(2023) 69:16
Kameyama et al. Journal of Wood Science
https://doi.org/10.1186/s10086-023-02088-z

ORIGINAL ARTICLE

Journal of Wood Science

Open Access

Unique characteristics of residual stress
distribution of large‑diameter keyaki (Zelkova
serrata) logs and examination of theirmeasurement method
Naohisa Kameyama1,4, Miyuki Matsuo‑Ueda1,2* , Shuoye Chen1,3 , Zhuoting Jiang1 , Tatsuya Ichiyanagi4,
Masato Yoshida1   and Hiroyuki Yamamoto1*   

Abstract
Large-diameter keyaki (Zelkova serrata Makino) logs have long been used in Japan as high-quality material especiallyfor traditional construction and furniture. However, unlike other major wood species in Japan, keyaki has been practi‑
cally considered as a wood species of high difficulty when processing. Keyaki frequently shows processing defects
due to sawing, such as warping, cracking, and so forth, which often reduces the production yield of logs. Furthermore,
there are drastic differences in those behaviors between individual logs, so causing unexpected processing defects. A
scientific approach is required to improve this situation, but data on the material characteristics of keyaki were scarce.
This study aimed to characterize the radial pattern of the residual stress as one of the processing-related characteris‑tics. The measurement method was also examined. This study used diametral planks obtained from large-diameter
keyaki logs to measure released strain of residual stress from pith to the bark sides. The results showed the distribu‑
tion of released strain often showed peculiar zig-zag patterns characterized with localized residual stress, unlike thesmooth bell curve pattern seen in typical logs of other species. Because of the unique characteristics, some practicalpoints related to the measuring methodology were investigated. The extent of influence of this residual stress in thelongitudinal direction was limited to within 30 cm from the site of measurement of the strain. In addition, the lengthof log necessary to measure the released strain without being affected by crosscutting was more than four times thelog diameter—greater than the diameter ratio theorized by previous studies.
Keywords Keyaki (Zelkova serrata Makino), Large-diameter log, Released strain of residual stress, Hardwood, Sawing*Correspondence:
Miyuki Matsuo‑Uedamatsuo.miyuki.4d@kyoto-u.ac.jp
Hiroyuki Yamamoto
hiro@agr.nagoya-u.ac.jp
1
Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya,Japan
2
Research Institute for Sustainable Humanosphere, Kyoto University, Uji,Japan
3
Graduate School of Agriculture, Kyoto University, Kyoto, Japan
4
Kameyama Construction Co., Ltd., Seki, Japan

Introduction
Zelkova serrata Makino, which is also called “Japanesezelkova” or “keyaki,” has long been used as a building andfurniture material in East Asia [1, 2]. Its use as a primary
lumber began in Japan around the twelfth century. Compared to other domestic wood species in Japan, such as
Japanese cypress (Chamaecyparis obtusa Endl.) and Japanese cedar (Cryptomeria japonica D. Don) keyaki has
been preferred owing to the high strength, the ease ofobtaining large-diameter logs, and the beauty of its grain[3]. Above all, it is highly sought after in the construction of traditional temples and houses as both structural© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Kameyama et al. Journal of Wood Science(2023) 69:16

and finishing materials owing to its good strength andlarge diameter (capable of creating large spaces inside of
the constructions) and its highly decorative appearance(Fig. 1).
However, keyaki wood is susceptible to warping andcracking during sawing. This often reduces the utilization
yield of harvested logs and frequently causes unexpected
and irregular warping and cracking in the primary lumber. Furthermore, there are drastic differences in thosebehaviors between individual logs, and it is difficult toevaluate the log quality beforehand. For the moment,
only highly skilled carpenters and craft persons canevaluate the quality of keyaki wood based on traditional
experience and intuition. To maximize the use of the precious forest resource of keyaki wood, it is necessary to
generalize the method for evaluating the quality. Therefore, an approach that is based on both experience andscience is required.
Generally speaking, the growth stress that occurs
with the formation of wood cell wall causes the residual

Fig. 1 Keyaki wood used in traditional temple construction (Shogo-ji
Temple, Miyazaki, Japan). Keyaki was used for main columns, beams
with carving, and block and bracket clusters called Tokyou at the topof the columns

