| As a key parameter,the textile permeability determines the impregnation of the textile reinforcement with liquid resin in liquid composite molding.Analysis of the complex correlation between permeability and mechanical response of multi-level structure of preform has been the research focus on resin flow mechanism.For structural composites,the nesting is a geometric and mechanical phenomenon caused by shifting of fabric layers.This effect not only affects the fabric compression responsive behavior,but also changes the resin flow path and contributes significantly to permeability variations.Therefore,it is an urgent issue to discuss the effect of nesting on compression and permeability of fabrics.The research is also helpful to to possess a deep understanding of the transport mechanism in porous media and optimize the injection process in iquid composite molding.In this diaaertation,the behavior of the compression response and the permeability of unidirectional cloth and 0°/ 90°NCF()are studied respectively.For the unidirectional cloth,first of all,according to the assumption that the cross section of fibers is elliptical,a modle of the thickness of fabrics related to layer shifting is built in relaxed state.Then based on the reasonable assumption on the change of the fiber bundle's cross-section and its constitutive model,several compression models of two-layer fabrics are established with different nesting.These modles associated fabric thickness with compression load,fabric structure and layer shifting can be used to describe the compressive behavior under any nested state.On this basis,the multilayer fabric compression response model is further established and the influence of the number of layers on compressive behavior is investigated.The main conclusions are as follows:(a)Different nesting results in the variation of fabric thickness in relaxed state.The differences of thickness are more obvious as the number of layers incresases.When the number of layers tends to infinity,the thickness of fabrics with maximum nesting is only 86.6% of the minimum nesting;(b)Nesting has great influence upon the compressive behavior of fabrics.Under the same forced applied conditions,the average thickness decreases with the increase of shifting ratio.The ratio of average thickness per layer decreases rapidly with the increase of pressure in the initial stage,and remains stable in the following stage;(c)The dimensionless ratio /nesting total(35)h(35)h decreases with the increase of pressure in the initial stage and remains stable in the following stage.Under the same forced applied conditions,the ratio increases with the increase of layer shifting and is up to 0.352 when the shifting ratio is equal to 1.(d)As the unpredictability of layer shifting in actual process of fiber placement,the statistical characteristics are analyzed theoretically.The results show that the distribution of average thickness per layer takes the shape close to a typical distribution and becomes narrow with the increase of number of layers.When n(29)10,the compaction properties of multi-layer fabrics can be described by the mathematical expectation of the thickness of inner layer as a function of external load.Based on the compression model of unidirectional cloth,the typical cell structure is divided into five different regions,and the permeability model of each region is established.Considering the resistance to the flow equal to1/ K,the permeation properties of different regions can be described as complex series and parallel connection.The in-plane and through-thickness permeability modles are then established as a function of fabric structure and layer shift.Due to the differences of layer shifting,each adjacent two layers is regarded as a porous medium with different permeability,and the equivalent permeability models of multi-layer fabrics are established.Different from unidirectional fabric,the effect of nesting on compression and in-plane permeability performance of 0°/ 90° NCF fabrics is very weak.The reason is that,0° yarns beween upper and lower layers are separated by 90° yarns and the fiber bundles in upper layer can not be embedded in the gaps between yarns in lower layers.Therefore,the compression model of 0°/ 90°NCF can be described by the minimal nesting model of unidirectional fabric.As the thickness of the fabric has nothing to do with the layer shift under the same pressure,the permeability model of single layer can be used to characterize the permeability of multilayer fabrics.However,layer shifting affects the flow path of the fluid in the fabric,especially the size of the pure spaces which have a great influence on the out-of-plane permeability.According to local fibre arrangement resulting from layer shifting in z direction,the typical cell structure is divided into 16 different regions.The prediction model of out-of-plane permeability can be established by using the series-parallel theory.Finally the equivalent permeability model of multilayer NCF is established by considering each two adjacent layers as different porous media.The results show that layer shifting has great effect on both the in-plane and out-of-plane permeability of unidirectional fabric,especially onyK andzK.With the increase of layer shifting,the permeabilityxK increases slowly.The reason is that the reduction of fiber volume fraction of bundles leads to the increase of its transverse permeability which determines the the resin flow in the x direction.As layer shifting increases,the change of yK is similar withzK.The permeability decreases sharply in initial stage and increases slowly in the following stage.The reason is that the space region disappears gradually in thickness direction in initial stage.This is the main factor affecting the permeabilityyK andzK.When layer shifting is equal to 0,the cross section of space region has the largest area,and the permeabilities yK andzK reach the maximum values.When layer shifting is equal to the width of gaps between bundles,the space region disappears and the permeabilities reach the minimum values.For 0°/ 90° NCF,layer shifting mainly affects the out-of-plane permeability.When the shifting is equal to 0,zK takes the maximum value,and when the shifting is equal to L/ 2,zK takes minimum value.As the unpredictability of layer shifting of multilayer fabrics in actual process,statistical characteristics are analysized theoretically.The results show that the permeability distribution tends to be close to a typical normal distribution.When the number of layers of 0°/ 90°NCF is less than 6,the out-of-plane permeability has a wide distribution across over three or four orders of magnitude.The distribution takes the shape close to a typical distribution when the number is more than 8.But for both of unidirectional fabric and 0°/ 90° NCF,the distribution becomes narrow with the increase of number of layers. |
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