摘要
随着膜结构建筑的广泛应用,建筑膜材料的力学性能研究愈来愈受到关注。相比经编类建筑膜材料,机织建筑膜材料的基布组织结构多为平纹或方平,织造工艺简单,尤其适合玻璃纤维织物类建筑膜材料的生产,应用更为广泛。建筑膜材料不同于一般的建筑材料,它是一种柔性的涂层织物复合材料,需要依赖膜的特定张拉形状才能承受载荷。建筑膜材料在各种拉伸载荷作用下的拉伸性能对于指导膜结构的设计和构建非常必要,因而,本课题围绕机织建筑膜材料在单轴、双轴和多轴拉伸载荷作用下的力学性能展开研究。
通过对机织建筑膜材料的单轴、双轴和多轴拉伸试验,研究膜材料在各种拉伸载荷作用下的力学性能,以期实现:(1)探索获得膜材料在各种载荷作用下的拉伸力学行为的测试方法,为建立膜材料在双轴和多轴拉伸载荷作用下拉伸测试标准提供参考;(2)把握建筑膜材料在各种载荷作用下的拉伸力学行为特征,为膜结构建筑的设计和施工提供更适用可靠的材料性能参数。
论文对机织建筑膜材料在单轴一偏轴拉伸力作用下的拉伸性能进行了分析,结果表明,在低拉伸应力和拉伸破坏应力作用下,膜材料的拉伸行为都表现出明显的各向异性特性和非线性特征。在低拉伸应力作用下,三次加/卸载循环后,膜材料的拉伸应力一应变关系线性程度大大提高。实验获得的膜材料弹性常数基本满足正交各向异性弹性材料在偏轴拉伸力作用下的本构关系,证明了在低拉伸应力作用下,在三次加/卸载后,机织建筑膜材料可以被看作为正交各向异性的弹性材料,为推导机织建筑膜材料的双轴拉伸弹性模量估算式提供了理论依据。从对机织建筑膜材料在拉伸破坏应力作用下的破坏模式分析中知,膜材料的破坏模式有三种:纯拉伸破坏、纯剪切破坏和拉伸一剪切混合型破坏等。利用Tsai-Hill强度准则可以对纯拉伸或纯剪切破坏模式下的膜材料断裂强度做出较好的预测,但该准则不适合对拉剪混合型破坏模式下膜材料断裂强度的预测。
在膜结构建筑应用过程中,膜材料的受力往往来自两个甚至多个方向,双轴拉伸试验成为深入了解膜材料拉伸力学特性的重要测试方法。然而,迄今仍未有国际认可的有关膜材料双轴拉伸试验的测试标准。论文通过合理设计并实施一系列的双轴拉伸试验,研究了拉伸测试中应变测试方法、应变测试位置和试样尺寸等测试因素对测试结果的影响。分析得出,在双轴拉伸测试中,应用接触式的针式应变计能正确反映测试结果;为避免测试位置对测试结果的可能影响,应变计的测试位置应尽可能地靠近试样的中心线位置;在测量精度较高的情况下,双轴拉伸试样尺寸可以缩小至中心区域尺寸为60mm×60mm,不会对测试结果造成明显影响,同时可节省试样材料。
论文分析了机织建筑膜材料在双轴拉伸循环载荷作用下的拉伸行为,以及在不同的双轴向应力加载比率下,拉伸残余变形的回复情况。研究表明,机织建筑膜材料的双轴拉伸行为受到加载条件影响显著,且随着拉伸循环载荷次数的增加,膜材料的线性化程度和弹性化程度都得到了较大的改善,但经纬两向拉伸行为(拉伸弹性模量和拉伸弹性回复性)存在明显的不对称性,这主要源自基布材料(织物)的结构和性能以及涂层过程中的拉伸作用对膜材料拉伸力学行为的影响。研究还表明,在双轴拉伸条件下,在膜材料的经纬向结构和力学性能如本文所示的不对称的情况下,经纬两向应力加载比率小于1的载荷条件不利于膜材料在膜结构中的应用,这在设计和施工中应尽力予以避免。
根据假设:低应力循环载荷作用后机织建筑膜材料可近似地看作为正交各向异性的线弹性材料,论文推导了机织建筑膜材料在双轴向拉伸载荷作用下拉伸弹性模量的估算方程式,结合双轴拉伸实验结果,验证得:用机织建筑膜材料的单轴拉伸弹性常数可以估算出膜材料在主应力方向(双轴向中应力加载较大的方向)上的弹性模量值。
论文还介绍了对机织建筑膜材料的多轴拉伸测试及多轴拉伸性能的研究。分析了多轴拉伸测试条件对拉伸测试结果的影响,如:试样的形状、尺寸和测试速度等。研究结果表明,在多轴拉伸测试中,拉伸试样形状和尺寸的选择对有效地把握膜材料多轴拉伸性能至关重要。分析得知,具有较大夹持臂宽度的齿轮型多轴拉伸试样有利于膜材料多轴拉伸性能测试研究;只有在较低的拉伸测试速度(低于20 mm/min)下,才能测得更为真实详尽的膜材料多轴拉伸性能曲线。
论文还对同一种机织建筑膜材料进行了单轴、双轴和多轴拉伸性能测试,分析对比了在单轴、双轴和多轴加载条件下的拉伸性能差异。分析表明,加载条件对机织建筑膜材料的拉伸性能影响显著。在膜材料的应用中,应全面考虑膜材料在各种加载条件下的拉伸性能,以便提高膜材料的使用效率,减少因此而带来的结构隐患。
从实验分析的角度,论文介绍了带有中心裂缝的机织建筑膜材料在多轴拉伸测试条件下的裂缝蔓延情况及破坏模式。结合分析带有中心裂缝的膜材料在多轴拉伸载荷作用下的拉伸断裂性能得知,由于机织建筑膜材料试样的经纬向的拉伸性能的明显差异,经向拉伸性能在膜材料多轴拉伸加载条件下起着主导作用。因而,在机织建筑膜材料的设计生产中,应尽可能平衡膜材料各向拉伸性能,使膜材料的多轴拉伸载荷作用下有较高的拉伸性能。
Lightweight structures,such as tension structures and air-supported structures, have been applied widely in the past thirty years.Woven membrane materials as roof materials used for this kind of structures play an important role in the application of lightweight structures.As a kind of flexible material,membrane materials have virtually little bending stiffness.To sustain a shape,the membrane materials must be in tension.Mechanical properties of the membrane materials in tension are therefore very important for structural design,installation and maintenance.Therefore,this study focuses on the tensile performance of woven membrane materials under uni-axial,bi-axial and multi-axial tensile loads.
