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Aging effects on physical and mechanical properties of spruce, fir and oak wood
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Original article
Aging effects on physical and mechanical properties of spruce, fir and oak wood Walter Sonderegger a,∗ , Katalin Kránitz a , Claus-Thomas Bues b , Peter Niemz a a
ETH Zurich, Institute for Building Materials (Wood Physics), Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland Dresden University of Technology, Faculty of Environmental Sciences, Institute of Forest Utilization and Forest Technology, Chair of Forest Utilization, Pienner Str. 19, 01737 Tharandt, Germany b
a r t i c l e
i n f o
Article history: Received 8 July 2014 Accepted 3 February 2015 Available online xxx Keywords: Aging Colour change Fir Mechanical properties Oak Sorption Spruce Swelling
a b s t r a c t Various aspects of natural aging on wood, such as the physical and mechanical property changes as well as colour changes, were investigated on wood of Norway spruce, silver fir and European oak. Thereby, aged wood from several historical or old, deconstructed buildings were compared with recent wood samples. It could be shown that aging modifies wood colour and causes a reduction of impact bending strength, whereas sorption and swelling as well as bending and fracture toughness do not, or only partly, show a modification over extended time. © 2015 Elsevier Masson SAS. All rights reserved.
1. Research aims Despite the properties of recent wood having been widely investigated and documented, there is still a lack of knowledge concerning the aging process of wood or the properties of the aged material. Such information is, however, crucial for the proper conservation of wooden cultural heritage objects or for the re-use of aged wood. The current study presents results from physical and mechanical investigations on wood from deconstructed buildings of different ages from several regions of Switzerland, Germany and the Czech Republic and thus will contribute to the knowledge of wood aging. 2. Introduction Aging processes in wood are mainly influenced by climatic factors (weathering) such as temperature, moisture content, climatic variability, precipitation and direct sunlight and by biotic factors such as insect and fungi degradation, as well as bacteria and marine borers [1]. Biotic deterioration is only relevant above a moisture
∗ Corresponding author. Tel.: +41 44 632 32 26. E-mail addresses: [email protected] (W. Sonderegger), [email protected] (K. Kránitz), [email protected] (C.-T. Bues), [email protected] (P. Niemz).
content of about 20%. Therefore, wood is most stable when stored indoors in dry air. When outdoors and exposed to direct sunlight, it undergoes chemical degradation caused by UV radiation, of which lignin is the most sensitive wood component to UV-light. Photodegradation of wood is a superficial process, but during aging, wood colour also changes over the whole cross section, which can be explained as a mild thermal oxidation. Photodegradation as well as oxidation processes can be simulated with artificial aging methods like the lightfastness test and thermal treatment, respectively [2–4]. The influence of aging on the physical and mechanical property changes is often ambiguous and depends, for example, on wood species, storage conditions and aging time. In contrast to thermally treated wood, which shows a clear reduction of swelling and shrinkage compared to untreated wood mainly due to its lower sorption behaviour [5], no distinct trend could be found for aged wood. Whereas Kohara and Okamoto [6] reported a swelling decrease for Japanese old timbers, Schulz et al. [7] determined an increase of swelling on a 300-year-old spruce beam, which may be due to its higher density. Other literature data show no significant differences in swelling or shrinkage of aged spruce or pinewood compared to recent wood [8,9]. Furthermore, the sorption change of aged wood is inconsistent. The sorption curves of aged Scots pine show no significant changes or even an increase compared to recent wood [9,10]. In contrast, other authors
http://dx.doi.org/10.1016/j.culher.2015.02.002 1296-2074/© 2015 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: W. Sonderegger, et al., Aging effects on physical and mechanical properties of spruce, fir and oak wood, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.02.002
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2 Table 1 Origin and provider of the aged material. Species
Age
Provider
Origin
Spruce
90–150
Chaletbau Matti AG, Gstaad, Switzerland Musée d’art et d’histoire de Genève, Switzerland Dresden University of Technology, Germany
Switzerland/Czech Republic Switzerland
Greiz and Pirna, Germany
Chaletbau Matti AG, Gstaad, Switzerland Dresden University of Technology, Germany
Switzerland/Czech Republic Greiz, Langhennersdorf and Meissen, Germany
Musée d’art et d’histoire de Genève, Switzerland Nägeli Holzbau AG, Gais, Switzerland Dresden University of Technology, Germany
Switzerland
–
Region of Zurich, St. Galler Rheintal and Appenzell, Switzerland
250
280–320
Fir
120–150 160–470
Oak
230
210–290 470 All
Recent
Appenzell, Switzerland Langhennersdorf, Germany
report a slightly decreased sorption for aged spruce and fir wood [11,12]. Several studies on softwoods used in construction, like spruce, fir or Scots pine, show similar or even increased strength values (bending, compression) of aged wood compared with recent wood [7,12–14]. In contrast, the elastic properties of aged wood show no consistent trends. Whereas Kránitz et al. [15] observed an increase in bending Young’s modulus of aged spruce wood, Lang [12] measured a decrease. No significant change or a decrease of Young’s modulus were found for aged Scots pine [9,12]. The decreased impact bending strength of aged wood [6,7,12,14,16] may be due to more brittleness or microcracks [12]. For this study, various aspects of natural aging on wood, such as the physical and mechanical property changes as well as colour changes, were investigated on aged wood from historic buildings. The experiments were performed on specimens of three different wood species commonly used in Europe, namely Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.) and European oak (Quercus robur L./Quercus petraea Liebl.). In every case, recent wood samples were investigated beside the aged samples to serve as a reference. The following investigations were carried out: • hygroscopic behaviour: sorption and swelling; • colour measurements; • bending tests: modulus of rupture (MOR), static and dynamic modulus of elasticity (MOE); • impact bending: standard and Dynstat tests and comparative dynamic MOE; • fracture toughness KIC . 3. Material Wood samples from deconstructed buildings of different ages from several regions of Switzerland, Germany and the Czech Republic were investigated. An overview is shown in Table 1. The woods provided by the Dresden University of Technology (TUD), Germany, were collected from historic buildings, such as the bell frame from Langhennersdorf, the castle in Greiz and the Albrechtsburg Castle in Meissen. All of them had a load-bearing role in indoor applications. Only small wood pieces were available, which were prepared for the sorption and swelling experiments. The Musée d’art et d’histoire de Genève, Switzerland offered a beam of spruce
wood and one of oak wood that had been part of an old Swiss house construction and probably had a load-bearing role. Both were used for determination of sorption and swelling parameters and the spruce beam was also used for colour measurements. The wood from Chaletbau Matti AG, Gstaad, Switzerland, where old wood is re-used to build traditional Swiss timber houses, originated from deconstructed buildings in Switzerland and the Czech Republic, however their origins could not be more precisely determined. The material was subjected to all of the experiments within the scope of this study. The oak wood provided by Nägeli Holzbau AG, Gais, Switzerland comes from a demolished residential house built in the region of Appenzell. The outer parts of the investigated beams were damaged by insects and were consequently excluded from the experiments. The specimens for all tests were cut from the sound parts of the beams. The recent wood of all three species originates from Switzerland from the regions of Zurich, St. Galler Rheintal and Appenzell. The dating of the aged samples was carried out at TUD using the dendrochronology method [17]. 4. Methods 4.1. Sorption and swelling Sorption and swelling was determined at a temperature of 20 ◦ C with relative humidity (RH) steps of 35, 50, 65, 80, 93, 80, 65, 50, and 35%. At the end of the cycle, the samples were oven dried at 103 ◦ C. At every step, mass and dimensions were recorded after equilibrium moisture content (EMC) was reached (mass change <0.1% in 24 hours). For the determination of swelling in the radial (R) and the tangential (T) directions, the specimens measured 20, 25 or 50 mm in the R and T directions and 10 mm in the longitudinal (L) direction. For the study of L-swelling, the specimens were sized 20 × 20 × 100 mm (R × T × L). The differential swelling coefficient (swelling in percent per change of moisture content [MC] in percent) was calculated in the range between 0 and 93% RH according to DIN 52184 [18]. It was determined by means of a regression curve taking into account the swelling values of all RH steps as a function of MC. 4.2. Colour changes To investigate colour changes, colour coordinates were determined according to the CIE Lab-system. The coordinates a*, b* and L* were calculated based on the colour intensities according to Eqs. (1–3):
a∗ = 500 ·
X 1⁄3 X0
∗
b = 200 ·
Y0
∗
L = 116 ·
Y 1⁄3
Y 1⁄3 Y0
−
−
Y 1⁄3
Y0
Z 1⁄3
(1)
Z0
(2)
− 16
(3)
a*: colour coordinate: position between red and green; b*: colour coordinate: position between yellow and blue; L*: lightness factor; X, Y, Z: CIE tristimulus values of the sample; X0 , Y0 , Z0 : CIE tristimulus values of the white reference. The measurements were carried out with the Minolta ChromaMeter CR-200 device, which works with diffuse illumination and has an 8 mm diameter measuring area and a viewing angle
Please cite this article in press as: W. Sonderegger, et al., Aging effects on physical and mechanical properties of spruce, fir and oak wood, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.02.002
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of 0◦ . The CIE standard illuminant D65 was used. On each specimen, three to five measuring points were chosen. The value recorded by the device corresponded to the average value of three measurements.
