Interface-related shear failure

Shear Behavior Along the Bed Joints The influence of mortar joints acting as a plane of weakness on the composite behavior of masonry is particularly relevant in case of strong unit-weak mortar joint combinations. Two basic failure modes can occur at the level of the unit-mortar interface tensile failure (mode I) associated with stresses acting normal to joints and leading to the separation of the interface and shear failure (mode II) corresponding to a sliding mechanism of the units or shear failure of the mortar joint. The preponderance of one failure mode over another or the combination of various failure modes is essentially related to the orientation of the bed joints with respect to the principal stresses and to the ratio between the principal stresses. 

The favorable results obtained for both dry and wet epoxy FRP wood samples with HMR in relation to the epoxy FRP-wood samples with RF are due mainly to the enhanced bond strength imparted by the coupling agent. The failure for the epoxy FRP-wood samples with HMR occurred primarily in the wood layer and, therefore, the response of these samples was governed by the shear strength of wood, and consistent percent material or cohesive failures for both the dry and wet samples (77.8% and 89.7%, respectively) were obtained. Moreover, these percent material failures are close to acceptable allowable values (approximately 80%) for wood failure. The failure for the epoxy FRP wood samples with RF happened mainly at the interface between the epoxy FRP and wood, which resulted in lower shear strength and lower percent material failure, especially for wet samples. 

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