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竹子具备高强度、高韧性等优点,并且生长速度快,可快速成材,被广泛应用于建筑、家具和造纸等领域[1−3]。作为21世纪最具开发潜力的生物质资源,竹材具有不可比拟的生态、经济以及社会效益[4],对促进碳达峰、碳中和具有极大的意义。竹材被认为是以薄壁组织为基体,嵌入维管束作为增强相的纤维增强生物复合材料[5−6]。维管束起着承担力学强度的骨架作用,基本组织填充其余部分,起着传递载荷的作用[7]。竹材优异的力学性能主要与纤维鞘组织比量密切相关[8]。目前,竹产业主要生产传统竹胶合板、竹席/竹帘胶合板、竹集成材和重组竹等或新型工程竹复合材料[9]、复合吸波材料[10]和竹缠绕复合管[11]等,大多以毛竹Phyllostachys edulis等大径级的竹材为原料[12−13],小径级的竹材应用较少。竹材的力学性能决定其利用价值与领域[14],竹材顺纹抗压强度作为评估竹材不同用途的适用性的重要指标[15−16],具有测试方便快速等优点[17]。小径级竹材物理力学性能测试并无相关标准,相关科学研究较少,基本上都参照GB/T 15780—1995《竹材物理力学性质试验方法》[18−19],缺乏评价小径级竹材的系统的物理力学实验方法。
本研究对不同长径比试件进行顺纹抗压试验,探索适合测试小径级竹材顺纹抗压强度的方法,为发掘小径级竹材工业应用提供科学依据[20]。同时利用基于YOLO深度学习算法开发的维管束识别模型对小径级竹材纤维鞘的组织比量和维管束密度作出统计并对其与抗压强度的关系进行线性拟合。通过引入人工智能用于竹材基础性质的研究,不仅可以解决人工处理耗时费力以及容易出错等问题,相较于传统图像处理方法还能提高处理精度和准确性。
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由表1可见:4种小径级竹材的顺纹抗压强度较为优异,采用2.0倍长径比试件测试,早园竹顺纹抗压强度最大,为82.91 MPa,水竹顺纹抗压强度最小,为67.01 MPa。在不同长径比的试件中,顺纹抗压强度测试结果平均值相差不大。方差齐性检验表明:4种小径级竹材的不同长径比试件的顺纹抗压强度差异不显著(P>0.05),故进行单因素ANOVA检验。检验结果显示:除苦竹外,其余3种小径级竹材的不同长径比试件对顺纹抗压强度没有产生显著影响(P>0.05)。然而,苦竹不同长径比试件对顺纹抗压强度产生显著影响(F=3.342,P=0.040<0.05)。进行LSD多重比较分析苦竹不同长径比试样之间的差异是否显著,结果显示:苦竹的1.0倍长径比试件与其他2种试件之间差异不显著,而1.5与2.0倍试件之间产生显著差异(P=0.012<0.05)。这可能是由于苦竹平均直径仅为12.04 mm,直径太小并且制备设备落后,对试件造成了不同程度的破坏。
表 1 顺纹抗压测试结果
Table 1. Test results of compressive strength
竹种 长径比 顺纹抗压强度 方差齐性
检验P单因素
ANOVA检验平均值/
MPa标准差/
MPa变异系
数/%F P 苦竹 1.0 71.64 ab 3.80 5.30 0.456 3.342 0.040 1.5 70.27 b 3.20 4.55 2.0 72.65 a 3.89 5.35 篌竹 1.0 74.57 a 8.02 10.76 0.989 0.239 0.788 1.5 74.01 a 7.68 10.38 2.0 73.19 a 7.59 10.37 水竹 1.0 67.09 a 5.75 8.57 0.776 0.222 0.801 1.5 67.78 a 6.01 8.87 2.0 67.01 a 5.12 7.64 早园竹 1.0 83.29 a 9.52 11.43 0.528 0.010 0.990 1.5 83.06 a 8.19 9.86 2.0 82.91 a 7.76 9.36 说明:不同小写字母表示同一竹种不同处理之间LSD检验差异显著(P<0.05)。 除去由于试件制备中引起的破坏而产生的测量误差,不同长径比的试样对顺纹抗压强度没有产生显著影响。根据表1结果所示:除苦竹外,其他3种竹材均是2.0倍长径比的试件测试的顺纹抗压强度的变异系数最小。其中,篌竹、早园竹和水竹2.0倍长径比试件的顺纹抗压强度变异系数分别为10.37%、9.36%和7.64%。苦竹的3种试件变异系数均较小,1.5倍长径比试件变异系数为4.55%。变异系数越小,说明数据越稳定。早园竹和篌竹的标准差在一定范围内随着长径比的增加而减少。具体而言,2.0倍长径比试件的标准差分别为7.76和7.59 MPa。水竹的2.0倍长径比试件最小,为5.12 MPa。苦竹标准差是1.5倍长径比试件最小,为3.20 MPa。标准差越小,试验数据的离散程度越小。综上所述,2.0倍长径比的试件测试的数据比较稳定,可靠性较好,变异系数小,意味着需要准备的最小试件数量就最少,整个试验的操作就相对容易,试验数据更加准确。因此,2.0倍长径比的试件用于测试小径级圆竹顺纹抗压强度最好。
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从长径比为2.0的试样的维管束参数 (表2) 可以看出:4种竹材的纤维鞘组织比量的标准偏差均较小,说明同一竹种不同竹株不同部位的纤维鞘组织比量相差不大。其中篌竹纤维鞘组织比量最大,为35.64%,最小的是水竹,为33.05%。苦竹的维管束分布密度最大,达7.94 个·mm−2,早园竹最小,为5.77 个·mm−2。
