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Suggestion of fracture toughness for controlling the ballast materials quality. Abstract . . . . . . . . Among the existing transportation systems, rail transport systems have special privileges including high capacity, safety, economy and etc. Structure of railways is composed of two main sections of substructure and Superstructure. Ballast is a layer between sleeper and sub-ballast that is composed of broken stones. These stones have uniform gradation. Since ballast bears heavy loads, its high quality is important. If the ballast is gradually crushed and fines are generated, stone permeability and drainage property will be damaged and finally, instability will be resulted. If the above problems occur at ballast, its substitution is necessary that has high costs. Ballast layer quality depends on materials and their density. Ballast must be resistant against applied load. For Ballast quality control several tests has been suggested in 301 railway magazine that after the test on Ballast, the results are compared with allowable values and Ballast quality was controlled. Ballast grains subjected to train loads, contain numerous cracks. From the fracture mechanic point of view, when stress intensity factor at the crack tip reaches the mixed-mode fracture toughness, crack growth begins and Ballast grains crush. Ballast crushing and abrasion are the main reasons for ballast contamination which is the main source of various defects, particularly geometric defects in the railway. Since disc-type specimens are among favorite test samples for determining mode I and mixed mode fracture toughness in brittle materials like rocks, Therefore it is suggested to add the determination of the mixed-mode fracture toughness I and II with the semi-circular bend specimens subjected to three-point-bend loading to the ballast quality control in 301 railway magazine. In this research, discussed the importance of fracture toughness in extraction, crushing and operation of Ballast, then the finite element method and experimental is used to analyze a semi- circular disc specimens under bending load and the fracture toughness of two Ballast mine, Anjilavand and Gaduk was determined. Anjilavand and Gaduk mine is two Ballast mines in Iran that using them for Ballast layers in railway substructure. The crack parameters KI, KII and T are calculated for different mixed-mode from pure mode I to pure mode II. YI, YII, and T* are the non-dimensional forms of KI, KII, and T, respectively. These parameters are functions of the crack length ratio, the crack angle and also the location of loading supports in the semi-circular bend specimen. The curves of YI, YII, and T* extracted for various combinations of modes I and II. Since the results for mixed mode fracture resistance of brittle materials are usually presented in a normalized form as KII/KIc versus KI/KIc, where KIc is a material constant called the pure mode I fracture toughness, using the mixed-mode fracture criteria, capped failure modes I and II for both ballast rocks were extracted and compared. Fracture toughness of Anjilacvand specimen is greater than Gaduk specimen that shows the Anjiavand Ballast for operation in railway is better than Gaduk Ballast.

Keywords: Fracture toughness, Ballast, Semi-circular bend specimen

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