Impact Damage Behavior Of 3D Hybrid Woven Composites

Unidirectional fiber-reinforced composites are widely used in the aerospace industry due to their excellent in-plane mechanical properties,high corrosion resistance,dimensional stability and fatigue life.Nevertheless,they exhibit poor delamination resistance and damage tolerance,particularly under impact.The lack of reinforcement in the through thickness direction makes them particularly vulnerable to out-of-plane threats caused by foreign objects,such as ice slabs or open-rotor blade fragments impacting on skin fuselages.A cost-effective alternative is the use of 3D woven orthogonal reinforcements,in which delamination resistance and damage tolerance are improved by weaving a yarn in the throughthickness direction.This technique allows the combination of several fiber types(hybridization)and enables the optimization of the composite properties by varying the fiber content.Preforms may be infused by using out-of-autoclave processing techniques,such as Vacuum Assisted Resin Transfer Molding(VARTM),leading to considerable cost savings,as opposed to autoclave consolidation.Despite of the potential of these materials,the use of hybrid 3D woven composites is limited by the lack of experimental data and reliable models able to predict the mechanical response of the material.This work analyzes the mechanical behavior of a hybrid 3D woven orthogonal composite made up of a thermoset polymeric matrix(TDE-86 epoxy)reinforced with carbon and Kevlar fibers.Two homogenous composites preform with Carbon and Kevlar and a hybrid3 D woven composite with a combination of both are analyzed.The mechanical behavior of the material was studied under tension,compression and shear loading in weft direction.The impact response and failure mechanisms were characterized under high velocity impact of asymmetric hybrid three-dimensional orthogonal woven composite by means of experimental and numerical study.The study includes an extensive inspection campaign carried out by means of optical microscopy to ascertain the failure micromechanisms under different loading conditions.The results provide a critical information about the failure micro-mechanisms involved in the damage process,which helps to explain the macroscopic properties of the composite.The influence of hybridization was discussed in detail under quasi-static and impact loading.Hybridization of carbon with Kevlar fibers did not affect the quasi-static transverse tensile behavior of the composite with 4% and 6% reduction in elastic modulus and strength respectively.However,considering the Carbon-Kevlar hybrid formulation,the elastic modulus and strength are increased by 67.4% and 49.5% respectively in comparison to the Kevlar 3D woven composite.The failure strain is also increased in the hybrid specimens as compared to the pure carbon composites due to the presence of ductile Kevlar fibers.A similar behavior was observed in the compressive response.The main failure mechanism in the tensile loading was fiber rupture and pull-out and in compression loading,was fiber-kinking and matrix crushing.The results of in-plane shear loading also depicted that the shear modulus and strength of Carbon composite did not decrease upon introducing the Kevlar fiber hybridization.Full-field strain analysis was also accomplished during each kind of loading condition and results gave an insight into the failure morphologies during the whole loading process.The high velocity results showed that the hybrid panels impacted on carbon face proved as best formulation in terms of energy absorption capability under high velocity impact because Kevlar fibers at the back side undergo large deformations before final fracture holding the projectile longer hence absorbing more energy.However,for Carbon and hybrid specimens impacted on Kevlar face,the energy dissipation was not significant.Macro and micro scale analysis of the impacted specimens showed that the major failure mechanisms are fiber tearing and matrix crushing.Delamination failure was not significant due to the extra binding force of z-yarns.Matrix crushing was the major failure mechanism in the carbon layers and the fiber pull-out and rupture was the main cause of failure in Kevlar layers.Moreover,the overall damage area at back side of the panels was bigger than the impacted side especially for low energy impacts.Strain data recorded at front and back side and in warp and weft directions simultaneously,showed that the Kevlar fibers underwent higher strain values during the impact process due to their ductile nature and better energy absorption capability.The strain values in warp direction were higher as compared to the weft direction especially for carbon layer which depicted the rigid behavior of the panel in the weft direction hence more damage was dispersed in this direction.A novel analytical predictive model named as Stiffness Model and FE based predictive model were developed and the engineering elastic constants and strength of hybrid 3Dwoven composites were determined.The analytical modeling method proposes a general methodology to predict the engineering elastic constants of 3D woven orthogonal composite material of the hybrid nature.This methodology is based on volume averaging method and iso-strain boundary condition.The 3D solid FE model of 3DWC has also been developed at multi scale level to understand the load carrying mechanism of various constituents and to determine the global engineering elastic constants.The stiffness model and the 3D solid FE models have been validated by comparing the predicted results with the experimental data.The predictions of global engineering elastic constants by both the models show satisfactory agreement with the experimental data.The relative difference of Young’s modulus in the weft direction for analytical model was 8.8% and for FE based model was 8.3%.Similarly,the prediction of Poisson’s ratio and shear modulus through analytical and FE model produces comparable results with the experimental data.Finally,the impact phenomenon was modeled using continuum shell element with embedded connector elements replacing the z-yarns.The inter-layer delamination is modeled using quadratic traction-separation law utilizing the cohesive contact scheme instead of cohesive 3D elements.The FE methodology led to a good approximation of different failure mechanisms of the composite and able to predict the residual velocity of the impactor when it is impacted on carbon or Kevlar face.The residual velocity of the impactor and failure mechanisms correspond well with the experimental data indicating the robustness of the proposed model.