Page 2 of 15

stress in the log after the repetition of the secondary
growth, leading to undesirable warping and cracking
during sawing [4]. Curiously, keyaki frequently forms
tension wood within the log regardless of whether thetree grows upright or inclined, while hard wood species generally generates tension wood on the upper sideof their inclined trunk [5]. Tension wood is composedof Gelatinous (G-) fibers and has specific physical properties. It is considered that the occurrence of specificgrowth stresses at tension wood causes the disturbanceof residual stresses [6–10]. Drying shrinkage in longitudinal direction is large at tension wood, which induces
the irregular deformation [e.g., 11]. These characteristics
could possibly contribute to the individual differences of
keyaki. However, the true nature of inside characteristics
of keyaki, such as residual stress, tension wood, and complex grain [12, 13], remains unclear.
Even though keyaki wood is considered to have specificcharacteristics, there are not many studies on it possibly
because keyaki wood has been used mainly in East Asiaand its use has been owned by the traditional experienceand intuition. Researches tried to establish the strength
criterion for the effective use of keyaki wood [14–16].
However, as these studies are fragmentary data on material properties, the characteristics of keyaki wood, especially the distribution of the properties inside the logs,
were still not clear. Yashiro et al. [17] measured themechanical properties and chemical composition using
a 5-cm-thick and 54-cm-diameter disk of keyaki wood.
They defined the tension wood based on the measured
values, and the area of tension wood in the disk wasalso determined. Using a tree of keyaki, Arakawa et al.
[18] measured the surface growth stress, longitudinal
Young’s modulus, and drying shrinkage and their relationships. They revealed that G-layer contributed thespecific properties of tension wood. Further, Abe et al.
[19] investigated the effects of the proportion of G-fiber
to normal fiber on the shrinkage of specimens owing toboiling and drying. On the residual stress of keyaki logs,
Okuyama et al. [20] and Huang et al. [21] measured the
distributions of released strain of residual stress (hereafter, “released strain”) using large-diameter logs aiming to
reduce the residual stress within the logs by heat treatment. The radial distributions of released strains in thelogs, two samples from Okuyama et al. and one sample
from Huang et al. [20, 21], showed unique shapes, notseen in other species. However, they did not mention indetails on the results.
While there are some data regarding mechanical orchemical properties and how to solve material issues of
keyaki wood (heat treatment, etc.) as mentioned above,
there are few reports that comprehensively reveal thecharacteristics of keyaki wood, such as morphologicalKameyama et al. Journal of Wood Science(2023) 69:16

Page 3 of 15

structure, mechanical and chemical properties, residual
stress, and the related issues, such as abnormal shrinkage,
warping, cracking, and their variations among individuals. Due to the lack of research, there is little examination
on measurement method for residual stress. Log-scale
investigation using sufficient samples and the examination of measurement method are needed.
The final goal of this study is to scientifically characterize the various properties of keyaki wood as well as thereasons for individual differences in wood properties.
In this paper, as the first step of a series of studies, thereleased strains of residual stresses were measured using
diametral planks of large-diameter keyaki wood. The
radial patterns of the residual stress and their diversity
among individuals were characterized. In addition, measurement issues, such as length of the plank and cutting
position, were discussed to establish the accurate method
to measure the residual stresses of keyaki wood. Sixtytwo large-diameter keyaki logs over 45 cm in top diameter, which had the sufficient sizes used as the primarybuilding material in the construction of traditional temples and houses in Japan, were used as materials. With
respect to quality of logs, this study primarily used logs
with a certain degree of commercial value in the market,
called *Kenta-zai.
*Kenta-zai: Market logs with approximately 2 m inlength and at least 30 cm in top diameter. While large
in diameter and long in log length, the wood may haveknots, be curved, or be too long to log; therefore, theselogs are cut down to lengths of 2 m and sold.

Materials and methods
Materials

This study used a total of 62 keyaki (Zelkova serrata
Makino) samples that were harvested from acrossJapan: 4 standing trees and 58 logs (Fig. 2). The sample information is summarized in Table 1. Logs with a
top diameter of 45 cm or greater were selected from thelog market. These logs satisfied the quality for primarystructural materials in traditional temples and houses,and for the purpose of experimenting with large quantities. The logs were cut during the dormant stage
between autumn and spring and stored in the market
field or warehouses for approximately six months until
experiments. The moisture content of the logs wasabove a fiber saturation point during the storage andthe experiments.
Preparation of diametral planks

Diametral planks were sawn from logs using a band saw
so that the planks contained a pith and their diameter
was as large as possible (Figs. 3 and 4a). As a result, theplanks often contained tension wood when the planks
contained eccentrically grown sections. Thickness of theplanks was 5 cm in tangential direction. The planks were
sawn as long as the log had no issues with branching,cracking, or rotting. In order to ensure a uniform thickness and to smooth the measurement surface, the surfaceof the planks was finished with an electric planer. Almost
all the planks included the pith, but due to the slight pithwinding along with the longitudinal direction, the pith
was located nearby one side of the plank surfaces. The
surface where the pith was located nearby was used as
the measurement surface. The dimensions of each diametral plank after sawing are shown in Table 1. The preparation of the diametral planks and the measurements of thereleased strain were carried out on the same day. The saw
mill work was conducted at Morijitsu Seizai lumber millin Kakamigahara, Gifu prefecture.

No.6

No.5
(a) standing tree (Tochigi pref.)(b) logs at the market (Gifu pref.)Fig. 2 Example of standing trees and logs at the market used for the experiments. a Standing tree (sample No. 4.), b Logs (sample Nos. 5 and 6)

Log

66.0 / 63.0 / 64.0

IbarakiShimane

Gifu

Saitama

Saitama

Saitama

Saitama

21

23

24

25

26

27

Ibaraki

20

22

Ibaraki

Ibaraki

18

19

Ibaraki

Ibaraki

16

Ibaraki

15

17

Saitama

Saitama13

14

Saitama

Saitama11

12

Saitama

Saitama

91068.5 / 71.0 / 74.5

Saitama

Autumn 2019 toSpring 2020

– / 57.6 / –51.2 / 53.1 / 59.4

215

158

144

186

210

211

170

207

228

270

209

280

175

290

232

Approx.210

215

199

177

181

Approx.180

Approx.260

Approx.300

196

248

202

201

May 20th 2020

May 19th 2020

July 9th to 10th 2019

January 28th to 29th
2020

Plank length (cm) Date ofmeasurement

●

●

●

●

●

●
●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

Experiment Experiment Experiment Experiment ExperimentFig. 3aFig. 3bFig. 3cFig. 3d
Fig. 3e(2023) 69:16

