The goal of this thesis is to develop an experimental setup to evaluate and analyze the effect of crack-parallel stress on the fracture response of materials. In standard fracture test configurations, the crack-parallel normal stress is negligible. However, a new type of experiment, briefly named the gap test, revealed that it was not the case for many quasibrittle and ductile materials. This experiment consists of a simple but new modification of the standard three-point-bend test. Plastic

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... with a near-perfect yield plateau are used to generate a constant crack-parallel stress, and the end supports are installed with a gap that closes only when the pads yield. This way, the test beam transits from one statically determinate loading configuration to another, making evaluation easy and accurate. In addition to the gap test, the size effect method, devised in the 1990s, was used to obtain an unambiguous fracture energy based on the geometric scaling of cracked structures with positive geometry. Unlike the work-of-fracture method which measures the total fracture energy on structures of one size, the size effect method was shown in 2014 to give a unique value of the initial fracture energy. It can be widely applied. For quasibrittle materials, i.e. heterogeneous materials consisting of brittle phases, concrete was used as a typical example. The gap test showed that a moderate crack-parallel compressive stress could increase up to ≈ 2 times the Mode I (opening) fracture energy of concrete, and reduce it to almost zero when approaching the compressive stress limit. Behavior with a similar trend can be observed within the characteristic length scale, which may be explained by the interplay between friction, interlocking and dilation of microcracks and microsurfaces. To explain this phenomenon, computational models were used, but not all of them could reproduce the results. In particular, the line-crack models, including the basic and enhanced phase-field models with one or two phase variables (PFM), cohesive crack models (CCM), and extended finite element method (XFEM) HPC at Texas Advanced Computing Center (TACC), and Carbon HPC at Argonne National Laboratory for providing computing resources. Above all, I would like to express my deepest gratitude towards my co-advisors, Prof. Zdeněk P. Bažant and Prof. Horacio D. Espinosa, for their guidance, support, and encouragement. For Prof. Bažant, I sincerely appreciate the amount of diligence and dedication he put into this project (and others that we have been working on). His demanding advising style encourages me to obtain more knowledge skills to prove or disprove a hypothesis, which leads me to a level I would never imagine. For Prof. Espinosa, I am indebted to him for teaching me a new language that my seven-year-ago-self would not appreciate-experimental mechanics. He encourages me to dig deeper into the physics of a problem and use the obtained knowledge to guide the model development. Both of my advisors help get me through new challenges that even they are not familiar with, making my time here a wonderful journey of finding what I am really excited about. I would like to send many thanks to the committee members: Prof. Gianluca Cusatis and Prof. Victor Lefèvre for spending their precious time to participate, read, and provide constructive feedback on my thesis. I'm also grateful to Prof. John Rudnicki for his 3.9 Simple asymptotic matching of small-to-large-scale yielding .