PSI - Issue 28

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

937

Author name / Structural Integrity Procedia 00 (2020) 000–000

5

Fig. 4. Experimental set-up for the dynamic direct shear test.

All the conditions for the application of the one-dimensional elastic stress wave propagation theory are fulfilled also in the here described dynamic shear MHB experiment; therefore it is possible to apply the following relationships in order to calculate the shear stress τ , shear slip δ and shear strain-rate ˙ γ of the sample.

A 0

τ ( t ) = E 0 ·

T ( t )

(1)

A ·

δ ( t ) = − 2 C 0

t

R ( t )

(2)

0

2 C 0

˙ γ ( t ) = −

R ( t )

(3)

L ·

where E 0 is the elastic modulus of the bars, A 0 is the cross section of the input and output bars, A is the shear cross section of the specimen within the gauge length L , C 0 is the elastic wave speed in the bar, T and R are the transmitted and reflected pulses, respectively. These pulse signals were obtained from semi-conductor strain-gauges stations on the input and out bars and they were acquired by means of a transient recorder HBM-Gen2 data acquisition system. The tests were filmed by a fast camera IDT-MotionPro Y4-S3 at 25kfps (see lower left part of Fig. 4). An example of the signals obtained during a dynamic shear test are shown in Fig. 6. The use of the specimen with a unique notch ensures better stability with respect to the variability of the boundary conditions. This geometry allows to identify the shear behaviour independently from other material failure in compression or tensile as shown in Fig. 5 where the numerical simulation of the specimen in dynamic is depicted.

4. Results and discussion

The results of the experimental campaign under direct-shear at high strain-rate are resumed in Table 3. The tests were carried out by imposing the same preloading condition of 50 kN in the pretensioned bar. In Fig. 7 the shear stress versus shear-slip curves for the four di ff erent UHPFRCs, are shown. Increasing the percentage of fibres an enhanced shear capacity is observed. The deformation capacity is incremented allowing higher