EUS = · c 2
(4)
where EUS is the MOE determined from ultrasound, the specimen density and c the sound velocity. The eigenfrequency (first flexural mode) was determined with the device Grindosonic MK 5 ‘Industrial’ (Lemmens N. V., Belgium) and the MOE was calculated according to Eq. (5) [20]: 4 · 2 · l4 · f 2 · · mn 4 · i2
1+
i2 · K1 l2
(5)
35 28
Bending MOR and MOE were determined in a three-point bending test according to the standard DIN 52186 [19]. Specimens’ dimensions were 20 × 20 × 400 mm (R × T × L). The measurements were carried out with the universal testing machine Zwick Z100 (Zwick GmbH & Co. KG, Ulm). The dynamic MOE was measured in advance by the two non-destructive testing methods, namely ultrasound and eigenfrequency. Prior to testing, the specimens were climatised at normal climate (20 ◦ C and 65% RH) until mass constancy was reached. Ultrasound was measured at a frequency of 50 kHz with the device BP-V (Steinkamp, Bremen). From this, the MOE was calculated according to Eq. (4):
EEf =
Ø 12,5
4.3. Bending tests
7,7
3
8, 1
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33,6
5
Fig. 1. Specimen size for fracture toughness tests according to DIN EN ISO 12737 [23], dimensions in mm.
in Fig. 1. The crack tip of each specimen was sharpened with a razor prior to testing to ensure a crack tip radius of < 0.25 mm. Velocity of the traverse was 1.5 mm/min in the case of oak specimens and for fir specimens with orientations RL and RT, while a velocity of 1 mm/min was applied for spruce specimens and fir specimens with orientations TL and TR. 5. Results 5.1. Sorption and swelling
EEf : Young’s modulus determined from eigenfrequency; f: eigenfrequency; : density; l: specimen length; i: radius of gyration in direction of the bending vibration; for rectangular cross sections: i2 = h2 /12 (h: specimen height); K1 , mn 4 : constants depending on the order of vibration; for vibrations of first order: K1 = 49.48, mn 4 = 500.6. 4.4. Impact bending The standard impact bending test was carried out according to DIN 52189 [21] with the pendulum machine 50 J (Mohr & Federhaff AG, Mannheim) to determine the impact bending strength. The specimens were sized 20 × 20 × 300 mm (R × T × L). As for the bending specimens (section 4.3), the dynamic MOE was calculated comparatively by ultrasound and eigenfrequency before the standard test. The impact bending strength was also determined with the Dynstat device according to DIN 53435 [22] with specimens sized 4 × 10 × 20 mm (T × R × L). All tests were carried out at normal climate (20 ◦ C, 65% RH) after the specimens reached EMC. The impact bending strength was calculated according to Eq. (6): w=
W b·h
The mean sorption and swelling coefficients of recent and aged woods are shown in Table 2. All sorption values are low compared with literature data [5,24], which may be due to low circulation within the climatic chamber, but are nonetheless comparable among each other. The sorption values of the aged woods are similar (spruce) or slightly lower (fir, oak) than those of the recent woods. For L-swelling, a reduction was measured for aged oak in contrast to recent wood, whereas the values for recent and aged spruce and fir are similar. Higher swelling values in the R- and T-directions were determined for aged spruce due to its higher density, which highly influenced the swelling behaviour [25]. Fig. 2a shows the R- and T-swelling of this species depending on density and separated by sample origin. Due to high variation within and between the different origins, no age influence could be located. However, the T-swelling is generally lower compared with the data of Sonderegger et al. [25]. In contrast, a clear reduction of the R- and T-swelling was measured for aged fir, which induced an improvement of dimensional stability. This is mainly attributed to the values of the two oldest origins ‘Greiz’ and ‘Langhennersdorf’ (Fig. 2b). For aged oak, a reduction in the T-direction was registered, which originates from values of ‘Langhennersdorf’ (Fig. 2c), whereas the slight increase in the R-direction can be explained by the higher density of the aged samples.
(6)
where w is the impact bending strength, W the work required for fracture of the specimen, b the width and h the thickness of the specimen. 4.5. Fracture toughness Fracture toughness was determined in mode I tension according to DIN EN ISO 12737 [23] using the MT300 Deben Microtest Stage Controller (Deben UK Ltd, Suffolk, UK) at RL, RT, TL and TR directions (first index = direction normal to the crack plane, second index = direction of crack propagation). The specimen size is shown
5.2. Colour changes A variance analysis of the colour behaviour between recent and aged samples showed a significant difference at a confidence level of 95% for L*, a* and b* by all species and for all samples (for more details, see [26]). Fig. 3 shows the colour changes according to wood age. The trends regarding spruce and fir show similar gradients at all coordinates and are in accordance with earlier observations [27]. The difference observed between aged and recent oak samples is remarkably smaller than those of the softwoods. However, the trend of the changes is in agreement with results that were obtained in simulated indoor sunlight exposure and in lightfastness
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Table 2 Differential swelling coefficients (q) in the radial (r), tangential (t) and longitudinal (l) directions and moisture content (u) at different relative humidities (RH) of recent and aged wood.
