The irreversible behaviour of crushable sands is investigated by means of a Discrete Element Method (DEM) analogue. The calculations have been performed using a recently developed procedure to simulate particle crushing that significantly enhances the computational efficiency of the method. The DEM analogue is subjected to a comprehensive set of stress probes in the triaxial plane in order to identify the basic mechanisms contributing to the total deformation. Three contributions to incremental strains are distinguished: elastic, plastic-structural and plastic-crushing. The behaviour observed appears to be consistent with the classic tenets of elasto-plasticity. The plastic mechanisms associated with fabric changes and with crushing are, however, different. In addition, the plastic flows for the two mechanism do not follow the normality rule. The same type of behaviour has been obtained under a wide variety of stress conditions and stress levels.