Issue 31

A. Abrishambaf et alii, Frattura ed Integrità Strutturale, 31 (2015) 38-53; DOI: 10.3221/IGF-ESIS.31.04 42 mounted on the specimen, whereas the “Top” and “Bottom” curves were obtained by averaging only the readouts of the LVDTs mounted, respectively, at the upper and lower specimen’s surface. From Fig. 5 it was visible that the readouts of the LVDTs mounted on the upper surface of specimens showed a relatively higher crack opening width compared to the one recorded by the LVDTs at the lower specimen surface. This denotes that the crack opened asymmetrically, which could be ascribed to the variation of effective fibres along the depth of the panel due to segregation. This aspect will be detailed further ahead, when the fibre distribution parameters are endorsed. Results and discussion: Uniaxial tensile test Figs 6(a) and 6(b) illustrate the average and envelope force-crack mouth opening relationship obtained from the uniaxial tensile test for the θ = 0º and 90 ° series, respectively. The value of the crack opening was determined by averaging the readouts of the four LVDTs. For both series ( θ = 0 ° and 90 ° ), the F - w curve was almost linear up to the crack initiation. After the crack onset, two distinct behaviours were observed for the θ = 0º and 90 ° series similarly to what was observed in splitting tensile tests, which could also be ascribed to the distinct number of fibres at the crack plane. 0.0 0.5 1.0 1.5 2.0 2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 Envelope Average Force [kN] w [mm] 0.0 0.5 1.0 1.5 2.0 2.5 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 Envelope Average Force [kN] w [mm] (a) (b) Figure 6 : Force – average crack width relationship, F- w , obtained from the uniaxial tensile tests: (a) θ = 0 ° and (b) θ = 90 ° . Regarding the θ = 0º series, fibres start to be slowly pulled-out being observed a semi-hardening response after the crack onset. Afterwards a plateau response was observed until a crack width of about 0.6 mm, and finally it was followed by a smooth reduction in the residual forces. Actually, during the uniaxial tensile testing of the θ = 0º specimens, once the peak load was achieved the sound of the fibre rupturing was clearly heard, which caused a rapid reduction in the value of the residual forces. This was also confirmed by inspecting the fracture surface visually after the specimen testing. Based on pull-out tests of hooked end fibres [14], fibre rupture may occur between the slip intervals of 0.6-1.0 mm, for the fibre inclination angle with the loading direction of 30 ° . As it will be discussed further ahead, for this series the most probable fibre orientation angle towards the cracking plane was around about 35º, this value was derived from the orientation probability distribution ascertained from an image analysis procedure. On the other hand, the θ = 90º series, after the crack initiation, shown a sudden force decrease up to a crack width of nearby 0.07 mm followed by a plateau. Cunha et al. [5] assessed the micro-mechanical behaviour of hooked end fibres by performing fibre pull-out test. It was verified that after a fibre sliding of nearby 0.1 mm, the fibre reinforcement mechanism was mainly governed by the hook plasticization during the fibre pull-out process. Additionally, in some specimens of the θ = 90º series, in particular those located closer to the casting point, shown a pseudo-hardening behaviour as it can be observed by the upper bound of the experimental envelope (Fig. 6(b)). Afterwards, beyond a crack width of about 0.9 mm, a reduction of the residual force was observed, which corresponded to the fibre rupture. Results and discussion: Evaluation of fibre distribution parameters In order to assess the distribution and orientation of fibres, an image analysis technique was implemented due to its simplicity and relatively low cost [15, 16]. This technique comprised of four main stages: in the first stage, the fracture surface of the specimen was grinded. Then the surface was polished by acetone in order to increase the reflective properties of steel fibres. Secondly, by using a high resolution digital photograph camera, a coloured image of the grinded surface was obtained. Finally, the achieved image was analysed using ImageJ [17] software to recognize steel fibres. The analysis procedure of an image was depicted in Fig.7. After analysing the results, the following parameters were derived out: Flow direction Notch direction Flow direction Notch direction