Mechanical behaviours of functionally graded graphene reinforced composite beams with open edge cracks

This thesis investigates the mechanical behaviours of functionally graded (FG) composite beams with open edge cracks. The parametric study considers the distribution patterns and weight fractions of graphene nanoplatelets (GPL) on the FG beam. Aspect ratio and width-to-thickness ratio reveals a limit tendency of the dimensions of GPL with the best mechanical characteristics. Crack on the FG beam is calculated in terms of crack depth ratio (CDR) and its crack location. Some traditional boundary conditions and elastic-ended restraints supplements on the edge-cracked FG beams reinforced by GPL. Temperature effect and step uniform load are also vital to study in it.

Crack is unavoidable existing in any structure or engineering products while applying the materials in our daily lives. Perfect materials cannot be found around us as the materials continue undergoing the loading, friction and wear etc. during the service lives. A known crack doesn't affect the performance of material but it is necessary to keep monitoring when the working environment and loading are changing rapidly. In the meantime, crack harms severely to the structure because of crack propagation by striking a sudden load or a prolonged servicing in adverse conditions such as under wide range of temperature and lead to the failure as a result. Crack appears in different shape, location and with an angle to the neutral axis to the structure. The influences of crack depth ratio (CDR) and crack location (L1/L) with single or double cracks on the behaviour of FG beam reinforced by GPL are investigated. An edge-crack is assumed perpendicular to the top surface of the FG beam in this study.

As users know that crack cannot be removed, the mechanical behaviour of the structure can be enhanced by changing the material constitutes along the thickness direction. A functionally graded material (FGM) is an excellent model with layer-wise structure by its own attribute such that it can upgrade and hence improve the mechanical behaviour of the beam. Every layer has equal thickness and is assumed that is perfectly bonded layer by layer without delamination. The performance of vibration, buckling, linear and nonlinear bending and dynamic response of FG beam are shown in the study as they critically affect the mechanical behaviours of the structure.

The structure reinforcement does not stop while having an outstanding performance of functionally graded material (FGM)on the beam structure. Nanofillers are widely used and with wide range of materials for the structure reinforcement. Graphene are used extensively as nanofillers reinforcing the FG beam. A remarkable result of adding a few content of graphene into the FG beam can achieve considerably reinforcement to reflect by the mechanical behaviours. GPL is usually employed when the distribution patterns, weight fraction and its geometry, are highly affected and favourable to reduce the bending effect on the FG beam with the reinforcement by nanofillers. The GPL distribution patterns area core investigation of improving the mechanical behaviour of FG beam comparing with the pure epoxy matrix. It is found that higher GPL contents concentrate on the top and bottom surface of the FG beam. It can achieve the best performance of mechanical behaviours of the beam in different aspects. GPL weight fraction follows as another study and finds the most economical amount applying for manufacture. For the next steps, the aspect ratio and geometry ratio of GPL are essential to the load transfer efficiently from the matrix to the nonfillers. Different aspect ratio and geometry ratio are studied to find out a comparative and effective ratio of the length, wide and thickness of GPL. In general, higher surface area and thinner GPL are the notable results throughout any aspects of mechanical behaviours. Slenderness ratio (L/h) is also concerned in some circumstances when the beam geometry leads higher ratio, the beam becomes longer and emerges higher central deflection of the FG beam reinforced by GPL.

Boundary conditions give very important insight to the cracked FG beam reinforced by GPL. The traditional boundary conditions in the structure are clamped-clamped, hinged-hinged, cantilever and so on at the beam with both ends. The degree of freedom is restricted the movement of the beam in some directions depending on nature of boundary conditions. A new kind of elastic-ended restraints is discovered and investigated as the above mentioned boundary conditions are not good enough and comprehensively to reflect the beam situated at the complicated and dynamic working environment. An elastic-ended restraint is a ratio of stiffness of the spring-end supports for both ends to the bending stiffness of the pure epoxy beam. The stiffness of the spring-end supports is simulated to the change of boundary conditions which helps understand the central deflection of the FG beam reinforced by GPL with an edge-crack.

Temperature is considered inevitably in the cracked FG beam reinforced by GPL as the structure is placed in the working environment. When the beam is under the room temperature, it indicates no temperature change (ΔT = 0). Temperature change is clearly identified in the study that has significant impact and cannot be prevented to the cracked FG beam reinforced by GPL. The results are observed that both cracked and uncracked beam with highest central deflection as temperature rise in the pure epoxy matrix. It is because the pure epoxy beam does not have any reinforcement in its structure and hence the severe bending effects to the entire structure are expected. If the crack exists in the FG beam reinforced by GPL with the temperature rise, it is again with higher central deflection of the beam comparing to the beam without temperature rise.

Step uniform load acting on the cracked FG beam reinforced by GPL demonstrating the effect of external force studies the dynamic response which follows the time change. Single and double cracks on the cracked FG beam are also studied together with the step uniform load showing higher deflection in specific CDR and its crack location.