木材静曲蠕变成套测试系统的研发

为解决木材变湿蠕变测试普遍存在的环境参数(温湿度和风速)控制方式和控制精度不足、蠕变变形量测试灵敏度和精度不足、数据智能采集和处理以及系统安全防护不足等问题,在集成现有技术的基础上研发出一套木材蠕变测试系统。该系统包括环境气候箱、蠕变测试机架、木材蠕变变形量及干缩湿胀测试单元、荷载同时装卸单元、数据采集和处理单元、系统保障和防护单元6个部分。该系统可在温度为0~70℃、相对湿度为10%~98%、风速为0.3~5.6 m/s时对木材或木基材料的普通蠕变和机械吸附蠕变进行长时间稳定的测试、记录,蠕变变形量及木材厚度方向的干缩湿胀检测精度为±0.01 mm,数据采样间隔在1 s~24 h范围内可调。与现有木材蠕变测试系统相比,本系统在多参数自动同步获取、数据采样精度、运行稳定性、安全性和环境风速可调性等方面有独特优势。通过前期对木材112 d循环变湿蠕变量、干缩湿胀量及环境温湿度参数的观测和测试结果分析,证实该套检测系统可对木材的蠕变挠度、干缩湿胀量、环境温湿度等参数进行长期连续稳定的检测、记录和显示,所有测试指标均能达到设计预期。同时,由于该系统具有多参数协同检测和精度较高等特性,使得实测木材变湿蠕变比采用喷蒸变湿处理试件所获蠕变测试结果在局部地方存在明显差异,如吸湿过程中的木材静曲挠度并不都是反向减小,而是取决于吸湿速率和外荷载的竞争关系,从而为木材静曲变湿蠕变机理的揭示提供了有力证据。该系统的研制为精确可控变温变湿环境下木材蠕变机理的研究提供了新的平台。

Research and development of testing system for static bending creep of wood

Under a continuous load,the deformation of wood increases with time,which is known as creep. Creep causes the wood to reduce its strength and overstretch its deformation,which would threaten structural safety during the service of wood components. Creep test equipment is a key assessment device for examining the creep properties of wood and investigating its creep mechanism. In order to overcome the drawbacks of the control mode and accuracy for testing environment parameters( temperature,relative humidity and airflow velocity),testing sensitivity and accuracy of creep deformation,insufficient of data intelligent collection and processing,lack of security protection measures for whole equipment,an auto monitoring and testing equipment for creep of clear wood was investigated and developed.This equipment included six components,which were climate chamber of Binder MKF720,creep test rack,wood creep deformation and shrink-swell recording unit,simultaneous load/unload unit,data collection and processing unit,and guarantee and protection unit. This equipment could be used for real-time testing and recording of the normal creep,mechanical-sorptive creep( MSC) and shrinkage and swelling of wood or wood-based materials in different test environments and requirements. The main technical indexes of this equipment were the test environment temperature ranging from 0 to 70 ℃( accuracy ±0.1 ℃),the relative humidity ranging from 10% to 98%( accuracy ±2%),ambient wind speed ranging from 0.3 to 5.6 m/s( accuracy ±0.1 m/s),the creep deformation and shrinkage-swelling of the specimen was 0-50 mm( precision ±0.01 mm),the recording interval was adjustable from 1 s to 24 h with automatic data logging and real-time graph line display. Compared with the existing wood creep test systems,this equipment had unique advantages in terms of multi-parameter automatic synchronous acquisition,data sampling accuracy,operational stability,safety and ambient wind speed adjustability. The previous observations and analysis of the results of the 112-day cyclic absorb-desorptive creep,shrinking and swelling of wood,and environmental temperature and humidity parameters proved that this creep testing system could continuously and reliably detect creep deflection,shrinkage and swelling,ambient temperature and humidity parameters. This system was capable of measuring,recording and displaying all test parameters to meet design expectations. At the same time,due to the system’s multi-parameter synergistic detection and high accuracy,the measured data was more accurate than the former spray steam moisture change system. The creep test results were differed significantly to that of the predecessor tests,especially during the initial moisture absorptive stage. It was found that the static deflection of wood during the moisture absorption was not always reversed but depended on the competitive relationship between the moisture absorption rate and the external load,thus providing a powerful evidence for the creep mechanism of wood. This system provided a new experimental platform for exploring fundamental principles of normal creep and MSC in wood under variable temperature-relative humidity environment.