Spruce Fir Oak
Age [years]
n
0 [g cm−3 ]
Recent 90–320 Recent 120–470 Recent 210–470
18 (10) 72 (30) 18 (10) 65 (20) 18 (12) 28 (8)
0.402 0.445 0.434 0.431 0.651 0.668
± ± ± ± ± ±
0.044 0.043 0.041 0.023 0.033 0.064
qr [%/%] 0.154 0.191 0.178 0.145 0.189 0.205
± ± ± ± ± ±
qt [%/%] 0.016 0.027 0.032 0.038 0.007 0.015
0.317 0.338 0.374 0.324 0.378 0.345
± ± ± ± ± ±
ql [%/%] 0.019 0.050 0.034 0.056 0.054 0.014
0.010 0.010 0.011 0.011 0.014 0.009
± ± ± ± ± ±
u35 [%] 0.003 0.004 0.002 0.003 0.003 0.003
6.3 6.5 6.4 6.1 6.7 6.0
± ± ± ± ± ±
0.2 0.6 1.0 0.6 1.2 1.1
u50 [%] 8.0 8.2 8.1 7.7 8.6 7.8
± ± ± ± ± ±
0.5 0.8 1.5 0.9 1.2 1.2
u65 [%] 10.4 10.6 10.6 10.1 10.9 10.1
± ± ± ± ± ±
u80 [%] 0.2 0.5 1.3 0.6 0.9 0.9
13.8 13.9 14.1 13.6 14.2 13.4
± ± ± ± ± ±
u93 [%] 0.7 0.8 0.7 0.8 1.1 0.4
19.4 19.1 19.5 18.9 19.3 18.7
± ± ± ± ± ±
0.9 1.6 0.4 1.6 0.6 0.7
0 : oven dry density; index of u: RH in percent; n: number of specimens (n for ql in parentheses); values with standard deviation.
tests, respectively [4,27]. Changes observed are similar to those that occurred during accelerated aging or at the beginning of thermal treatment [5,28–30]. This, and the fact that the whole cross section was affected, support the theory of Matsuo et al. [3], who claim that colour changes during aging are a consequence of a slow and mild oxidation process.
Fig. 2. Differential swelling of recent and aged wood depending on density and according to their origin and wood age. R-swelling: large symbols, T-swelling: small symbols; a: spruce (linear trends according to [25], R: drawn out, T: dashed); b: fir; c: oak.
5.3. Bending tests Table 3 shows the results of the bending tests. No trend could be found between the recent and aged specimens for MOR as well as the static and dynamic MOE, which agrees with divers data from literature [7,14]. Whereas for spruce, the mean values of the aged specimens were higher and a contradictory behaviour was shown
Fig. 3. Colour changes according to wood age and corresponding trend lines (spruce: solid line, fir: dashed, quercus: dotted); a: lightness (L*); b: redness (a*); c: yellowness (b*).
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Table 3 MOE determined by ultrasound (EUS ), eigenfrequency (EEf ) and by bending (Eb ) and MOR by bending ( b ).
Spruce Fir
Age [years]
n
[g cm−3 ]
Recent 120–150 Recent 120–150
45 44 27 14
0.411 0.428 0.475 0.441
± ± ± ±
EUS [MPa]
0.022 0.033 0.023 0.025
12,800 14,700 16,300 14,800
± ± ± ±
EEf [MPa] 1840 1650 1380 1090
11,000 12,700 13,700 12,800
± ± ± ±
b [MPa]
Eb [MPa] 1570 1490 1090 1030
11,000 11,900 14,800 13,900
± ± ± ±
2170 1500 1660 2100
75.6 81.2 86.6 76.9
± ± ± ±
9.5 9.6 8.9 16.0
n: number of specimens; : density; values with standard deviation.
Table 4 Impact bending strength (w) by standard tests according to DIN 52189 [21] and MOE determined by ultrasound (EUS ) and eigenfrequency (EEf ).
Spruce Fir Oak
Age [years]
n [–]
[g cm−3 ]
Recent 120–150 Recent 120–150 Recent 210
41 52 35 15 21 16
0.408 0.428 0.484 0.443 0.668 0.719
± ± ± ± ± ±
MC [%] 12.4 12.2 13.2 12.5 15.8 13.4
0.024 0.043 0.033 0.027 0.029 0.025
± ± ± ± ± ±
EUS [MPa] 0.4 0.5 0.3 0.4 0.4 1.3
12,400 14,400 16,200 14,200 10,600 13,700
± ± ± ± ± ±
w [kJ m−2 ]
EEf [MPa] 1790 1890 1660 1520 1950 1360
10,500 12,000 12,900 11,300 8600 11,200
± ± ± ± ± ±
1530 1670 1260 1240 1570 1240
36.9 35.2 41.2 38.6 35.7 32.2
± ± ± ± ± ±
10.7 11.3 6.8 9.3 13.0 12.2
n: number of specimens; : density; values with standard deviation.
for aged fir wood. This is mainly due to the different densities and is unlikely due to an aging effect. In contrast, Kránitz et al. [15] determined higher MOE values from ultrasound measurements for aged wood in the L-direction (tests on structural timber and clear specimens) and in the R-direction (tests on clear specimens).
5.4. Impact bending The results of the standard impact bending tests are shown in Table 4. The impact bending strength was higher for all recent woods compared with the aged woods, which coincides with the tendencies of previous studies [7,12,14]. Only fir exhibited density as the main influencing factor for this trend, rather than the aging effect. For oak, a clearly higher MC was shown for the recent wood compared with the aged wood. However, according to Koch [31], no significant differences of impact bending strength could be measured between normally conditioned and green samples. Therefore, the influence of MC can be neglected. Although the impact bending strengths of the recent spruce and oak woods were higher than their aged wood counterparts, their dynamic MOE was clearly lower. This contradictory behaviour was illustrated in Fig. 4 (dependent on density, which strengthens the effect). It can be explained probably by microcracks in the aged wood, which have a much greater influence on the impact bending strength than on the dynamic MOE [12,32]. The values of the Dynstat tests are shown in Table 5 and were clearly lower than the values of the standard tests. These differences in the values of the tests correspond with measurements by Bäucker et al. [33]. A similar aging effect to the standard tests was only found for oak.
5.5. Fracture toughness Table 6 shows the fracture toughness KIC of the different loads. The values of the two softwoods, spruce and fir, are similar in all directions. For both species, the aged samples showed a clear reduction of the KIC value only in the RT direction. The fracture toughness was in the same range as by Märki et al. and by Reiterer et al. [34,35], but was clearly higher than that of Keunecke et al. [36]. The oak values were more than twice as high compared with those of the
Table 5 Impact bending strength (w) determined by the Dynstat device according to DIN 53435 [22]. Age [years] Spruce Fir Oak
Recent 120–150 Recent 120 Recent 210
n [–] 110 114 42 65 42 35
[g cm−3 ] 0.414 0.443 0.506 0.476 0.681 0.733
± ± ± ± ± ±
0.020 0.041 0.016 0.021 0.028 0.024
n: number of specimens; : density; values with standard deviation.
w [kJ m−2 ] 16.4 23.8 32.6 27.5 14.1 12.9
± ± ± ± ± ±
4.9 4.2 2.9 2.9 6.2 3.7
Fig. 4. a: impact bending strength depending on density; b: MOE depending on density, determined by ultrasound (EUS ) prior to the impact bending test. The regression curves pass through the zero-point: dashed lines: recent wood; continuous lines: aged wood; bold lines: fir samples.
