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A Strain Rate Constitutive Model for Fiber Reinforced Concrete Based on Split Hopkinson Pressure Bar Tests

Bagher Shemirani, Alireza | 2017

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 50615 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Naghdababi, Reza
  7. Abstract:
  8. In this thesis, experimental and numerical investigations are performed on dynamic behavior of fiber reinforced concrete (FRC) by split Hopkinson pressure bar (SHPB) and penetration tests. In the experimental part, effects of parameters such as quasi-static compressive strength of matrix (36 and 48 MPa), strain rate (3*10-5 up to 100 s-1), type of fibers (crimped steel, hooked-end steel, micro polypropylene and micro polypropylene) and volume fraction of the fibers (0%, 1% and 2%) are studied by employing SHPB apparatus on testing more than 150 specimens. Also, different types of FRC panels with 5 cm thickness were casted to perform 20 penetration tests with compressed gas gun apparatus. The panels are subjected to impact by 4340 steel ogive-nose projectiles. Depth of penetration, perforation thickness, initial impact velocity of projectile and ballistic limit velocity of different FRC panels are measured experimentally. In order to accurately determine dynamic properties of FRC by the SHPB tests, specimens should be subjected to particular incident pulse loading that can be generated by using pulse shapers. To this end, the SHPB tests are performed and effective parameters on shaping pulses such as striker bar velocity, diameter and thickness of the pulse shaper are studied experimentally and numerically. Results show that using a relatively small diameter and thick pulse shaper is appropriate for testing FRC specimens. Numerical simulation results validated by the experimental data are used to provide general guidelines to properly choose dimensions of the pulse shapers for the FRC specimens in the SHPB test. In this regard, dynamic compressive strength, modulus of elasticity, stress-strain curves, energy absorption capacity (toughness) and behavior of the FRC specimens are determined at different strain rates. Also, dynamic increase factors (DIFs) of compressive strength, critical strain and modulus of elasticity are calculated and compared with the empirical formulas of the FIB model code 2010. The impact loading results show that the hooked-end steel fibers are more suitable than other types of fibers for increasing the dynamic compressive strength of specimens. Also, to increase dynamic compressive strength, the volume fraction of the fibers is more effective than type of the fibers. The proper volume fraction of the fibers is 2% and 1% for steel and polypropylene fibers. The failure patterns of FRC specimens with different types of fibers are approximately the same, but the specimens with different volume fractions of the fibers have different failure patterns. Using the experimental results, a constitutive model for the FRC is proposed which is an extension of Holmquist-Johnson-Cook constitutive model. The FRC constitutive model takes into account effects of pressure dependence, strain rate, volume fraction of the fibers and damage evolution. Also, dynamic behavior of the FRC is simulated using the constitutive model. Considering results of the projectile penetration in the FRC panels such as the ballistic limit velocity, the dimensions of the rear face cavity (due to the spalling phenomenon) and the front face cavity (due to the scabbing phenomenon) indicate a good agreement between the experimental data and the simulation results, so the proposed model is suitable for FRC structures design under dynamic loadings. Based on the results, to improve the performance of the concrete targets against projectile penetration, increasing the compressive strength of the concrete has little effect, while adding fibers to the concrete admixture is more effective for energy absorption. For example, in the penetration test, use of plain concrete with two times higher compressive strength only improves the ballistic limit velocity up to 10%, but FRC reduces the failure area at the back face of the target and increases the ballistic limit velocity up to 50%
  9. Keywords:
  10. Fiber Reinforced Concrete ; Impact Loading ; Constitutive Model ; Split Hopkinson Pressure Bar (SHPB)Test ; Strain Rate ; Penetration ; Wosh Out ; Compression Strength

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