Multi-layer riveted structures are widely applied to aircraft.During the service,cracks may appear within these structures due to stress concentration of the riveted holes.The guided wave monitoring has been proved to...Multi-layer riveted structures are widely applied to aircraft.During the service,cracks may appear within these structures due to stress concentration of the riveted holes.The guided wave monitoring has been proved to be an effective tool to deal with this problem.However,there is a lack of understanding of the wave propagation process across such kinds of structures.This study proposes a piezoelectric guided wave simulation method to reveal the propagation of guided waves in multi-layer riveted structures.Effects of pretension force,friction coefficient,and cracks that might influence wave characteristics are studied.The guided wave simulation data is compared with the experimental results and the results verify the simulation model.Then the guided wave propagation in a more complex long-beam butt joint structure is further simulated.展开更多
Carbon fiber-reinforced polymer(CFRP)is widely used in aerospace applications.This kind of material may face the threat of high-velocity impact in the process of dedicated service,and the relevant research mainly cons...Carbon fiber-reinforced polymer(CFRP)is widely used in aerospace applications.This kind of material may face the threat of high-velocity impact in the process of dedicated service,and the relevant research mainly considers the impact resistance of the material,and lacks the high-velocity impact damage monitoring research of CFRP.To solve this problem,a real high-velocity impact damage experiment and structural health monitoring(SHM)method of CFRP plate based on piezoelectric guided wave is proposed.The results show that CFRP has obvious perforation damage and fiber breakage when high-velocity impact occurs.It is also proved that guided wave SHM technology can be effectively used in the monitoring of such damage,and the damage can be reflected by quantifying the signal changes and damage index(DI).It provides a reference for further research on guided wave structure monitoring of high/hyper-velocity impact damage of CFRP.展开更多
The progressive failure in the top-hat stiffened composite panel under four-point bending damage was analyzed.The cohesive element was applied to simulate the debonding between the fuselage skin and stringer.In additi...The progressive failure in the top-hat stiffened composite panel under four-point bending damage was analyzed.The cohesive element was applied to simulate the debonding between the fuselage skin and stringer.In addition,the quadratic stress criterion and B–K criterion were adopted to predict initiation and propagation of the debondings.The propagation process,failure mode and ultimate load of the stiffened composite panel under four-point bending were predicted and compared with the test results.Specimens with co-bonded and co-cured processes were considered in this study.In regard to the co-bonded specimens,there existed only delamination between skin and stringer.The error between the predicted and experimental results of debonding load was about 2–8.8%.In terms of the co-cured specimens,the debonding between skin and stringer and the intra-layer cracks both can be observed.展开更多
As the use of composite materials in aerospace is growing fast,more metal-composite hybrid structures come into being and thermal stress becomes increasingly a concern that may affect structural safety.In this paper,e...As the use of composite materials in aerospace is growing fast,more metal-composite hybrid structures come into being and thermal stress becomes increasingly a concern that may affect structural safety.In this paper,experimental and numerical studies are conducted on the mechanical strain induced by thermal stress in an AL/CFRP hybrid structure subjected to a heating-cooling-heating cycle.The studied hybrid structure consists of a metal plate and a composite laminate fastened by three bolts.The experimental results show that the mechanical strain in either metal or composite exhibits a hysteresis as the structure undergoes the temperature cycle,which implies the existence of structural nonlinearities.Finite element analysis,which incorporates details of the bolt joint,reproduces the hysteretic responses that reach a reasonable agreement with the experimental ones.Numerical studies disclose the effects of the structural parameters,i.e.,friction coefficient,clamping force,fastener-hole clearance and bolt spacing,on the hysteresis and provide insights into the physical events during the thermal cycling.The reported work reveals that the movement of the bolts inside the surrounding holes is the key mechanism that drives the hysteretic thermal stress in the tested structure and sheds light on further investigations of structural safety of such hybrid structures under cyclic thermomechanical conditions.展开更多
This study demonstrates a homogenization approach via a modified state-based peridynamic(PD)method to predict the effective elastic properties of composite materials with periodic microstructure.The procedure of model...This study demonstrates a homogenization approach via a modified state-based peridynamic(PD)method to predict the effective elastic properties of composite materials with periodic microstructure.The procedure of modeling the PD unit cell(UC)of continuous fiber-reinforced composite is presented.Periodic boundary conditions are derived and implemented through the Lagrange multiplier method.A matrix-dominated approach for modeling the interphase properties between dissimilar materials is proposed.The periodicity and continuity assumptions are employed to determine the stress and strain fields,as well as the effective elastic properties.The PD-UCs of square and hexagonal packs as well as the 0/90 laminate microstructure are modeled and compared with the analytical,numerical and experimental results from the literature.Good agreement of predicted effective properties can be observed.Unlike other PD homogenization approaches,the effective material properties can be directly and individually obtained from simple loading conditions.展开更多
