RESEARCH ON NON-DESTRUCTIVE TESTING TECHNOLOGY IN CONSERVATION REPAIR PROJECT OF ANCESTRAL TEMPLE IN MUKDEN PALACE

Due to the use of wood and other non-permanent materials, traditional Chinese architecture is one of the most fragile constructions in various heritage objects today. With the increasing emphasis on the protection of cultural relics, the repair project of wooden structure has become more and more important. There are various kinds of destructions, which pose a hidden danger to the overall safety of the ancient buildings, caused not only by time and nature, but also by improper repairs in history or nowadays. Today, the use of digital technology is a basic requirement in the conservation of cultural heritage. Detection technology, especially non-destructive testing technology, could provide more accurate records in capturing detailed physical characteristics of structures such as geometric deformation and invisible damage, as well as prevent a man-made destruction in the process of repair project. This paper aims to interpret with a typical example, Ancestral Temple in Mukden Palace, along with a discussion of how to use the non-destructive testing technology with ground penetrating radar, stress wave, resistograph and so on, in addition to find an appropriate protection method in repair project of traditional Chinese wooden architecture.


Introduction
With the increasing emphasis on the conservation of cultural relics, the restoration work of historic buildings has become more and more important.Basically, most of the ancient buildings in China are wooden structure.Wood is a flexible material which could resist deformation in the earthquake.The drawback, however, is that they are susceptible to temperature, humidity and other external influences (which will lead to rot, cracks, tilt, foundation settlement, etc.), and poses a hidden danger to the entire architecture.Besides, after years of use, the decline in mechanical properties could affects their bearing capacity.
Moreover, traditional testing technology and restoration methods, determining restoration and demolition in the absence of accurate and effective detection and evaluation methods, like samping, eyeballing, knocking, which are simple and depend largely on 1 Supported by the National Natural Science Foundation of China, No. 51508376.manual experience and lack accuracy, will cause the loss of historical information of ancient construction, and even cause a certain degree of damage to wooden structure.Today, digital technology, especially non-destructive testing technology, could provide accurate records in capturing detailed physical characteristics of structures, such as geometric deformation and invisible damage, will prevent secondary manmade destruction in the repair process.This paper, in the form of case study of Ancestral Temple in Mukden Palace, aims to introduce non-destructive testing technology (ground penetrating radar, stress wave, resistograph and so on) and explore an appropriate protection method in the repair project of traditional Chinese wooden architecture.

Mukden Palace
Early construction of Mukden Palace began in 1625 by Nurhaci, the founder of the Qing dynasty.In 1631, additional structures were added during the reign of Huangtaiji, Nurhaci's successor.
The Mukden Palace was built to imitate Beijing's Forbidden City.
However, the palace also shows the cultural characteristics of Manchu and Tibetan (Figure 1) [6] .
In 1780, the Qianlong Emperor further expanded the palace.
Every year since then, successive Qing emperors usually stayed at Mukden Palace for some time.

Ancestral Temple in Mukden Palace
Ancestral Temple, located on the east side of the Great Qing Gate of Mukden Palace, is an antique-courtyard-style building (Figure 1).The temple is off the beaten track as it doesn't belong to the main part of the palace.
Ancestral Temple was built on an independent two-meters high platform, and formed a typical enclosed courtyard (四合院).This building complex consists of six buildings: main hall (Figure 2), temple gate (Figure 3), two side halls with one penthouse each (Figure 4).The main hall is an extends five(五间) building with front porch and Xieshan(歇山) roof covered with yellow glazed tiles.To the southwest of the main hall, located the Fen Jin pavilion (焚帛楼), which was used for sacrifice.To the east and west of the main hall, there are two side hall, which are extends three(三间), Yingshan(硬山) roof with yellow glazed tile.There is a penthouse in the south of each side hall.On the south of the