表 2 4种小径级竹材长径比为2.0试样的维管束参数
Table 2. Vascular bundle parameters of four species of small diameter bamboo at the length-to-diameter ratio of 2.0
竹种 纤维鞘体积分数/% 维管束分布密度/(个·mm−2) 平均值 标准偏差 平均值 标准偏差 苦竹 34.61 2.18 7.94 0.73 篌竹 35.64 1.51 6.32 0.92 水竹 33.05 2.15 7.01 0.31 早园竹 34.65 5.53 5.77 1.07 如图4所示:纤维鞘体积分数 (y) 与顺纹抗压强度 (x) 之间呈线性正相关,即纤维鞘体积分数越大,顺纹抗压强度越大。基于最小二乘法对其进行线性拟合,得到线性方程:y = 260.44x−18.26,R2=0.60,r=0.778,P<0.001。维管束分布密度和顺纹抗压强度之间的相关性并不强,可能原因是不同竹种的维管束面积以及类型不同。换而言之,不同的竹种即使维管束分布密度相同,也有可能单位面积内所含的纤维含量不同。
Compressive strength of small-diameter bamboo and its influencing factors
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摘要:
目的 探究测试小径级圆竹顺纹抗压强度的方法,以及纤维鞘体积分数和维管束分布密度对顺纹抗压强度的影响。 方法 以苦竹Pleioblastus amarus、篌竹Phyllostachys nidularia、水竹Phyllostachys heteroclada和早园竹Phyllostachys propinqua 等4种小径级竹材(胸高直径50 mm以下)为研究对象,采用圆竹形式的试件探究小径级竹材的顺纹抗压性能测试方法,测试不同长径比(试件长度与直径的比值)试件对顺纹抗压强度的影响。同时,利用基于YOLO深度学习算法开发竹材维管束检测模型对维管束数量和纤维鞘面积进行测定,以探索其对顺纹抗压强度的影响。 结果 不同长径比试件测试结果并无显著性差异,以数据稳定性为准,则2.0倍长径比试件测试结果较为合理。采用该长径比试件测试,早园竹顺纹抗压强度最大,为82.91 MPa,水竹顺纹抗压强度最小,为67.01 MPa。篌竹纤维鞘组织比量最大,为35.64%,水竹最小,为33.05%。苦竹的维管束分布密度最大,达7.94 个·mm−2,早园竹最小,为5.77 个·mm−2。将不同种竹材作为整体对象的研究表明:纤维鞘体积分数正向影响顺纹抗压强度,而维管束分布密度对顺纹抗压强度的影响较小。 结论 小径级竹材顺纹抗压强度的测试宜采用2.0倍长径比试件,控制试件在(90±30) s内压溃。该试验选用的小径级竹材的顺纹抗压性能较为优异,纤维鞘体积分数(y)与顺纹抗压强度(x)之间存在$ {y}\text{=260.44}{x}{-}\text{18.26} $线性相关性。图4表2参33 Abstract:Objective This study is aimed to investigate the testing method for determining the compressive strength of small-diameter bamboo culms as well as the impact of fiber sheath volume fraction and distribution density of vascular bundles on it. Method Four species of small-diameter bamboo, namely Pleioblastus amarus, Phyllostachys nidularia, Phyllostachys heteroclada, and Phyllostachys propinqua, with a diameter at breast height of less than 50 mm, were selected as research subjects before bamboo culm samples were utilized to investigate the compressive strength testing method for small-diameter bamboo and to examine the impact of varying length-to-diameter ratios on compressive strength. At the same time, a bamboo vascular bundle detection model based on the YOLO deep learning algorithm was employed to determine the number of vascular bundles and fiber sheath area so as to investigate their influence on compressive strength. Result There were no significant differences