Experimental Study And Strength Model For Envelope Materials And Structures

Given its unique features such as light self-weight,high load-bearing capacity,excellent weathering resistence,filling gas retaining,inexpensive cost and convenience in processing and stransportion,fabric composite has been widely employed as envelope material and applied in the manufacture of envelope structures.Considering their extraordinary mechanical behaviors and favoribal physical properties,the typical application scenes of the envelope structures are high altitude payload platforms including stratospheric airhips and super-pressure balloons.Especially,the research and development of stratospheric airships are of great importance while many countries paied attention on this field.Different from fixed wing aircraft,gyropter,spaceship,rocket and other aerocraft,the high altitude retaining of airships relies on the buoyancy generated by the density contrast between inner filling gas and environmental ambience.Based on this principal,stratospheric airships could hover at an altitude higher than 20 kilometers with relative low fuel consumptions.Theoterically,the hovering duration could reach up to hundreds of days.For aforementioned reasons,envelope structures have been widely applied in land observation,disaster warning,national defence and other aspects.However,previous research work focusing on the mechanical beheviors of envelope materials maily depends on the mono-uniaxial tensile tests and biaxial cyclic tensile tests under low stress level.Considering the authentic stress state of envelope material and practical working condition of envelope structure,structural design and numerical simulation based on previous research could not meet the requirement of both scientific research and practical engineering.To solve these problems,systematic experimental tests to determine the mechanical behaviors of the envelope material were innovately developed.Specifically,the biaxial tensile strength test was firstly proposed which significantly fill the gap in thi field.At the same time,computational analysis and numerical simulation towards the envelope structures were performed based on the mechanical model of welding seams,the mechanical model of natural weathering material,integrate biaxial constitutive law and the biaxial strength criterion.To start with,the investigation focused on the micro-structure of the envelope material and the mechanical behavior at the yarn level was carried out.Based on the decomposition of a typical envelope materal,the structural layer(also named the base fabric)manufactured by warp and weft yarns interweaving and functional layers made by high polymers were obtained.To observe the geometrical features of the interweaving micro-structures and the physical characteristics of the yarn cross sections,optical microscope and scanning electron microscope(SEM)were employed.Furthermore,the base fabric of the envelope material was disassembled where individual warp and weft yarns were acquired.After intact yarn specimens preparation,mono-uniaxial tensile tests were performed to study the mechanical properties of the yarns.Based on the test results,it was found that ratio between the uniaxial tensile strength Pu and the ideal yarn strength integration only reached 87.59 %.In line with the test results and microscope observation,the numerical models to simulate the microstructure of the envelope material and further its tensile performance where developed.The relationship between the macroscopic tensile behavior of the envelope material and the mechanical properties of the micro yarns was established.Secondly,the mechanical degradation of the envelope materials after their long-term service duration was studied.Since authentic stratospheric airship structures at oprating altitude normally undergo sereve environmental conditions including high-low cyclic temperature,roentgen radiation and ozone oxidation,their after-service mechanical properties would inevitably degrade.Because the appropriate test object which could integrally reflect different weathering factors was hard to get,previous research in this direction utilized artificial acceleration weathering to simulate the influence of only a single weachering factor.In this study,envelope specimens sampled from a decommissioned airship caplue structure where mono-uniaxial tensile test,uniaxial cyclic tessile test,biaxial cyclic tensile test and biaxial shear test were systematically performed.According to the test results,the uniaxial tensile strength decreased 46.02 and 27.04 % in warp and weft respectively.Meanwhile,the reduction of biaxial tensile moduli emerged larger than 50 %.Characterizeding the naturally weatherd envelope material laied the foundation for the long-term behavior analysed of the envelope structures.In Chapter 3,research work aiming at the weld seams,the widely used joint in envelope structures,was fulfilled.For stratospheric airships and other envelope structures,the basic structural layout was accomplished by sereveal cut-parts reflecting the design.At the joint location of different cut-parts,the general treatment means is heat welding generating the welding seams.On the other hand,mature and rational test method for the mechanical performance of the weld seams were absent in current test standarnd.In this study,three kinds of most employed weld seam form were summarized while representative specimens were proposed and prepared.By conducting uniaxial tensile tests,the tensile strengths were determined as 52.66,50.37 and 51.08 N/mm respectively.At same time,SEM equipment was employed to conduct scanning upon the cross sections of welding interface which could effectively illustrate the failure mechanism of the weading seams.Afterwards,a numerical simulation was conducted to reapprear the tensile behavior of the weld seams.On top of that,research work focusing on the strength criterion of the envelope material was carried out.Because of the limitation of the experimental method,previous study in this field could only illustrate the uniaxial tensile strength of the envelope material,which significantly deviated from the authentic biaxial stress state.Current cruciform specimens of envelope materials suggested in Japanese and Germany test standards could be adapted at low biaxial stress range,while infavorble specimen failure would occure at high stress level due to stress concenstration.In this study,an innovative experimental method was proposed to solve the biaxial tensile strength of the envelope material.Following the experimental study,two kinds of specimen was developed and manufactured as biaxial failure tests were fulfilled.It was found that the biaxial tensile strength of three typical envelope materials under a 1:1 stress ratio were 89.04,83.81 and 115.59 N/mm.Combined with high-speed camera and self-assembled biaxial tester,the biaxial failure mode of the envelope material was captured for the first time.Based on the test results,it was found that the biaxial tensile strengths were larger than the uniaxial tensile strength.In Chapter 5,series of biaxial failure tests with different warp-to-weft stress ratios was performed utilizing the test method proposed in this study.As noncontact measurement based on visual image correlation technology was employed,integral biaxial stress-strain curves of envelope materials were drawn from loading initiantion to biaxial failure.Furthermore,a novel biaxial strength criterion for the envelope materials was developed which was named five-parameter failure model.The criterion was compared to classical Tsai-Hill criterion,Yeh-Stratton criterion and Norris criterion.Based on the mechical models of the envelope materials proposed in this study,numerical models to analyze the inflation strenghth of the envelope structures were established.Different from traditional simulation,fine modling was introduced where the patterning effect in the manufacture process and the weld seam behavior were both taken into consideration.When the inflation pressure reached certain level,envelope structure would fail.The numerial simulation adapted software Abaqus with VUMAT subroutine.The failure load and failure mechanism of the inflation models were firstly proposed.The simulation results were analyzed and discussed.In the final Chapter,the research findings were summarized while the direction of the future study was illustrated.