13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- According to the ISO 34 standard method A, the samples used for tear fracture tests are processed into trouser test pieces. Each piece has a cut of depth 40mm, which is made with a razor blade, at the center of the width of the test piece in the direction indicated in Fig.2. Fig.1 Test piece for uniaxial tension Fig.2 Trouser sample for tear fracture testing 2.2. Measurements We examined the tensile and tear fracture properties of CB filled natural rubbers which have been subjected to isothermal aerobic aging. The thermal aging was conducted with a circulated air oven at temperatures of 70oC and 120oC for periods of time ranging from 10 hours to 144 hours. Uniaxial tension and tear fracture tests were conducted with a 5943 Instron single column materials testing system. The effects of aging were measured by the changes in tensile strength, stretch to fracture, initial elastic modulus and tear strength. The tensile strength is defined by the ratio of the peak force of the force-stretch curve to the reference cross section area. The Initial elastic modulus is the slop of the stress-stretch curve at very small stretches. The tear strength sT is given by the formula s F T d = (1) where F is the median force acting on the arm of the test piece, calculated in accordance with ISO 6133, d is the median thickness of the test piece. 3. Results 3.1. Tensile stress-strain properties Uniaxial tension tests up to fracture under a constant rate of 100mm/min were conducted at 23oC. Fig.3 shows the stress-stretch curves of rubbers with 5 vol% CB and 15 vol% CB, aged at 120oC for various periods of time. It is obvious that the tensile strength, stretch to fracture and initial elastic modulus change with aging time. Figs. 4-6 show the variations of these tensile properties with aging time. It is found that the initial elastic modulus increases with increasing aging time, while the tensile strength decreases with the increase in aging time up to 100 hours and then increases when the material aged over 144 hours. Moreover, the stretch to fracture decreases with increasing aging time, this phenomenon was reported in the literature [3]. Compared to 5 vol% CB filled rubber, there are more pronounced changes in the tensile properties for 15 vol% CB filled rubber. In vulcanizates the bond energy between the sulfur crosslink atoms and the polymer backbone greatly differ. There are three types of crosslinks in the vulcanizates: polysulfidic, disulfidic and monosulfidic [4]. The maximum service temperature of natural rubber is about 70oC, i.e., the mechanical properties of natural rubber aged at 70oC would not change significantly, however, the carbon black filled natural rubbers would experience severer thermal degradation at temperature above 70 oC [5]. As seen in Figs.7-9 the change in tensile properties are found to be dependent on aging temperature, the higher the aging temperature, the more pronounced decrease in tensile
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