Assessment of UMTS and averaged SED fracture criteria for U-notched specimens Hannelore Filipescu 1,a , Cristiana Caplescu 1,b and Niculai Pasca 1,c 1 ”Politehnica” University of Timisoara, Victoriei Square no.2, 300006, Timisoara, Romania a hannelore.filipescu@yahoo.com, b cristinacaplescu@yahoo.com, c nicu_pasca21@yahoo.com Keywords: notch fracture, UMTS and averaged SED criteria, polyurethane. Abstract. A set of mixed-mode I/II fracture tests was carried out on a polyurethane material. Using a single edge notched bend (SENB) type specimen, containing straight and respectively inclined U notches, the full range from pure mode I to dominant mode II was achieved. Two different fracture criteria were employed for the prediction of the fracture conditions: the UMTS criterion and the averaged SED criterion. Using these two criteria a set of curves were derived in terms of the fracture load vs. notch radius. The agreement between the experimental results and those predicted based on these two criteria is discussed finally. Introduction Mixed-mode I/II brittle fracture occurs frequently in notched components weakened by sharp and blunt notches. Therefore, brittle failure was studied theoretically using different fracture criteria. Recently, a study of mixed-mode behaviour of two types of polyurethane materials based on MTS and SED criteria was presented in [1].In this paper the mixed-mode fracture of a polyurethane material is investigated using U-notched SENB specimens based on two criteria: UMTS criterion proposed by Ayatollahi [2] and the averaged SED criterion developed by Gómez [3]. Experiments The material chosen for the experimental program was the polyurethane Necuron 1020 used in various industrial applications. The experiments were carried out at room temperature, the polyurethane showing a linear elastic behaviour up to fracture [4]. The mechanical properties for Necuron 1020 at room temperature are presented in Table 1. The elastic modulus E and the ultimate tensile strength  u were determined by tensile testing, the Poisson’s ratio was obtained using the impulse excitation technique, and the mode I toughness K Ic was determined using SENB specimens, according to ASTM D5045-99 requirements. Table 1. Mechanical properties of the polyurethane Necuron 1020   MPa E   MPa σ u   ν   m MPa K Ic   m MPa K ) a Ic   m MPa K ) b IIc Necuron 1020 3340 49.75 0.34 2.30 2.24 2.45 a) determined by extrapolating the experimental results, obtained on SENB U-notched specimens for mode I; b) determined by extrapolating the experimental results for dominant mode II (M e = 0.03). The geometries of the SENB specimens are presented in Fig. 1. In order to achieve the full range from pure mode I to mode II, two types of SENB specimens were investigated, as recommended in [5]: beams with straight U-notches (Fig. 1a) and beams with tilted U-notches (Fig. 1b), for all the specimens the thickness was 10 [mm]. To study the influence of the U-notch radius, four different root radii were considered: 0.5, 0.75, 1 and 2 [mm]. The tests were performed under displacement control at   min mm 1 , on a Zwick testing machine with maximum force of 5 [kN]. The recorded load-displacement curves were linear, without significant non-linearity, and the fracture occurred Key Engineering Materials Vols. 577-578 (2014) pp 113-116 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.577-578.113 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 79.114.4.138-30/07/13,17:33:35)