Please cite this article in press as: W. Sonderegger, et al., Aging effects on physical and mechanical properties of spruce, fir and oak wood, Journal of Cultural Heritage (2015), http://dx.doi.org/10.1016/j.culher.2015.02.002
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Table 6 Fracture toughness KIC at RL, RT, TL and TR directions.
Spruce Fir Oak
Age [years]
[g cm−3 ]
Recent 120–150 Recent 120–150 Recent 210
0.419 0.411 0.418 0.418 0.561 0.653
± ± ± ± ± ±
0.035 0.018 0.010 0.016 0.073 0.025
KIC [MPa m0.5 ] n
TR
15 44 17 16 13 14
0.273 0.258 0.292 0.307 0.601 0.593
± ± ± ± ± ±
0.049 0.030 0.026 0.074 0.066 0.042
n
RT
15 38 24 30 6 12
0.223 0.230 0.220 0.206 0.753 0.735
± ± ± ± ± ±
0.032 0.038 0.022 0.024 0.152 0.118
n
TL
38 57 28 20 28 12
0.362 0.326 0.292 0.296 0.765 0.673
± ± ± ± ± ±
0.071 0.050 0.036 0.039 0.162 0.111
n
RL
34 36 27 26 28 16
0.418 0.343 0.408 0.337 0.923 0.897
± ± ± ± ± ±
0.048 0.063 0.064 0.070 0.165 0.121
n: number of specimens; : density; values with standard deviation.
investigated softwoods and laid between values of beech [37] and sycamore maple [38]. Although the density of the aged oak wood was clearly higher than that of the recent wood, the KIC values were lower in all load directions. To verify whether this is attributable to an aging effect or to variations within the species needs further investigations. 6. Conclusion It could be shown that notable outcomes of aging are colour modification and a reduction of impact bending strength, whereas sorption and swelling as well as bending and fracture toughness do not, or only partly, alter as a result of aging. One problem was the high variation of the physical and mechanical properties within a species, which superimpose some possible aging effects. Therefore, more investigations are necessary. However, a better understanding of the aging process contributes to the conservation of wooden cultural heritage and, through promotion of recycling and longterm applications of wood, to the preservation of the environment as well. Acknowledgement Thanks go to the ETH Zurich for its financial support (ETHIRA grant, 2009-2013). Special thanks go to Dr. Björn Günther (Dresden University of Technology) for dating the aged samples, to Dr. Kilian Anheuser (Musée d’ethnographie de Genève) for his scientific support and to Nägeli AG (Gais, Switzerland) and Chaletbau Matti AG (Saanen, Switzerland) for providing aged wood. References [1] A. Unger, A.P. Schniewind, W. Unger, Conservation of wood artifacts, Springer Verlag, Berlin Heidelberg, 2001. [2] H.T. Chang, S.T. Chang, Correlation between softwood discoloration induced by accelerated lightfastness testing and by indoor exposure, Polym. Degrad. Stabil. 72 (2001) 361–365. [3] M. Matsuo, M. Yokoyama, K. Umemura, et al., Aging of wood: analysis of color changes during natural aging and heat treatment, Holzforschung 65 (2011) 361–368. [4] J. Miklecic, A. Kasa, V. Jirous-Rajkovic, Colour changes of modified oak wood in indoor environment, Eur. J. Wood Prod. 70 (2012) 385–387. [5] P. Bekhta, P. Niemz, Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood, Holzforschung 57 (2003) 539–546. [6] J. Kohara, H. Okamoto, Studies of Japanese old timbers, Sci. Rep. Saikyo Univ. 7 (1a) (1955) 9–20. [7] H. Schulz, H. von Aufsess, T. Verron, Eigenschaften eines Fichtenbalkens aus altem Dachstuhl, Holz Roh Werkst. 42 (1984) 109. [8] D. Holz, Zum Alterungsverhalten des Werkstoffes Holz – einige Ansichten, Untersuchungen, Ergebnisse, Holztechnologie 22 (1981) 80–85. [9] D. Erhardt, M.F. Mecklenburg, C.S. Tumosa, T.M. Olstad, New versus old wood: differences and similarities in physical, mechanical, and chemical properties, in: J. Bridgeland (Ed.), International Council of Museums-Committee for Conservation 11th Triennial Meeting, James&James, London, 1996, pp. 903–910. [10] L. García Esteban, F. García Fernández, A. Guindeo, P. Palacios, J. Gril, Comparison of the hygroscopic behaviour of 205-year-old and recently cut juvenile wood from Pinus sylvestris L., Ann. For. Sci. 63 (2006) 309–317.
[11] A. Kurtoglu, The properties of sorption of old spruce timber, Holzforsch. Holzverw. 35 (6) (1983) 125–126. [12] A. Lang, Charakterisierung des Altholzaufkommens in Deutschland, Dissertation, Universität Hamburg, Hamburg, 2004. [13] H.-J. Deppe, H. Rühl, Zur Beurteilung alter Bauhölzer. 1. Rohdichte und Druckfestigkeit, Holz Roh Werkst. 51 (1993) 379–383. [14] T. Baron, Untersuchungen an ungeschädigten und durch Pilzbefall Nadelholzbauteilen mit ausgewählten Prüfverfahren. geschädigten Dissertation, Technische Universität Dresden, Dresden, 2009. [15] K. Kránitz, M. Deublein, P. Niemz, Determination of dynamic elastic moduli and shear moduli of aged wood by means of ultrasonic devices, Mater. Struct. 47 (2014) 925–936. [16] M. Yokoyama, J. Gril, M. Matsuo, H. Yano, J. Sugiyama, B. Clair, S. Kubodera, T. Mitsutani, M. Sakamoto, H. Ozaki, M. Imamura, S. Kawai, Mechanical characteristics of aged Hinoki wood from Japanese historical buildings, C. R. Phys. 10 (2009) 601–611. [17] B. Günther, Erarbeitung einer Methode zur Erfassung von dendrochronologisch relevanten Jahrringmerkmalen der Trauben-Eiche (Quercus petraea [Matt.] Liebl.) auf der Grundlage der Röntgendensitometrie, Dissertation, TU Dresden, Dresden, 2012. [18] DIN 52184, Prüfung von Holz: Bestimmung der Quellung und Schwindung, 1979. [19] DIN 52186, Prüfung von Holz: Biegeversuch, 1978. [20] R. Görlacher, Ein neues Messverfahren zur Bestimmung des Elastizitätsmoduls von Holz, Holz Roh Werkst. 42 (1984) 219–222. [21] DIN 52189, Prüfung von Holz: Schlagbiegeversuch – Bestimmung der Bruchschlagarbeit, 1981. [22] DIN 53435, Prüfung von Kunststoffen – Biegeversuch und Schlagbiegeversuch an Dynstat-Probekörpern, 1983. [23] DIN EN ISO 12737, Metallic materials – Determination of plane-strain fracture toughness, 2011. [24] P. Niemz, W. Sonderegger, Sorptionsisothermen ausgewählter heimischer und fremdländischer Holzarten bei Adsorption und Desorption, Bauphysik 29 (2007) 381–382. [25] W. Sonderegger, P. Alter, P. Niemz, Untersuchungen zu ausgewählten Eigenschaften von Fichtenklangholz aus Graubünden, Holz Roh Werkst. 66 (2008) 345–354. [26] K. Kránitz, Effect of natural aging on wood, Dissertation, ETH Zurich, Zurich, 2015 [in press]. [27] L. Oltean, A. Teischinger, C. Hansmann, Wood surface discolouration due to simulated indoor sunlight exposure, Holz Roh Werkst. 