Research On Strengthening Mechanism Of T-stiffened Composite Panels Reinforced By Z-pin

Stiffened composite structure has been widely used in aerospace while the debonding of stiffener and skin is the main failure mode.Being suitable for composite structures made from prepreg,Z-pin technology can be used to reinforce the connection between stiffener and skin.Based on the exploration of processing technique,this paper presents an experimental and numerical study into the loading capacity of T-stiffened composite panels reinfoced by Z-pin and elaborates enhancing effect of Z-pins resulted from the complicated loading condition due to different structure dimensions.And a solid foundation was laid for practical application of Z-pin technology in composite stiffened structures.(1)The processing technique was explored and optimized.Firstly,the curing process of T-joints was accomplished with specially designed moulds by autoclave.However the general process led to defects such as Z-pin could not penetrate prepreg perform and the fillet was not compact enough after cured.Through increasing the flange thickness,magnifying the arc radius and the extent of fillet,samples with good quality were manufactured successfully.(2)Out-of-plane tensile and shear experiments were conducted to determine the connect property of stiffener and skin and analyze the strengthening mechanisms.Results reveal that the initial failure strength is not affected by Z-pin,while the ultimate strength and residual strength is improved greatly.Under tensile load,the actual stress situation of Z-pin tends to be complicated with the deflection of skin.As a result,a mixed mode I/II stress condition exists at the delamination crack tip during tensile loading,and an enhanced frictional zone is located near the delamination surface.The out-of-plane shear experiments show that the stiffener and skin separated instantaneously.And the intial failure strength is improved by Z-pin,while the enhancement for the residual mechanical capacity after abrupt initial failure was not obvious.(3)A parametric study was conducted to investgate the enhancing efficiency of Z-pin for the tensile capacity of T-joints.Compared to unpinned samples,the tensile capacity has got increased with Z-pin volume fraction and decreased when the fraction exceeds 1%.And the ultimate load can be improved by up to 70.4%.Subsequently,the effect of Z-pin reinforcement on the damage tolerance of composite T-joints was experimentally studied.It indicates that Z-pin exhibits bridging traction for joints with the damage in the range of 10%~30% and the tensile capacity is improved by more than 90%.Futhermore,the dimension of flange was altered to explore the optimum design for Z-pinned T-joints.Results show that the trendency of capacity for unpinned samples accords with Z-pinned ones when the dimension of flange altered.When samples reinforced by Z-pin,the optimal dimension of flange is reduced from 60 mm to 40 mm,which may give significant implication for weight reduction.(4)A 2D finite element model was established to analyze the effect of Z-pin for composite T-joints with different geometry dimension.Numerical methods were used to obtain strength and stiffness of the cohesive model rationally.The modeling involved modified simplification for delamination crack growth in the pinned joint using a cohesive zone model.The FE model was validated using experimental results for a pinned T-joint under tensile loading.Parametric analysis shows that stress ratio between pullout and shear declines with the span ratio(and rises with the thickness ratio),which enforces the failure mode of a single pin from pull-out to fracture,so that the tensile capacity of pinned T-joint is weakened gradually.