Micromechanics of engineered interphases in nacre-like composite structures

Although the composition of polymer matrix is only 5% by weight, the presence of polymer as an interlayer between the brittle aragonite bricks significantly enhances the toughness characteristics of the natural nacre. In this study, the mechanics of nacre-like brick-mortar structures is investigated through numerical simulations. The shear stress distribution in the matrix of nacreous composites is found to be non-homogeneous primarily due to: (i) elastic properties mismatch between brick and mortar and (ii) the periodic microstructure. This non-homogeneous stress distribution is successfully reduced by grading the elastic modulus of the matrix material so as to considerably enhance the performance of the nacreous composite. A framework has been developed here to design nacre-inspired composites incorporating a functionally modulus graded interphase material. The different parameters influencing the non-uniform stress distribution, such as: interphase thickness, elastic modulus and overlap length are studied, elucidating how such parameters can be effectively controlled to reduce the non-homogeneous stress distribution and reduce the peak stresses. The peak stresses in the interphase are observed to exponentially increase up to 100%, when overlap length is 10% of the brick length. The strength and peak stresses in the interphase are observed to be higher for thin interphases, where a 50% decrease in thickness resulted in a 40% increase in the peak shear stress, and an 80% reduction yielded 150% increase. On the other hand, the elastic modulus is observed to scale with the strength, for instance, when the modulus is increased by 20% and 50%, the increase in peak shear stresses in the interphase are observed to be 10% and 22%, respectively. Furthermore, the shear stresses in the interphase are made uniform by varying the parameters such as: interphase thickness, elastic modulus and overlap length. The developed methodology has been extended to design a nacre-like structure based on the material combination used in metal matrix composites, where the shear strength of the proposed nacre-like composite structure is found to be 32% higher than the natural nacre. The results provide a guideline for the design of nacreous composites.