重組竹的單軸與純剪應力應變關系

盛寶璐，周愛萍，黃東升

(南京林業大學 土木工程學院，南京 210037)

摘要：重組竹是將竹絲束平行組坯、經高壓膠合而成的一種生物質復合材料，是一種極具潛勢的建筑結構材料。研究重組竹的基本力學性能和應力應變關系，是建立此類材料本構關系和進行重組竹結構非線性分析的基礎。將重組竹理想化為橫向各向同性復合材料，通過試驗，給出了重組竹各主軸方向的單軸與各主平面的純剪力學參數，建立了各種應力狀態下的應力應變關系。結果表明，重組竹力學性能優于常用的結構用木材，且變異性較小。重組竹順紋受拉強度約是順紋受壓強度的2倍；橫紋受拉強度遠遠低于橫紋受壓強度；橫切面內的剪切模量及強度遠遠低于另外兩個方向，且橫紋剪切強度是順紋剪切強度的3倍。重組竹的應力應變關系和破壞模式與纖維參與受力程度密切相關。順紋受拉時，拉應力完全由纖維承擔，破壞表現為纖維的脆性拉斷，強度最高，應力應變為完全線性關系；其他應力狀態下，破壞均發生在基體或纖維-基體界面，若裂紋的擴展受到纖維限制，破壞呈漸進性，強度較低，應力應變曲線由早期的線性關系轉入后期的非線性關系；當裂紋的擴展未受到纖維限制，破壞強度最低，應力應變呈線性關系。

關鍵詞：重組竹；應力應變關系；破壞模式；橫觀各向同性復合材料

Received:2015-07-30

Foundation item：National Natural Science Foundation of China (No.51378263)；National Promotion Program of Forestry Science and Technology Achievements(No.[2015]21).

竹材是一種可再生、可降解的天然生物質復合材料，一般3~5年就可以成材，具有硬闊葉樹材的諸多優良特性，其比強度和比剛度高于鋼材，是一種理想的綠色高強建筑材料。竹結構建筑的環境負荷遠小于鋼材、混凝土等傳統建筑。但原竹材壁薄中空，幾何變異性大、含糖高易蟲蛀、質地不均勻、耐久性較差，不能滿足現代工程結構對構件的力學性能和幾何構形要求。原竹剖割成約長2 m、寬15 mm、厚3 mm的竹蔑，經80 ℃恒溫烘干至含水率低于11%，再將竹篾碾壓成竹絲束，同方向平行組坯并浸漬酚醛樹脂膠，通過高溫熱壓膠合，制成重組竹[1-3]。由于在工業化制造的過程中，竹材經過篩選，剔除了原竹的缺陷，且含水率低，故重組竹力學性能均勻、變異性小、強度高、耐久性好，很少出現收縮、翹曲、開裂等現象。重組竹可以加工成不同截面的型材，適用于房屋的梁、柱及跨度較大的構件[4-6]。因此，重組竹可以滿足現代建筑結構對材料的力學、環保與耐久性等方面的性能要求。研究表明，重組竹結構在節能環保、工業化生產、裝配式施工等方面有著傳統材料不可替代的優勢，以重組竹作為結構材料建造多層甚至高層建筑是極具潛勢的發展方向。

此外，結構服役期內，材料常常處于復雜應力狀態。作為復雜應力狀態下纖維定向復合材料的破壞準則，常包含多個材料常數。如，Hill[11]提出的正交各向異性材料破壞準則，包涵了6個彈性常數，由于Hill準則忽略了材料拉、壓性質的差異，故不適用于重組竹材料強度分析。Hoffman[12]考慮了材料拉、壓力學性能的差異，提出了如式(1)的破壞準則。

(1)

1重組竹力學性能的描述

度S23、S31和S12。上述所有彈性常數和強度參數均可通過重組竹3個主方向的單軸拉、壓和3個主平面純剪試驗得到。

圖1　重組竹3個材料主方向Fig.1　Main direction of PSB

2重組竹基本力學性能試驗

試驗竹材取自浙江安吉5年生毛竹(phyllostachys)，按照重組竹標準制作工藝制成試驗材料，試件的密度和含水率分別為11.0 kN/m3和 8.0%。每個試驗類型的試件為30個，試件構形見表1。單軸試驗參照ASTM D143-09[17]、剪切試驗參照ASTM D7078標準[18]進行。采用TDS-530多通道數據采集箱同時采集荷載、變形和應變，采集頻率為1 Hz。各類型試驗過程與現象詳見表2。

表1　試件的尺寸、形狀和相關參數計算公式

續表1

表2　試驗過程與現象

續表2

3試驗結果與分析

3.1　主要力學性能參數

統計分析表明，各類型試驗的力學參數基本呈正態分布，圖2給出了橫紋受壓力學參數的正態檢驗結果。表3為試驗測得的力學參數統計分析結果?？梢钥闯?，重組竹順紋拉、壓模量差異很小，順紋受拉強度約為順紋受壓強度的兩倍；橫紋受拉彈性模量約是橫紋受壓彈性模量的兩倍，橫紋受拉強度遠遠低于橫紋受壓強度；順紋剪切彈性模量與橫紋剪切彈性模量非常接近，順紋剪切強度遠遠低于橫紋剪切強度，而橫切面內的剪切模量及強度遠遠低于另外2個方向。

