Contents lists available at ScienceDirect Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech Parameters identification and validation of plastic-damage model of 304 stainless steel by small punch test at ambient temperature Amirreza Lotfolahpour, Nasser Soltani, Mehdi Ganjiani ⁎ , Davoud Baharlouei Department of Mechanical Engineering, University of Tehran, Tehran, Iran ARTICLE INFO Keywords: Small punch test Plastic properties Gurson-Tvergaard-Needleman Model Genetic algorithm Neural networks ABSTRACT Small punch test is an advantageous alternative to standard tests which need a substantial amount of material and are quite time-consuming. Finite element simulation along with an op- timization algorithm can be employed in order to identify the elastic, plastic and damage parameters of a material from force-displacement curve. In this research, the plastic properties and Gurson-Tvergaard-Needleman damage model parameters have been identified for 304 stainless steel at ambient temperature using force–displacement curve obtained from small punch test. For identification of parameters, an optimization procedure by involving Genetic Algorithm and Neural Networks has been proposed. To validate the identified parameters, tensile test on notched specimen has been performed. The experimental force–displacement curve and the si- mulation one, obtained by implementing the identified properties in ABAQUS, have been com- pared. These two curves are rather similar in most regions, however, there are some differences especially in the final stage. Differences in the nature of small punch test and tension test, and also differences in the stress triaxiality and friction coefficient effects, are the main reasons the two curves do not completely match. 1. Introduction Elastic and plastic properties play a crucial role in design and manufacturing of mechanical parts. There are diverse standard tests in order to obtain the mechanical properties of materials. But in some situations due to paucity of material or time limitations, performing standard tests is rather impossible. In these situations, Small Punch Test (SPT) could be a useful alternative. In SPT a miniature disk which is clamped between two cylindrical dies, is bi-axially loaded by a spherical punch until engender of macro cracks. In SPT, when a constant load is applied, the result of test is displacement-time curve which represents the creep behavior of the specimen [1,2] and when a controlled displacement is applied, the test result is force-displacement (F-D) curve which represents the elastic and plastic behavior of the specimen [3]. In 1984 Manahan et al. [4] Adopted SPT in order to investigate the radiation effects on the behavior of a material used in nuclear plants. They compared the F-D curve of a radiated specimen with an undamaged specimen of the same material. Some researchers used SPT to find correlation between SPT results and mechanical properties of materials [5–7]. By advances in computers speed, the SPT curves are directly used for identification of material models parameters. Usually, an optimization procedure along with inverse FEM are adopted to identify the models parameters. In 2017 Yang et al. [8] identified mechanical properties of Incoloy800H by SPT. For this purpose, they used inverse FEM along with a novel inverse methodology based on least squares-support vector machine (LS-SVM). Also, they compared the efficiency of Gold section and the proposed methodology https://doi.org/10.1016/j.engfracmech.2018.07.007 Received 1 February 2018; Received in revised form 4 July 2018; Accepted 4 July 2018 ⁎ Corresponding author at: P.O.B. 14155-6619, Tehran, Iran. E-mail address: ganjiani@ut.ac.ir (M. Ganjiani). Engineering Fracture Mechanics 200 (2018) 64–74 Available online 05 July 2018 0013-7944/ © 2018 Published by Elsevier Ltd. T