47.7 / 47.7 / 49.1

51.5 / 49.8 / 51.5

54.0 / 54.0 / 59.5

62.1 / 59.8 / 60.4

68.0 / 75.0 / 85.0

69.0 / 61.0 / 61.5

49.4 / 47.0 / 52.0

58.4 / 54.5 / 58.3

60.9 / 62.3 / 73.0

69.0 / 71.3 / 98.2

66.5 / 64.4 / 87.0

62.3 / 64.3 / 62.4

55.0 / 60.8 / 79.5

61.0 / 65.3 / 72.6

54.1 / 62.2 / 86.0

58.5 / 58.5 / 64.5

71.0 / 72.5 / 90.0

– / 72.0 / –8March 15th 2019

Autumn 2019 toSpring 2020

Tochigi– / 71.0 / –SaitamaMid. of February 20196

Tochigi

5– / 78.0 / –– / 76.0 / –– / 89.0 / –7Tochigi

Tochigi

3

4

Tochigi

2

November 18th to20th 2019– / 62.5 / –Tochigi

Tree

1

Plank width (top/middle/bottom)(cm)

Form Sample No. Prefecture Harvesting dateharvested

Table 1 Sample information

Kameyama et al. Journal of Wood Science
Page 4 of 15

Saitama

Shimane55

Saitama52

Saitama

Saitama

51

53

Saitama

50

54

Shimane

Saitama48

49

Saitama

Saitama46

47

Saitama

Saitama43

Saitama

Saitama42

44

Toyama

41

45

Hyogo

Hyogo

39

40

Hyogo

Hyogo

37

38

IbarakiTochigi

35

Ibaraki

34

36

Ibaraki

Ibaraki

32

33

Ibaraki

Ibaraki

30

31

Saitama

Ibaraki

28

29

Autumn 2019 toSpring 2020

Form Sample No. Prefecture Harvesting dateharvested

Table 1 (continued)

64.6 / 67.3 / 79.0

234

201

202

192

323

215

173

172

193

229

402

206

216

210

294

186

230

221

233

248

201

205

261

150

164

204

194

212

June 16th to 17th
2020

Plank length (cm) Date ofmeasurement

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

Experiment Experiment Experiment Experiment ExperimentFig. 3aFig. 3bFig. 3cFig. 3d
Fig. 3e(2023) 69:16

58.0 / 59.5 / 65.6

56.2 / 59.0 / 61.4

65.6 / 63.0 / 66.0

58.0 / 54.8 / 56.6

61.5 / 66.8 / 83.6

72.0 / 78.4 / 81.0

61.5 / 63.3 / 62.6

66.0 / 68.0 / 73.5

64.3 / 70.6 / 90.5

66.6 / 68.0 / 70.0

62.6 / 66.9 / 71.0

69.0 / 71.0 / 82.6

57.8 / 57.6 / 71.0

62.9 / 62.6 / 83.8

49.0 / 49.0 / 51.1

50.6 / 50.7 / 62.2

49.8 / 52.0 / 56.3

49.4 / 47.0 / 49.7

69.1 / 68.8 / 69.2

46.8 / 47.0 / 46.8

49.5 / 51.5 / 53.1

64.0 / 63.6 / 67.5

43.4 / 43.5 / 46.0

49.0 / 49.0 / 52.5

52.5 / 51.0 / 55.0

50.1 / 50.2 / 50.1

48.2 / 46.6 / 48.9

Plank width (top/middle/bottom)(cm)

Kameyama et al. Journal of Wood Science
Page 5 of 15

Gifu

Hyogo

Shimane

Fukui

59

60

61

62

Ibaraki

Chiba

57

58

Ibaraki56

Spring 2019

Form Sample No. Prefecture Harvesting dateharvested

Table 1 (continued)

44.6 / 45.7 / 47.0

66.3 / 65.3 / 73.8

59.6 / 61.4 / 66.5

49.5 / 54.1 / 66.0

61.0 / 59.0 / 66.8

66.0 / 67.1 / 73.5

70.2 / 68.5 / 74.6

Plank width (top/middle/bottom)
(cm)

216

202

214

314

208

206

187

Plank length (cm) Date ofmeasurement

●

●

●

●

●

●
●

●

●

●

●

●

●

●

Experiment Experiment Experiment Experiment ExperimentFig. 3aFig. 3bFig. 3cFig. 3d
Fig. 3eKameyama et al. Journal of Wood Science(2023) 69:16
Page 6 of 15

Kameyama et al. Journal of Wood Science(2023) 69:16

Page 7 of 15

No. 1

bottomdiametral plank0(a) 40 samples(b) 15 samples

Strain-gauges were pasted at theStrain-gauges were pasted at thecenter of plank; the gauge-edge

center of plank; both ends of theline (approximately 1 cm from theplank were repeatedly crosscut

gauge edges) was crosscut, then

10 cm, and released strains werereleased strain was measured.

measured at each crosscut.

