Issue 24

Y. Petrov et alii, Frattura ed Integrità Strutturale, 24 (2013) 112-118; DOI: 10.3221/IGF-ESIS.24.12 112 Special Issue: Russian Fracture Mechanics School Temporal peculiarities of brittle fracture of rocks and concrete Y. Petrov, I. Smirnov, A. Evstifeev, N. Selyutina St. Petersburg State University, Universitetsky prospekt, 28, Peterhof, 198504, St. Petersburg (Russian Federation) yp@YP1004.spb.edu ; ivansmirnov@math.spbu.ru A BSTRACT . When we want to compare the strength of two materials, we compare the table values of quasi- static strength. However results of some experiments show that the strength ratio of the materials can change with increase in rate of loading. Such "substitution effect" of material strength at different strain rates is studied in this paper. It is shown that one material can have a lower dynamic strength for a higher static strength compared to the other material. Tests of two different materials, tests of mortar and concrete, and tests of concrete with different water content are considered. It is shown that load-carrying capacity of materials and the substitution effect of material strength in a wide range of loading rates can be predicted by the incubation time criterion. K EYWORDS . Static strength; Dynamic strength; Incubation time; Substitution effect. I NTRODUCTION n selecting a material of construction an engineer is usually guided by the values of mechanical parameters obtained in quasi-static tests. A construction material is selected on the basis of its ability to withstand a certain stress (as one of the defining parameters). There is a set of test standards governing determination of the ultimate strength of a material under quasi-static tension, compression, bending, etc. However tests under dynamic loading conditions show essential differences of dynamic strength characteristics in comparison with those of quasi-static tests. These differences relate to the dependence of the mechanical characteristics on the history (duration, intensity, shape of the stress pulse, etc.) and method of loading [1]. A critical value can be considered as a material constant only under quasi- static loading conditions. Under dynamic loading, the critical properties are characterized by very strong instabilities and can vary by several orders of magnitude. As a result, the dynamic system behaviour often appears unpredictable. Importance of studying rocks and concrete dynamic strength is not in doubt. These materials are highly suitable materials for building structures in the energy sector, for instance solar energy storage water tanks, nuclear containment vessels, etc. However, such civil and military infrastructures (e.g. buildings, barracks, bridges, tunnels, containment structures for hazardous, toxic and inflammable materials, etc) are susceptible to catastrophic failure under intense sudden overloading (e.g. due to blasts, explosions) for which they are not designed. A correct and rational analysis of dynamically loaded structures requires an understanding of the behaviour of the mechanical properties of materials at high strain rates. Therefore a study of the behaviour of rocks and concrete at high rates of deformation and fracture is necessary. Dependence of rocks on the extraction quarry, as well as numerous admixtures of concrete makes the research of strength characteristics a complex task. Moreover, various heterogeneity and impurity content can lead to various dynamic effects of fracture. One of such effects is the change of the dominant strength between the two materials. A material, which has a lower strength compared to another material in quasi-static tests, can have greater strength under dynamic loading. I