Study On The Compressive Mechanical Propertyof Fiberreinforced Composites With Waviness Defect

As a representative of the new advanced composite materials,fiber reinforced plastic composites(FRP)are widely used in wind energy,marine engineering,aerospace and other high-end fields due to their specific high modus,high strength and excellent anticorrosion performance.However,defects are inevitable during the manufacturing process because of the internal or external factors.Fiber waviness / wrinkling is a kind of very common defects in the manufacture of composites,which can change the geometry or cause the misalignment of fibers on the microscopicscale,leading to the significant reduction of stiffness and compressive strength,resulting in the restriction of extensive use of FRP.In this study,a uniform waviness is selected as a research object based on unidirectional composites.Effects of waviness on the stiffness and compressive properties of FRP are accurately quantified and assessed by the FEA and experiment for prediction and validation.Firstly,the model that fiber arranged straightly and regularly in the matrix is selected to build the Representative Elements(RVE)for research with two kinds of material systems(AS4 carbon/3501-6 epoxy and E-glass/8084 vinyl ester)and three kinds of fiber arrays,homogenization method is also used to calculate the effective modus by ANSYS.Then the experimental data is compared with the numerical simulation results.The model for stiffness prediction whose data fit best with the experimental results is selected for the next research.The effects of fiber volume fraction on the stiffness of the composites are also explored by this way.Secondly,based on hexagonal random arrays the unit cell(UC)with the periodic boundary conditions,the stiffnessof unidirectional composite with uniform waviness is determined at different waviness ratio under the fixed fiber volume fraction.By this way,the differences of effects on different material systems are analyzed.At the same time,the UC is divided into several sub unit cell,then the average normal stresses of the sub unit cell are calculated by the homogenization method.So,acting as one of the macroscopicstrengththeory for orthotropic material,Tsai-Wu Tensor Failure Criterion is employed to predict the compressive strength of unidirectionalcomposites with uniform waviness.Thirdly,a two dimensional unit cell with uniformed waviness is built,and then the compressive strength of the material is determined by two critical stresses: the matrix shear failure stress and the instability in its actual sense before the shear failure is reached,which is called fiber buckingfailure theory based on micro-mechanics theory.The failure modes are also determined by the way: at the low waviness ratio,the lamina is damaged because of the instability,and in the high waviness ratio,the damage is determined by the shear failure of the matrix.The last,the experimental model of out-plane waviness is constructed,and then the numeric simulation results can be demonstrated by this way.The experimental results show that the longitudinal young's modulus and Poisson's ratiomatch well with numeric simulation results.It proves that the FEM for stiffness prediction is reasonable.While the experimental results for compression strength match well with the simulation results only at the high waviness ratio,at the low waviness ratio,the simulation results for compression strength are far away from the experimental results.