Fracture Toughness of Fiber Reinforced Concrete.
Annual rept. 1 Jun 83-1 Nov 84,
NORTHWESTERN UNIV EVANSTON IL DEPT OF CIVIL ENGINEERING
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The primary accomplishment during the reporting period was the development of nonlinear fracture criteria for cement-based composites. These fracture criteria accurately predict two important aspects of crack growth in concrete, fiber reinforced concrete and other cement-based composites 1 nonlinear process zone and 2 quasi-stable crack growth beyond the peak load. A preexisting crack in a concrete structure propagates in two stages subcritical crack growth, and--beyond the peak load--post critical crack growth. The second stage can be observed only if the structure is loaded in a displacement-controlled mode. As a result of nonlinear subcritical crack growth, when the fracture toughness of a concrete specimen for example, notched-beam is determined using conventional linear elastic fracture mechanics, different values are observed depending upon the size of the specimen. A size-independent critical stress intensity factor and critical crack-tip opening displacement are proposed as fracture toughness criteria for concrete. To evaluate these two parameters, nonlinear deformations must be extracted from the total displacement. How to evaluate these two parameters from the three-point notched beam test is detailed. Notched-beam specimens of three different sizes and five different materials composites were tested in a closed-loop testing system using the crack mouth opening displacement as a feed-back control. Crack growth was microscopically monitored.
- Ceramics, Refractories and Glass