单向及双轴向压缩载荷下的圆竹径向力学性能

摘要

研究圆竹在单向(x)和双轴向(x,y)压缩载荷下的径向力学行为.利用斜率法,估算双轴向载荷下竹间的长度尺寸效应；采用数字散斑方法,对应变场信息及长径比对环向应变的影响进行表征.结果表明:双轴向压缩强度是单向的2.4 ～ 2.5倍,压缩时间比单向减少30％ ～ 45％；双轴向载荷下竹节处强度是竹间的2.38倍,从结构方面看,竹节对竹筒是增强体而非缺陷.利用二参数Weibull分布函数,估算出双轴向压缩下的长度尺寸效应系数为0.056(x方向)和0.069(y方向),双轴向载荷下破坏首先发生在与x/y轴大约成45°位置处.应变场分析表明:x方向应变分布较均匀,应变值变化区间为-0.138 ～ 0.096,均值为-0.006;y方向应变呈现较大波动,应变值变化区间为-0.372 ～0.437,均值为0.002；在45°位置处,竹环内侧靠近竹黄部位受压应力,竹环外侧靠近竹青部位受拉伸作用.在双轴向载荷0 ～ 300 N下,随着长细比增加,应变的25％ ～ 75％分位值和均值的绝对值总体上呈减小趋势.%Mechanical behaviours of bamboo at uniaxial (χ) and biaxial (χ, y) directions were investigated by diametric compression tests, and the long-size effects of bamboo under biaxial compression loadings were estimated by using the slope method. Meanwhile, the information of strain-filed and the effects of ration length/radius on strain were characterized by the digital speckle correlation method. The strength with biaxial texts was 2. 4 ~ 2. 5 times as hard as that of the uniaxial while compression time was 30% ~45% less than that of the uniaxial. The strength of bamboo joint under biaxial compression loadings was 2. 38 times as hard as its near parts at inter-modal region, so, it acts as reinforcement for the bamboo and not a defect in its structure. The size effects of length under biaxial compression loading were 0. 056 (χ-direction ) and 0. 069 (y-direction ) with the help of 2P-Weibull distribution functions. Failure occurs at an about 45° position with x/y axial firstly when conducting biaxial compression. The χ-strain distribution was uniform. The range of variation was -0.138 ~0. 096, and the mean value was -0. 006. However, variation of y-strain field presented a more fluctuation compared with χ-strain filed, as the range of variation was —0. 138 ~0.096, the mean was -0.006. The location at 45° near the inner circle of bamboo ring suffered compression stress, conversely, the position close to the outside of the ring was given stretched effect. As the increase of length-diameter ratio, 25th ~ 75th percentiles of strain value and absolute value of mean, as a whole, presented a decreasing trend under biaxial loads 0 -300 N.