水热环境中马尾松早材和晚材的轴向拉伸力学行为

Longitudinal Tensile Mechanical Behavior of Earlywood and Latewood of Pinus massoniana in the Hydrothermal Environment

【目的】探究水热环境中马尾松早材(EW)和晚材(LW)的轴向拉伸力学行为,在生长轮尺度上揭示木材软化行为发生规律,为木材弯曲、塑化、大变形加工等实际应用中工艺参数的选择和优化提供参考。【方法】以马尾松心材区域第17生长轮的早材和晚材为研究对象,采用X射线剖面密度仪测定早材和晚材的绝干密度;利用轮转式切片机在早材和晚材区域的径切面(LR)制取LR-EW和LR-LW组织切片,在早材与晚材区域的弦切面(LT)制取LT-EW和LT-LW组织切片;应用X射线衍射仪和动态力学分析仪在30、40、50、60、70和80℃条件下测定LREW、LR-LW、LT-EW和LT-LW组织切片的微纤丝角、拉伸弹性模量、拉伸强度和拉伸应变速率。【结果】1)晚材的绝干密度(0.836 g·cm^(3))是早材(0.388 g·cm^(3))的2.2倍。任一温度时,早材的微纤丝角大于晚材,弦切面的微纤丝角大于径切面;饱水早材和晚材在径切面或弦切面的微纤丝角均随温度升高而减小。2)30℃时,晚材径切面的拉伸弹性模量是早材的2.1倍,晚材弦切面的拉伸弹性模量是早材的3.3倍。在4种组织切片中,LT-LW的拉伸强度最大,LR-LW的拉伸强度是LR-EW的2.0倍。绝干密度和微纤丝角是影响早材和晚材径切面、弦切面拉伸力学行为的2个关键因子。3)当温度由30℃升至80℃时,早材和晚材在径切面、弦切面的拉伸弹性模量和拉伸强度呈降低趋势,其降幅均为早材大于晚材、弦切面大于径切面。早材和晚材在径切面或弦切面拉伸弹性模量和拉伸强度降幅的拐点温度均为60℃,与木材细胞壁中木质素发生软化有关。4)任一加载速率(0.5、1.0和2.0 N·min^(−1))时,饱水早材和晚材在径切面、弦切面的拉伸应变速率均随温度升高而增大;当温度和加载速率一定时,早材径切面的拉伸应变速率是晚材的2.0倍,早材弦切面的拉伸应变速率是晚材的3.3倍。当加载速率倍增时,早材和晚材径切面、弦切面的拉伸应变速率均呈倍增变化趋势。【结论】饱水早材和晚材在径切面或弦切面的微纤丝角、拉伸弹性模量和拉伸强度均随温度升高而减小(或降低)。与晚材相比,温度变化对早材拉伸弹性模量和拉伸强度的影响更大。温度升高引起微纤丝角变小,抑制拉伸弹性模量和拉伸强度降低,但在60℃及以上水热环境中,木质素软化是引起拉伸弹性模量和拉伸强度降低的主要因素。

【Objective】This paper is proposed to investigate the longitudinal tensile mechanical behavior of earlywood(EW)and latewood(LW)of Pinus massoniana in the hydrothermal environment,with an aim to reveal the mechanism about the softening behavior of wood at the scale of growth ring.【Method】The absolute dry density of EW and LW in the 17th growth ring of the heartwood of Pinus massoniana was measured by X-ray profile densimeter.LR-EW and LR-LW tissue sections were obtained from radial(LR)section of EW and LW,and LT-EW and LT-LW tissue sections were obtained from tangential(LT)section of EW and LW by a slide slicing machine.The microfiber angle(MFA),tensile elastic modulus,tensile strength and tensile strain rate of LR-EW,LR-LW,LT-EW and LT-LW specimens were measured by X-ray diffractometer and dynamic mechanical analysis at 30,40,50,60,70 and 80℃,respectively.【Result】1)The absolute dry density of LW(0.836 g·cm^(3))was 2.2 times higher than EW(0.388 g·cm^(3)).At each temperature,the MFA of EW was larger than that of LW,and the MFA of specimen in the LR section was larger than that in the LT section.The MFA of water-saturated EW and LW in the LR and LT sections decreased with the increase of temperature.2)At temperature level of 30℃,the tensile elastic modulus of LW in the LR section was manifested that LW was 2.1 times higher than EW,the tensile elastic modulus of LW in the LT section was manifested that LW was 3.3 times higher than EW,respectively.Among the four tissue sections,the tensile strength of LT-LW was the highest;the tensile strength of LR-LW was 2.0 times higher than LR-EW.Absolute dry density and MFA are two key factors affecting the tensile mechanical behavior of EW and LW in the LR and LT sections.3)When the temperature rose from 30℃to 80℃,the tensile elastic modulus and tensile strength of LR-EW,LR-LW,LT-EW and LT-LW were decreased,the decline of tensile elastic modulus and tensile strength of EW was greater than that of LW,and the decline of tensile elastic modulus and tensile strength of specimen in the LT section was greater than that of in the LR section.The tensile elastic modulus and tensile strength of LR-EW,LR-LW,LT-EW and LT-LW of the temperature of turning point was 60℃,which was related to the softening of lignin in the wood cell wall.4)At each loading rate(0.5,1.0,2.0 N·min^(−1)),the tensile strain rate of LR-EW,LR-LW,LT-EW and LT-LW were decreased with the increase of temperature.At the same temperature and loading rate,the tensile strain rate of EW in the LR section was about 2.0 times higher than that of LW,the tensile strain rate of EW in the LT section was about 3.3 times higher than that of LW,respectively.When the loading rate was doubled,the tensile strain rate of specimens showed a doubling trend.【Conclusion】The MFA,tensile elastic modulus and tensile strength of LR-EW,LR-LW,LT-EW and LT-LW decreased with the increase of temperature.Compared with LW,the influence of temperature on the tensile modulus and tensile strength of EW was greater.The decline of tensile modulus was inhibited by the decrease of MFA with the increase of temperature,and while the lignin softening was the main factor affecting the decrease of the tensile modulus and tensile strength in the hydrothermal environment above 60℃.