PSI - Issue 28

Ezio Cadoni et al. / Procedia Structural Integrity 28 (2020) 933–942 Author name / Structural Integrity Procedia 00 (2020) 000–000

935

3

Table 2. Quasi-static results of UHPFRCs in bending and compression on prisms (40x40x160 mm 3 ). Material Density Flexural strength

Compression strength

(kg / m 3 )

(MPa)

(MPa)

7 days

28 days

7 days

28 days

7 days

28 days

UHPFRC 1% 2355 ± 37 UHPFRC 2% 2384 ± 32 UHPFRC 3% 2497 ± 27 UHPFRC 4% 2542 ± 23

2362 ± 33 2414 ± 33 2473 ± 35 2549 ± 43

22.0 ± 1.0 27.4 ± 2.7 30.2 ± 4.6 39.4 ± 2.5

23.6 ± 1.3 26.8 ± 1.4 37.0 ± 1.5 42.0 ± 6.7

137.2 ± 3.4 144.2 ± 1.0 153.6 ± 4.5 162.3 ± 3.4

177.6 ± 3.1 187.2 ± 2.6 197.0 ± 5.3 203.0 ± 4.5

Fig. 1. Scheme for shear tests

Observing Fig. 2a it is possible to note: i ) a combination of tensile and shear stresses in the measurement area; ii ) a concentration of the contact pressure towards the axis; iii ) a progressive rotation of the sample during the test with localisation of the contact pressures; iv ) a superposition of the e ff ects on the resulting signal to the gauge due to shear and compression into interfaces. The variability of the conditions of contact (geometric tolerances in conjunction with the fragility of the material) can lead to important di ff erences in the dynamic shear loading condition, hardly uncoupled in response to shear the material. At the contrary, the second solution (see Fig. 2b) highlights important advantages compared to the previous one and can be resumed as follows: i ) more uniform contact pressure; ii ) uniformity of stress in the measurement area; iii ) axial-symmetry and the absence of bending; iv ) lower contact pressures and limited risk of failure in the contact zones; v ) accidental non-symmetric contact can be compensated more easily; vi ) easy preparation of the specimen. On these basis the second configuration was selected as solution for the specimen shape in dynamic direct-shear test. In order to analyse the optimal shape of the specimen three di ff erent depth of notch have been studied as depicted in Fig. 3. In Fig. 3a is shown the symmetric specimen. The numerical analysis has highlighted the following results: • fracture is a ff ected by inertial forces on the specimen, • the resulting gauge on the output is a ff ected by the propagation of the fracture shear / tension, • the problem is very coupled with the loading rate, • the identification of the material properties can be di ffi cult. The solution in Fig. 3b is a much better solution but the tensile failure is still present. Finally, the best solution respect to the previous one is represented by Fig. 3c. Tensile failure is present but it occurs after the shear failure. The support flange can be changed due to production requirements. The changes have no influence on the response to the gauge.