Research On Stitching Reinforcement For Laminates Containing A Large Hole Based On Micromechanics Model

The carbon fiber reinforced composite materials have been widely used in the aerospace,ship,medical and sports industries due to notable performance,such as high specific strength and modulus,low density,easy to design and good fatigue resistance.In particular,the amount of application of carbon in the field of aerospace is regarded as a standard to measure the level of national science and technology.However,the carbon fiber reinforced composites(CFRP)have many problems to be solved due to the limitations of CFRP itself while realizing lightweight of equipment.In the field of aerospace,in order to install and maintain the equipment,it is often necessary to open holes on the laminates.In general,the diameter of holes is larger,and most of them are laminates with large holes(when the ratio of width D of the laminates to radius R of the holes is less than 6,they are called large holes).The holes will not only inevitably lead to the decrease of strength of laminates,but also to the problem of stress concentration at the edge of holes.More seriously,due to the edge effect of holes,delamination cracking damage will occur near the edge of holes.In the process of loading,delamination damage will further expand,which will seriously affect the mechanical properties and is inclined to cause the early failure of the laminates.In order to solve this problem,a relatively new reinforcement method——stitching reinforcement has been applied in this paper.The stitching reinforcement can effectively restrain the delamination of the edge around the hole and enhance the interlaminar mechanical properties of the edge around the hole without changing the geometrical shape and mass of the laminates.However,there are some inevitable limitations of stitching reinforcement.During the process of stitching reinforcement,stitching and needles will penetrate through the laminates,which will lead to damage of the laminates in plane,mainly including in-plane fiber bending,fiber fracture,out-of-plane fiber bending and resin-rich defect.These damages will have a greater impact on in-plane mechanical properties of the laminates.And different stitching parameters bring about different effects on the laminates.Therefore,it is particularly important to select suitable stitching parameters for the stitching reinforcement for composite laminates containing a large circular hole.On one hand,it is necessary to ensure that the in-plane mechanical properties should not be changed too much,on the other hand,it is also necessary to effectively enhance the interlaminar mechanical properties.In order to provide a reasonable guidance theory and method for choosing suitable stitching parameters,the damage of stitching was studied from the perspective of micromechanics.Firstly,an unit cell model was established for stitch damage of the stitching reinforcement for composite laminates containing a large circular hole.In order to find out the change of the materials properties of laminate after stitching effectively,the cells were divided into many parts:the stitching affected zone,the non-affected zone and the resin-rich zone.The fiber state function of each region was established by mathematical theory,and then the fiber volume fraction and the fiber bending angle state functions of the fiber bending region were deduced.MALTAB was used to plot the nephogram to show the distribution status of volume content and bending angle of fibers.The study showed that the maximum bending angle of the fibers in the cell surface was about 18 degrees,which is in good agreement with the actual situation.The state functions of elastic engineering constants in each region of a cell are solved by using the theory of composite micro-mechanics,and the average elastic engineering constants of a cell are obtained by using the homogenization algorithm.In addition,the modified coefficient K_i is selected to characterize the bending defects of out-of-plane fibers.The results show that the longitudinal Young’s modulus decreases while the transverse Young’s modulus,shear modulus and Poisson’s ratio increase by not more than-8%~15%.Secondly,ABAQUS/Standard module was used to establish a finite element model for the stitching reinforcement for composite laminates containing a large hole to simulate the effects of different stitching parameters on the mechanical properties.In order to effectively simulate the actual defects of stitching on laminates,the laminates with a large hole are divided into P and Q regions,where Q region is the stitching area.The elastic engineering constants of the stitching area are calculated by the theory in previous chapters.The P region is far from the edge of the hole and is not affected by the stitched defects,while the elastic engineering constants of the laminates are calculated by the composite micromechanics theory.At the same time,in order to effectively simulate the stress concentration problem caused by resin enrichment in stiching hole,a novel modeling method was established in this paper.The stiching hole model was introduced into each cell.The stiching hole was filled with resin and the stiching was embedded in resin.Tensile and compressive displacement loads were applied to the stitching reinforcement for composite laminates model containing a large hole.And the USDFLD subroutine of three-dimensional Hashin failure was introduced.The effects of different stitching parameters on the mechanical properties of laminates and the progressive damage process were studied.The results show that stitching can reduce the in-plane mechanical properties of laminates and induce early failure of the edge around holes of laminates.However,the expansion of delamination can be effectively suppressed under tension load.Finally,the method of choosing stitching parameters is given by using the research theory above.According to the stiffness value of the laminate required under the working condition,the appropriate stitching parameters under the current working condition are deduced by using the calculation method above.