The effect of micro-void on transverse stiffness and strength for fiber-reinforced composites subjected to load perpendicular to fiber is studied using the state-based peridynamic(PD)theory.The heterogeneous microstru...The effect of micro-void on transverse stiffness and strength for fiber-reinforced composites subjected to load perpendicular to fiber is studied using the state-based peridynamic(PD)theory.The heterogeneous microstructure with micro-voids are discretized with irregular and non-uniform grids on mesoscale.The PD representative volume element with randomly distributed and sized micro-voids has been established with periodic boundary conditions.A parametric study was performed to evaluate the effects of void fraction and void size on transverse properties.It was found that the transverse modulus decreases with the void fraction in the range of 0.5–2%.A smaller void has a greater impact on the transverse modulus than a larger void under the same void content.The crack growth path for the multi-fiber model subjected to transverse tension was analyzed.The PD predictions show that cracks initiate at the interface between fiber and matrix,then deflect to the micro-voids nearby.The PD predictions match well with the analytical and experimental observations available in the literature.展开更多
Nearly equiatomic NiTi shape memory alloy exhibits superelasticity,i.e.,it can be strained up to~7%and recover completely upon unloading,and consequently,the stress-strain response forms a closed hysteresis.The mechan...Nearly equiatomic NiTi shape memory alloy exhibits superelasticity,i.e.,it can be strained up to~7%and recover completely upon unloading,and consequently,the stress-strain response forms a closed hysteresis.The mechanical behavior of superelastic NiTi is characterized by significant tension-compression asymmetry,which leads to complexity in the stress-strain responses and deformation patterns of thin-walled superelastic NiTi tubes loaded by axial force and internal pressure simultaneously.In the reported biaxial experiments,the NiTi tube exhibits hardening responses and essentially homogeneous deformation in a neighborhood of equibiaxiality.In other cases,its stress-strain responses trace stress plateaus associated with localized deformation patterns,and the level of plateaus,magnitude of transformation strains,and orientation of the localization bands are strongly dependent on the axial-to-hoop stress ratio.In this paper,finite element modeling is performed to analyze numerically the mechanical response of biaxially loaded superelastic NiTi tube.A numerical feedback control scheme is developed to maintain the stress ratio to followthe target value.The simulations reproduce successfully the observed phenomena in the experiments,such as the localization of helical bands,the variation of band angles with stress ratio,as well as the hardening and uniform deformation near the state of equibiaxial stress.In addition,the variation of axial and hoop stress-strain responses with different stress ratios are also studied,which are reasonably close to the experimental ones.The presented work demonstrates the validity of the developed finite element analysis framework and paves the way for analysis of superelastic shape memory alloy structures under multiaxial loading.展开更多
In this paper,the shear stability of a composite hat-stringer stiffened panel was studied by the means of both shear frame test and theoretical analysis.The test specimen is a typical flat composite stiffened panel co...In this paper,the shear stability of a composite hat-stringer stiffened panel was studied by the means of both shear frame test and theoretical analysis.The test specimen is a typical flat composite stiffened panel composed of skin,five hat-shaped stringers,two Z-shaped transverse frames and reinforcement layers.Firstly,a method that can quantitatively capture the buckling load and buckling morphology was proposed.Then,considering the shear-loading fixture as an elastic system with hinged and bolted connections,a finite element model including both shear-loading fixture and specimen was established.The linear buckling analysis was carried out using the subspace method.The first-order buckling mode was in good agreement with the buckling morphology obtained from the test.Furthermore,the deformed configuration of the first buckling mode was multiplied by the mode scale factor,and then introduced into the model as the initial defect.Based on this model,the nonlinear buckling analysis was performed via arc length method.The analysis results were in good agreement with the test.The relative errors between the predicted buckling loads and the test results were 7.0% and−3.8% from linear and nonlinear buckling analyses,respectively.Nonlinear buckling analysis has higher accuracy and tends to be conservative than linear buckling analysis.展开更多
文摘Multi-layer riveted structures are widely applied to aircraft.During the service,cracks may appear within these structures due to stress concentration of the riveted holes.The guided wave monitoring has been proved to be an effective tool to deal with this problem.However,there is a lack of understanding of the wave propagation process across such kinds of structures.This study proposes a piezoelectric guided wave simulation method to reveal the propagation of guided waves in multi-layer riveted structures.Effects of pretension force,friction coefficient,and cracks that might influence wave characteristics are studied.The guided wave simulation data is compared with the experimental results and the results verify the simulation model.Then the guided wave propagation in a more complex long-beam butt joint structure is further simulated.
