Wednesday  |  Phipps  |  11:50 AM–12:10 PM

#13515, Gap Test Consequences for Fracture Mechanics of Quasibrittle Materials and Plastic-Hardening Metals

In all the standard fracture test specimens, the crack-parallel stresses, σxx, σzz, σxz are negligible, and so their effect has been obscured. However, such an effect on the material fracture energy is strong, as revealed in 2020 by a new type of experiment, called the gap test [1,2]. Its basic features are: (i) plastic support pads with near-perfect plastic yielding introduce first notch-parallel compression σxx; (ii) rigid supports are installed with gaps and engage only after σxx reaches a yield plateau in the pads; (iii) conducting the tests on geometrically scaled notched specimens of sufficiently different sizes, one can evaluate the fracture energy Gf as well as the effective width of the fracture process zone (FPZ). Thanks to this novel yet simple setup, the test beam transits from one statically determinate loading configuration to another, making evaluation unambiguous. For concrete, the gap test shows that crack-parallel compressive stress can double the fracture energy Gf of concrete or reduce it to almost zero. Similar scaled gap tests are conducted on aluminum, which is more complex to evaluate3 since one must separate the well-known effect of the millimeter-scale plastic-hardening yielding zone that surrounds the micrometer scale FPZ. Preliminary test results for aluminum show that σxx has not only the well-known interactions of σxx (= T-stress) with the yielding zone but also a significant micrometer-scale effect on the Gf (or Jcr). By the time of the conference, further gap test results should be available for gas shale and fiber composites, as well as microscopic observations of FPZ width. Asymptotic properties are discussed, finite element simulations are presented, and various practical consequences are pointed out.

References 1, 2 and 3 are freely downloadable as # 612, 613 and 620 from http://www.civil.northwestern.edu/people/bazant

Zdeněk Bažant   Northwestern University
Hoang Nguyen   Northwestern University
Abdullah Dönmez   Istanbul Technical University