Effects Of Waviness Defect On The Mechanical Properties Of Fiber Reinforced Composites:Theoretical Prediction And Experimental Investigation

Advanced Composite Materials?ACMs?can offer high specific modulus and strength,and has rapid development and widespread applications.Fiber reinforced plastic composite?FRP?constituted of fibers and polymeric matrix is one of the most typical ACMs,which has a large proportion and wide applications.Moreover,FRP represents the future development of new materials.However,defects are inevitable due to the subjective and/or objective reasons during the process of FRP manufacture.Defects have greatly adverse influence on the mechanical performance of FRP such as strength reduction,pre-failure,and lead to many potential safety problems.Mismatch of coefficient of thermal expansion?CTE?,non-uniform distribution of the interlaminar pressure,and operation mistakes would cause the deviation,deformation and/or curvature of fibers,which is termed as waviness defect.Waviness defect is one of the frequently encountered defects in composite structures.Essentially,mismatch of CTE and non-uniform distribution of the interlaminar pressure may be the inherent inducements.Waviness defect is thus impossible to be avoided and eliminated,completely.Furthermore,fill and warp strands are interlaced to fulfill the structure design of textile composites,which forms a particular fiber curvature.Consequently,waviness becomes the inherent characteristic of textile composite.Formation mechanism of waviness,waviness acceptance/rejection criteria for ensuring structure safety,assessment of mechanical properties and investigation of failure mechamism for composite materials with waviness,and so on,are one of the hot topics in the FRP field.In this paper,effects of waviness on the mechanical properties of FRP are accurately quantified and assessed.Theoretical prediction and experimental investigation are employed to study the mechanical performance of the traditional laminated composite and the balanced plain weave fabric composite embedded waviness.Firstly,the geometric characteristics of out-of-plane and in-plane waviness are presented and distinguished,and the trigonometric function is employed to describe the characteristics of waviness defect.Based on classical lamination theory?CLT?,the iso-stress assumption are used to establish the model of stiffness prediction for composite laminates with out-of-plane waviness?MPM-OP?.The parameters associated with the out-of-plane waviness such as the waviness ratio,the off-axis angle,the waviness zone and the waviness pattern,are introduced during the derivation of stiffness matrix of the wavy lamina.Similarly,the parameters associated with the in-plane waviness such as the waviness ratio,the off-axis angle,and the waviness pattern are introduce dring the derivation of compliance matrix of the wavy lamina.By use of iso-strain assumption,the basic model is established.Then the position and the number of wavy lamina are included in the basic model,and the model of stiffness prediction for composite laminates with in-plane waviness?MPM-IP?.Secondly,based on the elastic properties of fibers and matrix and finite element model,the homogenization method is employed to evaluate the elastic constants of AS4/3501-6 and E-glass/8084 lamina at a range of fiber volume fractions.With employment of the MPM-OP to the AS4/3501-6 unidirectional laminates embedding out-of-plane waviness,the effects of waviness on the elastic properties studied.For different fiber volume fractions and material systems,the unidirectional laminates with out-of-plane wavienss are also investigated.The results show that the waviness ratio and the waviness zone have large effects on E_x,E_z,G_yz,G_xz,v_yx,v_zx of AS4/3501-6 laminates with uniform waviness of out-of–plane?UW-OP?as well as E_x,G_xz,v_yx,v_zx,v_zy of E-glass/8084 laminates with graded waviness of out-of–plane?GW-OP?.The GW-OP has smaller effects on laminates than UW-OP.In-plane waviness has smaller effects on mechanical properties of wavy laminates for the closer distance between the wavy ply and the geometric central plane.The loss of mechanical performance is significant if a ply contributes more stiffness in the laminates with in-plane waviness.Thirdly,CLT is employed to determine the elastic constants of the AS4/3501-6laminates with lay-up of[±45,?0,90?₂]_s.Based on the developed stiffness model of laminates with in-plane waviness?MPM-IP?,the influence of waviness parameters on the AS4/3501-6 laminates with lay-up of[±45,?0,90?₂]_s is discussed.It is shown that the elastic properties of the multi-direction laminates nearly change when the waviness ratio is larger than 0.5.In-plane waviness has smaller effects on mechanical properties of wavy laminates for the closer distance between the wavy ply and the geometric central plane.The loss of mechanical performance is significant if a ply contributes more stiffness in the laminates with in-plane waviness.Fourthly,the validation of the predictions of MPM-OP above and a prospective study of tensile and bending of laminates with out-of-plane waviness are carried out by experiments.Based on the dry E-glass preforms,a“sandwich”configuration and the“two-step method”are developed for fabrication of laminates with out-of-plane waviness.First,the“core”is fabricated by the vacuum assisted resin infusion?VARI?process.Second,the dry performs viewed as the“skin”are laid up and down of the“core”.Then the“sandwich”is infused again by VARI and the laminates with graded waviness are manufactured.Additionally,the tensile and bending specimens are fabricated and tested.Experimental data shows that the mechanical properties of the laminates fabricated by“two-step method”is the same as the laminates by normal VARI.The predictions of E_x?v_yx agree well the experimental data for GW-OP and UW-OP.Out-of-plane waviness leads to the large reduction of tensile and bending strength.The fiber curvature at a small angle induces“stiffing effect”concave-up laminates with a/?=0.02.Interlaminar shear damage and delamination tend to occur in the vicinity of waviness and cause the final failure of wavy laminates.Fifth,the fiber curvature,also called waviness in this study,is the inherent characteristic for plain weave composite where warp and fill strands are interlaced in two mutually orthogonal directions to one another.As a consequence,the similar methodology employed in the investigation of the traditional laminated composites with waviness is adapted to study the waviness within plain weave composite.First of all,a particular plain weave composite,the balanced plain weave fabric composite is selected and a new mathematical description method is developed for characterizing the geometric morphology of the balanced plain weave fabric composite.Second,based on CLT and iso-stess assumption,the morphology of warp and fill strands and pure resin are respectively considered,then the 3D model of plain weave fabric composite is developed.The elastic constants of the balanced plain weave composite are predicted and the results agree well with others'theoretical values and experimental data.The influence of waviness parameters such as waviness ratio of the warp?or fill?strand and the the warp?or fill?strand cross section,the length of the udulation,and the gap between the adjacent warp?or fill?strands on the elastic constants of the balanced plain weave fabric composite are predicted.