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The tensile failure procedures of remoulded mudstone samples are experimentally studied using a designed deformation field inspection system. The samples are pulled to failure with the speed of 0.06 mm/min in the process of Brazilian split load. The mudstone is modified by Portland cement in order to enhance its tensile strength and the failure procedure of modified sample under tensile stress is researched. The fracture mechanism of mudstone before and after modification under tensile stress is investigated. The results show: (1) the tensile strength increases linearly with the Portland concrete content of modified samples after curing for 24 h at room temperature; (2) the deformation field localizes on the surface of unmodified samples at the early stage of loading, when the load is 58.2% of peak load, localization band occurs by means of merging deformation regions, and it forms macro crack that fails the sample; (3) the quantity and size of pores between aggregates decrease after modification with Portland cement, and the tensile strength of mudstone increases obviously because of the cementation of silicate; (4) with the increase of Portland concrete content, the position in which localization band occurs by means of merging deformation regions moves towards the load peak; (5) because of more elastic strain energy stored in modified samples, the macro crack extends rapidly when the load exceeds the peak load. [ABSTRACT FROM AUTHOR]