Damage mechanisms analysis of a multi-scale fibre reinforced cement-based composite subjected to impact and fatigue loading conditions

摘要

For several years, Laboratoire Central des Ponts et Chaussees (LCPC) has worked on the development of new cement composites in order to obtain materials sufficiently tough and ductile to be used in structures or structural elements without any other reinforcement that fibres. Then a multi-scale fibre reinforced cement-based composite (MSFRCC) has been developed and patented. It is principally characterized by a high percentage of fibres, percentage equal to 11 percent per m~3. Three fibre dimensions are used in this composite. In the present article, a qualitative analysis of damage mechanisms of this material under impact and fatigue loadings is proposed. Concerning impact loading condition, the main conclusions are: Apparent fibre-matrix adherence, which increases with the loading rate, leads to an increase in material modulus of rupturel, an increase much greater than for all existing cement-based materials due to high percentage of fibres used; Mechanical homogenization of composite with loading rate is the result of cracks delocalization during cracking process. This delocalization results from viscous effects generated within the matrix and around the fibre-matrix interfaces. Concerning fatigue loading condition, the main conclusions are: Intermediate fibre length (high percentage of meso-fibres) that is highly and positively involved in material static tensile strength, corresponds to scale of fibre that is sensitive to fatigue loading. As a matter of fact, meso-fibres become rapidly inactive and composite can no longer behave as a multi-scale reinforcement material. Material strength is then greatly affected. If the initial cracking state of the material before fatigue loading corresponds to a state of tensile strain that is less than or equal to 1.27 10~(-3), meso-fibres perfectly play their role with respect to relevant cracks (i.e. meso-cracks whose opening corresponds to their mechanical efficiency domain, that means less than 100 mu m), material fatigue behaviour being then good (fatigue rupture after 2 millions of cycles). Specimens that did not break before 2 millions of cycles have better residual bending behaviour (gain of 6.5 percent) than reference specimens (specimens which were not previously loaded in fatigue) This result is the consequence of a morphological modification of cracks due to fatigue loading. Indeed, fatigue cycles lead to a gradual "blunting" of crack tips, cracks that subsequently become less dangerous with respect to their potential propagation.