PSI - Issue 47

Ezio Cadoni et al. / Procedia Structural Integrity 47 (2023) 331–336 Author name / Structural Integrity Procedia 00 (2023) 000–000

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actual condition. The 3D-Modified Hopkinson Bar (3D-MHB) was developed (Cadoni and Albertini (2011); Cadoni et al. (2015, 2018)) in order to study the dynamic behaviour of materials in confined conditions. It is well-known that loading rate a ff ects quasi-brittle material fracturing mechanism (Cadoni et al. (2001); Albertini et al. (1997)) and mechanical properties, including strength and absorbed energy. Dynamic compressive behaviour of quasi-brittle materials is a ff ected by many factors, such as porosity, water content, boundary condition at the interface (specimen - bar), and confining pressure.

2. Experiments

2.1. Materials

Commercially available Ultra-High Performance Fibre-Reinforced Concrete products were analyzed. These prod ucts are known for their exceptional strength and durability, achieved through a highly resistant matrix (with a wa ter / cement ratio of 0.17) and high steel fiber reinforcement. The addition of 3% high carbon straight steel fibers (13mm long and 0.20mm in diameter with an aspect ratio of 65) resulted in an elastic modulus of 51 GPa, density of 2473 ± 35kg / m 3 , compression strength of 197 ± 5.3 MPa and flexural strength of 37 ± 1.5 MPa after 28 days. Three cylinder specimen geometries with diameters of 30 mm and heights of 30, 45 and 60 mm were used. For simplicity, the matrix material is hereafter referred to as UHPC, and the fiber-reinforced material as UHP(FR)C. The compressive tests were conducted using the 3D-MHB apparatus (Cadoni et al. (2015, 2018)) located at the DynaMat SUPSI Laboratory of the University of Applied Sciences and Arts of Southern Switzerland. This device consists of a pre-tensioned cylindrical bar, two bars with equal square cross-sections (50 mm sides), all bars are made of Maraging steel. The pre-compression state can be imposed with a 2 MN hydraulic actuator at the end of the output bar. The three bars’ dimensions are shown in Fig.1, and the total length of the 3D-MHB apparatus is 7.82 m along the impact axis. The bumper system increases the length to 8.80 m. A rectangular loading pulse of 2 MN amplitude and 800 µ s duration can be generated by pulling the pre-tensioned bar using a 4 MN hydraulic actuator. 2.2. Experimental set-up

Fig. 1. Experimental 3D-MHB device (Cadoni et al. (2018))

According to Fig.1, the steps for conducting a test with the 3D-MHB are as follows: (i) Apply a quasi-static stress state to the UHP(FR)C specimen by using the hydraulic actuator at the output bar (7); (ii) Pull the pretensioned bar (2) with the main hydraulic actuator (1) to the desired preload value, which is held by the fragile bolt, pretensioned bar, and contrast ring (3). Due to the elastic movement of the system, it is possible to check and adjust the preload on the specimen using actuator (7); (iii) A mechanical system ensures that the bolt is loaded without a ff ecting the rest of the apparatus. As a result of the failure of the fragile bolt in the 3D-MHB, a rectangular square pulse travels through the input bar (4) and output bar (6), loading the specimen (5) dynamically until it fails; (iv) The strain gage on the input and output bars record the incident, reflected, and transmitted waves, allowing for the calculation of stress-strain curves. Fig. 2 shows the load versus time curve of a 60 mm specimen.