The relationship between the mechanical properties of
In China, wood has been utilized since antiquity as building material for civil and cultural heritage architecture. Especially in the South of the country, Chinese fir (
At present, in the country the performance of the materials used for historical building construction and the structural and mechanical behaviour of the wooden buildings are determined principally during emergency reparations and strengthening research projects. Instead, it would be highly desirable to assess the performance of wood based on the knowledge of both the inherent durability and the climatic conditions (climate and microclimate) where the building is placed. This can also help planning the proper interventions of maintenance during the service life (
The aim of this study is to find the relationships between the mechanical properties of Chinese fir wood and the development of fungal decay, aimed at the implementation of a statistical model useful as a non-destructive and a fast method for determining the state of conservation of in-service timber structures.
In the past,
In the same context,
Little research has been done on the development of a constitutive model for decayed wood.
In this paper, the relationship between different degrees of wood decay and the corresponding mechanical properties has been explored and a model for fungal decayed Chinese fir was proposed. At the same time, the deterioration mechanism of wood decay was investigated by combining the FT-IR, electron-scanning microscope, and XRD analysis.
The wood specimens were then prepared from the logs according to the pattern provided in the Chinese standard GB/T1929-2009 (
The fungus
At first, the culture dishes inoculated with the fungus were put into electro-heating standing-temperature cultivator for one week; the related wood specimens were sterilized in a high-pressure steam sterilization pot in which the normal temperature is 138 °C. The sterilized specimens were then put in contact with the fungus hyphae, maintained in a climatic chamber (T = 28 ± 2 °C; RH = 75-85%) and checked periodically.
A pre-test was performed to assess the time needed for the mycelium to degrade Chinese fir wooden specimens.
The measurements and the tests performed are summarized in
Before the tests, all the specimens were kept in the constant temperature-humidity chamber (T = 20 ± 2°C; RH = 60-70%); all specimens were marked and weighted. Only the specimens aimed to the calculation of the mass loss were formerly dried in the oven (T = 100 ± 5 °C) to determine the anhydrous weight and then put in the constant temperature-humidity chamber again to reach the equilibrium moisture content. Afterwards, specimens were inoculated with the fungus and put at constant conditions (T = 28 ± 2 °C; RH = 75-85%) for the scheduled period according to the test scheme.
At the end of each planned decay period, 3 specimens “compressive type” were took out from the chamber, cleaned from superficial mycelium, dried in the oven (T = 100 ± 5 °C), and weighted. The mass loss was calculated as (
where
At the same time, 6 specimens for each decay period from group 2 and 3 were at first conditioned at T = 20 ± 2 °C and RH = 60-70% until constant mass, then underwent mechanical and chemical analysis.
The specimens “compression type” underwent the compression test and finally the determination of chemical composition and cellulose crystallinity.
The mechanical tests were performed using a universal testing machine (WDW-20KN®, HST, Jinan Hensgrand Instrument Co., Jinan, China) with an accuracy of 10 N. The compressive load was applied slowly with a loading rate between 200 and 300 kPa per second, in order to reach the failure within 1.5-2.0 minutes. Compression strength parallel to grain was calculated as the load at failure divided by the cross specimen area measured at the testing time. Compression modulus of elasticity (MOE) was determined in the straight line portion of the load-deformation curve.
The modifications of the chemical composition after the compression test were analysed by FT-IR spectrograms (infrared spectroscopic analysis), using the infrared spectrometer Varian® 670-IR+610-IR (Agilent, Santa Clara, CA, USA). For each decay period, 3 specimens “compression type” were chosen randomly and put into the oven up to their dry condition, then ground to powder and briquetted to the transparent ingot by potassium bromide pellet technique, so that the FT-IR spectrograms could be obtained.
The cellulose crystallinity was measured by D8 Advance X-ray Powder Diffractometer (Bruker, Billerica, MS, USA). The powder was scanned by the XRD in the scan range 5°-45° and at the 0.1° sec-1 scan speed in triplicates.
As for the specimens of group 3, six specimens per decay period were tested in tension parallel to the grain using the same machine described above for the compression tests. The two ends of the specimens were clamped in the grips of the testing machine so that the wider side was in contact with the grips and at a distance of at least 20-25 mm from the gauge portion. The specimens were mounted vertically. The load was applied slowly with a loading rate between 200 and 300 kPa per second. Tensile strength parallel to grain was calculated as the load at failure divided by the cross area of the gauge portion measured at the testing time; tensile modulus of elasticity was determined in the straight-line portion of the load-deformation curve.
Finally, one specimen “compression type” per each decay period was kept for the anatomical analysis, performed by an environmental scanning electron microscope (XL-30 ESEM, FEI/Philips, Hillsboro, OR, USA). A transverse section (1 mm thick) was cut from the decayed surface of the specimens, polished and observed by the ESEM.