The aim of this study is to discuss the testing conditions under bi-axial and multi-axial loads and tries to find out something important for the establishment of experimental standards of membrane materials.The objective of this study is to get a better understanding of the tensile performance of flexible membrane materials and try to guide the design and the application of the membrane structure.
The study focuses on the tensile performance of woven membrane materials under uni-axial loads.One group of off-axis tensile experiments,with off-axial angles of 0°,15°,30°,45°,60°,75°and 90°,are conducted under low tensile stresses.The anisotropic behaviors on elastic constants of the membrane materials are analyzed with the application of off-axial constitutive response of the orthotropic and elastic materials.It shows that the experiments agree well with the prediction results.It is proved that woven membrane materials could be regarded as orthotropic and elastic materials within 20%of the ultimate tensile stress.To analyze the anisotropic behaviors at the tensile failure stage,another group of off-axial tensile experiments with the same off-axial angles are carried out.Three types of failure mechanisms, pure tensile failure,pure shear failure and a mixed failure of tensile and shear,have been observed by analyzing the fracture configuration of the specimens under each bias tensile loading.For the prediction of the anisotropic failure strength of woven membrane materials,Tsai-Hill strength criterion is used.The criterion has shown its great efficiency,except that in the off-axial direction of about 15°,which is due to the difference on the yarn crimp between the warp and the fill directions resulting from weaving and coating processes.
Factors that might affect test results of membrane material under uni-and bi-axial tensile loads are examined.After series of tensile tests on PVC-coated membrane materials,it is demonstrated that to measure the strains in the two perpendicular directions,the contact method by the needle extensometer does not interfere the correct data recording;the positions where the strains are measured on specimens have a great influence on the test results of the stiffness and Poisson's ratio in warp direction under uni-axial load;to perform bi-axial tensile tests the size of the cruciform specimen in bi-axial tensile test can be much smaller than those suggested in the literature.It is also shown that the residual strains of woven membrane materials are affected not only by the properties of the constituent yams and woven structure but also by loading conditions during the coating process.
To find a feasible method to estimate the elastic modulus for woven membrane materials under bi-axial loads,modeling equations has been deduced.During the model development,woven membrane materials are treated as orthotropic,elastic and linear.Based on the constitutive relationships for orthotropic and elastic materials, two equations sets are deduced for the estimation of the elastic modulus in the warp and the fill direction respectively when suffering from bi-axial loads.From the comparison of the experiments and theoretical prediction,it can be noticed that the model can achieve ideal results when it comes to the estimation of the modulus in the main direction,which is the direction with a higher stress than the other one. Therefore,based on the hypothesis of orthotropic,elastic and linear materials,there is an easy way to estimate the bi-axial tensile properties of woven membrane materials in the main direction only through the uni-axial tensile tests.
The study also focuses on the experimental investigation of tensile performance of membrane materials under multi-axial tensile loads.Factors,such as specimen configuration and loading speed,which could affect the correct interpretation of testing results,are investigated.The configuration of the specimen for the multi-axial tensile tests is identified as a gear-shape with large arm widths.A loading speed of lower than 20 mm/min is suggested to obtain the tensile properties of membrane materials under multi-axial loads.The tensile performances of membrane materials under uni-,bi-and multi-axial loads are compared.It shows that the tensile strength and elastic modulus under bi-and multi-axial loads are much greater than those under uni-axial loads.Therefore,for the application of membrane materials in lightweight structures,bi-axial or multi-axial loading conditions will be necessary.
Experiments on the specimens with an initial crack in the center under multi-axial loads show that,by comparison with other loading directions,the tensile properties in warp direction of woven membrane materials play an important role in the failure performance and crack propagation under multi-axial loads.To eliminate the dependence on the mechanical properties in warp direction,the balance of the two principle directions of membrane materials should be improved.
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