基金supported by the National Natural Science Foundation of China(Nos.51921003,52275153)the Fundamental Research Funds for the Central Universities(No.NI2023001)+2 种基金the Research Fund of State Key Laboratory of Mechanics and Control for Aero-space Structures(No.MCAS-I-0423G01)the Fund of Pro-spective Layout of Scientific Research for Nanjing University of Aeronautics and Astronauticsthe Priority Academic Program Development of Jiangsu Higher Education Institu-tions of China.
文摘Carbon fiber-reinforced polymer(CFRP)is widely used in aerospace applications.This kind of material may face the threat of high-velocity impact in the process of dedicated service,and the relevant research mainly considers the impact resistance of the material,and lacks the high-velocity impact damage monitoring research of CFRP.To solve this problem,a real high-velocity impact damage experiment and structural health monitoring(SHM)method of CFRP plate based on piezoelectric guided wave is proposed.The results show that CFRP has obvious perforation damage and fiber breakage when high-velocity impact occurs.It is also proved that guided wave SHM technology can be effectively used in the monitoring of such damage,and the damage can be reflected by quantifying the signal changes and damage index(DI).It provides a reference for further research on guided wave structure monitoring of high/hyper-velocity impact damage of CFRP.
基金supported by National Natural Science Foundation of China under Grant Nos.U2241266,11902197 and 11972234.
文摘The progressive failure in the top-hat stiffened composite panel under four-point bending damage was analyzed.The cohesive element was applied to simulate the debonding between the fuselage skin and stringer.In addition,the quadratic stress criterion and B–K criterion were adopted to predict initiation and propagation of the debondings.The propagation process,failure mode and ultimate load of the stiffened composite panel under four-point bending were predicted and compared with the test results.Specimens with co-bonded and co-cured processes were considered in this study.In regard to the co-bonded specimens,there existed only delamination between skin and stringer.The error between the predicted and experimental results of debonding load was about 2–8.8%.In terms of the co-cured specimens,the debonding between skin and stringer and the intra-layer cracks both can be observed.
基金supported by National Science Foundation of China(Grant No.12072199).
文摘As the use of composite materials in aerospace is growing fast,more metal-composite hybrid structures come into being and thermal stress becomes increasingly a concern that may affect structural safety.In this paper,experimental and numerical studies are conducted on the mechanical strain induced by thermal stress in an AL/CFRP hybrid structure subjected to a heating-cooling-heating cycle.The studied hybrid structure consists of a metal plate and a composite laminate fastened by three bolts.The experimental results show that the mechanical strain in either metal or composite exhibits a hysteresis as the structure undergoes the temperature cycle,which implies the existence of structural nonlinearities.Finite element analysis,which incorporates details of the bolt joint,reproduces the hysteretic responses that reach a reasonable agreement with the experimental ones.Numerical studies disclose the effects of the structural parameters,i.e.,friction coefficient,clamping force,fastener-hole clearance and bolt spacing,on the hysteresis and provide insights into the physical events during the thermal cycling.The reported work reveals that the movement of the bolts inside the surrounding holes is the key mechanism that drives the hysteretic thermal stress in the tested structure and sheds light on further investigations of structural safety of such hybrid structures under cyclic thermomechanical conditions.
基金This work is supported by the National Natural Science Foundation of China under Grant Nos.1190219711972234 and is sponsored by Shanghai Sailing Program under Contract No.19YF1421700.
