重組竹的單軸與純剪應(yīng)力應(yīng)變關(guān)系

盛寶璐，周愛(ài)萍，黃東升

(南京林業(yè)大學(xué) 土木工程學(xué)院，南京 210037)

摘要：重組竹是將竹絲束平行組坯、經(jīng)高壓膠合而成的一種生物質(zhì)復(fù)合材料，是一種極具潛勢(shì)的建筑結(jié)構(gòu)材料。研究重組竹的基本力學(xué)性能和應(yīng)力應(yīng)變關(guān)系，是建立此類(lèi)材料本構(gòu)關(guān)系和進(jìn)行重組竹結(jié)構(gòu)非線(xiàn)性分析的基礎(chǔ)。將重組竹理想化為橫向各向同性復(fù)合材料，通過(guò)試驗(yàn)，給出了重組竹各主軸方向的單軸與各主平面的純剪力學(xué)參數(shù)，建立了各種應(yīng)力狀態(tài)下的應(yīng)力應(yīng)變關(guān)系。結(jié)果表明，重組竹力學(xué)性能優(yōu)于常用的結(jié)構(gòu)用木材，且變異性較小。重組竹順紋受拉強(qiáng)度約是順紋受壓強(qiáng)度的2倍；橫紋受拉強(qiáng)度遠(yuǎn)遠(yuǎn)低于橫紋受壓強(qiáng)度；橫切面內(nèi)的剪切模量及強(qiáng)度遠(yuǎn)遠(yuǎn)低于另外兩個(gè)方向，且橫紋剪切強(qiáng)度是順紋剪切強(qiáng)度的3倍。重組竹的應(yīng)力應(yīng)變關(guān)系和破壞模式與纖維參與受力程度密切相關(guān)。順紋受拉時(shí)，拉應(yīng)力完全由纖維承擔(dān)，破壞表現(xiàn)為纖維的脆性拉斷，強(qiáng)度最高，應(yīng)力應(yīng)變?yōu)橥耆€(xiàn)性關(guān)系；其他應(yīng)力狀態(tài)下，破壞均發(fā)生在基體或纖維-基體界面，若裂紋的擴(kuò)展受到纖維限制，破壞呈漸進(jìn)性，強(qiáng)度較低，應(yīng)力應(yīng)變曲線(xiàn)由早期的線(xiàn)性關(guān)系轉(zhuǎn)入后期的非線(xiàn)性關(guān)系；當(dāng)裂紋的擴(kuò)展未受到纖維限制，破壞強(qiáng)度最低，應(yīng)力應(yīng)變呈線(xiàn)性關(guān)系。

關(guān)鍵詞：重組竹；應(yīng)力應(yīng)變關(guān)系；破壞模式；橫觀(guān)各向同性復(fù)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(zhì)復(fù)合材料，一般3~5年就可以成材，具有硬闊葉樹(shù)材的諸多優(yōu)良特性，其比強(qiáng)度和比剛度高于鋼材，是一種理想的綠色高強(qiáng)建筑材料。竹結(jié)構(gòu)建筑的環(huán)境負(fù)荷遠(yuǎn)小于鋼材、混凝土等傳統(tǒng)建筑。但原竹材壁薄中空，幾何變異性大、含糖高易蟲(chóng)蛀、質(zhì)地不均勻、耐久性較差，不能滿(mǎn)足現(xiàn)代工程結(jié)構(gòu)對(duì)構(gòu)件的力學(xué)性能和幾何構(gòu)形要求。原竹剖割成約長(zhǎng)2 m、寬15 mm、厚3 mm的竹蔑，經(jīng)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(shù)脂膠，通過(guò)高溫?zé)釅耗z合，制成重組竹[1-3]。由于在工業(yè)化制造的過(guò)程中，竹材經(jīng)過(guò)篩選，剔除了原竹的缺陷，且含水率低，故重組竹力學(xué)性能均勻、變異性小、強(qiáng)度高、耐久性好，很少出現(xiàn)收縮、翹曲、開(kāi)裂等現(xiàn)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(gòu)件[4-6]。因此，重組竹可以滿(mǎn)足現(xiàn)代建筑結(jié)構(gòu)對(duì)材料的力學(xué)、環(huán)保與耐久性等方面的性能要求。研究表明，重組竹結(jié)構(gòu)在節(jié)能環(huán)保、工業(yè)化生產(chǎn)、裝配式施工等方面有著傳統(tǒng)材料不可替代的優(yōu)勢(shì)，以重組竹作為結(jié)構(gòu)材料建造多層甚至高層建筑是極具潛勢(shì)的發(fā)展方向。

此外，結(jié)構(gòu)服役期內(nèi)，材料常常處于復(fù)雜應(yīng)力狀態(tài)。作為復(fù)雜應(yīng)力狀態(tài)下纖維定向復(fù)合材料的破壞準(zhǔn)則，常包含多個(gè)材料常數(shù)。如，Hill[11]提出的正交各向異性材料破壞準(zhǔn)則，包涵了6個(gè)彈性常數(shù)，由于Hill準(zhǔn)則忽略了材料拉、壓性質(zhì)的差異，故不適用于重組竹材料強(qiáng)度分析。Hoffman[12]考慮了材料拉、壓力學(xué)性能的差異，提出了如式(1)的破壞準(zhǔn)則。

