重组竹的应力—应变关系与强度准则

发布时间：2018-03-27 05:30

本文选题：重组竹　切入点：力学性能　出处：《南京林业大学》2015年硕士论文

【摘要】：重组竹是将原竹截断、软化、疏解成纤维束后再进行干燥、施胶、组坯、热压成型的一种生物质复合材料,具有高强轻质、绿色环保等优点,是集社会、经济、生态效益为一体的极具发展潜力的新型建筑用材。由于对重组竹材料性能缺乏了解,重组竹在工程运用之前需要进行大量的针对性试验,用于解决设计中所涉及到的材料强度、构件承载力与刚度等问题。试验研究增加了工程的建造周期与成本,不利于重组竹结构的推广与普及。以为工程设计服务为出发点,本文对重组竹的基本力学性能,包括抗拉、抗压以及抗剪性能进行试验,获得了材料在不同受力状态下的破坏形态,力学参数以及应力—应变曲线,运用复合材料力学、弹塑性力学以及统计学的相关理论对试验数据和现象进行分析,得到如下结论:(1)重组竹的破坏是由于竹纤维束与基体结合界面的粘结失效造成的,提高竹纤维束与基体结合界面的粘结强度是改善重组竹力学性能的关键。(2)重组竹的顺纹拉、压力学性能优于横纹,横纹受剪时由于发挥了竹纤维束的抗拉性能使得横纹剪切反而优于顺纹与面内的性能;材料同一方向的拉、压、剪切性能也存在较大差异,证明了重组竹是一种由竹纤维束定向增强的复合材料,只要是有利于发挥竹纤维束性能的受力状态,其力学性能就相应提高。(3)裂纹的开展影响材料的刚度进而影响到应力—应变曲线的发展趋势,裂纹开展速率快、路径短,应力—应变关系为弹性,试件呈无明显征兆的脆性破坏;裂纹开展速率慢、路径长,应力—应变关系会出现明显的塑性段,试件呈具有延性的塑性破坏。重组竹不同受力状态下的应力—应变曲线可简化为三种模型,由模型的解析表达式得到的曲线与实测曲线的拟合效果良好。(4)将重组竹力学试验获得的强度参数按Tsai-Hill、Norris和Hoffman三种强度理论得到了材料在1-2平面的屈服和破坏包络曲线,其中Tsai-Hill准则得到的包络线最为平滑,在不同应力状态下基本介于Norris和Hoffman准则之间,在缺乏大量多轴试验研究的现阶段,采用Tsai-Hill准则是比较保守的做法,而Norris准则和Hoffman准则关于材料双轴受压或受拉会使材料强度提高的结论还需进一步的试验论证。(5)重组竹力学试验测得的强度样本服从正态分布,在小样本情况下由正态分布单侧置信下限系数法计算得到的具有90%保证率的材料强度标准值与按可靠度规范得到的具有95%保证率的强度标准值几乎一致,而保证率相差5%,说明小样本下应采用本文提出的正态分布单侧置信下限系数法确定材料强度标准值。(6)以重组竹清材小试件强度试验值为基础,考虑实际工程中的不利因素引入折减系数,得到了重组竹容许设计应力;借鉴木结构极限状态设计法对重组竹清材小试件强度标准值进行折减得到重组竹构件的强度设计参考值。
[Abstract]:The reconstituted bamboo is a kind of biomass composite material which is cut off, softened, dredge into fiber bundles and then dried, sizing, forming billet and hot pressing molding. It has the advantages of high strength, light weight, green environmental protection, and so on. It is a social and economic gathering. Because of the lack of understanding of the properties of the reconstituted bamboo materials, a large number of targeted tests need to be carried out before the application of the reconstituted bamboo materials to solve the strength of the materials involved in the design. The experimental study has increased the construction cycle and cost of the project, which is not conducive to the popularization and popularization of the reconstituted bamboo structure. In this paper, the basic mechanical properties of the reconstituted bamboo, including tensile resistance, are considered as the starting point of the engineering design service. The compressive and shear properties of the material were tested, and the failure patterns, mechanical parameters and stress-strain curves of the materials under different loading conditions were obtained. The experimental data and phenomena are analyzed by elastic-plastic mechanics and statistical theory. The following conclusion is drawn: the failure of the reconstituted bamboo is caused by the bond failure of the interface between the bamboo fiber bundle and the matrix. The key to improve the mechanical properties of reconstituted bamboo is to improve the bonding strength of the interface between bamboo fiber bundles and matrix. Due to the tensile properties of bamboo fiber bundles, the transverse shear is superior to the in-plane and in-plane properties, and the tensile, compression and shear properties of the materials in the same direction are also different. It is proved that the reconstituted bamboo is a kind of composite reinforced by bamboo fiber bundles, so long as it is favorable to give full play to the mechanical state of bamboo fiber bundles. The mechanical properties of the crack increase accordingly. The crack development affects the stiffness of the material and then affects the development trend of the stress-strain curve. The crack development rate is fast, the path is short, and the stress-strain relationship is elastic. The specimens show brittle failure without obvious signs, the crack developing rate is slow, the path is long, the stress-strain relationship will appear obvious plastic segment, The stress-strain curve of reconstituted bamboo under different stress states can be simplified into three kinds of models. According to the strength theory of Tsai-Hilli Norris and Hoffman, the yield and failure envelope curves of the material in the 1-2 plane were obtained according to the strength theory of Tsai-Hillen Norris and Hoffman. The envelope obtained by Tsai-Hill criterion is the smoother, which is basically between Norris and Hoffman criterion under different stress states. At the present stage, it is more conservative to adopt Tsai-Hill criterion in the absence of a large number of multiaxial experimental studies. However, the conclusion of Norris criterion and Hoffman criterion that biaxial compression or tension can increase the strength of materials needs further experimental demonstration. 5) the strength samples obtained from the mechanical tests of reconstituted bamboo are normally distributed. In the case of small sample, the standard value of strength of material with 90% guarantee rate calculated by the method of one-side confidence lower bound coefficient of normal distribution is almost the same as the standard value of strength with 95% guarantee rate obtained according to reliability specification. The difference of guarantee rate is 5, which indicates that the standard value of material strength should be determined by the method of lower confidence limit coefficient of normal distribution on one side under the small sample.) based on the strength test value of small specimen of reconstituted bamboo clear wood, The allowable design stress of reconstituted bamboo was obtained by introducing the reduction coefficient into consideration of the unfavorable factors in the actual project. Using the limit state design method of wood structure for reference, the strength standard value of reconstituted bamboo clear small specimen was reduced to obtain the strength design reference value of reconstituted bamboo component.
【学位授予单位】：南京林业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TU599

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