in the test results among specimens with different length-to-diameter ratios and the test results for the specimen with a length-to-diameter ratio of 2.0 were more reasonable. Of specimens at the length-to-diameter ratio of 2.0, Phyllostachys propinqua exhibited the highest compressive strength at 82.91 MPa while Phyllostachys heteroclada demonstrated the lowest strength at 67.01 MPa. The volume fraction of fiber sheath was highest in Phyllostachys nidularia at 35.64% and lowest in Phyllostachys heteroclada at 33.05%. The density of vascular bundles in Pleioblastus amarus was highest at 7.94 pieces·mm−2, while that of Phyllostachys propinqua was the lowest at 5.77 pieces·mm−2. Studies that treated various species of bamboo as a unified entity have shown that the positive effect of the volume fraction of fiber sheath on compressive strength was significant while the influence of vascular bundle distribution density on compressive strength was relatively minor. Conclusion A specimen with a length-to-diameter ratio of 2.0 is an ideal choice for testing the compressive strength of small-diameter bamboo and the specimen should be controlled for a period of (90±30) seconds before it collapses. Furthermore, the small-diameter bamboo selected for this experiment exhibited excellent compressive performance while there was a direct correlation between the volume fraction of fiber sheath (y) and compressive strength (x), as shown in the equation y=260.44x−18.26. [Ch, 4 fig. 2 tab. 33 ref.] -
表 1 顺纹抗压测试结果
Table 1. Test results of compressive strength
竹种 长径比 顺纹抗压强度 方差齐性
检验P单因素
ANOVA检验平均值/
MPa标准差/
MPa变异系
数/%F P 苦竹 1.0 71.64 ab 3.80 5.30 0.456 3.342 0.040 1.5 70.27 b 3.20 4.55 2.0 72.65 a 3.89 5.35 篌竹 1.0 74.57 a 8.02 10.76 0.989 0.239 0.788 1.5 74.01 a 7.68 10.38 2.0 73.19 a 7.59 10.37 水竹 1.0 67.09 a 5.75 8.57 0.776 0.222 0.801 1.5 67.78 a 6.01 8.87 2.0 67.01 a 5.12 7.64 早园竹 1.0 83.29 a 9.52 11.43 0.528 0.010 0.990 1.5 83.06 a 8.19 9.86 2.0 82.91 a 7.76 9.36 说明:不同小写字母表示同一竹种不同处理之间LSD检验差异显著(P<0.05)。 表 2 4种小径级竹材长径比为2.0试样的维管束参数
Table 2. Vascular bundle parameters of four species of small diameter bamboo at the length-to-diameter ratio of 2.0
竹种 纤维鞘体积分数/% 维管束分布密度/(个·mm−2) 平均值 标准偏差 平均值 标准偏差 苦竹 34.61 2.18 7.94 0.73 篌竹 35.64 1.51 6.32 0.92 水竹 33.05 2.15 7.01 0.31 早园竹 34.65 5.53 5.77 1.07 -
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