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Dynamic compressive and flexural behaviors of ultra-rapid-hardening mortar containing polyethylene fibers

Chun Booki, Shin Wonsik, Oh Taekgeun, Yoo Doo-Yeol

Archives of Civil and Mechanical Engineering

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2021

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Vol. 21, no. 2

576--592

EN

In this study, the dynamic, compressive, and flexural behaviors of ultra-rapid-hardening mortar (URHM) containing 2% polyethylene fiber are investigated. The results confirm the robust strain-hardening behavior of URHM at an early age of 4 h. Its tensile strength, strain capacity, and g value at 4 h were found to be 7.3 MPa, 5.1%, and 297.5 kJ/m3, respectively. The compressive and flexural strength and toughness of URHM increased with the strain rate. A higher loading rate led to a greater increase in the strength; the rate sensitivity was higher during flexure compared to that during compression. The highest dynamic increase factor (DIF) of the compressive strength was 1.75 up to a strain rate of 115/s; the highest DIF of the flexural strength was 3.34 up to a strain rate of 96/s. Its deflection-hardening behavior was converted to deflection-softening behavior under impact loads having a potential energy of 392 J or greater. Furthermore, the greater potential energy led to a lower energy dissipation rate, and more energy remained in the system. The rate sensitivity of the URHM under compression was similar to that of other fiber-reinforced concretes; however, its flexural strength was less sensitive to the strain rate than that of the others.

2

Spacing and bundling effects on rate-dependent pullout behavior of various steel fibers embedded in ultra-high-performance concrete

Kim Jae-Jin, Yoo Doo-Yeol

Archives of Civil and Mechanical Engineering

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2020

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Vol. 20, no. 2

218--235

EN

This study examines the effects of fiber geometry, spacing, and loading rate on the pullout resistance of steel fibers in ultra-high-performance concrete (UHPC). For this, three different types of steel fibers, four different fiber spacings, and three different loading rates ranging from 0.018 to 740 mm/s were considered. Test results indicated that the single straight fiber in UHPC was most rate sensitive for pullout resistance, followed by the single twisted and then hooked fibers. The bond strengths and pullout energy of specimens with multiple straight fibers were improved by increasing the loading rate but were not affected by fiber spacing. Closer fiber spacing had a detrimental effect on the dynamic pullout resistance of multiple hooked steel fibers in UHPC, while no enhancement of average bond strength of multiple twisted fibers was observed as fiber spacing and loading rate varied. The average bond strengths of single and bundled hooked and twisted steel fibers in UHPC were clearly improved by increasing the loading rate. Bundling of fibers enhanced the impact pullout resistance of all the steel fibers in UHPC. The highest dynamic increase factors for the bundled straight, hooked, and twisted fibers were approximately 3.78, 1.57, and 1.41, respectively, at the impact loads.

3

An experimental study on rate sensitivity of J-integral and its evaluation by small punch test for TRIP steel

Shi L., Iwamoto T., Hashimoto Sh.

Engineering Transactions

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2013

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Vol. 61, iss. 2

119--136

EN

Recently, much attention has been paid to TRIP steel since it indicates both high ductility and strength by strain induced martensitic transformation. This transformation allows TRIP steel to oﬀer larger energy absorption than other steel at the same strength level. Therefore, it is expected to be applied to automobiles as security components that absorb energy upon collision. To produce the best performance of TRIP steel, the J-integral of TRIP steel should be investigated with respect to a various deformation rates for an evaluation of energy absorption. In the present study, the three point bending (3B) test is conducted for investigating the Jintegral until the crack growth of TRIP steel. Then, in order to determine the energy absorption characteristic by the J-integral value at various locations in the components of TRIP steel, the size of the specimen should be very small. Thus, an SP test is introduced and conducted by using the newly established apparatus based on the SHPB method. By using the result of the SP test in conjunction with the result of a 3B test, the evaluation of the J-integral of TRIP steel subject to various deﬂection rates is attempted. The correlation between the J-integral and the equivalent fracture strain of the SP test for TRIP steel is challenged to be redeﬁned.