66 (2008) 51–56. [28] C. Brischke, C.R. Welzbacher, K. Brandt, A.O. Rapp, Quality control of thermally modified timber: Interrelationship between heat treatment intensities and CIE L*a*b* color data on homogenized wood samples, Holzforschung 61 (2007) 19–22. [29] L. Denes, E.M. Lang, Photodegradation of heat treated hardwood veneers, J. Photoch. Photobio. B 118 (2013) 9–15. [30] W. Sonderegger, D. Mannes, A. Kaestner, J. Hovind, E. Lehmann, On-line monitoring of hygroscopicity and dimensional changes of wood during thermal modification by means of neutron imaging methods, Holzforschung 69 (2015) 87–95. [31] G. Koch, Sekundäre Veränderungen im Holz dynamisch beanspruchter Fichten (Picea abies [L.] Karst.) aus immissionsbelasteten und windexponierten Hochlagenbeständen, Mitteilungen der Bundesforschungsanstalt für Forst und Holzwirtschaft Hamburg Nr. 192, Hamburg, 1999. [32] W. Sonderegger, P. Niemz, The influence of compression failure on the bending, impact bending and tensile strength of spruce wood and the evaluation of non-destructive methods for early detection, Holz Roh Werkst. 62 (2004) 335–342. [33] E. Bäucker, A. Bemmann, C.T. Bues, G.-J. Nuys, Holzeigenschaften stark immissionsgeschädigter Fichten (Picea abies [L.] Karst.) in den Hochlagen des Osterzgebirges, Holz Roh Werkst. 54 (1996) 251–258. [34] C. Märki, P. Niemz, D. Mannes, Vergleichende Untersuchungen zu ausgewählten mechanischen Eigenschaften von Eibe und Fichte, Schweiz. Z. Forstwes. 156 (2005) 85–91. [35] A. Reiterer, G. Sinn, S.E. Stanzl-Tschegg, Fracture characteristics of different wood species under mode I loading perpendicular to the grain, Mat. Sci. Eng. A 332 (2002) 29–36.
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[36] D. Keunecke, P. Niemz, M. Tollert, A. Hansel, Mechanical properties and failure behaviour of yew and spruce determined with a compact tension test and digital image correlation, Wood Res. 57 (2012) 669–675. [37] T. Ozyhar, S. Hering, P. Niemz, Moisture-dependent elastic and strength anisotropy of European beech wood in tension, J. Mater. Sci. 47 (2012) 6141–6150.
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[38] W. Sonderegger, A. Martienssen, C. Nitsche, T. Ozyhar, M. Kaliske, P. Niemz, Investigations on the physical and mechanical behaviour of sycamore maple (Acer pseudoplatanus L.), Eur. J. Wood Prod. 71 (2013) 91–99.
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Available online at
ScienceDirect www.sciencedirect.com
Original article
Aging effects on physical and mechanical properties of spruce, fir and oak wood Walter Sonderegger a,∗ , Katalin Kránitz a , Claus-Thomas Bues b , Peter Niemz a a
ETH Zurich, Institute for Building Materials (Wood Physics), Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland Dresden University of Technology, Faculty of Environmental Sciences, Institute of Forest Utilization and Forest Technology, Chair of Forest Utilization, Pienner Str. 19, 01737 Tharandt, Germany b
a r t i c l e
i n f o
Article history: Received 8 July 2014 Accepted 3 February 2015 Available online xxx Keywords: Aging Colour change Fir Mechanical properties Oak Sorption Spruce Swelling
a b s t r a c t Various aspects of natural aging on wood, such as the physical and mechanical property changes as well as colour changes, were investigated on wood of Norway spruce, silver fir and European oak. Thereby, aged wood from several historical or old, deconstructed buildings were compared with recent wood samples. It could be shown that aging modifies wood colour and causes a reduction of impact bending strength, whereas sorption and swelling as well as bending and fracture toughness do not, or only partly, show a modification over extended time. © 2015 Elsevier Masson SAS. All rights reserved.
1. Research aims Despite the properties of recent wood having been widely investigated and documented, there is still a lack of knowledge concerning the aging process of wood or the properties of the aged material. Such information is, however, crucial for the proper conservation of wooden cultural heritage objects or for the re-use of aged wood. The current study presents results from physical and mechanical investigations on wood from deconstructed buildings of different ages from several regions of Switzerland, Germany and the Czech Republic and thus will contribute to the knowledge of wood aging. 2. Introduction Aging processes in wood are mainly influenced by climatic factors (weathering) such as temperature, moisture content, climatic variability, precipitation and direct sunlight and by biotic factors such as insect and fungi degradation, as well as bacteria and marine borers [1]. Biotic deterioration is only relevant above a moisture
∗ Corresponding author. Tel.: +41 44 632 32 26. E-mail addresses: [email protected] (W. Sonderegger), [email protected] (K. Kránitz), [email protected] (C.-T. Bues), [email protected] (P. Niemz).
content of about 20%. Therefore, wood is most stable when stored indoors in dry air. When outdoors and exposed to direct sunlight, it undergoes chemical degradation caused by UV radiation, of which lignin is the most sensitive wood component to UV-light. Photodegradation of wood is a superficial process, but during aging, wood colour also changes over the whole cross section, which can be explained as a mild thermal oxidation. Photodegradation as well as oxidation processes can be simulated with artificial aging methods like the lightfastness test and thermal treatment, respectively [2–4]. The influence of aging on the physical and mechanical property changes is often ambiguous and depends, for example, on wood species, storage conditions and aging time. In contrast to thermally treated wood, which shows a clear reduction of swelling and shrinkage compared to untreated wood mainly due to its lower sorption behaviour [5], no distinct trend could be found for aged wood. Whereas Kohara and Okamoto [6] reported a swelling decrease for Japanese old timbers, Schulz et al. [7] determined an increase of swelling on a 300-year-old spruce beam, which may be due to its higher density. Other literature data show no significant differences in swelling or shrinkage of aged spruce or pinewood compared to recent wood [8,9]. Furthermore, the sorption change of aged wood is inconsistent. The sorption curves of aged Scots pine show no significant changes or even an increase compared to recent wood [9,10]. In contrast, other authors
http://dx.doi.org/10.1016/j.culher.2015.02.002 1296-2074/© 2015 Elsevier Masson SAS. All rights reserved.
