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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DIF fct was observed in the case of specimen No. 4 (plain). All specimens with fibres showed lower values of DIF fct , see Table 3. The fibre network formed by the presence of the fibres in the matrix probably reduces the inertia effect and also slows crack velocities, which also influences the DIF fct . The lowest DIF fct values were achieved in the case of specimen with aramid fibres.

Table 3. The quasi-static and dynamic properties and bulk density. Designation

Indirect tensile strength (MPa)

Dynamic indirect tensile strength (MPa)

DIF fct ( (-)

Critical strain (Y axis) (%)

1 (aramid)

5.22 3.04 4.94 2.34

34.0 27.6 33.0 32.7

6.5 9.1 6.7

2.1 1.7 1.3 0.5

2 (PP)

3 (carbon) 4 (no fibre)

13.9

Fig. 2. Load-deformation curves for: specimen with PP fibres (top left); Aramid fibres (top right); Carbon fibres (bottom left) and without fibres (bottom right) In order to obtain more detailed information on the fracture process during the Brazilian test, optical non-contact technique, the digital image correlation (DIC) method, was applied on selected specimens to evaluate the deformations and strain distribution. The maximal strain values were evaluated in the x, xy and y direction – their courses in time are plotted in Fig. 3 and 4. As a result, the maximum of the strain in the y direction at the moment of fracture initiation was found. The values of this strain, which can be considered as the fracture strain (critical strain), are summarized in the Table 3. Fig. 5 depicts values of the calculated indirect tensile strength of all tested specimens versus loading rate. The effect of the particular fibre reinforcement differs. Dynamic indirect tensile strength was generally affected by fibres addition only in low extent, the calculated values were only up to 4% higher compared to plain specimen (aramid fibre), and even lower in the case of specimen reinforced with polypropylene fibre. But the maximal stress peak was achieved at higher strain (critical strain) when the fibres were incorporated. Aramid fibre possesses the highest values of tenacity and elongation, see Table 2, compared to other investigated fibres. Using aramid fibre, the highest values of dynamic tensile strength and critical strain were achieved, even though those fibres were shorter than the other tested fibres and thus critical crack opening, when no tensile stresses can be transferred, should have come earlier. Good result can be attributed to high tenacity and elongation of the fibres. Another reason could be strong bond between fibres and matrix. The critical strain in case of the specimen No. 1 (with aramid fibre) was determined 2.1%,