表4將重組竹與常用的結構用木材性能[19]進行了比較，可以看到，重組竹順紋彈性模量高于大部分的常用的結構用木材，而強度遠高于所有常用的

結構用木材；重組竹材料力學參數的變異性比木材小。因此，重組竹的性質優于木材，是一種理想的建筑結構材料。

圖2　橫紋受壓正態分布檢驗圖Fig.2　Gaussian distribution test for compressionin

試驗類型彈性模量均值/MPaCV/%泊松比均值/MPaCV/%比例極限應力應變均值/MPaCV/%均值/MPaCV/%峰值強度應力應變均值/MPaCV/%均值/MPaCV/%極限強度應力應變均值/MPaCV/%均值/MPaCV/%順紋受拉受壓受剪橫紋受拉受壓受剪垂直于平面內剪切10296140.37118.4201.26131189081733.0930.28665.5391.401955.7682.07221361188.21190.77253066280.34.43150.28271365147.68100.59923.14153.50151446139.73150.78523.44103.7634746143.64100.6214

表4　重組竹與結構木材的力學性能比較

3.2　應力-應變關系

(2)

(3)

(4)

式中:系數λi和βi(i=1, 2, 3), 可以由下式確定

(5)

圖3表示上述解析公式得到的曲線與試驗曲線的比較?？梢钥闯觯瑑烧呶呛狭己谩?/p>

圖3　應力應變曲線曲線的比較Fig.3　Compare the calculating stress-strain curves to those obtained by

4結論

重組竹順紋拉、壓彈性模量相差無幾，順紋抗拉強度大約是橫紋抗壓強度的2倍，橫紋抗拉強度低于橫紋抗壓強度；順紋、橫紋剪切彈性模量相近，大約是順紋抗拉彈性模量的1/8；順紋抗剪強度大大低于橫紋抗剪強度，橫切面內的抗剪彈性模量與強度都低于另外兩個方向的剪切強度與剪切模量。

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(編輯胡玲)

Author brief：Sheng Baolu(1990-)，PhD candidate,main research interest:mechanics performance analysis of bamboo/wood structure,(E-mail)baolu.sheng@yahoo.com。

Zhou Aiping(corresponding author)，associate professor,PhD,(E-mail)zaping2007@163.com。

Stress-strain relationship of parallel strand bamboo under uniaxial or pure shear load

Sheng Baolu, Zhou Aiping, Huang Dongsheng

(College of Civil Engineering, Nanjing Forestry University, Nanjing 210037, P.R. China)

Abstract:Parallel Strand Bamboo (PSB) is a biocomposite composed of long narrow parallel bamboo strands which are adhesively bonded under high pressure. It has more and more attractive structural applications in building and construction engineering. It’s important to well understand the stress-strain relationship to develop the constitutive law and conduct the nonlinear analysis of PSB structures. The PSB was treated as an transversely isotropic composite in the experiment and the uniaxial parameters in each main material axis and the pure shearing parameters in each main material plane were proposed, and the corresponding stress-strain relationships of each stress state were also established. The results show that compared with common used woods in construction engineering, PSB has higher strengths with less variability. Strength of tension parallel to grain is nearly as twice as that of compression parallel to grain. In perpendicular to grain direction, the strength of tension is much lower than the that of compression. Shearing in transverse-to-grain plane presents lowest modulus and strength than those shearing in other two directions. The shearing strength in perpendicular to grain is as about 3 times as that of shearing in parallel to grain direction. The stress-strain relationships and the failure modes of PSB are significantly depended on the way in which the fiber participated. In parallel to grain direction, tensile damage almost entirely contributes to the broken of fibers, which shows the highest strength and brittle behavior among all stress states. In other cases, when the expending of failure cracks are restricted by fibers, the damage presents progressive process and higher strength, and the stress-strain relationships exhibit linearity in the earliear life while turn to nonlinearity in the later life of the specimens. When damages take place in matrix or in fiber-matrix interface without fibers involved in, the material shows lower strength, and the stress-strain curves present linear and brittle behavior.

Key words:Parallel strand bamboo;stress-strain relationship;failure mode;transversely isotropic composite

通信作者周愛萍()，副教授，博士，(E-mail)zaping2007@163.com。

作者簡介：盛寶璐(1990-)，博士生,主要從事現代竹木結構研究，(E-mail)baolu.sheng@yahoo.com。

基金項目：國家自然科學基金(51378263)；林業科學技術成果國家級推廣項目([2015]21號)。

收稿日期：2015-07-30

中圖分類號：TU502

文獻標志碼：A

文章編號：1674-4764(2015)06-0024-08

doi:10.11835/j.issn.1674-4764.2015.06.004