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2 Table 1 Origin and provider of the aged material. Species
Age
Provider
Origin
Spruce
90–150
Chaletbau Matti AG, Gstaad, Switzerland Musée d’art et d’histoire de Genève, Switzerland Dresden University of Technology, Germany
Switzerland/Czech Republic Switzerland
Greiz and Pirna, Germany
Chaletbau Matti AG, Gstaad, Switzerland Dresden University of Technology, Germany
Switzerland/Czech Republic Greiz, Langhennersdorf and Meissen, Germany
Musée d’art et d’histoire de Genève, Switzerland Nägeli Holzbau AG, Gais, Switzerland Dresden University of Technology, Germany
Switzerland
–
Region of Zurich, St. Galler Rheintal and Appenzell, Switzerland
250
280–320
Fir
120–150 160–470
Oak
230
210–290 470 All
Recent
Appenzell, Switzerland Langhennersdorf, Germany
report a slightly decreased sorption for aged spruce and fir wood [11,12]. Several studies on softwoods used in construction, like spruce, fir or Scots pine, show similar or even increased strength values (bending, compression) of aged wood compared with recent wood [7,12–14]. In contrast, the elastic properties of aged wood show no consistent trends. Whereas Kránitz et al. [15] observed an increase in bending Young’s modulus of aged spruce wood, Lang [12] measured a decrease. No significant change or a decrease of Young’s modulus were found for aged Scots pine [9,12]. The decreased impact bending strength of aged wood [6,7,12,14,16] may be due to more brittleness or microcracks [12]. For this study, various aspects of natural aging on wood, such as the physical and mechanical property changes as well as colour changes, were investigated on aged wood from historic buildings. The experiments were performed on specimens of three different wood species commonly used in Europe, namely Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.) and European oak (Quercus robur L./Quercus petraea Liebl.). In every case, recent wood samples were investigated beside the aged samples to serve as a reference. The following investigations were carried out: • hygroscopic behaviour: sorption and swelling; • colour measurements; • bending tests: modulus of rupture (MOR), static and dynamic modulus of elasticity (MOE); • impact bending: standard and Dynstat tests and comparative dynamic MOE; • fracture toughness KIC . 3. Material Wood samples from deconstructed buildings of different ages from several regions of Switzerland, Germany and the Czech Republic were investigated. An overview is shown in Table 1. The woods provided by the Dresden University of Technology (TUD), Germany, were collected from historic buildings, such as the bell frame from Langhennersdorf, the castle in Greiz and the Albrechtsburg Castle in Meissen. All of them had a load-bearing role in indoor applications. Only small wood pieces were available, which were prepared for the sorption and swelling experiments. The Musée d’art et d’histoire de Genève, Switzerland offered a beam of spruce
wood and one of oak wood that had been part of an old Swiss house construction and probably had a load-bearing role. Both were used for determination of sorption and swelling parameters and the spruce beam was also used for colour measurements. The wood from Chaletbau Matti AG, Gstaad, Switzerland, where old wood is re-used to build traditional Swiss timber houses, originated from deconstructed buildings in Switzerland and the Czech Republic, however their origins could not be more precisely determined. The material was subjected to all of the experiments within the scope of this study. The oak wood provided by Nägeli Holzbau AG, Gais, Switzerland comes from a demolished residential house built in the region of Appenzell. The outer parts of the investigated beams were damaged by insects and were consequently excluded from the experiments. The specimens for all tests were cut from the sound parts of the beams. The recent wood of all three species originates from Switzerland from the regions of Zurich, St. Galler Rheintal and Appenzell. The dating of the aged samples was carried out at TUD using the dendrochronology method [17]. 4. Methods 4.1. Sorption and swelling Sorption and swelling was determined at a temperature of 20 ◦ C with relative humidity (RH) steps of 35, 50, 65, 80, 93, 80, 65, 50, and 35%. At the end of the cycle, the samples were oven dried at 103 ◦ C. At every step, mass and dimensions were recorded after equilibrium moisture content (EMC) was reached (mass change <0.1% in 24 hours). For the determination of swelling in the radial (R) and the tangential (T) directions, the specimens measured 20, 25 or 50 mm in the R and T directions and 10 mm in the longitudinal (L) direction. For the study of L-swelling, the specimens were sized 20 × 20 × 100 mm (R × T × L). The differential swelling coefficient (swelling in percent per change of moisture content [MC] in percent) was calculated in the range between 0 and 93% RH according to DIN 52184 [18]. It was determined by means of a regression curve taking into account the swelling values of all RH steps as a function of MC. 4.2. Colour changes To investigate colour changes, colour coordinates were determined according to the CIE Lab-system. The coordinates a*, b* and L* were calculated based on the colour intensities according to Eqs. (1–3):
a∗ = 500 ·
X 1⁄3 X0
∗
b = 200 ·
Y0
∗
L = 116 ·
Y 1⁄3
Y 1⁄3 Y0
−
−
Y 1⁄3
Y0
Z 1⁄3
(1)
Z0
(2)
− 16
(3)
a*: colour coordinate: position between red and green; b*: colour coordinate: position between yellow and blue; L*: lightness factor; X, Y, Z: CIE tristimulus values of the sample; X0 , Y0 , Z0 : CIE tristimulus values of the white reference. The measurements were carried out with the Minolta ChromaMeter CR-200 device, which works with diffuse illumination and has an 8 mm diameter measuring area and a viewing angle
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of 0◦ . The CIE standard illuminant D65 was used. On each specimen, three to five measuring points were chosen. The value recorded by the device corresponded to the average value of three measurements.
EUS = · c 2
(4)
where EUS is the MOE determined from ultrasound, the specimen density and c the sound velocity. The eigenfrequency (first flexural mode) was determined with the device Grindosonic MK 5 ‘Industrial’ (Lemmens N. V., Belgium) and the MOE was calculated according to Eq. (5) [20]: 4 · 2 · l4 · f 2 · · mn 4 · i2
1+
i2 · K1 l2
(5)
35 28
Bending MOR and MOE were determined in a three-point bending test according to the standard DIN 52186 [19]. Specimens’ dimensions were 20 × 20 × 400 mm (R × T × L). The measurements were carried out with the universal testing machine Zwick Z100 (Zwick GmbH & Co. KG, Ulm). The dynamic MOE was measured in advance by the two non-destructive testing methods, namely ultrasound and eigenfrequency. Prior to testing, the specimens were climatised at normal climate (20 ◦ C and 65% RH) until mass constancy was reached. Ultrasound was measured at a frequency of 50 kHz with the device BP-V (Steinkamp, Bremen). From this, the MOE was calculated according to Eq. (4):
EEf =
Ø 12,5
4.3. Bending tests
7,7
3
8, 1
W. Sonderegger et al. / Journal of Cultural Heritage xxx (2015) xxx–xxx
33,6
5
Fig. 1. Specimen size for fracture toughness tests according to DIN EN ISO 12737 [23], dimensions in mm.