20 cm(c) 6 samples(d) 1 sample(e) 21 samples

Strain-gauges were pasted at

Strain-gauges were pasted at theAfter the gauge-edge line wasthe center of plank and 50 cm

center of plank and 30 and 60 cm fromcrosscut, the areas between thefrom the center toward boththe center toward both ends; bothstrain-gauges were also cut along

ends; both ends of the plankends of the plank were repeatedly

the grain. Then released strain was

were repeatedly crosscut 10

crosscut 10 cm, and released strains

measured.

cm, and released strains were

were measured at each crosscut.

measured at each crosscut.

Fig. 3 Production procedure of diametral planks and five measurement patterns of released strain

Measurement of released strain

For all 62 samples, five measurement patterns, (a)–(e) in
Fig. 3, were tried for each objective.
For 40 samples, a strain measurement position was
set at the center of diametral plank, where strain gauges
were pasted at 2-cm intervals from pith to each bark side
(Fig. 3a). Crosscuts were made approximately 1 cm fromboth edges of the strain gauges (hereafter, “gauge-edgeline”), and the released strain in the longitudinal direction was measured.
For 15 samples, the strain measurement position
was set at the center of diametral plank (Fig. 3b). Bothends of the plank were repeatedly crosscut by 10 cm
in length until approaching the gauge-edge line of theKameyama et al. Journal of Wood Science(2023) 69:16

Page 8 of 15(a) Saw-mill work(d) Crosscutting a gauge-edge line(b) After pasting strain-gauges(c) Strain-gauges connected to data loggers(e) Cutting between strain-gauges

Fig. 4 Sawing process of diametral planks and measurement of released strainstrain measurement position, and released strains were
measured at each crosscutting. The relationship between
the released strain and the distance from the strain measurement position to the crosscut end was examined.
For six samples, the strain measurement position was
set at the center of diametral plank and 50 cm fromthe center toward both ends, where strain gauges were
pasted at 2-cm intervals from the pith to each bark side
(Fig. 3c). Gauge-edge lines of strain measurement positions were crosscuts, and the released strains werecompared between the different strain measurement
positions in the same diametral plank.

Kameyama et al. Journal of Wood Science(2023) 69:16

For one sample, five strain measurement positions wereset at the center of diametral plank, and at 30 and 60 cm
from the center toward both ends. At each measurement
position, 3 strain gauges were pasted at 2-cm intervals
around the pith and other strain gauges were pasted at
4-cm intervals toward each bark side (Fig. 3d).
For 21 samples from Fig. 3a and b, after the gauge-edgelines were crosscut, the areas between the strain gauges
were also cut along the grain and measured to examine
the effect on strain values by releasing the residual stressin the radial direction (Fig. 3e).
For all the patterns, the released strain was measured
as follows:At each strain measurement point, strain gauges(Kyowa Electronic Instruments KFGS-10–120-C1-11
L3M3R and L5M3R, three-wired, gauge length of 10 mm,
cable lengths of 3 or 5 m) were pasted using cyanoacrylate quick-setting glue (Kyowa Electronic Instruments
CC-33A) along the longitudinal direction (Fig. 4b). Strain
gauges were connected to data loggers (Kyowa Electronic
Instruments UCAM-65C-AC, EDX-10B, EDX-14A, DBS120B-8), and initial strain values were measured (Fig. 4c).
All types of crosscuts were made using an electric circular saw. Finally, crosscuts were made at the gauge-edgelines of the strain measurement positions (Fig. 4d). For
pattern Fig. 3e, after crosscutting the gauge-edge lines,
an electric reciprocating saw was used to cut between

Page 9 of 15

adjacent strain gauges with the depth of approximately
1.0–1.5 cm along the grain (Fig. 4e).

Results and discussion
Characteristics of residual stressCharacteristics of radial distributions of released strains
in logs

Figures 5 shows examples of the distribution of released
strain from the pith to both bark sides, measured in thecenter of each log. Other data of all logs are shown insupplementary information. The horizontal axis represents the distance of the strain measurement point fromthe pith (cm), with the pith is set at ± 0; the vertical axis
represents released strain (microstrain) (hereafter, “μs”).
Released strain is represented as positive with elongation
owing to compressive stress release and negative with
contraction owing to tensile stress release.
Generally speaking [22, 23], the distribution of released
strain in a typical upright tree has a smooth bell shape(Fig. 5, Japanese cedar, “Smooth (No spike) and Bellshape” [24]), with the compressive stress reaching amaximum near the pith and mechanical neutrality (zero)at a distance of approximately two-thirds of the radiusfrom the pith. The tensile stress increases approaching
the bark. For the plank, the mechanical neutrality (zero)
becomes at a distance of approximately one-third to halfof the radius from the pith because residual stress in theFig. 5 Typical distribution of released stain in an upright tree of Japanese cedar (Cryptomeria japonica D. Don) [24]) and four representative
types of distribution pattern of keyaki wood measured in this study (sample Nos. 1, 5, 8, and 23). Plus and minus signs of the horizontal axis wereoccasionally given depending on the experimental situation

