Formation Mechanism Of In-plane Fiber Waviness And Its Effect On Performance Of Composites In Automated Fiber Placement

Due to its high efficiency,flexibility,quality and low material waste,automated fiber placement?AFP?process has distinct advantages over other kinds of automated manufacturing processed for large and complex composite parts,such as S shaped in-let,fuselage and central wing box.However,the development of AFP in China is still in early stage and most of studies on AFP are concerned about basic areas,including AFP machine configuration,process parameters optimization and trajectory generation.Currently,influences of process induced defects on the performance of composite have not been fully researched.This paper mainly studied the generation mechanism of process induced defects and its influences on the performance of composite parts,especially for the in-plane fiber waviness.Firstly,possible kinds of process induced defect were obtained by a systematic analysis of mechanical characteristics and deformation of prepreg tow used in AFP process.Compared with the formation condition of other defects,in-plane fiber waviness is worthy of further study.The distribution of single buckled fiber was described as sine curve.Based on the relationship between fiber off-axial angle?_max and fiber axial compressive strain?,as well as that of amplitude-wavelength ratio A/L and?,axial compressive strain can be defined as a macroscopic parameter to characterize the magnitude of in-plane fiber waviness.What can be found is that the larger axial compressive strain?is,the larger value of A/L or?_max is,the more severe in-plane fiber waviness is.Secondly,to study the effect of fiber waviness on the performance of composite,four models in accordance with possible distribution of micro-buckled fibers are proposed and defined as uniform scale and iso-phase model,uniform scale and random-phase model,gradient scale and iso-phase model,gradient scale and random-phase model,respectively.Equations of laminate engineering elastic constants containing different scales of in-plane fiber waviness are derived for above models,combining with maximum stress theory and two-dimensional lamina theory.Results show that the effect trend of fiber waviness on engineering elastic constants is same in different models.The longitudinal tensile modulus,in-plane shear modulus and Poisson's ratio of lamina with in-plane fiber waviness fade with increasing scale of fiber waviness,while the influence of in-plane fiber waviness on transversal tensile modulus can be ignored.Moreover,in common scale level of fiber waviness in AFP,differences of engineer elastic constants obtaining by different models with same scale of fiber waviness is too low to take into consideration.Therefore,the actual distribution of fiber waviness can be described by uniform scale and iso-phase model to simplify the computational.Results of finite element analysis validate the theoretical analysis on the influences of fiber waviness on composite performance.Thirdly,a new approach to prepare the test specimen with in-plane fiber waviness is proposed in consideration of the mismatch between the current test standard and actual fiber trajectory.According to the generation mechanism of in-plane fiber waviness,two-dimensional bending deformation of prepreg tow is replaced by one-dimension axial compressive deformation by a experimental set-up.Specimens with different scales of in-plane fiber waviness are prepared by this experimental set-up and the distributions of fiber are obtained by optical microscope.Results show that the desired fiber waviness can be generated by the experimental set-up,which validate that the magnitude of in-plane fiber waviness can be characterized by axial compressive strain.Furthermore,experiment results have good agreements with theoretical analyses.Fourthly,the scale of fiber waviness arisen during the AFP process can be evaluated by the relationship between geodesic radius of trajectory?and critical buckling radius?_cri corresponding to the set axial compressive strain of prepreg.If?is larger than?_cri,actual compressive strain?is smaller than set compressive strain?_cri.Thus no wrinkles induced by excessive deformation.The layup quality of this fiber path is good.Otherwise,the layup quality of fiber trajectory becomes worse with declining value of?,when?is smaller than?_cri.This evaluation criterion can be verified by he tlayup qulity of fixed angle trajectory on a cone.Finally,some methods to reduce or eliminate the negative effect of process induced defects on composite parts are also been proposed based on above results.For fiber waviness,a set of lateral bending layup experiments show that fiber grade,width and thickness of prepreg,compaction force,layup temperature and rate influence the generation of fiber waviness.So the layup quality can be enhanced by parameters optimization,reasonable selection of material and fiber trajectory.If the width of prepreg tow is uniform and accurate and the distributions of gap or overlap are homogeneous,mechanical properties of composite parts will dramatically improve.