Research on Dynamic Response and Failure Behavior of Architectural Laminated Glass under Hard Body Impact
Author:Wang Xing Er
As one type of safety glass,laminated glass(LG)is increasingly used in curtain wall and structural purpose.The brittle cracking characteristic of glass under impact renders great risk during service.However,impact resistance design models is not available in most design codes or standards so far,and the impact damage mechanism lacks adequate insightful information supported with sufficient experimental and numerical data.It is therefore of importance to conduct studies on the impact resistance of laminated glass.This work concerns on the architectural laminated glass using polyvinyl butyral(PVB)and SentryGlas(?)Plus(SGP)interlayers,and carries out the investigation into the impact damage mechanism and the numerical prediction model with both drop weight impact tests and combined finite-discrete element analysis.A drop weight impact test system encompassing high speed filming and dynamic impact response acquisition system is firstly devised and manufactured.An experimental investigation into the damage behavior of full sized PVB and SGP laminated glass panels is then carried out.A mean minimum breakage velocity(MMBV)test approach has been employed in testing 83 specimens of 23 groups,to determine the breakage energy that triggers glass breakage.The crack initiation and crack propagation process is captured using high speed photos.This study carefully examines the effects of six design variables,that are,quantity of glass panels,glass make up,glass type,panel size,support condition,interlayer thickness,on the impact resistance and damage mechanism of laminated glass.The key factors that influence the breakage energy,breakage stiffness,impact force,equivalent dynamic stiffness and energy dissipation behavior are then identified.In order to overcome the problems in modelling the impact failure of laminated glass,a comparative review on the available numerical approaches(finite element method,FEM;extended finite element method,XFEM;discrete element method,DEM;combined finite-discrete element method,FEM/DEM)is carried out to illustrate their fundamental principles,modelling techniques and applications by using several example cases.An example of monolithic glass beam under impact is examined to identify the weakness and advantages of each approach.The most feasible approach,i.e.FEM/DEM,is determined and further examined and their results were compared with the experimental data for modelling the high speed and oblique impact tests on glass.By addressing the key problems of FEM/DEM in modelling the delamination of glass-interlayer interface,this work develops a model encompassing the formulation of the discrete crack model(DCM)for glass,the Mooney Rivlin model to represent the hyperelasticity of PVB interlayer,and the adapted Xu and Needleman model to describe the irreversible combined damage-plasticity behavior of interface.The comparison between the simulation and experimental results for several glass make-ups validates the applicability of the proposed FEM/DEM model and identifies the shortcoming and resulting errors.This work investigates the damage mechanism of architectural laminated glass under hard body impact through experimental and numerical studies,it can provide insightful information and suggestions for impact resistance design of laminated glass.The concluding remarks are collarated and listed as follows:(1)Crack/deformation pattern:The crack pattern of laminated glass is featured with the radial cracks and circumferential crack network in inner glass panel,the rippled cracks and petal shaped cracks in outer glass panel.An abnormal crack pattern is commonly seen in triple layered LG.Smaller fragments coincide with higher strengthening level.The deformation patterns are featured with cross-type yield line(in LG made of same glass types),single lateral yield line(in HSG-FTG)and x-type yield line(FTG-HSG).If glass panels experiences simulatation cracking in all constituent panels,the crack commonly initiates in the inner panel,and the lagging time of the following cracks is less than 0.16 ms(PVB LG)or 0.96 ms(SGP LG).The propagation speed of rippled crack is higher than that of radial crack.(2)Impactor motion:The effects of glass type and interlayer type on the impactor motion are negligible,support condition only shows obvrious influence when glass panel is larger The peak impact displacement is close in each LG type.It is less than 4 mm for intact glass,is less than 6 mm at single glass panel breakage,and is larger than 30 mm when both panels break.(3)The effects of design variables investigated are summarized as follows:a)Interlayer thickness:The use of 1.52 mm PVB interlayer in LG can obtain better impact resistance than 0.76 or 3.04 mm PVB.The interlayer thinner than 0.76 mm will reduce the critical impact velocity that triggers the stiffness degradation and increases the risk of both panels breaking at the same impact.Increasing the SGP thickness presents no improvement of impact resistance and even has negative effect on dissipating impact energy.b)Support condition:Edge clamping can provide better impact resistance than bolted connection.The corresponding increase ratio may be larger than 100%.It also provides better ductility in the post breakage stage when subjected to repeated impacts.c)Glass type/glass make up:FTG-HSG configuration can evidently improve impact resistance and critical velocity.Placing HSG in the outer side will weaken impact resistance.The effects of glass types on energy dissipation characteristics can be omitted.In addition,reducing the ratio of outer-inner panel thickness without increasing total thickness of LG can enhance impact resistance and critical velocity,while it can also improve impact resistance but tends to have brittle failure in the same impact and has no enhancement in breakage stiffness if only increasing inner panel thickness.The glass thickness has negligible influence on dissipating energy.One conclusion can be found that glass type and total glass thickness will significantly affect the amplitude of impact force,that is,if LG panel adopts more FTG panels or FTG panel with greater thickness,it can sustain greater impact force;d)Glass panel quantity/glass panel size:Increasing glass panels cannot improve impact resistance,it may even cause higher risk of inner glass breakage.It can be found that the middle glass panel contributes more in LG stiffness than both outer and inner glass panel.Increasing panel size will cause negative effects on impact resistance,and its effects on improving breakage stiffness depend on breakage sequence.(4)Critical velocity that triggers the stiffness degradation:for double layered LG,critical velocity is found to be less than 3.1 m/s(PVB)and 2.5 m/s(SGP)before the first breakage,3.1 m/s(PVB)and 2.9 m/s(SGP)before the second breakage.For triple layered LG,no stiffness degradation can be seen before breakage.The critical velocity is less than 3.1 m/s(PVB)and 2.9 m/s(SGP)in the post breakage stage.The corresponding impactor mass is 13.5 kg.(5)Two types of delamination can be found,that are,outmost delamination and inter delamination.Edge clamping leads to continuous delamination under repeated impacts,while bolted connection will maintain constant size of delamination zone of which the diameter is less than 100 mm.(6)DEM and FEM/DEM approach can satisfactorily simulate typical crack pattern and fracture growth.However,DEM presents difficulty in calibrating realistic properties such as energy release rate.It may lead to a high degree of unpredictability in modelling energy dissipation features of materials,and further results in the remarkable deviation of global response.On the contrary,FEM/DEM can successfully produce the pattern and the featured size of glass surface damage,and predicts contact duration in the high-speed impact cases.However,FEM/DEM will overestimate the energy dissipation during the high-speed impact because of the small element size setting in the contact zone.XFEM shows limitations in modelling dynamic fracture and obtaining typical crack pattern.FEM with element erosion method can adequately model typical crack pattern,whereas the element erosion method will underestimate the global impact response.(7)The developed FEM/DEM model can adequately reproduce the characteristics of stress wave propagation that map well with experimental and analytical results.The results reveal that crushing cracks and delamination will cause dramatic delay of wave propagation.The delamination model proposed in this work can successfully produce the Mode I and Mode II dominate delamination behavior near impact point.This work also identifies the shortcoming and resulting errors of discrete crack model for glass when modelling thermally strengthened glass.