Kameyama et al. Journal of Wood Science(2023) 69:16

log would release slightly when sawing [25]. On the other
hand, the distribution of released strain in keyaki exhibited various patterns, following not to a smooth curve,
but rather a zig-zag pattern with spike-shaped released
strains (hereafter, ‘spike-shaped strain”), indicating generation of large, localized stresses (hereafter, “spikeshaped stress”) (Fig. 5, sample Nos. 1, 5, 8, and 23). Thispattern has not been reported with Japanese cedar, larch(Larix sp.) and Japanese cypress [20, 22, 24, 26] as wellas buna (Fagus crenata), teak (Tectona grandis), mangium (Acacia mangium), and falcata (Paraserianthes falcataria) which were hardwoods [27–31], more difficultfor processing than softwoods. This spike-shaped strain
was considered to be distinguishing characteristics of
keyaki wood.
Focusing on the overall shape of the distribution of
released strain and the appearance of spike-shaped
strain, four representative types were extracted from theresults (Fig. 5 and Additional file 1: Fig. S1). The first type
was that the overall shape was bell shaped and the spikeshaped strains appeared entirely over radial direction(Fig. 5 No.5). The second type also has a bell-shaped distribution but its spike-shaped strains appeared partially(Fig. 5 No.1). Other two types were not bell shaped andclassified to the third type with the spike-shaped strainsthat appeared entirely (Fig. 5 No.23) and to the fourth
type with the spike-shaped strains that appeared partially(Fig. 5 No.8). Some samples were intermediate between
these types. This classification of the spike-shaped strain
would be useful for discussing the variation among individuals and among the tree origins. To investigate therelationship between the distribution patterns and thetree origins, all the distributions were classified based on
the prefecture harvested (Additional file 1: Fig. S2). The

Page 10 of 15

relationship was not so clear; however, it would be worth
continuing the discussion. Spike-shaped strain would bealso related to heterogeneous distribution of morphological structure or physical properties. Further observation
and analysis of factors, such as local tension wood, interlocked grain, Young’s modulus, annual ring width, andmicrofibril angle, were required.
Extent of the influence of spike‑shaped stressin the longitudinal direction

For measurement patterns Fig. 3b and c, both ends ofthe plank were repeatedly crosscut by 10 cm in length
until approaching the gauge-edge line of the strain measurement position with strain values recorded at every
crosscut. Based on the changes in the strain values, it
was discussed how far the effects of locally generated
spike-shaped stresses reached in the longitudinal direction (Fig. 6). When the crosscut position has a sufficientdistance from the strain gauges, the distribution of strain
values assumed a smooth arched shape, with the absolute value of strain gradually increasing the closer the
cutting position was to the gauge. However, when the
distance from the gauge became less than 30 cm, spikeshaped strain began to appear, and after crosscuts weremade at the gauge-edge lines, the spike shape completely
appeared. The spike shape first appeared at 30 cm in two
samples, 20 cm in four samples, 10 cm in 11 samples, and1 cm (at the gauge-edge lines) in three samples. For one
sample, the spike shape first appeared at a large distanceof 60 cm.
Based on the above, spike-shaped strain appearedin locations close to strain measurement points. The
extent of influence of spike-shaped stress in the longitudinal direction, which was the cause of the spike-shaped

Fig. 6 Typical examples showing longitudinal range of influence of spike-shaped stress. (Legends indicate the distance between the crosscutting
position and the strain gauges.)Kameyama et al. Journal of Wood Science(2023) 69:16

Page 11 of 15

strain, could be limited to the area surrounding thestrain measurement points. The extent of the effect of
the spike-shaped stress would be related to the peculiar
characteristics of tension wood. Further investigation is
expected to reveal the reasons.

confirmed [e.g., 14]. As there are few reports about theradial distribution of Young’s modulus of large-diameter
keyaki log, the measurement of Young’s modulus andsubsequent analysis will be necessary to discuss thisissue.

Difference of released strain distribution among different
longitudinal positions in the same log

Required log length for measuring released strainof the residual stress

The results of measurement patterns Fig. 3c and drevealed the differences in released strain distribution at
different longitudinal positions in the same plank (Fig. 7).
As indicated in each graph in Fig. 7, the distribution
pattern “without spike” or “with spike,” respectively,
appeared in the similar radial area regardless of the longitudinal positions. Namely, the radial range where thespike-shaped strain appeared were always similar at different longitudinal positions, and the radial range where
the spike-shaped strain “did not” appear were also similar.
Within the range with spike-shaped strain, each measurement point where the spike-shaped strain appeared
did not correspond completely at different longitudinal
positions.
Based on the above, it was found that the radial pattern of residual stress in the same log was similar along
longitudinal direction in the trunk, although it was notmatched completely when focusing each local measurement point.

Residual stress is completely released near the crosscut
end of the log, and the residual stress distribution is disturbed at a position apart from the crosscut end in a certain distance: this is called “end effect” [23, 35]. Therefore,
it is necessary to consider the length of the log required
to measure the released strain of the residual stress without being affected by the “end effect.”
Wilhelmy and Kübler [36, 37] not only predicted thechange of distribution of the residual stress in a log due
to crosscut using the finite element method (FEM) but
also tried to verify obtained predictions based on an
experiment using red oak (Quercus rubra L.). They introduced a parameter “L/D ratio” (= L divided by D), where
L is the distance from the crosscut end to the measurement position of the residual stress; D is the log diameter. They theoretically predicted that if the L/D ratio
exceeded 1.3, the crosscut did not disturb the value of theresidual stress. However, they also reported that based on
the experiment an L/D ratio of approximately 2 was necessary to detect the residual stress without the end effect.
Thereafter, Kübler and Chen [38] improved their calculation using FEM and finally concluded that an L/D ratio of
approx. 1.5 was required.
In the present study, logs with various lengths anddiameters were used, and the radial distribution of
released strain of the residual stress was measured in thecenter of each log. It was considered that residual stressmight have been partially released in some logs when thetree trunks were felled and logged. Then, the measurement patterns Fig. 3b and c were examined to determine
the minimum log length that allowed the measurement
without end effect.
Figure 9b shows the relationships between strain values
and the L/D ratios, provided that the strain value before
crosscut is set as 0. Measurements of strains were taken
at each 10-cm crosscut at the selected five measurement
points indicated in Fig. 9a [on the pith, bark sides (+, −),and the midpoint between them (+, −)]. The strain began
to change obviously when the crosscut was made at a certain distance to the position of the strain gauges. In thepresent study, this position of L/D ratio was defined as"change point" (shown by a dashed line in Fig. 9b) and
was detected for each sample. The relationship between
the change point and the value of the initial L/D ratio,
which is calculated as half of the plank length divided by

