被动式红外热像法研究古代建筑壁画裂隙病害动态成因

Study on Dynamic Causes of Cracks in Ancient Architectural Murals by Passive Infrared Thermography

红外热像法是文物保护领域广泛应用的无损检测方法之一。在目前的文物保护工作中,常利用主动式红外热像法对文物本体的病害状况和结构缺陷进行评估,而对被动式红外热像法的重视程度不足,且极少直接考察文物与赋存环境之间的动态作用关系。以辽宁义县奉国寺元代建筑壁画裂隙病害为例,探索被动式红外热像法研究“文物本体-赋存环境”交联耦合作用的可行性。在拍摄壁画表面红外热像照片从而获得更全面檐墙结构信息的基础上,分别对东、西墙中部纵向长裂隙以及北墙N1壁画整体开展连续红外热成像监测,并对裂隙、普通墙体表面和木骨处墙体表面进行温度测量;采用累积距平法和小波分析法对这些特征位置温度的变化趋势和周期波动性进行分析,从传热学角度对监测和分析结果进行定性讨论,区分壁画不同位置的传热影响模式,并结合裂隙产状特征对其形成过程进行初步判断。结果表明,壁画属多层不均匀结构,存在传热通路和结构强度上的差异;东、西墙的温度变化趋势具有相似性,而在波动信号及波动能量方面存在明显不同;裂隙的总体波动能量大于墙体,并与发育程度呈正相关;第一主周期的波动能量来源于气温的日变化,而次级主周期的波动能量可能与传热过程中的随机扰动有关,白天及日落后一段时间内的能量波动在传热影响中起到了关键作用;木骨与裂隙处的传热过程更快,随时间推移形成高温中心与横向传热趋势。裂隙病害的产生是材料对温度机械应变差异的结果,而传热影响途径有二:一是以檐墙为媒介的导热,二是通过檐柱处空气通道的对流换热;前者主要诱导生成间口内上下平行若干层的横向短裂隙组,后者则促进檐柱处纵向长裂隙的产生。该研究可为评估其他类型文物在复杂环境下的传热影响过程提供参考。

Infrared thermography is a widely used nondestructive testing method in cultural relic protection.In current works,active infrared thermography is often used to evaluate cultural relics'disease conditions and structural defects.In contrast,passive infrared thermography is not paid enough attention,and the dynamic relationship between cultural relics and their environment is rarely directly investigated.Taking the crack disease of Yuan Dynasty architectural murals in Fengguo Temple,Yixian County,Liaoning Province,as an example,this paper explored the feasibility of passive infrared thermography to study the cross-coupling effect between cultural relics and their existing environment.Based on obtaining more comprehensive information on the eave wall structure by taking thermal infrared images,continuous infrared thermography monitoring was carried out on the longitudinal long cracks in the middle of the east and west walls as well as the whole N1 mural of the north wall,and temperature measurement was carried out on the cracks,the ordinary wall surface and the wall surface where the wood keels located.Then,the cumulative anomaly and wavelet analysis methods were used to analyze the changing trend and periodic temperature fluctuation at these characteristic positions.The monitoring and analysis results were qualitatively discussed from the perspective of heat transfer,and the influence modes at different positions of the mural were distinguished.Combined with the characteristics of the cracks,the formation process was preliminarily judged.The results show that the mural has an uneven multi-layer structure,and there are differences in the heat transfer pathway and structural strength.The temperature trends of the east and west walls are similar,but there are obvious differences in fluctuation signal and energy.The overall fluctuation energy of cracks is greater than that of walls and is positively correlated with the degree of development of the cracks.The fluctuation energy of the first main cycle comes from the diurnal variation of temperature.In contrast,the secondary main cycles may be related to random disturbances in the heat transfer process.The energy fluctuation during the daytime and after sunset plays a key role in the heat transfer effect.The heat transfer process at the wood keels and the cracks is faster,so the high-temperature centre and the transverse heat transfer trend are formed over time.The occurrence of crack disease results from the mechanical strain differences of material under the influence of temperature change.There are two heat transfer pathways:one is the heat conduction with the eave wall as the medium,and the other is the convective heat transfer through the air channel at the eave column;the former mainly induces the formation of several parallel transverse short crack groups in the units,while the latter promotes the formation of longitudinal long cracks at the eave columns.This paper can provide a reference for evaluating the heat transfer process of other cultural relics in complex environments.