(1)

1重組竹力學(xué)性能的描述

度S23、S31和S12。上述所有彈性常數(shù)和強(qiáng)度參數(shù)均可通過(guò)重組竹3個(gè)主方向的單軸拉、壓和3個(gè)主平面純剪試驗(yàn)得到。

圖1　重組竹3個(gè)材料主方向Fig.1　Main direction of PSB

2重組竹基本力學(xué)性能試驗(yàn)

試驗(yàn)竹材取自浙江安吉5年生毛竹(phyllostachys)，按照重組竹標(biāo)準(zhǔn)制作工藝制成試驗(yàn)材料，試件的密度和含水率分別為11.0 kN/m3和 8.0%。每個(gè)試驗(yàn)類(lèi)型的試件為30個(gè)，試件構(gòu)形見(jiàn)表1。單軸試驗(yàn)參照ASTM D143-09[17]、剪切試驗(yàn)參照ASTM D7078標(biāo)準(zhǔn)[18]進(jìn)行。采用TDS-530多通道數(shù)據(jù)采集箱同時(shí)采集荷載、變形和應(yīng)變，采集頻率為1 Hz。各類(lèi)型試驗(yàn)過(guò)程與現(xiàn)象詳見(jiàn)表2。

表1　試件的尺寸、形狀和相關(guān)參數(shù)計(jì)算公式

續(xù)表1

表2　試驗(yàn)過(guò)程與現(xiàn)象

續(xù)表2

3試驗(yàn)結(jié)果與分析

3.1　主要力學(xué)性能參數(shù)

統(tǒng)計(jì)分析表明，各類(lèi)型試驗(yàn)的力學(xué)參數(shù)基本呈正態(tài)分布，圖2給出了橫紋受壓力學(xué)參數(shù)的正態(tài)檢驗(yàn)結(jié)果。表3為試驗(yàn)測(cè)得的力學(xué)參數(shù)統(tǒng)計(jì)分析結(jié)果。可以看出，重組竹順紋拉、壓模量差異很小，順紋受拉強(qiáng)度約為順紋受壓強(qiáng)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(qiáng)度遠(yuǎn)遠(yuǎn)低于橫紋受壓強(qiáng)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(qiáng)度遠(yuǎn)遠(yuǎn)低于橫紋剪切強(qiáng)度，而橫切面內(nèi)的剪切模量及強(qiáng)度遠(yuǎn)遠(yuǎn)低于另外2個(gè)方向。

表4將重組竹與常用的結(jié)構(gòu)用木材性能[19]進(jìn)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(jié)構(gòu)用木材，而強(qiáng)度遠(yuǎn)高于所有常用的

結(jié)構(gòu)用木材；重組竹材料力學(xué)參數(shù)的變異性比木材小。因此，重組竹的性質(zhì)優(yōu)于木材，是一種理想的建筑結(jié)構(gòu)材料。

圖2　橫紋受壓正態(tài)分布檢驗(yàn)圖Fig.2　Gaussian distribution test for compressionin

試驗(yàn)類(lèi)型彈性模量均值/MPaCV/%泊松比均值/MPaCV/%比例極限應(yīng)力應(yīng)變均值/MPaCV/%均值/MPaCV/%峰值強(qiáng)度應(yīng)力應(yīng)變均值/MPaCV/%均值/MPaCV/%極限強(qiáng)度應(yīng)力應(yīng)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(nèi)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(jié)構(gòu)木材的力學(xué)性能比較

3.2　應(yīng)力-應(yīng)變關(guān)系

(2)

(3)

(4)

式中:系數(shù)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(xiàn)與試驗(yàn)曲線(xiàn)的比較。可以看出，兩者吻合良好。

圖3　應(yīng)力應(yīng)變曲線(xiàn)曲線(xiàn)的比較Fig.3　Compare the calculating stress-strain curves to those obtained by

4結(jié)論

重組竹順紋拉、壓彈性模量相差無(wú)幾，順紋抗拉強(qiáng)度大約是橫紋抗壓強(qiáng)度的2倍，橫紋抗拉強(qiáng)度低于橫紋抗壓強(qiáng)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(qiáng)度大大低于橫紋抗剪強(qiáng)度，橫切面內(nèi)的抗剪彈性模量與強(qiáng)度都低于另外兩個(gè)方向的剪切強(qiáng)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(ài)萍()，副教授，博士，(E-mail)zaping2007@163.com。

作者簡(jiǎn)介：盛寶璐(1990-)，博士生,主要從事現(xiàn)代竹木結(jié)構(gòu)研究，(E-mail)baolu.sheng@yahoo.com。

基金項(xiàng)目：國(guó)家自然科學(xué)基金(51378263)；林業(yè)科學(xué)技術(shù)成果國(guó)家級(jí)推廣項(xiàng)目([2015]21號(hào))。

收稿日期：2015-07-30

中圖分類(lèi)號(hào)：TU502

文獻(xiàn)標(biāo)志碼：A

文章編號(hào)：1674-4764(2015)06-0024-08

doi:10.11835/j.issn.1674-4764.2015.06.004