Experimental determination of critical strain energy density of ductile materials

https://doi.org/10.23939/ujmems2019.01.039
Надіслано: Травень 15, 2019
Переглянуто: Червень 20, 2019
Прийнято: Червень 26, 2019
1
Karpenko Physico-mechanical Institute of the NAS of Ukraine
2
Karpenko Physico-mechanical Institute of the NAS of Ukraine
3
Karpenko Physico-mechanical Institute of the NAS of Ukraine
4
Lviv Polytechnic National University
5
Національний університет «Львівська політехніка»
6
Lviv Polytechnic National University

The method of experimental determination of strain energy density of plastic materials is developed. The technique for complete true stress-strain curves plotting is formulated. The standard hydraulic testing machine is equipped with specially designed experimental setup for Bridgman specimens testing at strain controlled tension loading with digital camera and light source for using digital image correlation method – a non-contact technique for strain and displacement measurement. The digital image correlation method was used to determine the local strain at the onset of fracture in the neck of Bridgman specimen. The technique takes into account the change of cross-section area in the neck of specimen due to internal crack propagation when calculating the true stresses. The complete true stress stress-curve of 40Kh alloyed steel is plotted end critical strain energy density of steel is determined from it. It is shown that the critical strain energy density of material, determined from the curve obtained by developed technique is 1.8 times higher than determined from the classical true stress-strain curve and is close to the value of the specific heat of fusion of steel. The curves built using the proposed technique can be used for setting material properties in stress-strain state calculations by finite element method at large scale yielding conditions, for instance at pressure vessels critical pressure calculation. The critical strain energy density value can be considered as a material property at fatigue life-time calculation using energy approach.

[1] Yu. Du, et al., “Analysis of the stress-strain state of the process zone of a plate with central crack under biaxial loading,” Materials Science, vol. 53, no. 1, pp. 86-92, 2017. https://doi.org/10.1007/s11003-017-0047-1

[2] Yu. V. Mol’kov, “Experimental determination of the specific strain energy of 65G steel under cyclic loading,” Materials Science, vol. 52, no. 4, pp. 522-529, 2017. https://doi.org/10.1007/s11003-017-9985-x

[3] H. J. Shindler, “Strain energy density as the link between global and local ap p roach to fracture”, in Proc. of 10th Int. Conf. on Fracture, Honolulu, 2001.

[4] L. F. Gillemot, “Criterion of crack initiation and spreading,” Engineering Fracture Mechanics, vol. 8, no. 1, pp. 239-253, 1976. https://doi.org/10.1016/0013-7944(76)90089-8

[5] A. Valiente, “On Bridgman’s stress solution for a tensile neck applied to axisymmetrical blunt notched tension bars,” J. Appl. Mech., vol. 68, no. 3, pp. 412-419, 2000. https://doi.org/10.1115/1.1360689

[6] M. A. Sutton, M. Cheng, W. H Peters, et al., “Application of an optimized digital correlation method to planar deformation analysis,” Image Vision Comput., vol. 4, no. 3, pp. 143-150, 1986. https://doi.org/10.1016/0262-8856(86)90057-0

[7] B. Pan, K. M. Qian, H. M. Xie, and A. Asundi, “Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review,” Meas. Sci. Technol., vol. 20, no. 6, pp. 062001–062007, 2009. https://doi.org/10.1088/0957-0233/20/6/062001

[8] Z. Wang, “On the accuracy and speed enhancement of digital image correlation technique,” J. Exper. Mech., vol. 26, no. 5, pp. 632-638, 2011.

[9] Yu. V. Mol’kov, “Otsiniuvannia opirnosti ruinuvanniu yemnostei pid tyskom iz vykorystanniam enerhetychnoho pidkhodu” [“Evaluation of pressure vessels fracture resistance using energy approach”], Ph.D. dissertation, Karpenko physico-mechanical institute of the NAS of Ukraine, Lviv, Ukraine, 2014. [in Ukrainian].

[10] Yu. V. Mol’kov, “Application of the method of digital image correlation to the construction of stress–strain diagrams,” Materials Science, vol. 48, no. 6, pp. 832-837, 2013. https://doi.org/10.1007/s11003-013-9576-4

[11] A. Kalup, M. Žaludová, S. Zlá, et al., “Latent heats of melting and solidifying of real steel grades”, in Proc. 23rd International Conference on Metallurgy and Materials (METAL-2014), Brno, Czech Republic, 2014, pp. 695-700.

[12] Th. H. Courtney, Mechanical behaviour of materials. Long Grove, IL: Waveland Press, 2005.

Yu. Molkov, Ya. Ivanyts’kyi, T. Lenkovs’kyi, A. Trostianchyn, V. Kulyk, R. Shyshkovskyy, "Experimental determination of critical strain energy density of ductile materials", Ukrainian Journal of Mechanical Engineering and Materials Science, vol. 5, no. 1, pp. 39-44, 2019.