Study On Tearing Property Of An Envelope Material For Airships

The stratospheric airship is a controllable aircraft floating at around 20km altitude.Generally,it possesses the ability of stating at a fixed point with long-term,great load capacity and reusable feature,etc.Therefore,it has invaluable prospect in the field of military and civilian application.As one of significant structural parts in an airship,envelope material possesses the following properties:high strength,light weight,low permeability and resistance to environmental degradation through temperature,etc.When it comes to the tearing resistance,the envelop fabrics of the semi-rigid or the rigid airships may be inevitably punctured or stabbed by the internal structures or the external sharp objects in the applications.As a result,the safety and servicing lifetime would be effected badly.In this paper,the mechanical properties of a new envelope material for stratospheric airships were studied.Firstly,several reinforcement methods for the clamped zone of membrane specimens were analyzed.Then the uniaxial tensile tests of the envelope material were carried out to obtain the basic mechanical properties of such textile.And then the tearing tensile properties of the new fabric subjected in uniaxial and bi-axial tension were researched.Four factors had been studied on the mechanical characteristics of the envelope material.Finally,four theories were applied to analysis experimental results of the above tearing tensile tests.Specific contents are as follows:Firstly,it is known that the envelope material is easily to break or slip in the clamp since its characteristics of high strength and ultrathin.Three different materials and six kinds of binders were selected to reinforce the uniaxial tensile specimens'clamps.The tests results show that applying aluminum as a reinforcement material and 3M glue as the adhesive is the optimum choice.Then the cruciform specimens'clamps were reinforced in same way and the results testify that the method is effective and practicable.Then un-axial tensile tests of the reinforced envelope material were carried out.Basic mechanical properties of the textile were obtained and the stress-strain curves were fitted in three-stages.Secondly,uniaxial and biaxial tearing tests of the membrane material with central cracks were carried out.The initial crack length,the crack orientation and the stretching rate were selected as three factors during uniaxial tearing tensile tests.Simple variable method was applied.However,the orthogonal resolution method was employed to analyze the initial crack length,the crack orientation and the loading ratio influences on the mechanical performance in biaxial tearing tests.Experimental results show that the ultimate failure of the specimen can be classified as“?”type and"z"type and all cracks propagate along the yarn direction.Whereas,there are another rupture type,i.e.,fracturing in specimen's limb region in the biaxial tearing tests.The results of uniaxial tests manifest that tiny effect can the stretching rate produced on the textile.However,the loading ratio has significant influence on the maximum tearing force in biaxial tests.It can lead to various failure types.The experimental data show that the maximum tearing force will increase if the loading ratio in such yarn direction is enlarged.Meanwhile,the maximum tearing force increases as the initial crack length getting larger or the orientation getting larger.What is more,the factors of the initial crack length and the orientation can be combined as the equivalent initial crack length after thoroughly observing and analyzing the experimental data.Finally,four tearing theories were applied to analyze the coated fabric.In this part,the traditional Griffith's energy theory,the Stress intensity factor theory,Thiele's empirical theory and the Stress field theory were specifically introduced.Then the abovementioned theories were concluded as two forms:the form of power function ?_c=k₁?^n₁ and the form of logarithmic function ?_c=k₂/?1+??n₂??.Then the Stress field theory was modified by considering the specimens'width infuluence.The form of power function theoretical results show a higher distance with the experimental data comparing the results deduced by logarithmic function form when the equivalent crack length is less than 10mm.However,both the two forms theoretical results have a good accordance with experimental data when the equivalent crack length is around 10mm to 40mm.but when the crack length is higher than 40mm,only the Stress intensity factor theory and the modified Stress field theory fit well to experimental data in the uniaxial tearing tensile tests.At last,the relationship between the stress concentration factor of crack tips and specimen's edge with equivalent initial crack length was studied based on the modified Stress field theory.Then,a method on confirming the minimum width of specimen in accordance to the equivalent initial crack length was proposed.