Investigations on crack behavior in lamellar and brick-mortar alumina/epoxy composites

Abstract

Due to the crucial need and importance of materials with high strength and toughness across numerous fields including aerospace, automobile, and protective systems such as sports and military armors, enormous efforts have been made to fabricate nacre-like high performance materials. Nacre (the inner layer of mollusc sea shell) has several structural features at different length scales. At mesoscale, the nacre looks like a structure with interlocked bricks due to the presence of dove-tail kind of features. At microscale, the micron-sized hard bricks are interlayered with few nano-meters thick soft phase to form a brick-mortar type structure. The hard bricks in the nacre have nano-asperities and pillars, that act as a frictional stop. The structural hierarchy present in the nacre generates multiple strengthening and toughening mechanisms that makes it highly strong and tough. Inspired from the fascinating mechanical behavior of the natural nacre, several attempts have been made in the past to mimic the structural features to develop stronger and tougher artificial materials. But, it is still challenging to surpass the properties of the nacre in the artificial materials. Although, improvements have been made using multitudes of fabrication techniques, the bi-directional freeze casting method emerged as a promising route to fabricate artificial composites that mimics several structural features of the nacre. In the bi-directional freeze casting, the polydimethylsiloxane (PDMS) wedge (low thermal conductivity) placed in between cold copper plate and ceramic suspension plays an important role to modulate the microstructural features such as lamellae thickness and interlamellar spacing and orientation of the microstructure. In this context, there is lack of understanding that how temperature gradients across the suspension depend on the PDMS wedge angles? What will be the freezing front velocity in the bi-directional freeze casting? How temperature gradients and freezing front velocities influence the microstructure of the ceramic scaffold? The freeze casted ceramic scaffolds have directional pores/lamellar channels. Hence, fundamentally it is expected that the lamellar/brick-mortar composites made using ceramic scaffolds as preform, will show anisotropic behavior. Consequently, it becomes essential to determine the anisotropic behavior of such composites. Because the development of tough materials is directly linked to the fracture mechanics aspect, it becomes vital to characterize the crack behavior in newly developed anisotropic lamellar/brick-mortar composites. This work explores the fabrication and characterization of lamellar/brick-mortar composites to address the above-mentioned key issues. Due to the crucial need and importance of materials with high strength and toughness across numerous fields including aerospace, automobile, and protective systems such as sports and military armors, enormous efforts have been made to fabricate nacre-like high performance materials. Nacre (the inner layer of mollusc sea shell) has several structural features at different length scales. At mesoscale, the nacre looks like a structure with interlocked bricks due to the presence of dove-tail kind of features. At microscale, the micron-sized hard bricks are interlayered with few nano-meters thick soft phase to form a brick-mortar type structure. The hard bricks in the nacre have nano-asperities and pillars, that act as a frictional stop. The structural hierarchy present in the nacre generates multiple strengthening and toughening mechanisms that makes it highly strong and tough. Inspired from the fascinating mechanical behavior of the natural nacre, several attempts have been made in the past to mimic the structural features to develop stronger and tougher artificial materials. But, it is still challenging to surpass the properties of the nacre in the artificial materials. Although, improvements have been made using multitudes of fabrication techniques, the bi-directional freeze casting method emerged as a promising route to fabricate artificial composites that mimics several structural features of the nacre. In the bi-directional freeze casting, the polydimethylsiloxane (PDMS) wedge (low thermal conductivity) placed in between cold copper plate and ceramic suspension plays an important role to modulate the microstructural features such as lamellae thickness and interlamellar spacing and orientation of the microstructure. In this context, there is lack of understanding that how temperature gradients across the suspension depend on the PDMS wedge angles? What will be the freezing front velocity in the bi-directional freeze casting? How temperature gradients and freezing front velocities influence the microstructure of the ceramic scaffold? The freeze casted ceramic scaffolds have directional pores/lamellar channels. Hence, fundamentally it is expected that the lamellar/brick-mortar composites made using ceramic scaffolds as preform, will show anisotropic behavior. Consequently, it becomes essential to determine the anisotropic behavior of such composites. Because the development of tough materials is directly linked to the fracture mechanics aspect, it becomes vital to characterize the crack behavior in newly developed anisotropic lamellar/brick-mortar composites. This work explores the fabrication and characterization of lamellar/brick-mortar composites to address the above-mentioned key issues. (XFEM) are carried out to compliment the experimental understanding of crack behaviour in lamellar composites. It is shown that the higher anisotropy makes the crack path more tortuous and thus increases the fracture toughness of lamellar composites having thicker alumina phase or higher crack orientation. Further, the effect of change in microstructure form lamellar to brick-mortar on the strength and fracture toughness of alumina/epoxy composites is analysed using experiments and an analytical model based on shear lag theory. The experimental results show that both the strength and fracture initiation toughness is comparatively higher for brick-mortar composites in comparison to the lamellar composites. However, the critical toughness Jc is maximum for lamellar composites due to higher crack tortuosity. The shear lag analysis suggests that the relatively shorter bricks carry uniform shear stress in comparison to longer lamellae, and hence shows maximum resistance to fracture initiation (corresponds to maximum elastic strain energy density) for an optimum overlap length. It is envisaged that the findings of the present study will be a significant step towards the development of nacrelike lamellar/brick-mortar composites for structural applications.