Methodology for residual stress measurement
Impact of between‑gauge cuts on released strain

The results of measurement pattern Fig. 3e demonstrated
that strain values slightly increased toward the compressed side in 10 of the 21 samples (Fig. 8a), with almost
no change observed in strain values in the other 11
samples (Fig. 8b). In nine of the former 10 samples thatshowed an increase, values increased especially near thepith; in only one sample values increased throughout theradial distribution. In addition, the maximum strain value
after the increase was 6145 μs (sample No. 49), followedby 4448 μs (No. 54)— only these two samples showed
extremely high values (Fig. 8c). Except for these two
samples, as no or slight change in strain was observed inmost of the planks, it was reasonable to assume that thereleased strain could be measured with almost complete
accuracy when crosscuts were made at the gauge-edgelines. Other data from 21 samples are shown in the Additional file 1: Fig. S3.
The reasons for which strain values increased near thepith may be deduced by the low longitudinal Young’s
modulus in the area around the pith. For example, theYoung’s modulus in Japanese cedar logs was lower near
the pith than near the bark [32–34]. However, for keyaki,
a clear decrease in Young’s modulus near the pith was notKameyama et al. Journal of Wood Science(2023) 69:16

Fig. 7 Released strain distribution among different longitudinal positions in the same log. For sample No. 3, radial ranges with/without
spike-shaped strain are indicatedPage 12 of 15

Kameyama et al. Journal of Wood Science(2023) 69:16(a) Strain increased near the pith
(10 of 21 samples)

Page 13 of 15(b) Strain did not change(11 of 21 samples)(c) Extremely high strain
increased near the pithFig. 8 Changes in strain due to cutting between the strain gauges. (It is noted that the maximum values of the vertical axis are not the same)

Fig. 9 Experiment to determine the minimum log length that allowed the measurement without end effect. a Strain measurement points on thepith, the outermost part of xylem, and the midpoints between them whose strain values were followed by 10-cm crosscutting (e.g., sample No. 5). b
Change of strain values at the five strain measurement points. Change point: L/D ratio of crosscut position where the strain values began to change
obviously

log diameter before crosscut, is shown in Fig. 10. The theoretically suggested “change point” value of 1.3 and 1.5
and an experimentally suggested “change point” value of
2.0 are also shown in Fig. 10 [36–38].
In the sample of which initial L/D ratio of the plank was
smaller than 2.0, the correlation between the initial L/D
ratio (x) and change point (y) theoretically should follow
a straight line of y = x; however, measured change point
was slightly smaller than the initial L/D ratio as shownin Fig. 10. This suggests that in the sample with an initial L/D ratio smaller than 2.0, some amount of residual
stress had been already released when the tree trunk
was logged. It was considered that in such a sample, thechange point must coincide with the initial L/D ratio;
however, the measured change point was underestimatedby the first 10 cm crosscut.
In the sample of which initial L/D ratio was larger than
2.0, change point was between 1.5 and 2.0 regardless of
the initial L/D ratio. The maximum value of change point

seemed to converge to 2.0, suggesting that the log length
is required to be larger than 4D to measure the released
strain of the residual stress without end effect. These
results could be understood by Saint–Venant’s principle[39]. Considering Saint–Venant’s principle, if a mechanically active position (i.e., crosscut end) is enough apart ata certain distance from the position of interest (i.e., theposition of strain measurement), there is little mechanical effect on the position of interest. The samples in thisstudy could be said to follow this theory and the certain
distance was around y = 2.0.
Even when initial L/D ratio was similar (i.e., L/D ratiosof 1.61–1.63 for four samples), there was a variation in
the values of the change point. This possibly came fromindividual differences, such as the tension wood andinterlocked grain.
Based on the above, it was clear that the length of thelog required to measure the released strain without end
effect was at least four times the diameter; L/D ratio wasKameyama et al. Journal of Wood Science(2023) 69:16

Page 14 of 15

Fig. 10 Relationship between the change point (y) and the value of initial L/D ratio (x) and the comparison with the thresholds of previous
researches [36–39]. Initial L/D was calculated as half of the plank length divided by log diameter2.0. This agreed with the experimental results of red oakreported by Wilhelmy and Kübler [37].

