A bio-inspired composite system for protecting critical structural components from extreme loads

© 2017 Dr. Abdallah GhazlanAccidental and deliberate loads on civil and military structures continue to cause severe damage worldwide, along with catastrophic losses of human life. The significance of this problem is evidenced by high government spending on counter-terrorism, which has steadily increased over the past decade to several billion dollars. There is little evidence on the efficiency of such expenditures, which highlights the need for scientific intervention. This study seeks a solution from nature, which has optimised its structure over millions of years of evolution to survive extreme loads that arise from the harsh conditions of its environment. The attractive feature of natural structures is that they tend to minimise their weight by employing highly brittle minerals during their assembly but the overall structure boasts a fracture toughness that is several orders of magnitude greater. This makes natural structures highly attractive to the protective structural engineering discipline. The overarching aim of this research project is to develop a lightweight bio-inspired composite system (BCS) for protecting critical structural elements from extreme loads by identifying and mimicking the key strengthening and toughening mechanisms facilitated by the structural features of nacreous seashells. This aim spans three disciplines, namely extreme loading, biomimicry and computational geometry. Firstly, a comprehensive literature review is conducted on these three key areas to understand: 1) the physics of blast loading and shockwave dynamics; 2) the key toughening mechanisms that are employed in the armour system of nacreous seashells to protect their soft tissues from extreme loads; and 3) computational geometry techniques that can be employed to mimic several key features of these natural structures and translate them to protective structural systems. Building on this knowledge, a computational framework is developed to generate nacre-mimetic composite structures in a format that is recognised by computer aided design (CAD) and finite element programs. This framework can automatically construct and manipulate the geometry of the nacre-mimetic composite structure, which saves significant time by automating the modelling and manufacturing process. The framework utilises geometry manipulation and meshing functionalities that are already implemented in popular software packages, and implements additional subroutines where specialised functions are not available. For example, a specialised subroutine is required to automatically insert cohesive elements between polygonal bricks to model the nacre-like mortar. The geometry was developed to be transferrable to a CAD format such that the nacre-mimetic structure can be manufactured using rapid prototyping technologies such as 3D printing or laser cutting. Several analytical models were subsequently developed at the unit cell level to gain preliminary insight into the parameters responsible for the superior load transfer efficiency found in nacre. By extending current analytical studies conducted by researchers in the area of biomimicry, which mainly investigated the behaviour of nacre’s brick and mortar structure under planar tension, a shear lag approach was employed, which assumes that the tensile force applied to the bricks is transferred via shear through the mortar. As such, parametric studies were conducted to investigate the significance of the interfacial geometry and the overlap length between the bricks in adjacent layers, with the objective of quantifying the effects of these features on the energy absorbing capacity and load transfer efficiency in the composite structure. This preliminary study showed that the waviness of the interface improves the shear transfer efficiency in the mortar and maximises the load sharing efficiency between the bricks and the mortar. To this end, a more comprehensive numerical model was developed to mimic nacre’s polygonal brick and mortar structure more closely and account for fracture in the mortar, which has been observed experimentally. Voronoi diagrams, which are well-known in computational geometry, were employed to automatically generate different nacre-mimetic polygonal structures. This facilitated several key parametric studies for understanding the behaviour of the nacre-mimetic composite under quasi-static loading, whereby the numerical model was validated using experimental data available in the literature. These studies showed that the constitutive behaviour of the ductile mortar was responsible for the high toughness of nacre, which accounted for the hardening phase observed in the experimental stress-strain curve. This result was contrasted to the abovementioned simplified unit cell model, which indicated that the waviness of the interface and the overlap length between adjacent bricks played a key role. The shape of the bricks was also found to be significantly influential on the crack deflection and arrest capabilities in the mortar. The voronoi approach was again employed to investigate the dynamic behaviour and failure modes of a bio-mimetic polygonal brick and mortar panel under blast loading using finite element modelling techniques. Several parametric studies were conducted to establish the influence of different geometric and material parameters on the damage and load distribution in the composite. This model utilised the automated geometric construction and manipulation capabilities of the computational framework mentioned earlier. It was found that an increased number of layers in the brick and mortar structure increased the energy dissipated throughout the composite, which was much more prominent than a monolithic panel of equal mass. A bio-inspired polygonal brick and mortar composite structure was then manufactured from medium density fibreboard (MDF), which was bonded by a ductile polyurethane adhesive. The single edge notched specimen (SENT) utilised several key features found in nacre, namely the polygonal brick and mortar structure, reinforcing bridges between the bricks and the soft ductile adhesive bonding. The MDF panel was manufactured by utilising the automated computational framework mentioned previously and its behaviour was investigated under quasi-static loading. Compared to a brittle monolithic MDF specimen of equal mass, the nacre-mimetic composite showed significant improvements in ductility and energy absorbed. This was achieved by deflecting cracks away from the brittle bricks and into the ductile polyurethane mortar, which did not fracture at the conclusion of the test. The high potential of this study in terms of protective structures is evident because brittle non-structural materials that are abundantly found in structures, such as ceramic tiles, can be toughened to protect critical structural elements from extreme loads