Characterization Of Microstructure In Stitched Composite Laminates And Simulation Of In-Plane Mechanical Properties

A serious problem with traditional fiber reinforced polymer(FRP) laminates is their relatively poor interlaminar fracture toughness and low impacted damage tolerance which makes them susceptible to delamination when subjected to interlaminar loading. Stitching as a cost-effective method has received recognition for the remarkable improvement in the through-the-thickness mechanical properties.The introduction of the through-the-thickness reinforcement can substantially improve the interlaminar properties of laminates,such as the interlaminar fracture toughness,low-velocity impact damage tolerance and compression-after-impact(CAI) strength.The significant improvement in the interlaminar fracture toughness implies that delamination might be no longer the main failure mode of stitched composite laminates.In contrast,for the stitched composite laminates,what calling attention of researchers is that the insertion of the stitch thread forces the fibers to distort and congregate,making the in-plane microstructure inhomogeneous and generating local damages,which will affect the in-plane mechanical properties of stitched composite laminates.In this study,we focus on the parameterized and quantificational characterization of the microstructure induced by stitching,the investigation of the inhomogeneous microstructure and material properties,development of a theoretical model to predict the in-plane mechanical properties of stitched composite laminates.And we try to uncover the inherent relationship of in-plane macro-mechanical properties and microstructure and obtain the quantified connection between the in-plane stiffness as well as strength of the stitched composite laminates under tensile loading and the material system,fiber content, stitching parameters such as diameter of stitch yarn and stitching density.Based on the experimental research on the microstructure around a stitch,a representative elementary volume is divided into such domains as the undistorted region, the fiber distortion region,the resin-rich pocket and the through-thickness reinforcement section.Four microstructure parameters such as distortion length,distortion width, minor axis and major axis of stitch hole are introduced to describe the local structure induced by stitching.The microstructure parameters are measured quantificationally based on the analysis of digital images.The averaged values and the distributions of four structural parameters are obtained by statistical analysis of the dimensions.Image analysis revealed that the structural parameters are statistically distributed and fitted Gaussian density functions.The proportions between the minor axis and the other parameters are obtained.The dimension of distortion length is about 12 times(thin yarn) and 20 times(thick yarn) as that of the minor axis,the dimensions of distortion width and major axis are about 3-4 times as that of the minor axis,and the minor axis of stitch hole is about 80%of the diameter of stitch yarn.A novel fiber distortion model(FDM) is developed based on the investigation of typical morphology to predict the in-plane fiber misalignment angle and inhomogeneous fiber content using the basic microstructure parameters.This will be the foundation for evaluating the inhomogeneous effective material properties of the stitched composite laminates.It is assumed that the path of a fiber in the distortion region can be represented by a cosine function.For simplicity,the fiber volume fraction is assumed to follow a liner variation along the y-axis.The analytical expressions of the fiber misalignment angle and fiber volume fraction are derived rigorously,and the spatial distributions of fiber misalignment angle and fiber content within the unit cell of the ply are obtained using fiber distortion model.Six kinds of tensile samples of stitched unidirectional composite laminates including difference stitch densities and difference diameters of the stitch thread as well as unstitched laminate are designed in this study.The failure modes and tensile strength are recorded during the experimental process.The results show that at the low stitch density, the failure mode is fiber breakage,and the tensile strength of the stitched composite laminate is reduced by about 10-15%compared with that of unstitched laminate.The tensile strength of stitched composite laminates decreases with the stitch density and diameter of stitch thread.When the stitch density is very high,the failure mode is splitting of the composite instead of fiber breakage,so the tensile strength is very low, about 70%of that of the strength of unstitched one.Because of the distortion and inhomogeneous distribution of the in-plane fibers induced by stitching,the deformation of a single fiber is not only in the longitudinal direction but also in the transverse direction even under unidirectional tensile loading. We establish a multiple curved micro-beam model and derive governing differential equations of the model,which take into account the interaction of adjacent fibers.The stress concentration around the stitch hole can be well predicted using this model compared with the results calculated by finite element method.A two-parameter Weibull distribution is adopted to describe the statistics of fiber strength.Considering fiber random breakage and matrix shear failure,a unified multiply micro-beam model is developed to simulate the failure process of stitched unidirectional composite laminate under tensile loading,coupling with Monte-Carlo simulation technique.The in-plane tensile strength of stitched unidirectional composite laminates is predicted using the unified multiply micro-beam model coupling with Monte-Carlo simulation technique.The results simulated by this model agree well with that of the experiment,which indicates that the theoretical model,the simulation method and the program are reliable.Then,the effects of stitch density and microstructure parameters including distortion length,distortion width and minor axis on the tensile strength and elastic modulus of stitched composite laminate are studied in this paper.The results show that the tensile strength increases with the decrease of stitch spacing and with the increase of stitch step;decreases with the distortion width and minor axis of stitch hole; the effect of distortion length on tensile strength is not monotonic.The effects of stitch density and microstructure parameters on the in-plane elastic modulus are very little. The tensile strength of the composite is dispersive because of the statistical distribution of fiber strength.It is found that the tensile strength of stitched unidirectional composite laminates can be described statistically by the two-parameter Weibull distribution when the strength of fiber obeys two-parameter Weibull distribution,and the shape parameter in the strength distribution of the composite is almost the same with that of the fiber.