Conclusion
The goal of this paper was to understand the characteristics of residual stress in large-diameter keyaki logs, whichhave so far been poorly understood. The measurement
method for keyaki wood was also examined. The released
strain of 62 sample logs from the pith to the bark sides
was measured using diametral planks with pith. The variation in strain distributions of released strain revealedin this paper provided some insight into the nature of
keyaki wood which were said to vary widely between
individuals.
The detailed conclusions were as follows:
One of the major characteristics was generationof the spike-shaped stress, which had never beenreported in other wood species.
Pattern of radial distribution of released strain was
uniform in the same plank regardless of the positions relative to the longitudinal direction.
The length of the log required to measure thereleased strain without being affected by the end
effect was at least four times of the diameter.
In the future, it will be necessary to quantitatively characterize spike-shaped stress and to examine the origins
of spike-shaped stress correlated with factors such as
local tension wood, interlocked grain, Young’s modulus,
annual ring width, microfibril angle, and so forth.

Abbreviations
μs		Microstrain
FEM		Finite element method
L/D ratio		L Divided by D, where L is the distance from the cross‑cut end to the measurement position of the residual
stress; D is the log diameter
Initial L/D ratio	Initial L divided by D, which is calculated as half ofthe plank length divided by log diameter D before
crosscutSupplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s10086-​023-​02088-z.
Additional file 1: Fig. S1. Results of the distribution of released strain
obtained from all samples.The results were classified based on the 4 typesof distribution patterns. Fig. S2. Results of the distribution of released
strain obtained from all samples.The results were classified by the prefec‑
ture of tree origin. Fig. S3. Changes in strain due to cutting between thestrain gauges.
Acknowledgements
This work was supported by Japan Society for the Promotion of Science
KAKENHI Grant Number JP19H05360. The authors are grateful to KAMEYAMACONSTRUCTION Co., Ltd. for their support. The authors would like to thank Mr.
Daizo Ishii for giving valuable knowledge on keyaki wood.
Author contributions
NK planned this study, conducted the experiment, analyzed the data, andwrote the manuscript. MM-U co-supervised this study, conducted the experi‑ment, analyzed the data, and revised the manuscript. SC, JZ, and TI conducted
the experiment and analyzed the data. MY co-supervised this study andprovided the specific advices to this study. HY co-supervised this study,
conducted the experiment, analyzed the data, and revised the manuscript. All
authors read and approved the final manuscript.
Funding
This study was funded with JSPS KAKENHI Grant Number JP19H05360 (Grantin-Aid for Scientific Research on Innovative Areas, Elucidation of the strategies
of mechanical optimization in plants toward the establishment of the bases
for sustainable structure system).

Kameyama et al. Journal of Wood Science(2023) 69:16

Availability of data and materials
The datasets used and/or analyzed during the current study are available fromthe corresponding author on reasonable request.

Declarations
Competing interests
The authors declare that they have no competing interests.
Received: 10 August 2022 Accepted: 10 February 2023

References
1. Ihara K (1989) Wood species and the positions where they are used inJapanese historical houses from the middle to modern ages. Sci Conserv
28:25–62 (In Japanese)
2. Ishimaru S, Ishimura S (2003) Studies of the effects of Chinese and Koreanclassic furniture styles in Japan –focusing on the Zelkova used for furni‑
ture (1). Bull Jpn Soc Sci Design 50(4):31–40 (In Japanese with English
summary)
3. Arioka T (2016) Keyaki. Hosei University Press, Tokyo (In Japanese)
4. Sasaki Y, Okuyama T, Kikata Y (1978) The evolution process of the growthstress in the tree. -the surface stresses on the tree. Mokuzai Gakkaishi
24:149–157
5. Onaka F (1949) Studies on compression and tension wood. Mokuzai
Kenkyu 1:1–88 (In Japanese)
6. Norberg PH, Meier H (1966) Physical and chemical properties of thegelatinous layer in tension wood fibres of aspen (Populus tremula L.).
Holzforschung 20:174–178
7. Okuyama T, Yamamoto H, Iguchi M, Yoshida M (1990) Generation processof growth stresses in cell walls II. Mokuzai Gakkaishi 36:797–803
8. Okuyama T, Yamamoto H, Yoshida M, Hattori Y, Archer RR (1994) Growthstresses in tension wood: role of microfibrils and lignification. Ann For Sci
51:291–300
9. Yamamoto H, Okuyama T, Sugiyama K, Yoshida M (1992) Genera‑tion process of growth stresses in cell walls IV. -action of the cellulose
microfibril upon the generation of the tensile stresses. Mokuzai Gakkaishi
38:107–113
10. Romberger A, Hejnowicz Z, Hill JF (1993) Plant structure: function andDevelopment: a treatise on anatomy and vegetative development, withspecial reference to woody plants. Springer-Verlag, New York, pp 155–156
11. Vilkovská T, Klement I, Čunderlík I (2019) Longitudinal contraction of reac‑tion and opposite wood during drying process. Wood Res 64:903–912
12. Johansson M, Ormarsson S (2009) Influence of growth stresses and mate‑rial properties on distortion of sawn timber–numerical investigation. Ann
For Sci 66:604
13. Ormarsson S, Dahlblom O, Johansson M (2009) Finite element study ofgrowth stress formation in wood and related distortion of sawn timber.
Wood Sci Technol 43(3):87–403
14. Hashizume H, Furukawa I, Sakuno T, Omori H (1987) On the utilizing vol‑ume and wood quality of Zelkova serrata Makino. Hardwood Res 4:49–59(In Japanese with English abstract)
15. Furukawa I, Fukutani S, Kishimoto J (1989) Characteristics of the variationof wood quality within keyaki (Zelkova serrata) trees -horizontal variationsof ring width, fiber length, vessel element length, specific gravity and lon‑
gitudinal compression strength. Hardwood Res 5:197–206 (In Japanesewith English abstract)
16. Ido H, Nagao H, Kato H (2012) Bending strength and compressive
strength parallel to the grain of Keyaki (Zelkova serrata) timber. Mokuzai
Gakkaishi 58:144–152 (In Japanese with English abstract)
17. Yashiro M, Igarashi Y (2000) Distribution of some physical, chemical andmechanical properties in a cross-section of a large zelkova tree (Zelkova
serrata Makino) and occurrences of tension wood and opposite wood.
Res Bull Niigata Univ For 33:19–39
18. Arakawa Y, Abe K, Yamamoto H, Okuyama T, Suzuki T (2003) Study on thephysical and functions of the gelatinous layer in tension wood of Zelkova
serrata Makino. Nagoya Univ For Sci 22:13–18

