【作者】 毛春见；

【导师】 许希武；

【作者基本信息】 南京航空航天大学 ， 工程力学， 2013， 博士

【摘要】 复合材料具有比强度高、比刚度高、可设计性等一系列优点，已经在航空航天等领域得到了广泛的应用。传统的层合复合材料由于自身的弱点，在受到低速冲击后容易产生分层损伤，导致冲击后的压缩强度大幅度下降，从而限制了其优势的发挥。缝合复合材料通过引入厚度方向的纤维既提高了层板的抗冲击性能和冲击后压缩性能又保留了传统层板的良好面内性能，因而具有广阔的应用前景。缝合复合材料层板低速冲击和冲击后压缩性能研究是缝合层板损伤容限和耐久性研究的基础，具有重要的理论意义和应用价值。本文对缝合层板进行了低速冲击和冲击后压缩性能研究，具体的研究工作包括：（1）采用落锤冲击法对不同类型缝合复合材料层板进行低速冲击试验，使用无损检测方法进行损伤探测，并对冲击后试件进行压缩试验，研究了不同类型缝合复合材料层板的冲击损伤特性和冲击后压缩性能。研究结果表明：与未缝合层板相比，缝合层板具有更好的抗冲击性能，更高的冲击后压缩强度；冲击能量越大，缝合的作用越明显；缝合密度越大的层板抗冲击性能越好，冲击后压缩强度越高。缝合方向和铺层顺序是影响低速冲击和冲击后压缩性能的重要因素。（2）使用三维动力学有限元法，采用三维实体单元模拟单层板，采用界面单元模拟层间界面的力学行为，以空间杆单元模拟缝线的增强作用，建立了缝合复合材料层板在低速冲击载荷作用下的渐近损伤分析模型。面内采用基于应变描述的Hashin失效准则进行损伤判断，采用刚度折减对面内的材料性能进行退化；层间采用二次应力准则进行初始损伤判断，采用双线性折减方法对界面的性能进行折减。针对不同类型的缝合层板，模拟了低速冲击载荷作用下的冲击响应和渐进损伤过程，数值结果与试验吻合较好，证明了该方法的合理有效性。（3）采用三维渐进损伤分析模型对缝合复合材料层板低速冲击的影响因素进行了研究。分别研究了层板厚度、缝合密度、缝线直径和缝线强度对缝合层板低速冲击响应和冲击损伤的影响规律。研究结果表明：厚度越小，缝合对提高抗冲击性能的作用也越小。冲击能量一定的情况下，当缝线直径和缝线强度小于某一个特定值时，低速冲击过程中缝线出现断裂，且缝线直径和缝线强度越小，缝线断裂越多，冲击损伤投影面积越大。（4）根据含损伤缝合层板在压缩载荷下的破坏模式和破坏机理，将冲击损伤等效为椭圆孔，利用杂交应力单元计算冲击后缝合层板的应力分布，采用基于特征曲线概念的点应力判据建立了缝合层板冲击后压缩强度的分析方法，与试验结果比较证明了该分析方法的正确性，并讨论了特征长度、损伤面积及椭圆孔参数等对冲击后压缩强度的影响规律。最后采用三维渐进损伤分析方法结合开口等效法对缝合层板低速冲击和冲击后压缩进行了一体化分析。更多还原

【Abstract】 Composite laminates have been widely used in aeronautics and astronautics industry because oftheir advantages of high specific strength, high specific stiffness and designable. For the traditionallaminates, internal delaminations can be easily produced by low–velocity impact, which leads tosignificant reduction of compression strength and limits the advantages of composite laminates. Byintroducing stitched lines in the thickness direction, stitched laminates not only improve theinter-laminar impact resistance and compressive characteristics after impact but also retain theadvantages of traditional laminates, which offers them broad application prospects. The research onlow-velocity impact damage and compression strength of stitched laminates has considerabletheoretical significance and application value in the sense that it is the foundation of studying on thedamage tolerance and durability of stitched laminates. In this thesis, the low-velocity impact andresidual strength of stitched laminates are studied. The main contents are as follows:(1) In the experiments of different stitched laminates, low-velocity impact was simulated by thedrop weight method, impact damage was detective by the non-destructive testing, and compressiontesting was completed by a material testing machine. In the thesis, influencing factors of impactdamages and compression strength after impact for different stitched laminates were studied. Theresults indicate that: compared with unstitched laminates, stitched laminates have better damageresistance for impact and higher compressive strength after impact. The effect of stitching is betterwhen impact energy is bigger. The impact resistance and post-impact residual strength are bothimproved as increasing the stitch density. Stitching direction and layup are important factors whichinfluence the impact resistance and compressive strength after impact.(2) A3D dynamic finite element model was proposed to predict the progressive damage ofstitched laminates under low-velocity impact, in which the laminate was simulated by brick element,the interface was modeled by cohesive element and stitching thread was realized by bar element. Thestrain-based Hashin criterion was employed to determine the inter-laminar damages and the stiffnessreduction was used to simulate the in-plane damage evolution. The stress-based quadratic criterionwas employed to evaluate the initial damage of interface, and the bilinear model was adopted todegrade the properties of damaged interface. The impact response and damage progression ofdifferent stitched laminates was calculated and numerical results coincided with the experimental results excellently, which verifies the effectiveness of the model.(3) The influencing factors of low-velocity impact on stitched laminates were studied by using a3D progress damage model. The effects of thickness of laminates, stitching density, diameter andstrength of stitching threads were researched in detail. The results indicate that the improvement ofimpact resistance decreases with increasing the thickness. When the diameter and strength of stitchingare lower than certain values under the same impact energy, the stitching will break. The smallerstitching diameter and strength are, the more stitching will be damaged.(4) Based on the damage modes and failure mechanisms of damaged stitched composites undercompression loadings, the impact damage was modeled by an elliptical hole. The stress field ofstitched composites after impact was calculated by hybrid stress elements. The residual compressivestrength was predicted by using the point stress criterion based on characteristic curve and comparedwith the experiment results to validate the proposed method. The influences of character distance,damage area and parameters of elliptical hole on the compression strength after impact were discussed.Finally, by using the3D progress damage model and equivalent hole method, integrated analysis onlow-velocity impact and compression strength after impact is conducted.更多还原