in Fig. 1. The crack tip of each specimen was sharpened with a razor prior to testing to ensure a crack tip radius of < 0.25 mm. Velocity of the traverse was 1.5 mm/min in the case of oak specimens and for fir specimens with orientations RL and RT, while a velocity of 1 mm/min was applied for spruce specimens and fir specimens with orientations TL and TR. 5. Results 5.1. Sorption and swelling
EEf : Young’s modulus determined from eigenfrequency; f: eigenfrequency; : density; l: specimen length; i: radius of gyration in direction of the bending vibration; for rectangular cross sections: i2 = h2 /12 (h: specimen height); K1 , mn 4 : constants depending on the order of vibration; for vibrations of first order: K1 = 49.48, mn 4 = 500.6. 4.4. Impact bending The standard impact bending test was carried out according to DIN 52189 [21] with the pendulum machine 50 J (Mohr & Federhaff AG, Mannheim) to determine the impact bending strength. The specimens were sized 20 × 20 × 300 mm (R × T × L). As for the bending specimens (section 4.3), the dynamic MOE was calculated comparatively by ultrasound and eigenfrequency before the standard test. The impact bending strength was also determined with the Dynstat device according to DIN 53435 [22] with specimens sized 4 × 10 × 20 mm (T × R × L). All tests were carried out at normal climate (20 ◦ C, 65% RH) after the specimens reached EMC. The impact bending strength was calculated according to Eq. (6): w=
W b·h
The mean sorption and swelling coefficients of recent and aged woods are shown in Table 2. All sorption values are low compared with literature data [5,24], which may be due to low circulation within the climatic chamber, but are nonetheless comparable among each other. The sorption values of the aged woods are similar (spruce) or slightly lower (fir, oak) than those of the recent woods. For L-swelling, a reduction was measured for aged oak in contrast to recent wood, whereas the values for recent and aged spruce and fir are similar. Higher swelling values in the R- and T-directions were determined for aged spruce due to its higher density, which highly influenced the swelling behaviour [25]. Fig. 2a shows the R- and T-swelling of this species depending on density and separated by sample origin. Due to high variation within and between the different origins, no age influence could be located. However, the T-swelling is generally lower compared with the data of Sonderegger et al. [25]. In contrast, a clear reduction of the R- and T-swelling was measured for aged fir, which induced an improvement of dimensional stability. This is mainly attributed to the values of the two oldest origins ‘Greiz’ and ‘Langhennersdorf’ (Fig. 2b). For aged oak, a reduction in the T-direction was registered, which originates from values of ‘Langhennersdorf’ (Fig. 2c), whereas the slight increase in the R-direction can be explained by the higher density of the aged samples.
(6)
where w is the impact bending strength, W the work required for fracture of the specimen, b the width and h the thickness of the specimen. 4.5. Fracture toughness Fracture toughness was determined in mode I tension according to DIN EN ISO 12737 [23] using the MT300 Deben Microtest Stage Controller (Deben UK Ltd, Suffolk, UK) at RL, RT, TL and TR directions (first index = direction normal to the crack plane, second index = direction of crack propagation). The specimen size is shown
5.2. Colour changes A variance analysis of the colour behaviour between recent and aged samples showed a significant difference at a confidence level of 95% for L*, a* and b* by all species and for all samples (for more details, see [26]). Fig. 3 shows the colour changes according to wood age. The trends regarding spruce and fir show similar gradients at all coordinates and are in accordance with earlier observations [27]. The difference observed between aged and recent oak samples is remarkably smaller than those of the softwoods. However, the trend of the changes is in agreement with results that were obtained in simulated indoor sunlight exposure and in lightfastness
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Table 2 Differential swelling coefficients (q) in the radial (r), tangential (t) and longitudinal (l) directions and moisture content (u) at different relative humidities (RH) of recent and aged wood.
Spruce Fir Oak
Age [years]
n
0 [g cm−3 ]
Recent 90–320 Recent 120–470 Recent 210–470
18 (10) 72 (30) 18 (10) 65 (20) 18 (12) 28 (8)
0.402 0.445 0.434 0.431 0.651 0.668
± ± ± ± ± ±
0.044 0.043 0.041 0.023 0.033 0.064
qr [%/%] 0.154 0.191 0.178 0.145 0.189 0.205
± ± ± ± ± ±
qt [%/%] 0.016 0.027 0.032 0.038 0.007 0.015
0.317 0.338 0.374 0.324 0.378 0.345
± ± ± ± ± ±
ql [%/%] 0.019 0.050 0.034 0.056 0.054 0.014
0.010 0.010 0.011 0.011 0.014 0.009
± ± ± ± ± ±
u35 [%] 0.003 0.004 0.002 0.003 0.003 0.003
6.3 6.5 6.4 6.1 6.7 6.0
± ± ± ± ± ±
0.2 0.6 1.0 0.6 1.2 1.1
u50 [%] 8.0 8.2 8.1 7.7 8.6 7.8
± ± ± ± ± ±
0.5 0.8 1.5 0.9 1.2 1.2
u65 [%] 10.4 10.6 10.6 10.1 10.9 10.1
± ± ± ± ± ±
u80 [%] 0.2 0.5 1.3 0.6 0.9 0.9
13.8 13.9 14.1 13.6 14.2 13.4
± ± ± ± ± ±
u93 [%] 0.7 0.8 0.7 0.8 1.1 0.4
19.4 19.1 19.5 18.9 19.3 18.7
± ± ± ± ± ±
0.9 1.6 0.4 1.6 0.6 0.7
0 : oven dry density; index of u: RH in percent; n: number of specimens (n for ql in parentheses); values with standard deviation.
tests, respectively [4,27]. Changes observed are similar to those that occurred during accelerated aging or at the beginning of thermal treatment [5,28–30]. This, and the fact that the whole cross section was affected, support the theory of Matsuo et al. [3], who claim that colour changes during aging are a consequence of a slow and mild oxidation process.
Fig. 2. Differential swelling of recent and aged wood depending on density and according to their origin and wood age. R-swelling: large symbols, T-swelling: small symbols; a: spruce (linear trends according to [25], R: drawn out, T: dashed); b: fir; c: oak.
5.3. Bending tests Table 3 shows the results of the bending tests. No trend could be found between the recent and aged specimens for MOR as well as the static and dynamic MOE, which agrees with divers data from literature [7,14]. Whereas for spruce, the mean values of the aged specimens were higher and a contradictory behaviour was shown
Fig. 3. Colour changes according to wood age and corresponding trend lines (spruce: solid line, fir: dashed, quercus: dotted); a: lightness (L*); b: redness (a*); c: yellowness (b*).
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Table 3 MOE determined by ultrasound (EUS ), eigenfrequency (EEf ) and by bending (Eb ) and MOR by bending ( b ).
Spruce Fir
Age [years]
n
[g cm−3 ]
Recent 120–150 Recent 120–150
45 44 27 14
0.411 0.428 0.475 0.441
± ± ± ±
EUS [MPa]
0.022 0.033 0.023 0.025
12,800 14,700 16,300 14,800
± ± ± ±
EEf [MPa] 1840 1650 1380 1090
11,000 12,700 13,700 12,800
± ± ± ±
b [MPa]
Eb [MPa] 1570 1490 1090 1030
11,000 11,900 14,800 13,900
± ± ± ±
2170 1500 1660 2100
75.6 81.2 86.6 76.9
± ± ± ±
9.5 9.6 8.9 16.0
n: number of specimens; : density; values with standard deviation.
Table 4 Impact bending strength (w) by standard tests according to DIN 52189 [21] and MOE determined by ultrasound (EUS ) and eigenfrequency (EEf ).
Spruce Fir Oak
Age [years]
n [–]
[g cm−3 ]
Recent 120–150 Recent 120–150 Recent 210
41 52 35 15 21 16
0.408 0.428 0.484 0.443 0.668 0.719
± ± ± ± ± ±
MC [%] 12.4 12.2 13.2 12.5 15.8 13.4
0.024 0.043 0.033 0.027 0.029 0.025
± ± ± ± ± ±
EUS [MPa] 0.4 0.5 0.3 0.4 0.4 1.3
12,400 14,400 16,200 14,200 10,600 13,700
± ± ± ± ± ±
w [kJ m−2 ]
EEf [MPa] 1790 1890 1660 1520 1950 1360
10,500 12,000 12,900 11,300 8600 11,200
± ± ± ± ± ±
1530 1670 1260 1240 1570 1240
36.9 35.2 41.2 38.6 35.7 32.2
± ± ± ± ± ±
10.7 11.3 6.8 9.3 13.0 12.2
n: number of specimens; : density; values with standard deviation.
for aged fir wood. This is mainly due to the different densities and is unlikely due to an aging effect. In contrast, Kránitz et al. [15] determined higher MOE values from ultrasound measurements for aged wood in the L-direction (tests on structural timber and clear specimens) and in the R-direction (tests on clear specimens).
