Mechanical Behavior Of Materials And Structures In The Form Of Brick And Mortar Structure

Survived over millions of years,nature creature formed lots of natural biological material with excellent properties.Nacre materials not only possess high strength but also present high toughness has attracted much attention from researchers.Study shows that the superior mechanical properties of nacre come from its unique structure,“Brick-and-mortar”(B-and-M)structure,in which Calcium Carbonate tablets arranged together with organic biopolymers.It makes Calcium Carbonate possess a tensile strength of 70-180 MPa and a fracture strain up to 1%.The high toughness is due to the delicate multilevel structure combined with mineral bridge,polygon grains,nano protrusions and “Brick-and-mortar” structure.The existence of these factors not only makes it difficult to fully explain its mechanic mechanism,but also greatly inspired people for the design and construction of high performance materials.Based on this,nacre of aquaculture Hyriopsiscumingii in Zhejiang were investigated by observation of microstructure and mechanical behavior,to explore the toughening mechanism.Based on it,the “B-and-M” structural laminated plates and cylindrical shells were constructed to study the influence of microstructure on mechanical properties,which is useful for the exploration and technical support for the material application in the fields of protective structure etc..The main contents of the current research are as following:1.Microstructure observation and mechanical properties tests of nacre were carried out;Nacre microstructure was observed by scanning electron microscopy(SEM)and its microstructure morphology;Nonindentation test system was used to test the hardness,stiffness,and elastic modulus of nacre in different directions,and the influence of loading depth,loading rate etc.were analyzed.The tensile,compression,three point bending test were carried out for nacre and the influence of water immersion on nacre was analyzes.Experimental results show that: nacre material is anisotropic and sensitive to the strain rate,and the elastic modulus and the hardness increase with the decreases of strain rate;but the strain rate sensitivity of properties in different directions is different,and the sensitivity of thickness direction of nacre is more obvious;the strength and toughness of nacre was increased by water infiltration,and increased with the increase of immersion time.2.A simplified theoretical model of equivalent elastic modulus was put forward.According to the microstructure form of the approximate periodic distribution of nacre,a simplified analytical model was established to calculate the equivalent elastic modulus of the pearl layer in different directions.On the basis of the above analysis,the influence of the material properties and size on the equivalent elastic modulus was analyzed.Results show that the elastic modulus of mortar has a great influence on the equivalent elastic modulus of the material.With the decrease of the elastic modulus of mortar,the equivalent modulus of “B-and-M” structure decreases.When elastic modulus of mortar decreases to 20% of elastic modulus of brick,the equivalent elastic modulus decreases sharply.Therefore,in the “B-and-M” structural composite material,the elastic modulus of the mortar material should not be less than 20% of the brick material for maintaining the excellent properties of the entire structure.3.The finite element model of “B-and-M” structure was built to study the basic mechanical properties and the dynamic response as well as the anti penetration ability of cylindrical shells and plates structures under impact.According to the microstructure of nacre,regular hexagonal and irregular random polygon “B-and-M” structural composite plate and shell models were established using ABAQUS software to simulate the structure response of “B-and-M” structural laminated composite plates and shells under bullet impact.Simulation results of composite plates show that the “B-and-M” structure can effectively improve the toughness of flat plates;the ceramic plate under impact load forms a conical failure shape,however the “B-and-M” structure under same load forms cylindrical failure shape.“B-and-M” structure has a larger deflection and high deformation;the impact performance of “B-and-M” structural composite plate is better than that of single material composite structure.In the same condition,the damage degree of “B-and-M” structural composite plate is smaller than ceramic plate,mainly displays in small hole,high-speed debris,low speed bullet residual velocity and high ballistic limit velocity;energy absorption of irregular “B-and-M” structural composite plate is better than the regular one,and increasing the volume fraction of brick in each layer,or reducing the volume fraction of brick layer can effectively improve the toughness of composite plates and energy absorbing ability.Simulation results of “B-and-M” structural composite cylindrical shells show: compared with pure ceramic cylindrical shell,B-and-M” structural composite cylindrical shells have lower stiffness and lower ballistic limit velocity,but only partial failure for composite cylindrical shell and maintain good structural integrity;the Impact resistance of regular “B-and-M” structural composite cylindrical shell is better than the irregular one,mainly reflected in the small extent of damage,the low residual bullet speed,and the less debris.