Research On Interlaminar Mechanical Behavior And Damage Suppression Mechanism Of Stitched Composites

Stitching is one of most effective methods to improve the interlaminar mechanical properties of composites,and is widely applied in aerospace fields.However,the interlaminar mechanical performance and damage mechanism of stitched composites are very complicated due to abundant stitching parameters,so that the reasonable design of stitching parameters is a challenging work.Therefore,it is significant to reveal the influences of stitching parameters on the interlaminar mechanical performance and the damage mechanism of stitched composites for the purpose of providing effective guidance in the reasonable design and engineering application of stitched composites.However,the mechanism about the effect of stitching spacing and pattern on interlayer bonding of stitched fabrics has not been reported yet.Besides,the damage suppression mechanism of stitching space and pattern under the quasi-static interlaminar loading has not been reported yet.Dynamic impact behavior and the corresponding damage suppression mechanism under different impact energies,and the damage toler ance enhancement mechanism of the compression after impact have not been investigated.Therefore,this thesis focused on systematically revealing the compaction behavior of stitched fabrics,the quasi-static short beam shear behavior of stitched composites,low-velocity impact behavior,and compression after impact behavior by experiments.The influences of stitching space and pattern on interlaminar mechanical properties and the damage suppression mechanism were explored in detail.Contents and innovational results are listed as follows:(1)The compaction behavior of stitched fabricsThe equilibrium locking structure formed by the stitching threads can significantly enhanced the interlayer bonding force between the fabrics,resulting in the increase of ability to resist the compaction deformation.In this thesis,the experiments of compaction deformation with different stitching spaces and patterns were conducted.It was revealed that the constraint effect of the stitching thread on the fiber yarns suppresses the yarn flattening and bending deformation,improving the ability to resist the compaction deformation,and enhancing the interlayer bonding force.The increase of interlayer bonding force is more obvious with small stitching spacing and the diagonal stitching pattern.Furthermore,a viscoelasti c compaction theoretical model of stitched fabrics that accurately describes the effects of stitching space and pattern was proposed.Then,the mechanism of the constraint of the stitching thread on the relative motion between mutually perpendicular and parallel fibers was clarified.(2)The quasi-static short beam shear interlaminar behavior of stitched compositesTo reveal the quasi-static interlaminar behavior and damage mechanism of stitched composites under different stitching spaces and patterns,this thesis conducted the short beam shear experiment of the stitched composite.The effects of stitching space and pattern on shear force-displacement curves were analyzed.Results shown that stitching can effectively improve the load ing change ratio after the damage initiation,and revealed that the small stitching space and stitching along the main load-bearing direction are more effective.Furthermore,the quantitative characterization of the damage was carried out using X-ray computed tomography.The results revealed that the damage is well suppressed as the stitching forms a closed loop and improves the interlayer bonding.The effects of stitching spaces and patterns on the damage volume and the maximum damage size in the cross-section were also investigated.(3)The low-velocity impact behavior of stitched compositesTo investigate the influence of stitching spaces and patterns on the dynamic mechanical behavior and damage of stitched composites under different impact energies,the low-velocity impact experiments of stitched composites were carried out.The evolutions of the impact force with time/center displacement,and the transferred energy/velocity with time,were deeply discussed.It was revealed that the diagonal stitching and small stitching spaces effectively enhance the impact r esistance.Furthermore,the quantitative damage characterization was carried out by C-scan technique.It was confirmed that the damage has the farthest extension along the stitching thread due to the uneven stress distribution caused by the stitching threa d.In addition,it was identified the damage suppression effect is better with diagonal stitching and the smallest stitching space,and is more obvious when the impact energy increases.(4)The damage tolerance mechanism under compression after impact of stitched compositesTo investigate the influence mechanism of the stitching spaces and patterns on the compression after impact(CAI)behavior,the CAI experiments of the stitched composites were carried out.The effect of stitching spaces and patterns on the compressive load-displacement was analyzed.Results shown that the stitching effectively improves the CAI strength and displacement.In addition,digital image correlation and optical microscopy methods were used to characterize the damage evolution and damage failure modes.The influence of different stitching parameters on the distribution of damage strain bands was analyzed,which confirms that the stitched composites have obvious progressive damage behavior.Besides,it was identified that the diagonal stitching inhibits the extension of kink damage,resulting in more and shorter kink damage.