Non-destructive testing technology
As a new digital technology, non-destructive testing technology plays an important role for the ancient building conservation.
Non-destructive testing is the adoption of physical mechanics or chemical properties to effectively test and inspect the relevant characteristics of the target object (such as shape, displacement, stress, optical properties, fluid properties, mechanics, etc.) without destroying the structure and operational performance of the detection target [3] .
With the research and development of new digital technology, non-destructive testing technology has integrated GPR, stress wave and resistograph to overcome the shortcomings of single testing technique and increase the accuracy of the test results.
Comprehensive comparison and data analysis would improve the accuracy of the location and condition of the damaged parts, and make it possible to explore a new way to repair damaged wooden structure.
Non-destructive testing technologies have overcome the shortcomings of the traditional detection method, and can be used to visually assess the condition, even internal, of timber structure.
Several common non-destructive testing technologies are applied in the research of Ancestral Temple in Mukden Palace.

Ground penetrating radar
GPR is a fast and efficient, high-precision non-destructive testing technology.In the beginning, radar detection technology was mainly used for underground exploration, while later, with the development of high-frequency electronic technology and computer data processing technology, GPR was also used in expanding fields.
GPR is a method of using radio waves at the frequency of 106-109Hz to determine underground media distribution.It is mainly composed of integrated frames, antenna and related accessories (Figure 5).GPR detects target by emitting high-frequency broadband short-pulse electromagnetic waves to the ground or target body through the transmitting antenna.The waves are then reflected by subsurface stratigraphy or target body, and received by the receiving antenna.The path, field strength and waveform of electromagnetic wave, while propagating in the media, will change with the changes of the dielectric and geometric properties of the media (Figure 6) [4] .As the difference in the dielectric constant of air and soil is significant, GPR will get intense reflected signals when the target body has obvious gaps or holes.As shown in the figure 7、8, from the south side of the main hall of the east gable, there is a void phenomenon below 200mm-650mm of the foundation at 10m of the wall.
The test result of GPR in ancient architecture is more accurate and feasible, providing a strong support for the protection and reinforcement of ancient architectures.However, it also has some

Stress wave
Stress wave technology, detecting wave velocity and vibrational spectrum through sensors, is one of the most common methods of non-destructive testing of wood and wood-based composites [7] .
Depending on the actual diameter of the detected object, we can arrange 6, 8, 10 or even more sensors around the detection section (Figure 9).The shorter the diameter is, the fewer sensors are process pseudo color image (Figure 11).As the image shows, the red part is in damage, in which the average value of the velocity of propagation is 709m / s and 818m / s, which is lower than the lowest average of health.The cracked area accounts for 14% of the total.

Resistograph
The wood resistograph mainly composed of probe (and its protection device), the microcomputer system and the storage battery (Figure 12).To detect damages, the micro-probe, 1.5mm in diameter, driven by motor, was stabbed into the interior of wood at a uniform rate.The computer records what the resistance pressure is for this process into the memory card, and print the output test photos, which were processed into two-dimensional graphics to indicate internal defects in the pillars section [2] (Figure 13).The resistograph maps decayed degrees and wood age through different resistance values, and form curve image.
Moreover, with the help of stress wave detection, which could find out the most damaged area with the probes radially stabbed in, the damaged condition of wooden structure would be determined more accurately through the data comparison between the resistograph and stress wave.

Testing results of Imperial Palace in Shenyang
After external observation, the use of ground penetrating radar,

Figure 4 Figure 1 .Figure 2 Figure 3
Figure 4 The general layout of Ancestral Temple in Mukden Palace

Figure 6 Figure 5
Figure 6 Working principle and recording diagram of GPR

Figure 7
Figure 8 GPR analysis of the main hall of the east gable

4. 1 Cracks4. 2 Decayed
Figure 11 Stress wave pseudo image of D5 pillar of the main hall

Figure 14
Figure 14 Wooden structure cracks of the temple gate

Figure 18 3
Figure 18 Resistograph two-dimensional plane detection chart of D5 pillar of the main hall