5.4. Impact bending The results of the standard impact bending tests are shown in Table 4. The impact bending strength was higher for all recent woods compared with the aged woods, which coincides with the tendencies of previous studies [7,12,14]. Only fir exhibited density as the main influencing factor for this trend, rather than the aging effect. For oak, a clearly higher MC was shown for the recent wood compared with the aged wood. However, according to Koch [31], no significant differences of impact bending strength could be measured between normally conditioned and green samples. Therefore, the influence of MC can be neglected. Although the impact bending strengths of the recent spruce and oak woods were higher than their aged wood counterparts, their dynamic MOE was clearly lower. This contradictory behaviour was illustrated in Fig. 4 (dependent on density, which strengthens the effect). It can be explained probably by microcracks in the aged wood, which have a much greater influence on the impact bending strength than on the dynamic MOE [12,32]. The values of the Dynstat tests are shown in Table 5 and were clearly lower than the values of the standard tests. These differences in the values of the tests correspond with measurements by Bäucker et al. [33]. A similar aging effect to the standard tests was only found for oak.
5.5. Fracture toughness Table 6 shows the fracture toughness KIC of the different loads. The values of the two softwoods, spruce and fir, are similar in all directions. For both species, the aged samples showed a clear reduction of the KIC value only in the RT direction. The fracture toughness was in the same range as by Märki et al. and by Reiterer et al. [34,35], but was clearly higher than that of Keunecke et al. [36]. The oak values were more than twice as high compared with those of the
Table 5 Impact bending strength (w) determined by the Dynstat device according to DIN 53435 [22]. Age [years] Spruce Fir Oak
Recent 120–150 Recent 120 Recent 210
n [–] 110 114 42 65 42 35
[g cm−3 ] 0.414 0.443 0.506 0.476 0.681 0.733
± ± ± ± ± ±
0.020 0.041 0.016 0.021 0.028 0.024
n: number of specimens; : density; values with standard deviation.
w [kJ m−2 ] 16.4 23.8 32.6 27.5 14.1 12.9
± ± ± ± ± ±
4.9 4.2 2.9 2.9 6.2 3.7
Fig. 4. a: impact bending strength depending on density; b: MOE depending on density, determined by ultrasound (EUS ) prior to the impact bending test. The regression curves pass through the zero-point: dashed lines: recent wood; continuous lines: aged wood; bold lines: fir samples.
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Table 6 Fracture toughness KIC at RL, RT, TL and TR directions.
Spruce Fir Oak
Age [years]
[g cm−3 ]
Recent 120–150 Recent 120–150 Recent 210
0.419 0.411 0.418 0.418 0.561 0.653
± ± ± ± ± ±
0.035 0.018 0.010 0.016 0.073 0.025
KIC [MPa m0.5 ] n
TR
15 44 17 16 13 14
0.273 0.258 0.292 0.307 0.601 0.593
± ± ± ± ± ±
0.049 0.030 0.026 0.074 0.066 0.042
n
RT
15 38 24 30 6 12
0.223 0.230 0.220 0.206 0.753 0.735
± ± ± ± ± ±
0.032 0.038 0.022 0.024 0.152 0.118
n
TL
38 57 28 20 28 12
0.362 0.326 0.292 0.296 0.765 0.673
± ± ± ± ± ±
0.071 0.050 0.036 0.039 0.162 0.111
n
RL
34 36 27 26 28 16
0.418 0.343 0.408 0.337 0.923 0.897
± ± ± ± ± ±
0.048 0.063 0.064 0.070 0.165 0.121
n: number of specimens; : density; values with standard deviation.
investigated softwoods and laid between values of beech [37] and sycamore maple [38]. Although the density of the aged oak wood was clearly higher than that of the recent wood, the KIC values were lower in all load directions. To verify whether this is attributable to an aging effect or to variations within the species needs further investigations. 6. Conclusion It could be shown that notable outcomes of aging are colour modification and a reduction of impact bending strength, whereas sorption and swelling as well as bending and fracture toughness do not, or only partly, alter as a result of aging. One problem was the high variation of the physical and mechanical properties within a species, which superimpose some possible aging effects. Therefore, more investigations are necessary. However, a better understanding of the aging process contributes to the conservation of wooden cultural heritage and, through promotion of recycling and longterm applications of wood, to the preservation of the environment as well. Acknowledgement Thanks go to the ETH Zurich for its financial support (ETHIRA grant, 2009-2013). Special thanks go to Dr. Björn Günther (Dresden University of Technology) for dating the aged samples, to Dr. Kilian Anheuser (Musée d’ethnographie de Genève) for his scientific support and to Nägeli AG (Gais, Switzerland) and Chaletbau Matti AG (Saanen, Switzerland) for providing aged wood. References [1] A. Unger, A.P. Schniewind, W. Unger, Conservation of wood artifacts, Springer Verlag, Berlin Heidelberg, 2001. [2] H.T. Chang, S.T. Chang, Correlation between softwood discoloration induced by accelerated lightfastness testing and by indoor exposure, Polym. Degrad. Stabil. 72 (2001) 361–365. [3] M. Matsuo, M. Yokoyama, K. Umemura, et al., Aging of wood: analysis of color changes during natural aging and heat treatment, Holzforschung 65 (2011) 361–368. [4] J. Miklecic, A. Kasa, V. Jirous-Rajkovic, Colour changes of modified oak wood in indoor environment, Eur. J. Wood Prod. 70 (2012) 385–387. [5] P. Bekhta, P. Niemz, Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood, Holzforschung 57 (2003) 539–546. [6] J. Kohara, H. Okamoto, Studies of Japanese old timbers, Sci. Rep. Saikyo Univ. 7 (1a) (1955) 9–20. [7] H. Schulz, H. von Aufsess, T. Verron, Eigenschaften eines Fichtenbalkens aus altem Dachstuhl, Holz Roh Werkst. 42 (1984) 109. [8] D. Holz, Zum Alterungsverhalten des Werkstoffes Holz – einige Ansichten, Untersuchungen, Ergebnisse, Holztechnologie 22 (1981) 80–85. [9] D. Erhardt, M.F. Mecklenburg, C.S. Tumosa, T.M. Olstad, New versus old wood: differences and similarities in physical, mechanical, and chemical properties, in: J. Bridgeland (Ed.), International Council of Museums-Committee for Conservation 11th Triennial Meeting, James&James, London, 1996, pp. 903–910. [10] L. García Esteban, F. García Fernández, A. Guindeo, P. Palacios, J. Gril, Comparison of the hygroscopic behaviour of 205-year-old and recently cut juvenile wood from Pinus sylvestris L., Ann. For. Sci. 63 (2006) 309–317.
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