PSI - Issue 13

Martina Drdlová et al. / Procedia Structural Integrity 13 (2018) 1731–1738 Drdlová and Čechmánek/ Structural Integrity Procedia 00 ( 2018) 000 – 000

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The force P is obtained using the recorded stress pulses mentioned above as follows: ( ) = 2 2 [ ( ) + ( ) + ( + 0 )] (3) R – radius of the bar (m), t 0 – transition time of the stress wave in the specimen (s), where R is the radius of the SHPB and t 0 is the transition time of the stress wave in the specimen. The course of the Brazilian test was recorded using a high-speed camera Photron Fastcam Mini AX 200. For brittle materials, the time from the deformation to the final failure under dynamic loading is very short. To obtain a detailed dynamic deformation and fracture proces, high frame rate is required. To record the fracture of the specimens as accurate as possible, the rate of 160,000 frames per second was used. Digital image correlation was performed to evaluate the dynamic deformation and strain fields in the specimens using the VIC-2D software. The critical strains (i.e. maximal strain at failure) for each specimen were evaluated. 2.3. Results and discussion The indirect tensile strength at quasi-static load was determined, the average value of six specimens is presented in Table 3. It is evident that the addition of fibres increases indirect tensile strength at quasi-static load, the rate of the increase depends on the mechanical properties of incorporated fibres. Load-deformation course was recorded for each specimen, see Fig. 2. It is evident, that addition of any type of fibre increases the quasi-static tensile strength, with the best result achieved using aramid reinforcement. The presence of the fibres affects also the course of deformation of the specimen during the load. The critical tensile deformation values are higher in the case of specimens with fibres, the highest approximate value of 0.9 mm was achieved again in the case of aramid fibre reinforced specimen (the critical tensile deformation value for specimen without fibre was calculated 0.64 mm, with PP fibre 0.82 and carbon fibre 0.76 mm). To determine the dynamic tensile strength, the splitting method (Brazilian test), described in Chapter 2.2, was used. Even it was originally proposed for determining the quasi-static tensile strength of brittle materials, Rodriguez et al. (1994) carried out experimental and numerical analysis of the fracture of ceramic specimens under a splitting force and concluded that the splitting tests can also be employed to determine the dynamic tensile strength of brittle materials, when the elastic behaviour and the equilibrium state is ensured, and the failure is produced in a predictable manner. All tested materials were found to be strain rate sensitive, however to different extents. The values of the indirect tensile strength obtained at the dynamic loading are higher than those captured at quasi-static loading, see Table 3 (the average value of at least 10 specimens per mixture is given). This increase can be described by DIF fct (dynamic increase factor), which can be calculated using the following equation: = (4) σ ctimp – impact indirect tensile strength (MPa), σ ct – quasi-static indirect tensile strength (MPa) . The DIF fct values are summarized in the Table 3. It is evident that the dynamic loading leads to significant increase in the tensile strength , which is in accordance with findings of other authors (e.g. Chen (2014) or Ožbolt (2014)) . Mechtcherine et al (2011) investigated the tensile properties of strain hardening cementitious composite reinforced with short PVA fibres and reported significant strain rate effect (DIF fct 6.7 at strain rate 180 s -1 ) which he attributed to the development of a greater number of micro-cracks parallel and orthogonal to the loading direction, as well as to extensive plastic deformations of the PVA fibres prior to and during fibre pullout. In the presented study, the influence of three types of polymeric fibres on dynamic indirect tensile properties were compared. The DIF fct value is higher in the case of specimens with lower initial value of tensile strength. Generally speaking, for concrete of a lower strength, the ITZ between the coarse aggregate and mortar matrix is usually the weakest link. When the concrete is subjected to a static loading, cracks are initiated and propagated through the ITZ (around the coarse aggregate, which then linked up to form larger cracks in the matrix. When subjected to high strain rate loadings, the cracks do not have time to propagate through the weak ITZ but are forced to go directly through the stronger coarse aggregates. This is one of the reasons for the increased concrete strength at higher strain rates. However, for concrete with higher strength, the matrix is much stronger and as a result, the coarse aggregates, rather than the ITZ, may be the weakest link in the concrete. Therefore, cracks may propagate through the coarse aggregate instead of the ITZ around the coarse aggregate, even under static loading conditions. Thus, the strength increase at high strain rates for higher strength concrete is usually less significant than that of lower strength concrete, as the coarse aggregates have already contributed to the static strength of the former (Wang et al. (2017)). In presented study, the highest value (13.9) of