The Mechanical Performance And Failure Mechanism Of Z-pin Reinforced Composite Skin/Stiffener Structures

The outstanding performance and weight saving advantages has imparted a widely application in aerospace sectors to composite materials.Thin wall panels reinforced by stiffeners such as ribs,beams and frames are the main structures to increase the structural efficiency used in loadbearing components.However,the weak bond at skin/stiffener interface is the main concern encountered by the users and designers,which forms via adhesive or co-cure.On one hand,the static adhesive strength is relatively low and largely depends on the processing quality;on the other hand,the bonding interface is susceptible to the harsh environment and impact load leading to interfacial crack initiation and quick propagation during the cyclic loading.Due to the lack of crack-stoppers,extremely conservative design method is usually employed to comply the no crack growth policy.As a result,the large safety factor reduces the freedom of performance maximization and the space for weight saving.In this study,Z-pin technique was used as a solution for interface reinforcement for stiffened panels.Based on experimental and finite element methods,the interlaminar bridging effect of Z-pins was analyzed.The failure of Z-pinned skin-stiffener interface and the crack-stopping mechanism of Z-pins were explored under quasi-static,cyclic and impact loading conditions.The results and conclusions would provide guidelines for the design and usage of Z-pin reinforced stiffened panels.The Z-pin reinforcing effect was analyzed based on the bridging force and energy absorption.In order to maximum the reinforcing efficacy,the single-pin model was built for a parametric study.Also the concept of twisted fiber reinforced Z-pins was brought up with an evaluation of the mechanical performance.The main mechanism of Z-pin reinforcemend were manifested in two aspects.On one hand,the bridging force was generated through Z-pins between the delaminated surfaces to preventing the relative motion.The results showed that the main factor determines the bridging force lied in the Z-pin/laminate bonding strength.On the other hand,the engaged energy in the deformation and fracture of Z-pins elevated the “apparent fracture tougness” of laminates.The factors effecting the energy absorption were normal residue contact force at pin/laminate interface and frictional coefficient while the former one was more crucial.When the Z-pin was pulled out at an angle to the through-thickness direction,it would squeeze the resin around it leading to a rise of the contact force.Helical grooves existing on the surface of twisted fiber reinforced Z-pins(TFR Z-pins)enabled an enlarged contact area with the laminates which contributed higher bridging force.Experimental results showed that when the fiber twist was 80 twist/m,the maximum bridging load increased by 19% at an affordable price of tensile properties loss.Based on the skin/stringer generic configuration,the interface debond performance of Z-pin reinforced interface was studied with 3 different distribution patterns of Z-pins under typical loading conditions.The conclusion was that,with the presence of Z-pins,the crack initiation could be hardly affected,however the load capacity was dramatically improved while the sudden failure was changed into a progressive failure mode.Compared to the control specimens,Z-pins with a volume fraction of 0.785% enabled about 1 time improvement on bending failure load and more than 10 times increase to the energy absorption while only 55% and 86% improvements on the skin tension debond load and energy absorption resp.The results on different distribution patterns demonstrated that the denser distribution in the more critical regions can optimize the reinforcement efficiency of Z-pins and lead to a further improvement on the debond resistance.Aiming at the optimal design of Z-pins distribution,the skin/stringer strip finite element model was built with the help of cohesive zone modeling and the simulation results agreed well with the experimental results for different distribution patterns.The parametric analysis showd that only when Z-pins were inserted within the 1mm distance to the flange edge,the crack initiation could be postponed,but it's hardly possible considering the existed insertion method.Also,a certain relationship between the reinforcement and the density of Z-pins does not exist because it depended on the exact distribution of Z-pins.For skin/stringer interface,more Z-pins should be implanted near the flange ends.Compared to higher energy absorption capacity,a higher bonding strength with laminates is more favorable for the debond resistance for skin/stiffener interface.Cyclic tension and bending tests were designed and conducted to investigate the influence of Z-pin reinforcement on the crack initiation life,crack propagation life and crack propagation rate.On the other hand,crack growth paths and history was observed to reveal the propagation mechanism at Z-pin reinforced interface.It was manifested that fatigue cracks forming at the resin rich corner at the flange edge can't be avoided or delayed by Z-pin reinforcement,but the propagation life can be obviously prolonged by 5-50 times.Also similar to the static testing results,Z-pins showed more noticeable improvement under cyclic bending load.For the optimized distribution pattern,Z-pins can change the continuous propagation into a propagation-suspension-propagation mechanism and the crack propagation rate is reduced by 1-3 orders of magnitude.During the crack suspension stage,cracks propagated at Z-pin/laminate interface with an extremely low rate.By means of the optimal design of Z-pins distribution,interfacial crack can be confined to a limited region after the initiation,called the safe crack propagation length,which can be referred as a guideline for the safety life assessment of damaged structures.The T-and hat-shaped stiffener reinforced skin widely used in aircraft structures were used to reveal and predict the improvement of Z-pinning on the impact resistance of stiffened panels by means of experiments,finite element anaylsis and ultrasonic C-scanning technique.Experimental results showed that,the impact damage resistance of stiffened panel was improved by Z-pin reinforcement.When the stiffener edge was subjected to a 6.36 J low velocity impact load,compared to non-pinned structures,28% increase of peak impact force and 68% decrease of total energy absorption were obtained for the T-stiffener reinforced skin,while 17% increase of peak impact force and 6.4% decrease of total energy absorption for the hat-stiffener reinforced skin.The Z-pins located near the ends of the stiffener flanges can suppress the crack propagation within the Z-pinned region which contributes to a 58% decrease of the delamination area for T-shaped stiffened skin and a 40% decrease for the hat-shaped stiffened skin.Based on the layer-by-layer failure assumption of Z-pin reinforcement,the Z-pinned T-stiffener and hat-stiffener reinforced skin finite element models were built with the in-plane and interlaminar progressive failure of skin laminates taken into consideration.The mechanical performance and delamination area were simulated showing that the impact force results agreed well with the experiments while the delamination area calculation was conservative with a deviation less than 15%.The further parametric analysis on the T-stiffener reinforced skin model showed that under 2~6.36 J impact,50~53% and 34~37% reductions on the delamination area could be achieved for T-and hat-shaped stiffener reinforced skin structures repectively due to Z-pins with a local volume fraction of 2.18%,which revealed the dramatic improvements on the impact damage resistance due to Z-pin reinforcement.Meanwhile,the increase of pullout energy absorption of Z-pin was favorable to reduce the impact delamilation area of Z-pinned structures.