| 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. |
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