ct displacement asuring eq ipme . For details of the validation p o e ure e Baer (2012). Copyright © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). eer-re ie er responsibility of the Scientific Com ittee of IGF Ex-Co. Keywords: Dynamic fracture mechanics; R curve; low blow; drop tower; ductile cast iron