文摘This study demonstrates a homogenization approach via a modified state-based peridynamic(PD)method to predict the effective elastic properties of composite materials with periodic microstructure.The procedure of modeling the PD unit cell(UC)of continuous fiber-reinforced composite is presented.Periodic boundary conditions are derived and implemented through the Lagrange multiplier method.A matrix-dominated approach for modeling the interphase properties between dissimilar materials is proposed.The periodicity and continuity assumptions are employed to determine the stress and strain fields,as well as the effective elastic properties.The PD-UCs of square and hexagonal packs as well as the 0/90 laminate microstructure are modeled and compared with the analytical,numerical and experimental results from the literature.Good agreement of predicted effective properties can be observed.Unlike other PD homogenization approaches,the effective material properties can be directly and individually obtained from simple loading conditions.
基金supported by the National Natural Science Foundation of China under Grant Nos.11902197 and 11972234is sponsored by Shanghai Sailing Program under Contract No.19YF1421700.
文摘The effect of micro-void on transverse stiffness and strength for fiber-reinforced composites subjected to load perpendicular to fiber is studied using the state-based peridynamic(PD)theory.The heterogeneous microstructure with micro-voids are discretized with irregular and non-uniform grids on mesoscale.The PD representative volume element with randomly distributed and sized micro-voids has been established with periodic boundary conditions.A parametric study was performed to evaluate the effects of void fraction and void size on transverse properties.It was found that the transverse modulus decreases with the void fraction in the range of 0.5–2%.A smaller void has a greater impact on the transverse modulus than a larger void under the same void content.The crack growth path for the multi-fiber model subjected to transverse tension was analyzed.The PD predictions show that cracks initiate at the interface between fiber and matrix,then deflect to the micro-voids nearby.The PD predictions match well with the analytical and experimental observations available in the literature.
文摘Nearly equiatomic NiTi shape memory alloy exhibits superelasticity,i.e.,it can be strained up to~7%and recover completely upon unloading,and consequently,the stress-strain response forms a closed hysteresis.The mechanical behavior of superelastic NiTi is characterized by significant tension-compression asymmetry,which leads to complexity in the stress-strain responses and deformation patterns of thin-walled superelastic NiTi tubes loaded by axial force and internal pressure simultaneously.In the reported biaxial experiments,the NiTi tube exhibits hardening responses and essentially homogeneous deformation in a neighborhood of equibiaxiality.In other cases,its stress-strain responses trace stress plateaus associated with localized deformation patterns,and the level of plateaus,magnitude of transformation strains,and orientation of the localization bands are strongly dependent on the axial-to-hoop stress ratio.In this paper,finite element modeling is performed to analyze numerically the mechanical response of biaxially loaded superelastic NiTi tube.A numerical feedback control scheme is developed to maintain the stress ratio to followthe target value.The simulations reproduce successfully the observed phenomena in the experiments,such as the localization of helical bands,the variation of band angles with stress ratio,as well as the hardening and uniform deformation near the state of equibiaxial stress.In addition,the variation of axial and hoop stress-strain responses with different stress ratios are also studied,which are reasonably close to the experimental ones.The presented work demonstrates the validity of the developed finite element analysis framework and paves the way for analysis of superelastic shape memory alloy structures under multiaxial loading.
基金supported by National Natural Science Foundation of China,Ye Qisun Fund underGrant No.U2241266sponsored by Commercial Aircraft Corporation of China Ltd.(COMAC).
文摘In this paper,the shear stability of a composite hat-stringer stiffened panel was studied by the means of both shear frame test and theoretical analysis.The test specimen is a typical flat composite stiffened panel composed of skin,five hat-shaped stringers,two Z-shaped transverse frames and reinforcement layers.Firstly,a method that can quantitatively capture the buckling load and buckling morphology was proposed.Then,considering the shear-loading fixture as an elastic system with hinged and bolted connections,a finite element model including both shear-loading fixture and specimen was established.The linear buckling analysis was carried out using the subspace method.The first-order buckling mode was in good agreement with the buckling morphology obtained from the test.Furthermore,the deformed configuration of the first buckling mode was multiplied by the mode scale factor,and then introduced into the model as the initial defect.Based on this model,the nonlinear buckling analysis was performed via arc length method.The analysis results were in good agreement with the test.The relative errors between the predicted buckling loads and the test results were 7.0% and−3.8% from linear and nonlinear buckling analyses,respectively.Nonlinear buckling analysis has higher accuracy and tends to be conservative than linear buckling analysis.
