In this paper, we explore the particle-scale origin of the additional shear strength of unsaturated granular materials in pendular states induced by the capillary effect by applying the Stress-ForceFabric (SFF) relationship theory into unsaturated granular material stress analysis. The work is based on Discrete Element simulations with the particle interaction model modified to incorporate the capillary effect. By decomposing the total stress tensor into a contact stress tensor originating from contact forces and a capillary stress tensor due to capillary effect, the directional statistics of particlescale information have been examined. The observations have been used to support the choice of the appropriate analytical approximations for the directional distributions associated with the solid skeleton and water bridges respectively. The SFF relationship for unsaturated granular materials is hence formulated, which has been shown matching with the material stress state in good accuracy and used to interpret the material strength in terms of the relevant micro-parameters. Macro and micro observations are carried out on both relatively dense and loose samples in triaxial shearing path to the critical state. The capillary force remains nearly isotropic during triaxial shearing. Anisotropy in the water bridge probability density, however, develops alongside the anisotropy in contact normal density, which gets smaller when the suction level gets lower and the water content becomes higher.The anisotropy effect in the water phase is much smaller than the solid skeleton and it coupling effect with the solid phase makes the fabric anisotropy in wet materials smaller than that of the dry ones.Combining with the SFF function, it can be clarified that the increased solid coordination numbers and mean contact forces by water bridge effect are more important factors for the suction induced shear strength.

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