The results of mechanical tests were used to calculate the loss of mechanical performance as the ratio between the mechanical property of sound specimens (before the inoculation with fungi) and the same property measured at several times along the decay period. The loss of performance was related to the mass loss, hereafter indicated as “decay ratio”. The values were fitted with linear and polynomial regression.
The stress-strain curves of each specimen were obtained directly during the mechanical test. Thus, the average stress-strain curve of specimens of each decayed condition for compression and tensile tests were calculated. The DCM consisted of the average stress-strain curves of 8 different decayed condition for “compressive type” and “tensile type”.
After decay induction, the mass loss rate at each decay period was calculated and the results are shown in
The percentage of mass loss is hereafter indicated as decay ratio (
The anatomical analysis showed the evolution of microstructure pattern of Chinese fir wood. Some longitudinal anatomical pictures of samples at 0, 4, 8 and 14 week of fungal exposure are shown in
There is a wide scientific literature about the use of infrared spectroscopic analysis as a tool for deep investigations on the decay of wood constitutive molecules by fungal rot (
The XRD diffraction analysis of the specimens after 0, 6, 10, 14 weeks of exposure to fungi degradation revealed that the cellulose crystallinity decreased rapidly during the test. The peak dropped more rapidly in later periods, because at that time the fungi all survived depolymerizing cellulose and hemicellulose, as already detected by FTIR analysis.
The relationship between mechanical performances (strength and modulus of elasticity) and relative decay ratio was fit based on the test data (
After 14 weeks there was 7.21% of mass loss. Correspondingly, the compression strength decreased by 19.1% and the tensile strength by 21%. The results are consistent with previous similar analysis on Chinese fir wood (
A constitutive model describes the stress-strain (
The constitutive model followed the basic relationships of wood material. In order to reflect the influence of the different decay grade on the stress-strain relationship, the decay ratio of the specimens along the entire experiment was included in the model. The decay ratios and the stress-strain curves observed in the mechanical tests were used to establish the mechanical degenerated rule in the so called Decay Constitutive Model. The model obtained is the following (
Here,
The model can be divided into four phases: when the values of strain and stress are positive, the wood material is in the stage of tension: (1) the relationship of
The model was based on the test data with a decay ratio between 0 and 7.21%, thus it should be applied for decay levels lower than 7.21%.
From DCM, the specimen for compressive strength parallel to grain were in the elastic stage under invariable original loads. At this time, the loads and the deformation were kept in a linear stage. After it reached the compressive yield stress, the specimen can turn into the elastic-plastic stage. Lastly, it was the failure stage when it reached the ultimate compressive strength with the obvious residual strength. Comparing the stress-strain curves of different decay situation, the ultimate compressive strength decreases during the test, so did the elastic modulus, and the decrease became faster after 6 weeks of exposure to brown rot fungus. However, the specimen for tensile strength parallel to grain were in the elastic stage under invariable original loads, and then turn into the failure stage when it reached the ultimate tensile strength with no residual strength. Comparing the stress-strain curves of different decay situation, the ultimate tensile strength and elastic modulus both decrease during the test, and the decrease became faster after 4 weeks of exposure to the fungus.
The decay test revealed that the mean mass loss percentage after 14 weeks was 7.21%, indicating that Chinese fir wood has a good intrinsic resistance against brown rot
The cell wall is seriously damaged by fungal rot, which drastically decreases hemicellulose and crystalline cellulose content, thus reducing the mechanical properties of wood.
The constitutive model of the decay specimen could be divided into four stages: compressive elastic stage, compressive elastic-plastic stage, compressive plastic stage, tensile elastic stage.
This paper is supported by the National Natural Science Foundation of China (Grant No. 51478409, 51338001), the State Administration of Foreign Experts Affair (Grant No. GDT20163200026), Program of 100 Foreign Experts in Jiangsu Province (Grant No. JSB2017029)
Changes in anatomical characteristics of a Chinese fir wood specimens inoculated with the fungus
FTIR spectrogram of the specimens for o (A), 6 (B), 14 (C) weeks of contact with brown rot fungi.
Relationship between decay and compressive or tensile strength parallel to the grain.
Relationship between decay and compressive or tensile modulus of elasticity.
Decay constitutive model of wood with incipient brown rot.
Number and type of specimens used for different tests. (N): Replicates per decayed period; (1): “compression type” specimen; (2): “tensile type” specimen.
Group | Test | Specimen type | N |
---|---|---|---|
1 | Mass loss | 1 | 3 × 8 |
2 | Compression parallel to the grain, Chemical components and cellulose crystallinity | 1 | 6 × 8 |
3 | Tensile parallel to the grain | 2 | 6 × 8 |
4 | Anatomical observations | 1 | 1 × 8 |
Average mass loss of the wood samples at different stages of decay after inolculation.
Decay period (weeks) | Mass loss rate (%) |
---|---|
0 | 0 |
2 | 0.24 |
4 | 0.9 |
6 | 1.4 |
8 | 2 |
10 | 3.34 |
12 | 5.36 |
14 | 7.21 |