European Journal of Wood and Wood Products ( IF 2.6 ) Pub Date : 2021-03-06 , DOI: 10.1007/s00107-021-01674-6 Choosak Rittiphet , Kamolwun Dumyang , Nirundorn Matan
To prevent defects caused by drying collapse, transverse pre-mechanical compression with lateral restraint of green oil palm wood (OPW) before kiln-drying was performed. OPW specimens were divided according to specific gravity into low density (LD 0.22 ± 0.05) and medium-density (MD 0.33 ± 0.02) groups. Effects of compressive strain (0–0.65) and strain rate (0.01–1.0/min) on moisture removal and wood recovery after the compression and on collapse and internal void formations after kiln-drying were investigated. Results indicate that pre-mechanical compression linearly removes moisture out of wood and linearly generates permanent thickness strain that is independent of the strain rate used. The pre-compression reduces the true cross-sectional strain in the kiln-dried LD OPW and MD OPW and the internal voids caused by drying collapse in the LD OPW. The collapse-induced voids, not observed in all kiln-dried MD OPW, disappear when compressive strain > 0.45 is applied to the LD OPW. Microstructural examination reveals that directional folding of parenchyma cells caused by the compression reduces intercellular micro-voids randomly generated during drying by the non-directional collapse of parenchyma cells caused by capillary force. However, above the compressive strains of 0.35 and 0.25 in the LD OPW and MD OPW, interfacial micro-cracks, generated at the boundaries between the parenchyma cells and the fiber bundles, link to form shear bands at the angle of approximately 30–40° to the compression axis. After kiln-drying, these shear bands expand into shear-induced voids. Finally, mechanical properties of the kiln-dried LD OPW pre-compressed at a 0.45 strain without collapse-induced voids are comparable to those that have been air-dried.
中文翻译:
机械前压缩对油棕自由脱水,干燥塌陷及相关内部空隙的影响
为了防止干燥塌陷造成的缺陷,在窑干之前进行横向预机械压缩,并在侧面施加绿色油棕木(OPW)的约束。根据比重将OPW样品分为低密度(LD 0.22±0.05)和中等密度(MD 0.33±0.02)组。研究了压缩应变(0–0.65)和应变速率(0.01–1.0 / min)对压缩后水分去除和木材恢复以及窑干后塌陷和内部空隙形成的影响。结果表明,机械前压缩可线性去除木材中的水分,并线性生成永久厚度应变,而与所使用的应变率无关。预压缩减少了窑干燥的LD OPW和MD OPW中的真实横截面应变,以及由于LD OPW中的干燥塌陷而导致的内部空隙。当对LD OPW施加大于0.45的压缩应变时,在所有窑干MD OPW中都未观察到的塌陷引起的空隙消失。显微组织检查表明,由压缩引起的薄壁细胞的定向折叠减少了干燥过程中由于毛细作用力引起的薄壁细胞的非定向塌陷而随机产生的细胞间微孔。但是,在LD OPW和MD OPW中,在0.35和0.25的压缩应变之上,在实质细胞和纤维束之间的边界处产生的界面微裂纹以约30–40°的角度链接形成剪切带。压缩轴。窑干后,这些剪切带扩展成剪切引起的空隙。最后,窑干燥的LD OPW的机械性能为0。