Page 15 of 15

19. Abe K, Yamamoto H (2007) The influences of boiling and drying treat‑ment on the behavior of tension wood with gelatinous layers in Zelkova
serrata. J Wood Sci 53:5–10
20. Okuyama T, Kanagawa Y, Hattori Y (1987) Reduction of residual stresses inlogs by direct heating method. Mokuzai Gakkaishi 33:837–843
21. Huang Y, Chen S, Kuo-Huang L, Lee M (2005) Growth stress of Zelkova
serrata and its reduction by heat treatment. For Prod J 55(5):88–93
22. Watanabe H (1967) A study of the origin of longitudinal growth stresses
in tree stems. Bull Kyushu Univ For 41:169–176 (In Japanese with English abstract)
23. Archer RR (1986) Growth stresses and strains in trees. Springer-Verlag,
New York
24. Matsuo-Ueda M, Tsunezumi T, Jiang Z, Yoshida M, Yamashita K, Matsuda
Y, Matsumura Y, Ikami Y, Yamamoto H (2022) Comprehensive study of
distributions of residual stress and Young’s modulus in large-diametersugi (Cryptomeria japonica) log. Wood Sci Tech 56:573–588
25. Bandyopadhyay N, Archer RR (1979) Relief of growth stresses in planks.
Holzforschung 33:43–46
26. Tejada A, Okuyama T, Yamamoto H, Yoshida M (1997) Reduction of
growth stress in logs by direct heat treatment. Assessment of a commer‑
cial-scale operation. For Prod J 47:86–93
27. Gril J, Jullien D, Bardet S, Yamamoto H (2017) Tree growth stress andrelated problems. J Wood Sci 63:411–432
28. Okuyama T, Yamamoto H (1992) The residual stresses in living tree. In:
Fujiwara H, Tabe T, Tanaka K (eds) Residual stresses- III. Elsevier, Barking, pp
128–133
29. Wahyudi I, Okuyama T, Sudo-Hadi Y, Yamamoto H, Watanabe H, Yoshida M(2001) Relationship between released strain and growth rate in 39 yearold Tectona grandis planted in Indnesia. Holzforschung 55:63–66
30. Wahyudi I, Okuyama T, Sudo-Hadi Y, Yamamoto H, Yoshida M, Watan‑abe H (1999) Growth stress and strains in Acacia mangium. For Prod J
49(2):77–81
31. Wahyudi I, Okuyama T, Sudo-Hadi Y, Yamamoto H, Yoshida M, Watanabe H(2000) Relationship between growth rate and growth stresses in Paraserianthes falcataria grown in Indonesia. J Trop For Prod 6:95–105
32. Watanabe H, Tsutsumi J, Matsumoto T, Ohta S (1964) Studies on Juvenile
wood. II. On distribution of specific compressive strength and specificmodulus of elasticity in stem of Sugi-tree. Mokuzai Gakkaishi 10:125–130(In Japanese with English abstract)
33. Zhu J, Takata K, Iijima Y, Hirakawa Y (2003) Growth and wood qualityof Sugi (Cryptomeria japonica) planted in Akita prefecture I. Variationof some wood quality indices within tree stems. Mokuzai Gakkaishi
49:138–145 (In Japanese with English abstract)
34. Tadooka N, Koizumi A, Zhu J (2005) The variation in anatomical charac‑
teristics and mechanical properties among the stem of Japanese cedar(Cryptomeria japonica). Res Bull Hokkaido Univ For 62:15–30 (In Japanesewith English abstract)
35. Kübler H (1987) Growth stresses in trees and related wood properties. For
Prod Abstr 10:61–119
36. Wilhelmy V, Kübler H (1973) Probe for measurement of strains inside solid
bodies. Exper Mech 13:142–144
37. Wilhelmy V, Kübler H (1973) Stresses and checks in log ends from relieved
growth stresses. Wood Sci 6:136–142
38. Kübler H, Chen T (1974) How to cut tree disks without formation ofchecks. For Prod J 24(7):57–59
39. Timoshenko SP, Goodier JN (1970) Theory of elasticity, 3rd edn. McGrawHill Book Company, New York

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in pub‑lished maps and institutional affiliations.